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Electronics => Metrology => Topic started by: quarks on January 12, 2013, 04:42:41 pm

Title: Ultra Precision Reference LTZ1000
Post by: quarks on January 12, 2013, 04:42:41 pm
While waiting for the ESI gear I ordered for my resistance measuring stuff, I started to look for the best possible voltage reference.
I already have a few good DC Sources (Valhalla 2701C, Burster 6406, Knick SJ300 and a few Geller SVRs), but I do not have a "real" Voltage Reference Standard (like Fluke 732B or a Datron or any similar) to compare them to.

Besides "Josephson-Voltage-Standard" the next best still seems to be LTZ1000 and this is the reference I would like to give a try.

I read a lot of datasheets, Application Notes, Nuts&Volts posts, Bop Pease stuff, Joe Gellers info's (incl. his Patent), discussions in different blogs and many more sources.

Here are a few links I found good and worth reading (and translating):

http://www.linear.com/product/LTZ1000 (http://www.linear.com/product/LTZ1000)
http://www.maxmcarter.com/vref/ (http://www.maxmcarter.com/vref/)
http://www.febo.com/pipermail/time-nuts/2008-November/034723.html (http://www.febo.com/pipermail/time-nuts/2008-November/034723.html)
http://www.hellocq.net/forum/read.php?tid=139719 (http://www.hellocq.net/forum/read.php?tid=139719)
http://www.amobbs.com/thread-3593996-1-1.html (http://www.amobbs.com/thread-3593996-1-1.html)
http://www.eefocus.com/lymexbg2vo/blog/09-12/181353_9b971.html (http://www.eefocus.com/lymexbg2vo/blog/09-12/181353_9b971.html)

The last one shows a lot of boards, so I think of trying to get one or more, if that is possible.

Now I would be interested if anyone here in the forum has already done a LTZ1000 DIY project and is willing to share the outcome?
Or maybe one of you know where to get a well designed board or plans to do one and can offer to buy it?

thx
quarks
Title: Re: Ultra Precision Reference LTZ1000
Post by: EEVblog on January 13, 2013, 07:29:22 am
The LTZ1000 is the 7V reference used in industry standard HP3458A 8.5 digit multimeter.
Part number 03458-66509
No oven, it just sits inside the meter case on it's own board with it's circuitry.
If it's good enough for the worlds best reference multimeter used in practically every cal lab, then it's good enough for any DIY project!

Dave.
Title: Re: Ultra Precision Reference LTZ1000
Post by: (In)Sanity on January 13, 2013, 08:01:34 am
The LTZ1000 is the 7V reference used in industry standard HP3458A 8.5 digit multimeter.
Part number 03458-66509
No oven, it just sits inside the meter case on it's own board with it's circuitry.
If it's good enough for the worlds best reference multimeter used in practically every cal lab, then it's good enough for any DIY project!

Dave.

This is a very interesting piece of information.  I had no idea the LTZ1000 reference was used in the 3458A.   I have a very nice condition 3457A which I believe uses the LM399 as a reference.   I have to wonder if their is any advantage to upgrading the 3457A to use an LTZ1000 or is it's 6 1/2 (7 1/2) digit resolution would see no real advantage?  It would of course need to be re-calibrated which I'm due for anyway.  I'm always looking to improve something just for the sake of doing so.   

On another note an interesting video might be one on temperature coefficients and techniques used to stabilize and/or compensate for temperature drift.   

Jeff
Title: Re: Ultra Precision Reference LTZ1000
Post by: PA4TIM on January 13, 2013, 08:15:04 am
Dave, it depends on how you look at it, like the LM399 the LTZ1000 has a heater resistor so it is inside his own little oven.

A member of the voltnuts list has just modded his solartron 7081 and replaced the zeners with a LTZ1000 board. He also did measurements on noise and stability. He is really good on this stuff. 

Voltnuts is a "sister" of timenuts. Several members made LTZ1000 references. Jim Williams has an appnote about a sub-ppm standard and he used a LTZ1000 too. I made his design but then using a LM399 because I have no source for a LTZ1000.

I have a Fluke 332, a guildline 4 Standardcell cabinet ( will all calibration reports since production) and several other Fluke, Philips and homebuild calibrators/references. (a gift from a company that closed down theirs calibration lab) Besides that ESI resistors, two Flukes and a ESI KV devider, standard capacitors and inductors ect. And the famous, still made GR-1620 capacitance bridge that was first used by the GenRad lab self, then when it closed moved together with a staff member to a national brittisch Cal-lab and later sold to a dutch call lab and finaly ended in my lab.

Network analyses, capacitors anf voltnutting is my thing. Nice to meet an other voltnut. Blackdog, here on the forum also has a lot of knowledge about voltreferences. If i remember well also with the LTZ1000 but i think we will see him here. LTZ1000 works like a magnet too voltnuts .

I'm very interested to see your design and the result. I read the hardest part is something with setting the heater current. But some say that is not, others say it is. As far as I found out it only was a problem on some old 3458 reference boards

The things that make it hard are the parts around it. My 10V LM399 bases reference is now powered on 24/7 for months. I monitored it for months and it becomes more stable every month but still it varies 2-4 ppm and that is a reaction on temperature and humidity. So one of my next projects Will be an oven for the reference and a relative humidity and temp meter that will serve to give me those values and work as a controller for the oven. ( as soon as i find out if it is nessaserry and possible to keep humidity in the oven constant) for temp I will use one of Williams early designs when he was stil working for MIT.
Title: Re: Ultra Precision Reference LTZ1000
Post by: (In)Sanity on January 13, 2013, 08:23:46 am
The things that make it hard are the parts around it. My 10V LM399 bases reference is now powered on 24/7 for months. I monitored it for months and it becomes more stable every month but still it varies 2-4 ppm and that is a reaction on temperature and humidity. So one of my next projects Will be an oven for the reference and a relative humidity and temp meter that will serve to give me those values and work as a controller for the oven. ( as soon as i find out if it is nessaserry and possible to keep humidity in the oven constant) for temp I will use one of Williams early designs when he was stil working for MIT.

So does this mean that my 1993 3457A should be far more stable then a fresh off the shelf LM399,  or do they degrade over time?   I fear becoming a volt nut,  I already fuss about stuff being out of tolerance by 10 uV,  let alone a fraction of that. 
Title: Re: Ultra Precision Reference LTZ1000
Post by: Mechatrommer on January 13, 2013, 08:44:47 am
$40 a pop hmmm...
http://www.ebay.com.my/itm/IC-LINEAR-TO-99-LTZ1000ACH-/250886390149?pt=LH_DefaultDomain_0&hash=item3a69fe8185 (http://www.ebay.com.my/itm/IC-LINEAR-TO-99-LTZ1000ACH-/250886390149?pt=LH_DefaultDomain_0&hash=item3a69fe8185)
Title: Re: Ultra Precision Reference LTZ1000
Post by: PA4TIM on January 13, 2013, 09:16:02 am
40 dollar is rather cheap, i would not trust that. They also have grades. I bought 1027 from Farnell, they only had C grade, then I asked a sample from LT, the sended me one, B grade. Allready a few ppm better but they have A grade too. But no clue how to get one if you are not named Agilent or so.

Every reference needs to be "burned" in a good brand meter uses burned in references, it takes about 1000 hours. The yearly calibration will show you the behaviour. Then there will be a moment the changes between calibration become very small and you can extend the time between them. So for hobby use a nice aged calibrated meter with call history is most times good for many years of fun without the need for calibration. A friend bought a 3457, had it calibrated ( in the states that is very affortable, here is is allmost the pricec of a new meter) and it was still spot on. My Prema came with a cal report ( just cal, no adjustments made) . All measurement were spot on. 10V was 10.000,000V so i know it is allready stable and little chance in changing much in time. My new Keitley 2000 now changed around 10 uV in two years. But most of that happend the first months.
Title: Re: Ultra Precision Reference LTZ1000
Post by: SeanB on January 13, 2013, 09:29:11 am
Look for used ones, and buy a dozen or so. Then you can sort them into 3 groups: dead, working but drift, and good.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on January 13, 2013, 10:06:06 am
The LTZ1000 is the 7V reference used in industry standard HP3458A 8.5 digit multimeter.
Part number 03458-66509

Hi Dave,
thanks for your reply. I knew that and I can add that Fluke also had a special version of it, called HP 3458A/HFL, where I believe they only changed one resistor on the LTZ1000 board to lower the heater temp but resulted in even better specs than the original HP meter.

About the part number, do you know or think that it is possible to order this as a spare part when you do not have the 3458 DMM?

thx
quarks
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on January 13, 2013, 10:37:32 am
Dave, it depends on how you look at it, like the LM399 the LTZ1000 has a heater resistor so it is inside his own little oven.

A member of the voltnuts list has just modded his solartron 7081 and replaced the zeners with a LTZ1000 board. He also did measurements on noise and stability. He is really good on this stuff. 

Voltnuts is a "sister" of timenuts. Several members made LTZ1000 references. Jim Williams has an appnote about a sub-ppm standard and he used a LTZ1000 too. I made his design but then using a LM399 because I have no source for a LTZ1000.

I have a Fluke 332, a guildline 4 Standardcell cabinet ( will all calibration reports since production) and several other Fluke, Philips and homebuild calibrators/references. (a gift from a company that closed down theirs calibration lab) Besides that ESI resistors, two Flukes and a ESI KV devider, standard capacitors and inductors ect. And the famous, still made GR-1620 capacitance bridge that was first used by the GenRad lab self, then when it closed moved together with a staff member to a national brittisch Cal-lab and later sold to a dutch call lab and finaly ended in my lab.

Network analyses, capacitors anf voltnutting is my thing. Nice to meet an other voltnut. Blackdog, here on the forum also has a lot of knowledge about voltreferences. If i remember well also with the LTZ1000 but i think we will see him here. LTZ1000 works like a magnet too voltnuts .

I'm very interested to see your design and the result. I read the hardest part is something with setting the heater current. But some say that is not, others say it is. As far as I found out it only was a problem on some old 3458 reference boards

The things that make it hard are the parts around it. My 10V LM399 bases reference is now powered on 24/7 for months. I monitored it for months and it becomes more stable every month but still it varies 2-4 ppm and that is a reaction on temperature and humidity. So one of my next projects Will be an oven for the reference and a relative humidity and temp meter that will serve to give me those values and work as a controller for the oven. ( as soon as i find out if it is nessaserry and possible to keep humidity in the oven constant) for temp I will use one of Williams early designs when he was stil working for MIT.

Do you remember the name of the voltsnuts guy or can give a hint how to identify him? I also read there from people planing transplating LTZ1000 into Valhalla, so that could be the next interesting project.

About the Jim Williams article, was it AN86 or the designfeature at edn you mean?  I think I have read most (if not all) of the relevant info's. It is so much theory that I feel I now have go practical to learn and understand it better.   

About your gear, you are a lucky man and now I can proof to my wife that I am not the craziest nerd on the planet  :-+ That helps a lot to get even more goodies.

thx
quarks

Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on January 13, 2013, 10:48:11 am
$40 a pop hmmm...
http://www.ebay.com.my/itm/IC-LINEAR-TO-99-LTZ1000ACH-/250886390149?pt=LH_DefaultDomain_0&hash=item3a69fe8185 (http://www.ebay.com.my/itm/IC-LINEAR-TO-99-LTZ1000ACH-/250886390149?pt=LH_DefaultDomain_0&hash=item3a69fe8185)

thx for the link. Have you tried this source? My feeling is unless one can confirm it is no fake I tend to stay away.

The min. order of 250$ and the price of 54.50$ (QTY 1-99) at Linear Technology is ok. But before that, I try to find a local (German/Europe) source to avoid shipping + tax + custom (which will easily double the sellers price). If anyone knows where to order, please share.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on January 13, 2013, 10:51:35 am
Look for used ones, and buy a dozen or so. Then you can sort them into 3 groups: dead, working but drift, and good.

do you know of a trustworthy source to buy from?

thx
quarks
Title: Re: Ultra Precision Reference LTZ1000
Post by: muvideo on January 13, 2013, 01:40:22 pm
A member of the voltnuts list has just modded his solartron 7081 and replaced the zeners with a LTZ1000 board. He also did measurements on noise and stability. He is really good on this stuff. 

Do you remember the name of the voltsnuts guy or can give a hint how to identify him? I also read there from people planing transplating LTZ1000 into Valhalla, so that could be the next interesting project.

I think Fred is referring to Mickle T., see:
http://www.febo.com/pipermail/volt-nuts/2013-January/002322.html (http://www.febo.com/pipermail/volt-nuts/2013-January/002322.html)
He posted also other articles before,
for Mickle the 7081 is a buggy imperfect machine,
so he decided to make it better  :o
Hats off to him!

For the little I undertand, the reference itself
is only a part of the things needed to have e
stable voltage reference, that is an assembly
that has to be characterized as a whole.
Just see all the crazyness that goes to the
selection of the right resistors,
Fred can give many info about these things.

My Prema came with a cal report ( just cal, no adjustments made) . All measurement were spot on. 10V was 10.000,000V so i know it is allready stable and little chance in changing much in time.

I'm so curious about these meters, what model
do you have? Do you have some docs like spec
sheer and or manuals? How does it perform?
If you can post some pics ( a teardown maybe?)
I think it will be the first I've seen on the net.

Thank you
Fabio.
Title: Re: Ultra Precision Reference LTZ1000
Post by: jnd on January 13, 2013, 01:51:24 pm
40 dollar is rather cheap, i would not trust that. They also have grades. I bought 1027 from Farnell, they only had C grade, then I asked a sample from LT, the sended me one, B grade. Allready a few ppm better but they have A grade too. But no clue how to get one if you are not named Agilent or so.

Every reference needs to be "burned" in a good brand meter uses burned in references, it takes about 1000 hours. The yearly calibration will show you the behaviour. Then there will be a moment the changes between calibration become very small and you can extend the time between them. So for hobby use a nice aged calibrated meter with call history is most times good for many years of fun without the need for calibration. A friend bought a 3457, had it calibrated ( in the states that is very affortable, here is is allmost the pricec of a new meter) and it was still spot on. My Prema came with a cal report ( just cal, no adjustments made) . All measurement were spot on. 10V was 10.000,000V so i know it is allready stable and little chance in changing much in time. My new Keitley 2000 now changed around 10 uV in two years. But most of that happend the first months.
I wouldn't say $40 is cheap enough to be not trustworthy, it's comparable to new prices from Linear and Chinese always get them for better price. I just checked Taobao and there are lot of them for around $20.

Right now there are 4 sellers of LTZ1000 on eBay. Two of them, Polida and yankee_electronic claim new and use the same generic pic from web. Polida is just bulk part seller, I've seen few comments that people got fake parts but can't say for sure, I'd say both of them are questionable as they don't even have the real photo. Other two, hifi-szjxic and bbshonic, sell used and they claim 100% tested. bbshonic even "Gurantee exchange if it is fault" and have some history of selling them. However I didn't find any feedback for any of those sales.

For myself I'd rather buy used and burned in, working parts since I don't have any instrument which could be used to track any changes in the reference. I guess someone will have to try and report back what they got, so who's first? >:D

For the board, I haven't seen any for sale, only the ridiculously priced meter boards like Dave showed. Ideally I want some kit or finished board like what's been on the Chinese forums because getting all the precision parts around is single quantities must be PITA. So if anyone wants to make small run, I'm interested for one or two.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on January 13, 2013, 02:41:17 pm
I think Fred is referring to Mickle T., see:
http://www.febo.com/pipermail/volt-nuts/2013-January/002322.html (http://www.febo.com/pipermail/volt-nuts/2013-January/002322.html)
He posted also other articles before,
for Mickle the 7081 is a buggy imperfect machine,
so he decided to make it better  :o
Hats off to him!
thx for the link.

About Prema, it is a German company located in Mainz (nearby where I live) but unfortunately no longer produce any DMMs.

I don't know the model you are talking about, but here are some hopefully interesting (but German) links:
http://www.amplifier.cd/Test_Equipment/other/Prema-DMM.html (http://www.amplifier.cd/Test_Equipment/other/Prema-DMM.html)
http://www.amplifier.cd/Test_Equipment/other/Prema-5000.html (http://www.amplifier.cd/Test_Equipment/other/Prema-5000.html)
http://www.amplifier.cd/Test_Equipment/other/Prema-5017.html (http://www.amplifier.cd/Test_Equipment/other/Prema-5017.html)
http://ohh.de/homepage.htm (http://ohh.de/homepage.htm)
http://ohh.de/5017.htm (http://ohh.de/5017.htm)
http://ohh.de/6048.htm (http://ohh.de/6048.htm)
http://ohh.de/8017.htm (http://ohh.de/8017.htm)

I just checked my archive I only have found Prema 5000, 5017 and 6040 documents (all in German).
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on January 13, 2013, 03:04:18 pm
For myself I'd rather buy used and burned in, working parts since I don't have any instrument which could be used to track any changes in the reference. I guess someone will have to try and report back what they got, so who's first? >:D

For the board, I haven't seen any for sale, only the ridiculously priced meter boards like Dave showed. Ideally I want some kit or finished board like what's been on the Chinese forums because getting all the precision parts around is single quantities must be PITA. So if anyone wants to make small run, I'm interested for one or two.

I still hope someone here in the forum has already gone through this and is willing to share. For myself I would buy all the needed components (also more than I need for myself, to later on share) but I will probably not even try to do my own board, because my last layout is maybe from 30 years ago and was not as professional as I want this one to be.
Title: Re: Ultra Precision Reference LTZ1000
Post by: TorqueRanger on January 13, 2013, 03:19:33 pm
For myself I'd rather buy used and burned in, working parts since I don't have any instrument which could be used to track any changes in the reference. I guess someone will have to try and report back what they got, so who's first? >:D

For the board, I haven't seen any for sale, only the ridiculously priced meter boards like Dave showed. Ideally I want some kit or finished board like what's been on the Chinese forums because getting all the precision parts around is single quantities must be PITA. So if anyone wants to make small run, I'm interested for one or two.

I still hope someone here in the forum has already gone through this and is willing to share. For myself I would buy all the needed components (also more than I need for myself, to later on share) but I will probably not even try to do my own board, because my last layout is maybe from 30 years ago and was not as professional as I want this one to be.

Do me a favor and just google LTZ1000 and look at all the pics...
Title: Re: Ultra Precision Reference LTZ1000
Post by: alm on January 13, 2013, 03:25:45 pm
This is a very interesting piece of information.  I had no idea the LTZ1000 reference was used in the 3458A.   I have a very nice condition 3457A which I believe uses the LM399 as a reference.   I have to wonder if their is any advantage to upgrading the 3457A to use an LTZ1000 or is it's 6 1/2 (7 1/2) digit resolution would see no real advantage?  It would of course need to be re-calibrated which I'm due for anyway.  I'm always looking to improve something just for the sake of doing so.   
The LTZ1000 is also used in various voltage references like the Fluke 732B and some Datron references. Note that HP ran the LTZ1000 at a very high temperature in the HP 3458A, which made the drift much higher than in voltage reference applications. Much lower drift could be achieved by lowering this temperature. Downside is smaller operating temperature range.

The accuracy of a a DMM is not just determined by the internal reference. It also depends on other factors like ADC linearity and the input resistor network drift. Note that the HP 3456a has better 90 day and 1y accuracy specs than the HP 3457a, even though they share the same selected LMx99 reference.

So does this mean that my 1993 3457A should be far more stable then a fresh off the shelf LM399,  or do they degrade over time?   I fear becoming a volt nut,  I already fuss about stuff being out of tolerance by 10 uV,  let alone a fraction of that. 
As some people already mentioned, the majority of the drift is in the first year or so. The reference should be much more stable than when it was produced. Note that HP would also have done a far amount of burn in and selection.

For the board, I haven't seen any for sale, only the ridiculously priced meter boards like Dave showed. Ideally I want some kit or finished board like what's been on the Chinese forums because getting all the precision parts around is single quantities must be PITA. So if anyone wants to make small run, I'm interested for one or two.
Someone used to be selling 3458a reference boards for around the $100 mark, probably from some surplus source. Haven't seen these for a while, though. PCB design (eg. guarding) is going to be critical, leakage currents can easily induce errors at the single ppm level that these circuits are operating.
Title: Re: Ultra Precision Reference LTZ1000
Post by: muvideo on January 13, 2013, 04:11:16 pm
About Prema, it is a German company located in Mainz (nearby where I live) but unfortunately no longer produce any DMMs.

I don't know the model you are talking about, but here are some hopefully interesting (but German) links:
http://www.amplifier.cd/Test_Equipment/other/Prema-DMM.html (http://www.amplifier.cd/Test_Equipment/other/Prema-DMM.html)
http://www.amplifier.cd/Test_Equipment/other/Prema-5000.html (http://www.amplifier.cd/Test_Equipment/other/Prema-5000.html)
http://www.amplifier.cd/Test_Equipment/other/Prema-5017.html (http://www.amplifier.cd/Test_Equipment/other/Prema-5017.html)
http://ohh.de/homepage.htm (http://ohh.de/homepage.htm)
http://ohh.de/5017.htm (http://ohh.de/5017.htm)
http://ohh.de/6048.htm (http://ohh.de/6048.htm)
http://ohh.de/8017.htm (http://ohh.de/8017.htm)

I just checked my archive I only have found Prema 5000, 5017 and 6040 documents (all in German).

Thank you for the links, I dont read German, but the web pages
can be translated by online translator, I previously have stumbled
in amplifier.cd  page about prema multimeters, but I forgot about that.
I found also PA4TIM's excellent page about his meter:
http://www.pa4tim.nl/?p=3624 (http://www.pa4tim.nl/?p=3624)

Fabio.
Title: Re: Ultra Precision Reference LTZ1000
Post by: saturation on January 13, 2013, 04:16:40 pm
There's a lot of discussion about the LTZ1000 in the archives.  One summarized some the work of this Chinese group, making, analyzing and dissecting the LTZ in an attempt to create a low cost improved reference, here one photo of an ambitious multiboard reference with an LTZ each.  The Chinese site also analyzed the 3458a reference board.

(http://[url=http://www.hellocq.net/forum/attachment/vbb/5/9/4/4/209071.attach?0]http://www.hellocq.net/forum/attachment/vbb/5/9/4/4/209071.attach?0[/url])

It spans 2006-2010, and the thread is 50+ pages.  Lots of photos and data sheets.

http://www.hellocq.net/forum/read.php?tid=139719&page=6 (http://www.hellocq.net/forum/read.php?tid=139719&page=6)
Title: Re: Ultra Precision Reference LTZ1000
Post by: PA4TIM on January 13, 2013, 04:57:40 pm
http://www.pa4tim.nl/?p=3624. (http://www.pa4tim.nl/?p=3624.) Prema 5017 7.5 digit benchmeter. Very solid build. The meter does not switch of totally uing the front powerbutton, the reference stays powered.
Teardowns not on this kind a stuff, only if i have to open it because of repairs. I do not like more dirt and dust inside just for pictures ;-)

I see the link to the modified 7081 allready is given by fellow Voltnut Fabio.

I have a Solartron 7061. I made some modifications too. I have not replaced the reference diode, if I remember well Fabio has done some experimenting in biasing it for better tempco. I just did a check up according the manual. And then I mounted a heatsinks on the outside with a fan on it. I made a temperature controller to drive the fan. The sensor is mounted inside de meter and the fan spins slow around 18 degrees roomtemp, speeding up if roomtemperature changes. It keeps the inside of the meter ( at the measure point) at 39 degrees.

The biggest problem with voltnutting is environment. My lab changes temp from about 17 to 33 degrees over a year and relative humidity from 20 to 40 %
If you want to keep everything tracking you need climate control. But that is not easy and i think costs a lot. Those of you in a all year long hot country probably have airco so maybe it then is possible.

I did not know the appnote number from the Williams reference , it is in my book Analog circuit design part 1, but i looked it up for you, according the book It should be number 86 a standards lab grade 20 bit DAC with 0.1 ppm/C drift. Look at the picture of the setup, a bunch of 3458, two KV dividers and his reference, that is a very expensive table.

My LM399 is made this way. I also used two KV's and the bufferamp. But I made a 10 turn potentiometer on the front that gives me about 800 uV. This for easy adjusting abd keep my KVs free for use with the 332. ( one for deviding, the second for checking)  This pot is the weakest part. The tempco is probably horrible. And mist of the drigpft at this time probably comes from this. If i am going to make the oven for it, the portentiometer will be changed.  I will make is so that 10 turns give me about  100 uV in that case tempco will not do much because one full turn (1K) will give 10 uV so if the tempco is 1000 ppm the resistance of the 10K pot will change 10 Ohm in total. So maybe this gives a 0.1uV fault / degree ( if i do the math correct, i have dyscalculus and no calculator at hand right now)
On the other hand if I  ditch the pot I maybe do not need the oven. Resistors I use are 0.05 to 0.01 % from dead a Fluke 8500 benchmeter.  There is guarding, i avoided mechanical stress ( the lm399 anf LTC1052 are mounted on sockets but without the ( leaking and dielectric) plastic. So no thermal stress to the components. After this everything is cleaned with ipa.
Only problem i have to solve is EMC ( if i power down everything except the prema or solartron and the LM or 332 or whatever and use a KV i get a nice linear behavour. So 1uV is 1uV on the meter and 10V is 10V. But if I leave the lights on ( halogene on a rail) everything goed wrong. There is about 150 uV deviation over 10V  in that case. And the strange thing the readings go down. It dies not matter if I use the 332, LM or whatever. The meter straight on the 332 and using its own divider gives no problem so it must be the cables from source to KV and from KV to meter ( but shielded, twisted, coaxial, triax ect all give the same problem.

Welcom to the darkside of voltnutting ;-)

Title: Re: Ultra Precision Reference LTZ1000
Post by: (In)Sanity on January 13, 2013, 05:17:03 pm
Thanks Alm for the replies.

This is a game I would love to get in to.   I thought about just getting a high grade LM399,  aging it for a long time and taking a measurement with my 3457A.   The idea being that the vref alone without any other components in play should in theory drift less then the combined circuit in my 3457.  I then of course run in to the issue of which one really drifted or did both,  etc.   It seams like you need 3 or 4 references to really figure out what's going on.  Also as I mentioned before a video on how all of this works and techniques to help compensate for it might be interesting.   Items like adding semiconductors in the right place to compensate for the drift of the resistors..etc.  I have a home brew power supply right now that's fairly stable,  it's only stable because the cheap op-amp just so happens to drift in the opposite direction as the cheap resistors,  etc.   I guess you could also say I just got lucky. 

So is a low grade LTZ1000 better then a high grade LM399 ?   

Jeff
Title: Re: Ultra Precision Reference LTZ1000
Post by: muvideo on January 13, 2013, 05:43:05 pm
I see the link to the modified 7081 allready is given by fellow Voltnut Fabio.

I have a Solartron 7061. I made some modifications too. I have not replaced the reference diode, if I remember well Fabio has done some experimenting in biasing it for better tempco. I just did a check up according the manual. And then I

Thank you for the onorific title, but I'm only a VoltNut
follower :) and unfortunately nothing to say about the
7061 stability, my lab is worse than yours and I'd like to
have at least another meter with similar or better specs
before trying anything on reference.

The biggest problem with voltnutting is environment. My lab changes temp from about 17 to 33 degrees over a year and relative humidity from 20 to 40 %
If you want to keep everything tracking you need climate control. But that is not easy and i think costs a lot. Those of you in a all year long hot country probably have airco so maybe it then is possible.

I throw in another element :)
I still dont have experience in building precision references,
it's a time (and money) consuming task, but one can learn
a lot. My question is if it's reasonable to build a temperature
and humidity controlled enclosure for the reference.
Once chosen the right components, and defined the layout
(quite a result, not easy ), the next thing to do should be
the charachterization, i.e. log the aging behaviour over time.
AFAIK the worse offenders remaining that screw up the
stability are temperature and humidity. Build an ovenized
enclosure can be done (and it's been done). I have never
seen a humidity controlled enclosure yet, but I've seen
around some solid state dehumidifiers, nothing cheap
unfortunately (google SPE dehumidifer).
Maybe that can compensate for poor lab enviroments :)

Fabio.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on January 13, 2013, 05:45:26 pm
There's a lot of discussion about the LTZ1000 in the archives.  One summarized some the work of this Chinese group, making, analyzing and dissecting the LTZ in an attempt to create a low cost improved reference, here one photo of an ambitious multiboard reference with an LTZ each.  The Chinese site also analyzed the 3458a reference board.

(http://[url=http://www.hellocq.net/forum/attachment/vbb/5/9/4/4/209071.attach?0]http://www.hellocq.net/forum/attachment/vbb/5/9/4/4/209071.attach?0[/url])

It spans 2006-2010, and the thread is 50+ pages.  Lots of photos and data sheets.

http://www.hellocq.net/forum/read.php?tid=139719&page=6 (http://www.hellocq.net/forum/read.php?tid=139719&page=6)

thx saturation,
your picture is missing but think I know which one you mean.
I have read all of this quite a while ago and I think you where the one who posted the link back then.
Title: Re: Ultra Precision Reference LTZ1000
Post by: (In)Sanity on January 13, 2013, 05:50:55 pm
Stupid question perhaps,   but is it possible to run one of these references inside of a vacuum to perhaps eliminate humidity from the picture?   Again it might be a stupid question.

Jeff
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on January 13, 2013, 05:54:32 pm
http://www.pa4tim.nl/?p=3624. (http://www.pa4tim.nl/?p=3624.) Prema 5017 7.5 digit benchmeter. Very solid build. The meter does not switch of totally uing the front powerbutton, the reference stays powered.

your website is just great I have walked through it several times already and always find new and interesting details.
Also the translation on the site works quite good for me.

Thx a lot

edit: I just saw this http://www.pa4tim.nl/?p=2733 (http://www.pa4tim.nl/?p=2733) so you are my man.
In another post I asked for infos/help https://www.eevblog.com/forum/reviews/esi-resitance-standard-sr1010-and-standard-resistor-sr104/msg171260/#msg171260 (https://www.eevblog.com/forum/reviews/esi-resitance-standard-sr1010-and-standard-resistor-sr104/msg171260/#msg171260) but unfortunately had no reply. And now I see you have the same Set ESI SR1010 (also the same values 10 Ohm to 100k).
Title: Re: Ultra Precision Reference LTZ1000
Post by: UPI on January 13, 2013, 05:59:24 pm
Reminds me of this relatively quick/inexpensive idea:

http://www.romanblack.com/xoven.htm (http://www.romanblack.com/xoven.htm)
Title: Re: Ultra Precision Reference LTZ1000
Post by: SeanB on January 13, 2013, 06:03:57 pm
Not a vacuum, but in a sealed enclosure with a large sachet of silica gel inside to adsorb water. Heat it up in an oven to dry it out well and then place in the dry enclosure hot and then seal it up while it is still hot. Then it will make a very low humidity interior. you will need to have IP68 rated connectors in the box, and preferably a smaller inner box to hold the reference in an insulated place. You can have heaters in the outer box to keep it at a stable temperature, and this will keep the inner stable. All you need on the outer is to keep it to within 2C, as the inner insulation will attenuate the variances. I have seen this ( without the dessicant) in an appnote to get a inner box to be temperature stable to within millidegrees.
Title: Re: Ultra Precision Reference LTZ1000
Post by: chrome on January 13, 2013, 06:08:32 pm
Can anyone tell my why 6.95V?

It seems like such an arbitrary value.

I'm sort of assuming it's inherit with the design maybe?
Title: Re: Ultra Precision Reference LTZ1000
Post by: SeanB on January 13, 2013, 06:11:58 pm
Reminds me of this relatively quick/inexpensive idea:

http://www.romanblack.com/xoven.htm (http://www.romanblack.com/xoven.htm)

I did that years ago, and they work well.

6.95V as it is a stable point where the voltage is not quite avalance breakdown and not quite zener action. Lower and higher voltages have drift with temperature, this is a sweet spot in the characteristic. Often you get a 6V2 zener diode internally in series with a silicon diode to make a 6V8 unit, as this is also very stable as the tempco's of the zener and the diode are nearly the same but of opposite direction so they cancel out nearly totally.
Title: Re: Ultra Precision Reference LTZ1000
Post by: (In)Sanity on January 13, 2013, 06:20:15 pm
Thanks for the replies about the vacuum,  Potting I assume would be subject to moisture absorption and too slow to respond to temperature changes or corrections ?  So if it got cold for example it would take far to long to stabilize and perhaps have too much hysteresis?

Also which would be better for a 10.0000 volt reference,   a 6.95 and an op-amp or two 6.95's and a voltage divider ?   Which would be more stable ?

I'm full of questions at the moment.

Thanks,

Jeff
Title: Re: Ultra Precision Reference LTZ1000
Post by: SeanB on January 13, 2013, 06:34:37 pm
Potting would also stress the references. the biggesst issue is the resistors you use, not the opamp. You can get an opamp that is chopper stabilised, which removes it's own drift, but the resistor drift determines the stability. Better to use a single reference and multiply it up to 10V, one less source of error.
Title: Re: Ultra Precision Reference LTZ1000
Post by: (In)Sanity on January 13, 2013, 06:53:08 pm
Potting would also stress the references. the biggesst issue is the resistors you use, not the opamp. You can get an opamp that is chopper stabilised, which removes it's own drift, but the resistor drift determines the stability. Better to use a single reference and multiply it up to 10V, one less source of error.

Thanks Sean,   that's what I was wondering.   I was looking to see which would be the lesser of the two evils.  To build one of these references correctly appears to be a very expensive endeavor with $90 references, $28 resistors and an equally expensive trim pot which should be avoided if possible,  ohh and let's not forget the chopper amp.   I may still pick up an LM399 just for experimentation sake and not breaking the bank.  I'm fortunate that I have a full finished basement and the temperature down here stays pretty constant relative to the rest of the house. So my 3457A also stays pretty stable.  I'm pretty good with long term projects,  so this could be fun.

Jeff
Title: Re: Ultra Precision Reference LTZ1000
Post by: SeanB on January 13, 2013, 07:04:24 pm
Often the resistors are selected to be close, then a variable is used to get the final adjustment and is replaced by a fixed value. Leads to using Vishay foil on glass resistors in series and with final trim being by soldering a few low TC metal film resistors and then cutting resistor leads to trim up to the right value.
Title: Re: Ultra Precision Reference LTZ1000
Post by: PA4TIM on January 13, 2013, 07:11:14 pm
http://www.pa4tim.nl/?p=2531 (http://www.pa4tim.nl/?p=2531) my LM399 ( the last one at the bottom, all my reference projecs are on this page)

Quarks: i have the ESIs but I did no stability tests with them. If i use them to calibrate I measure all off thecterminals and then take the avarage. Then measure all in serie and that must be the same value as the 12 separate. I have the bar that sets them parallel and I'm told this is enhances the precision by averaging out the differences.

Mine all measure very constant. No significant differences between terminals of one unit.
Problem of monitoring drift, tempco ect ( Vrefs, resistors, capacitors ect) is you never know what is drifting, the DUT or the instrument or both  :)

Is it because australia is very far away or is it just me. This forum is very slow, it takes ages for pages to open, before I can type comments and many notifications never seem to arrive. ( do I made wrong settings 's ? )
It would be great is i could set notifications on permanent z
Title: Re: Ultra Precision Reference LTZ1000
Post by: SeanB on January 13, 2013, 07:15:14 pm
Server is in the USA. They must be awake and watching movies............
Title: Re: Ultra Precision Reference LTZ1000
Post by: (In)Sanity on January 13, 2013, 07:55:27 pm
Server is in the USA. They must be awake and watching movies............

Haaa,  the only movies I watch are electronics related for the most part.  Sadly I can't stream anything in HD anymore,  even though I have a 20 Meg connection.  Everything is going to crap lately..but that's another topic.

Thanks for the information about not using a trim pot.  I kind of knew that in the back of my head,  but somehow was in denial about it actually being done that way.   I wonder if anyone has made any good references by balancing out the positive and negative temperature coefficients of the devices in the circuit?   I know most DMM's and such have a thermistor or ten to obtain somewhat of this behavior.  I guess it would be no better then just using an oven no matter which way you rolled it.   Perhaps a combination of the methods.   I'm such a newb at temperature stability issues.   

Also can't everything be corrected in software :)

Jeff
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on January 13, 2013, 08:14:50 pm
http://www.pa4tim.nl/?p=2531 (http://www.pa4tim.nl/?p=2531) my LM399 ( the last one at the bottom, all my reference projecs are on this page)

Quarks: i have the ESIs but I did no stability tests with them. If i use them to calibrate I measure all off thecterminals and then take the avarage. Then measure all in serie and that must be the same value as the 12 separate. I have the bar that sets them parallel and I'm told this is enhances the precision by averaging out the differences.

Mine all measure very constant. No significant differences between terminals of one unit.
Problem of monitoring drift, tempco ect ( Vrefs, resistors, capacitors ect) is you never know what is drifting, the DUT or the instrument or both  :)
Thx again,
I do also have  the optional parallel, serial-parallel compensation networks and the shorting bars and have measured allmost all possible  combinations.
What I am still interested in, is how resistance transfer works. If you could share some knowledge that would be great. But this is OT here  and I can, if there is any interest, update the other post with more details.
Tomorrow I will also pick up some more related ESI gear from custom.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Conrad Hoffman on January 15, 2013, 06:02:34 pm
FWIW, my Analogic 8200 6 1/2 digit voltage/current reference uses the LM399 and it's remarkably good. If you can find the circuit on-line, it uses analog switches to change the output from a master divider chain and is definitely worth studying.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on January 15, 2013, 06:31:41 pm
Hello Conrad,
analog switching sounds interesting and with LM199/299/399 I remember a Bop Pease article where he used multiple of them together. I will definitely try to have a look at it.

A first quick search for a Analogic 8200 schematic unfortunately showed no result for manual and service manual.
But the search lead to
https://www.eevblog.com/forum/projects/kelvin-varley-divider-(and-precision-voltage-source)/msg71081/?PHPSESSID=c8a26b98cc233fa3e62a0ec3c9f47bb1#msg71081 (https://www.eevblog.com/forum/projects/kelvin-varley-divider-(and-precision-voltage-source)/msg71081/?PHPSESSID=c8a26b98cc233fa3e62a0ec3c9f47bb1#msg71081)
so I tried also Data Precision also with no luck. Do you maybe have a link or can post a photo/scan?

thx
quarks
Title: Re: Ultra Precision Reference LTZ1000
Post by: muvideo on January 15, 2013, 06:46:38 pm
quarks:
http://www.ko4bb.com/Manuals/09%29_Misc_Test_Equipment/DataPrecision_8200_6.5_Digit_Calibrator_Service_Manual.pdf (http://www.ko4bb.com/Manuals/09%29_Misc_Test_Equipment/DataPrecision_8200_6.5_Digit_Calibrator_Service_Manual.pdf)

I'm reading it now, it's a wonderful manual, almost an application note :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: robrenz on January 15, 2013, 06:46:48 pm
I believe this (http://www.google.com/url?sa=t&rct=j&q=&esrc=s&frm=1&source=web&cd=1&cad=rja&ved=0CDYQFjAA&url=http%3A%2F%2Fwww.ko4bb.com%2FManuals%2F09)_Misc_Test_Equipment%2FDataPrecision_8200_6.5_Digit_Calibrator_Service_Manual.pdf&ei=XaP1ULKJEbK00AG7n4GoBQ&usg=AFQjCNHrdJ-98Kdm5OYIdMycj7Wo1IVtJg&sig2=dZ2zElyh6XqcawXVnk1lnQ) is it

Edit: Sorry muvideo I was too slow
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on January 15, 2013, 06:58:05 pm
great, thx a lot
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on January 17, 2013, 01:40:36 pm
in the Wavetek 1271 I just saw, they use a LM399 and a LTZ1000 Board
Title: Re: Ultra Precision Reference LTZ1000
Post by: Mechatrommer on January 17, 2013, 02:06:43 pm
$40 a pop hmmm...
thx for the link. Have you tried this source? My feeling is unless one can confirm it is no fake I tend to stay away.
yup if you cant trust it then be pleased to stay away. i havent tried it myself and perharps wont be in near future. verifying ultra ultra thing is not easy. imagine you have 6.95V very stable nvm verification. and then what? reducing it to useful value using a divider? the resistors must be equally good. and then what? ADC? DAC? etc, as some people already highlighted it, all of them must be equally good. so you add up the cost to what? nvm the cost of time and pcb prototype. i'm not trying to kill some hope here but, analog is a kick arse thing its not suitable for a person esp like me (i'm thinking of regretting it :(), $40 is expensive already for me there must be reasons why those high end stuff cost multi $K. i believe i have no point here, just talking. but the most important part is... YMMV.
Title: Re: Ultra Precision Reference LTZ1000
Post by: free_electron on January 17, 2013, 03:11:39 pm
I believe bob pease had a circuit using a few lm399s to make a nanovolt reference.
Title: Re: Ultra Precision Reference LTZ1000
Post by: PA4TIM on January 17, 2013, 05:01:32 pm
The Czar of bandgap references himself, that must be a decent design.
Do you know where it was publiced ?
Title: Re: Ultra Precision Reference LTZ1000
Post by: muvideo on January 17, 2013, 05:35:05 pm
I remember only one of his "What’s All This ___ Stuff, Anyhow?" series,
this one: http://electronicdesign.com/archive/what-s-all-long-term-stability-stuff-anyhow (http://electronicdesign.com/archive/what-s-all-long-term-stability-stuff-anyhow)
But I'd like to know if there is some more in-depth article...

Fabio.
Title: Re: Ultra Precision Reference LTZ1000
Post by: free_electron on January 17, 2013, 06:34:08 pm
i think it's in one of his Bob Pease Show video's... hang on ...

Precision Current Source (https://www.youtube.com/watch?v=2N6cjGS7lUE#)

or it may be in his book analog troubleshooting. he uses like 10 of em ...
Title: Re: Ultra Precision Reference LTZ1000
Post by: jnd on January 18, 2013, 12:16:41 am
i think it's in one of his Bob Pease Show video's... hang on ...

Precision Current Source (https://www.youtube.com/watch?v=2N6cjGS7lUE#)

or it may be in his book analog troubleshooting. he uses like 10 of em ...
I saw this one just week or two ago, there is one LM399. BTW TI's channel on Youtube have them reuploaded in better quality (I tried to search there for "anyhow") which is interesting. The new version is here, it's still pretty relevant to the precision talk here: Whats All This Current Source Stuff, Anyhow? (https://www.youtube.com/watch?v=Sv21wD3FRmY#)
Title: Re: Ultra Precision Reference LTZ1000
Post by: nukie on January 18, 2013, 01:57:02 am
These are 3 of my LTZ1000 references and geller svr 5V. Along with two Motorola SZA263 used in the Fluke 8840a and Fluke 731B voltage standard and Fluke 732A reference standard. Still waiting for free time to attack those SZA263 with solder iron. Please note, without high precision supporting passive components these parts are as good as nothing.

(http://img232.imageshack.us/img232/9430/77451258.jpg)
(http://img36.imageshack.us/img36/8523/83517661.jpg)

You can read more about it here
http://home.51.com/jj3055/diary/item/10053954.html (http://home.51.com/jj3055/diary/item/10053954.html)

Schematics here
http://bbs.38hot.net/read-htm-tid-36472.html (http://bbs.38hot.net/read-htm-tid-36472.html)

And finally here how to convert 7V to 10V
http://www.crystalradio.cn/thread-229749-1-1.html (http://www.crystalradio.cn/thread-229749-1-1.html)
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on January 18, 2013, 08:19:42 am
@ Mechatrommer, thx for your reply. I know it will be hard to get all the ideal parts but I at least want to try and find out.

@ nukie, that looks very promising. What you already have is exactly what I thinking to do. thx a lot for sharing.
I will read through your links and come back to you.
Title: Re: Ultra Precision Reference LTZ1000
Post by: PA4TIM on January 18, 2013, 08:59:53 am
Hmm, i think I am not good enough in Chinese (or Japanese ?)  to read the links from nukie.
Title: Re: Ultra Precision Reference LTZ1000
Post by: nukie on January 18, 2013, 09:35:57 am
Use Google Chrome there's auto translate. I can't read Chinese but the schematics and diagrams teaches a lot.


Title: Re: Ultra Precision Reference LTZ1000
Post by: muvideo on January 18, 2013, 09:50:57 am
nukie, the LTZ1000 is pretty costly, probably it
would be fun and cheaper form me to experiment
with cheaper references like the lm399, that will
also set a starting point to make comparisons.

What do you think about the SZA263?
How do they compare with LM399? I've seen them (the 263)
used in old fluke standards, and meters, there are
the schematics online, but it seems that
the 399 is simpler to use, am I correct?
Is there any cheap source for SZA263 references,
other than dismantling old fluke meters?

Thanks,
Fabio.
Title: Re: Ultra Precision Reference LTZ1000
Post by: nukie on January 18, 2013, 10:16:48 am
Most sza263 you can find today are mostly reclaimed from recycling Flukes. You can contact Fluke to see if they stock any surplus, they do it for military program maybe?

As for LM399 vs 263, I would settle for the LM399, its widely used and lots of example around. I bought 2 more than 15 months ago and has been burning in since, performance seems to be pretty stable after 4000hours. I am planning a 5x LM399 in parallel when I have more time.

The LTZ1000 I have are also from used equipment. They are sourced directly from a electronics recycling market in China. There are people who are willing to go out and source for the right equipment that carries the specific component.   The components are removed carefully and resold. They are not expensive. Same case with the foil resistors.

There are fake LTZ1000 floating around even the Chinese locals have encounter them. There are also companies that repackage these chips with new gold leads and new case, I suspect that's what you get from EBay.

Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on January 18, 2013, 10:52:35 am
Use Google Chrome there's auto translate. I can't read Chinese but the schematics and diagrams teaches a lot.

I read all through it. That's is really helpfull. Could you or anybody else download the schematics and can share it? Have you bought the boards and/or can you share where to get them?
Title: Re: Ultra Precision Reference LTZ1000
Post by: muvideo on January 18, 2013, 04:36:41 pm
The LTZ1000 I have are also from used equipment. They are sourced directly from a electronics recycling market in China. There are people who are willing to go out and source for the right equipment that carries the specific component.   The components are removed carefully and resold. They are not expensive. Same case with the foil resistors.

There are fake LTZ1000 floating around even the Chinese locals have encounter them. There are also companies that repackage these chips with new gold leads and new case, I suspect that's what you get from EBay.

This is interesting, can you share more details, also in PM if you want.
I'd like to know where to ask for these parts (LTZ1000 and metal foil resistors)
and the order of magnitude for the price I could expect for these parts.
Talking about ebay, what you say is that used parts are more trustable
than new ones?

Thanks,
Fabio.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Christe4nM on January 19, 2013, 11:21:24 am
You can read more about it here
http://home.51.com/jj3055/diary/item/10053954.html (http://home.51.com/jj3055/diary/item/10053954.html)

Schematics here
http://bbs.38hot.net/read-htm-tid-36472.html (http://bbs.38hot.net/read-htm-tid-36472.html)

And finally here how to convert 7V to 10V
http://www.crystalradio.cn/thread-229749-1-1.html (http://www.crystalradio.cn/thread-229749-1-1.html)

Looking at the pcb I can't stop wondering why these slots are milled. The voltage isn't that high that you need the extra creepage distance, and right now it seems to me it's only weakening the pcb's structural integrity. Worst case it introduces unknown offsets as the pcb will bend a bit either when moved, or with temperature changes. Am I overlooking something here?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Rerouter on January 19, 2013, 12:54:01 pm
i would imagine to maintain the accuracy required the leakage currents (even if absolutely tiny) have to be mitigated somehow, and the slot probably is one of the easier ways to accomplish it,
Title: Re: Ultra Precision Reference LTZ1000
Post by: SeanB on January 19, 2013, 01:10:14 pm
More there for thermal isolation, as the references are all enclosed in an insulating cover and this removes the path for heat leakage out. The thin connecting traces also reduce heat transfer down them.
Title: Re: Ultra Precision Reference LTZ1000
Post by: PA4TIM on January 19, 2013, 01:19:33 pm
Slots in pcbs are for limiting mechanical stress. A pcb is screwed in a cabinet so there is mechnical stress, due to movement, gravity, thermal stress ect. That is why the often make a sort of U slot around a reference.  ( i ould not see it sharp on the pictures so maybe you alk about other slots, because i work mobile)
There is app note about references, can not remember of it was about the LM399 of from LT. They lso describe the best locations on the pcb in relation to stress and return current paths.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Christe4nM on January 19, 2013, 01:37:54 pm
I indeed talked about the L-shaped slots around the LTZ1000. Thanks for your reply's.

As far as appnotes go I found at least these ones:
Linear AN42 Voltage Reference Circuit Collection (http://cds.linear.com/docs/Application%20Note/an42.pdf)
Intersil AN177 Voltage Reference Application and Design Note (http://www.intersil.com/content/dam/Intersil/documents/an17/an177.pdf)

Yet it seems they are not the one you (PA4TIM) mentioned. Might this be the one?
Linear AN82 Understanding and Applying Voltage References (http://cds.linear.com/docs/Application%20Note/an82f.pdf)

@everyone: the literature references mentioned at the last page of application notes are a great way to delve deeper into any subject.
Title: Re: Ultra Precision Reference LTZ1000
Post by: PA4TIM on January 19, 2013, 01:45:46 pm
Yes AN82 is the one that talks about the slots
Title: Re: Ultra Precision Reference LTZ1000
Post by: muvideo on January 19, 2013, 01:53:40 pm
Also here linear talks about mechanical stress problems pag 19-20:
http://cds.linear.com/docs/Datasheet/1460fc.pdf (http://cds.linear.com/docs/Datasheet/1460fc.pdf)

I learned about the strain problems watching pics of internals
of the datron meters, there is an example on these
pics (excellent ones) from tekfan, 4th pic, there are the
two matched zeners:
https://www.eevblog.com/forum/reviews/what-do-you-want-to-see/msg41015/#msg41015 (https://www.eevblog.com/forum/reviews/what-do-you-want-to-see/msg41015/#msg41015)

Fabio.
Title: Re: Ultra Precision Reference LTZ1000
Post by: keen101 on February 07, 2013, 11:15:49 am
Quote
For the board, I haven't seen any for sale, only the ridiculously priced meter boards like Dave showed. Ideally I want some kit or finished board like what's been on the Chinese forums because getting all the precision parts around is single quantities must be PITA. So if anyone wants to make small run, I'm interested for one or two.

I'm helping my dad produce a few replacement boards for the HP 03458-66509 boards. He is working on his prototypes now. I don't know much about it, but he went to a lot of trouble ordering a bunch of precision resistors and stuff. He is now working on getting the ltz1000 chips from linear. He worked for HP/Agilent for 45 years and he calibrated instruments like the HP 3458A. He is recently retired. He is thinking of sell these for cheaper than what the official HP boards would cost. I am helping him make the boards and he is doing the complicated stuff. These boards should be pin compatible. Is there any interest in them here?

(http://i1010.photobucket.com/albums/af224/keen101/electronics/DSCF1508_zpse7e72cf5.jpg)

(http://i1010.photobucket.com/albums/af224/keen101/electronics/DSCF1510_zpsae43bba7.jpg)
Title: Re: Ultra Precision Reference LTZ1000
Post by: saturation on February 07, 2013, 01:10:41 pm
Just thoughts: a worthwhile project, but a very limited market.  Owners of working 3458a are not likely to pop in a new board  for fear or having to re-calibrate it, which is not cheap.  Owners of ones requiring service and thus are likely to experiment with new boards are much fewer, reason being if you own this DMM chances are you have it in working order and calibrated, and if you are storing it for the day you'll repair it, meaning you also don't need it right now, its worth $3000+ not working, so its an easy sell on eBay.

You'd have to prove that your aftermarket voltage reference boards are >= the factory boards in terms of accuracy and at least equal in quality.  Until that is known, I'm not sure an owner will risk having their units drift out of spec due to unforseen issues with the boards stability, since just checking the unit via a cal lab is $500-600, and the gold calibration runs past $1300.




Quote
For the board, I haven't seen any for sale, only the ridiculously priced meter boards like Dave showed. Ideally I want some kit or finished board like what's been on the Chinese forums because getting all the precision parts around is single quantities must be PITA. So if anyone wants to make small run, I'm interested for one or two.

I'm helping my dad produce a few replacement boards for the HP 03458-66509 boards. He is working on his prototypes now. I don't know much about it, but he went to a lot of trouble ordering a bunch of precision resistors and stuff. He is now working on getting the ltz1000 chips from linear. He worked for HP/Agilent for 45 years and he calibrated instruments like the HP 3458A. He is recently retired. He is thinking of sell these for cheaper than what the official HP boards would cost. I am helping him make the boards and he is doing the complicated stuff. These boards should be pin compatible. Is there any interest in them here?

(http://i1010.photobucket.com/albums/af224/keen101/electronics/DSCF1508_zpse7e72cf5.jpg)

(http://i1010.photobucket.com/albums/af224/keen101/electronics/DSCF1510_zpsae43bba7.jpg)
Title: Re: Ultra Precision Reference LTZ1000
Post by: krivx on February 07, 2013, 04:27:09 pm
You'd have to prove that your aftermarket voltage reference boards are >= the factory boards in terms of accuracy and at least equal in quality.  Until that is known, I'm not sure an owner will risk having their units drift out of spec due to unforseen issues with the boards stability, since just checking the unit via a cal lab is $500-600, and the gold calibration runs past $1300.

Wow, is it really that expensive? I had no idea. I assume this is because of the high requirements on the calibrator/source equipment? How does this compare to having, say, a 3.5 or 4.5 digit meter calibrated?
Title: Re: Ultra Precision Reference LTZ1000
Post by: ve7xen on February 07, 2013, 04:33:44 pm
Just thoughts: a worthwhile project, but a very limited market.  Owners of working 3458a are not likely to pop in a new board  for fear or having to re-calibrate it, which is not cheap.  Owners of ones requiring service and thus are likely to experiment with new boards are much fewer, reason being if you own this DMM chances are you have it in working order and calibrated, and if you are storing it for the day you'll repair it, meaning you also don't need it right now, its worth $3000+ not working, so its an easy sell on eBay.
I think the idea is more for volt-nut types to build a decent house standard, not people trying to repair 3458's. Depends on cost, but if he can get it significantly cheaper than what the HP boards go for on the used market I expect he'll sell some. Not a large market, but large enough to make it worthwhile I think.
Title: Re: Ultra Precision Reference LTZ1000
Post by: saturation on February 08, 2013, 11:31:03 am
The 3458a is very popular metrology grade top of the line DMM; I wouldn't be surprised if nearly all metrology labs use one in lieu of its competitors, making it a defacto standard top line DMM.

Its thus, a bit more difficult to test the extremes of its range.  As a ball park, calibrating a 5.5 digit DMM is $100-200  in the US, and $200-300 for 6.5 digits, Agilent prices.  A reason for the range is that fully automated DMMs like say the 1272a are just plug-and-play calibration, while older DMM may need more labor.

The $500 calibration is a typical one but for its best specifications, the $1500 cal comes only from Agilent, AFIAK.  Even at $500 most folks will send it to Agilent to insure its done properly.

https://service.tm.agilent.com/infoline/Product-Service.aspx?pn=3458A&countryid=114

https://service.tm.agilent.com/infoline/product-service-query.aspx


You can type your Agilent device above and the website will quote you a charge, it varies by country.

The cost is proportionate to the labor, and at the lowest cost its basically mostly the cost of the calibrator and or references, buying and maintaining it.

You'd have to prove that your aftermarket voltage reference boards are >= the factory boards in terms of accuracy and at least equal in quality.  Until that is known, I'm not sure an owner will risk having their units drift out of spec due to unforseen issues with the boards stability, since just checking the unit via a cal lab is $500-600, and the gold calibration runs past $1300.

Wow, is it really that expensive? I had no idea. I assume this is because of the high requirements on the calibrator/source equipment? How does this compare to having, say, a 3.5 or 4.5 digit meter calibrated?
Title: Re: Ultra Precision Reference LTZ1000
Post by: keen101 on February 09, 2013, 09:28:57 pm
I think the idea is more for volt-nut types to build a decent house standard, not people trying to repair 3458's. Depends on cost, but if he can get it significantly cheaper than what the HP boards go for on the used market I expect he'll sell some. Not a large market, but large enough to make it worthwhile I think.

Okay i was just curious. I thought maybe something like this project might work for some of you, but like i said before i don't know much about it.

http://www.maxmcarter.com/vref/ (http://www.maxmcarter.com/vref/)
Title: Re: Ultra Precision Reference LTZ1000
Post by: alm on February 09, 2013, 09:59:12 pm
This is exactly the volt-nuts kind of application that ve7xen was referring to. A good implementation of the LTZ1000 is non-trivial and requires several fairly exotic components. For the right price, I think you will get some buyers on this forum. No idea how many. I would certainly consider it.

I think the street price for a used 3458A reference was under $100 last time I saw one for sale on eBay. One advantage that the real Agilent units would have is that the LTZ1000 would be selected and burned in. I seem to recall a fair amount of variation in noise of the LTZ1000. This assumes that the references for sale on eBay were not factory rejects.

If he has access to a calibrated 3458A or similar DMM, than he might be able to offer calibration, as a high-end equivalent to the Geller Labs / Voltage Standard products. The reference would need to be burned in for bests results, which is likely to expensive/complex to make it worthwhile.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on February 12, 2013, 06:48:11 pm
Use Google Chrome there's auto translate. I can't read Chinese but the schematics and diagrams teaches a lot.

I read all through it. That's is really helpfull. Could you or anybody else download the schematics and can share it? Have you bought the boards and/or can you share where to get them?

So far nukie has not responded. Does anyone else know, where to get the boards nukie uses and can share the schematic?
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on February 12, 2013, 06:50:37 pm
I'm helping my dad produce a few replacement boards for the HP 03458-66509 boards. He is working on his prototypes now. I don't know much about it, but he went to a lot of trouble ordering a bunch of precision resistors and stuff. He is now working on getting the ltz1000 chips from linear. He worked for HP/Agilent for 45 years and he calibrated instruments like the HP 3458A. He is recently retired. He is thinking of sell these for cheaper than what the official HP boards would cost. I am helping him make the boards and he is doing the complicated stuff. These boards should be pin compatible. Is there any interest in them here?

that looks good. Do you and your Dad want to offer/sell these boards?
Title: Re: Ultra Precision Reference LTZ1000
Post by: keen101 on February 12, 2013, 11:19:23 pm
that looks good. Do you and your Dad want to offer/sell these boards?

He said he will probably try listing them on ebay soon. Maybe by next week, but i'm not sure. I honestly don't know if anyone will want to buy them, but we will see.

(http://i1010.photobucket.com/albums/af224/keen101/ltz1000_clone1_zps8a206777.jpg)

(http://i1010.photobucket.com/albums/af224/keen101/ltz1000_clone2_zps6454debe.jpg)
Title: Re: Ultra Precision Reference LTZ1000
Post by: JBeale on February 16, 2013, 12:59:50 am
I might be interested in a LTZ1000 reference board, but my interest level would depend on the price :-)   I am just starting to test a batch of LM399 references and finding that each one is quite individual as far as microvolts of noise/drift in any given frequency range. I care most about frequencies of 1 Hz and lower, so maybe better to call it "drift".

"...now working on getting the ltz1000 chips from linear."   From this link http://www.linear.com/purchase/LTZ1000 (http://www.linear.com/purchase/LTZ1000) and then
http://shopping.netsuite.com/s.nl?ext=F&c=402442&sc=2&category=&search=LTZ1000 (http://shopping.netsuite.com/s.nl?ext=F&c=402442&sc=2&category=&search=LTZ1000)
it looks like "Linear Direct" currently has 14 of the LTZ1000ACH#PBF parts in stock, but none of the other types.  But they also note "a minimum buy may apply".
Then again, they also said that on the order form about the LM399's I got, but then shipped me the six parts I requested anyway.
Title: Re: Ultra Precision Reference LTZ1000
Post by: amspire on February 16, 2013, 06:10:17 am
The device I use as my reference is a Thaler VR305A hybrid IC. 5V 0.01% initial accuracy. 0.6 pmm/C without an oven or on-chip heater. If you want to put them in an oven, it includes a diode junction for as a temp sensor. That means they are better then total 0.001% stability as the temperatures varies from 17 degC to 33 degC and no warm up time - good enough for me.  The LTZ1000 can obviously get an order of magnitude better temperature stability with its built in heater but you do have to give it over 5 minutes to stabilize after turn on.

The Thaler references use a buried zener based reference,  and I think resistors and thermistors on the ceramic substrate are then laser trimmed to zero the temp coefficient.

Long term stability is worse then the LTZ1000 - 6ppm in 1000 hours. Does it stabilize after 1000 hours? It seems to, but I don't know. I don't have any figures in how they actually perform long term after they have burnt in. There is much less in a  LTZ1000, so I guess there are much fewer components that can drift.

The problem with the Thaler references is they used to be very affordable - I think I got 5 for about $20 each from Thaler over 10 years ago.

Thaler were bought by Cirrus Logic (Apex Microtechnology) and since then, every time I look, the price has gone higher. A couple of years ago, they were $70 and Digikey have them currently for $93. Apex Microtechnology website does seem to offer samples. http://www.apexanalog.com/products/voltage-sine-wave-references/ (http://www.apexanalog.com/products/voltage-sine-wave-references/)
Title: Re: Ultra Precision Reference LTZ1000
Post by: JBeale on February 16, 2013, 04:20:33 pm
I made a table of the specs on all the precision references I found referenced on the "volt-nuts" list. I lost interest in the VRE305 after I saw the price on DigiKey. It is by far the most expensive one on the list; twice as expensive as a LTZ1000 from Linear Tech (although to be fair, the LTZ1k needs several other expensive parts in the standard circuit).  Both the Apex product page and the data sheet actually call the VRE305 a "low-cost" part, which is actually true by reference to some of their other parts... Digikey lists the VRE102CA at $262.   I would be impressed if they were willing to sample any of these gold-plated parts to a hobbyist.
Title: Re: Ultra Precision Reference LTZ1000
Post by: amspire on February 16, 2013, 04:38:14 pm
I made a table of the specs on all the precision references I found referenced on the "volt-nuts" list. I lost interest in the VRE305 after I saw the price on DigiKey. It is by far the most expensive one on the list; twice as expensive as a LTZ1000 from Linear Tech (although to be fair, the LTZ1k needs several other expensive parts in the standard circuit).  Both the Apex product page and the data sheet actually call the VRE305 a "low-cost" part, which is actually true by reference to some of their other parts... Digikey lists the VRE102CA at $262.   I would be impressed if they were willing to sample any of these gold-plated parts to a hobbyist.
The datasheet Apex used is the Thaler datasheet with the name Thaler replaced. Nothing else was ever changed except for a fivefold increase in price. No new devices have been designed. Sad really.
Title: Re: Ultra Precision Reference LTZ1000
Post by: fmaimon on February 16, 2013, 04:39:36 pm
There is someone on ebay selling VRE305KS for less than $30 but there is no picture of the device. I've sent him a message asking for a photo of the IC.
Title: Re: Ultra Precision Reference LTZ1000
Post by: amspire on February 16, 2013, 11:10:56 pm
There is someone on ebay selling VRE305KS for less than $30 but there is no picture of the device. I've sent him a message asking for a photo of the IC.
If it is the ebay sale for US$22, it looks a good deal. They are the devices that can go down to -40C. Mine are only rated down to 0C.

The IC's are surface mount versions of the DIP8 package, like mine. Here is a photo, if it helps:

(https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=39567)

Also I just found there very  cute 8mm x 8mm peltier devices for $4 inc postage that can shift up to 1/3W of heat. I thought they could be good for making a small temp controlled "oven" for voltage references, crystals, etc. It means an operating temp of something like 30 degC can be chosen, rather then having to pick something well above normal ambient, like 50 degC. The total heat you have to dissipate from the peltier is about 1.2W so that can be a small heatsink, or directly to a metal box. I have ordered a few to play with.

(https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=39569)
http://www.ebay.com.au/itm/TES100701-DC-0-8V-0-33W-Peltier-Cooler-Thermoelectric-Cooler-Cooling-8-x-8-mm-/110900388714? (http://www.ebay.com.au/itm/TES100701-DC-0-8V-0-33W-Peltier-Cooler-Thermoelectric-Cooler-Cooling-8-x-8-mm-/110900388714?)
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on February 17, 2013, 12:02:39 pm
LM399H and LM399AH can be bought at www.electronic-search.de (http://www.electronic-search.de) in germany. I did that a few months ago and made a small board with this milled slots to keep away mechanical stress and minimize thermal drop between the pins resulting in thermo voltages. Keep in mind that LM399 is available from LT and National Semiconductor, both with some small difference in spec. You could also ask them for the LTZ1000.

I'm sensitized with references at ebay. I bought a few MAX6350MJA (Buried Zener in a CERDIP package) from ebay and hoped that they were also used and recycled, because MAXIM don't stock them anymore. CERDIP because of the humidity influence to references plastic package is sensitive to.
The brand printing was the first indication, didn't look like MAXIMs original logo. The package marking on the bottom showed some code Linear Technology uses for their products (N12345) instead of a 7 digit letter combination plus a 4 digit date code, what is common. And also measurements showed that there is nothing but a bandgap reference inside, presumably a Ref02 or similar.
MAXIM now knows about that asian seller.

BTW: If some of you have a few (used) MAX6350MJA in private stock and want to sell them send me a message.
Title: Re: Ultra Precision Reference LTZ1000
Post by: amspire on February 17, 2013, 01:42:07 pm
branadic,

Have you looked at the Thaler/Cirrus/Apex VRE305KS that was mentioned above?

http://www.ebay.com/itm/Low-Cost-Precision-Reference-IC-VRE305-VRE305KS-NEW-/221065482973 (http://www.ebay.com/itm/Low-Cost-Precision-Reference-IC-VRE305-VRE305KS-NEW-/221065482973)

SOP-8 Ceramic package and almost compatible with the Maxim device. Most specs as good or better then the Maxim chip. Major difference is that the VRE305KS has a seperate GND and Reference GND, so that the reference output is not affected by the IC supply current. Other differences are that the VRE305 required 13.5V minimum compared to Maxim's 8V and consumes slightly more current, but has better long term stability figures. The VRE also has a temp sensor junction in case you want to use it in an oven.
Title: Re: Ultra Precision Reference LTZ1000
Post by: alm on February 17, 2013, 02:00:35 pm
Any idea how it compares to the much cheaper LM399 in real world figures? The spec for long term stability of the LM399 is much worse (20 ppm / 1k hrs typ), but we know from the many devices designed around it (including the far majority of the 6.5 digit DMMs) that it can perform much better. Is the same true for the VRE305? I guess it should be better since it runs at a much lower temperature, everything else being equal. But of course everything else is not equal.
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on February 17, 2013, 02:52:30 pm
branadic,

Have you looked at the Thaler/Cirrus/Apex VRE305KS that was mentioned above?


Yes, I once did, but I only payed 5.80€ for an new LM399H, you can guess what I prefered to buy.

"...Fairic is a Hong Kong based company with China offices in different provinces..."

ebay + China + Precision Voltage References --> all my alarm bells are ringing. Thanks for the link, but I would never again move any finger to click on the "buy it now" button. The last seller also had nearly 100% positive feedback. I'm not sure if the guys who bought the MAX6350MJA too where able to measure their devices? They should have noticed that the device behaves contrary the datasheet.

This is a typical example of such a fake:
http://www.ebay.com/itm/Maxim-MAX297-MAX293MJA-MAX294MJA-MAX297MJA-8-Cerdip-IC-/260653697527?pt=LH_DefaultDomain_0&hash=item3cb02bc9f7 (http://www.ebay.com/itm/Maxim-MAX297-MAX293MJA-MAX294MJA-MAX297MJA-8-Cerdip-IC-/260653697527?pt=LH_DefaultDomain_0&hash=item3cb02bc9f7)
Notice the brand, this is how the MAX6350MJA fake was looking like.
Title: Re: Ultra Precision Reference LTZ1000
Post by: fmaimon on February 19, 2013, 01:11:59 pm
The guy sent me this photo. It has some strange things.
So, what do you think?
Title: Re: Ultra Precision Reference LTZ1000
Post by: amspire on February 19, 2013, 02:45:28 pm
The ThC logo is the older logo of Thaler Corp, and my Thaler datasheet is Rev D 2001. I have posted it below.

The USA marking and omitting the "S" off the end is in line with the way Thaler do it. I thought I had purchased mine in the early 2000s, but it looks like I got it in 2007, so I have a late model with the newer TC logo.

The case style is unusual, so there may not be any other reference ICs in an identical case. Mine has no visible ceramic cement between the upper and lower half of the case, but that many be a refinement after 6 years. The metal contact construction in this 2001 IC are slightly different to my 2007 IC.

Why has he got a photo of a more accurate "A" model when he is offering a "K" model? I do not know the answer to that question, except for the fact that to quote the part as a VRE305KS, he is not reading it off a chip - that may be the code of a tube, box, bag, or somewhere else. The IC will have the part number VRE305K.

Given the market price, it is possible that someone has got a REF02A chip (or similar) and put it in the SOP-8 ceramic package to make a fake. It would work in the same circuit, except the initial accuracy would be over 10 times worse, and the long term drift over 20 times worse. In the photo, the IC is sitting on a PCB, and may be soldered onto the PCB. It is odd.

I did an image search on Google, and no match for that photo was found, so the seller may have actually taken the photo.
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on February 23, 2013, 10:47:32 am

He said he will probably try listing them on ebay soon. Maybe by next week, but i'm not sure. I honestly don't know if anyone will want to buy them, but we will see.

(http://i1010.photobucket.com/albums/af224/keen101/ltz1000_clone1_zps8a206777.jpg)

(http://i1010.photobucket.com/albums/af224/keen101/ltz1000_clone2_zps6454debe.jpg)

Any news about that? Couldn't find the offer yet, but was interested in the price too. Have them seen a burn-in test or are they just new and directly soldered to the board?
Strange, can't find any "LTZ1000" with it as keyword on e**y, only by searching for "HP 3458" and similar I get a few hits. What's going on there?

I made a table of the specs on all the precision references I found referenced on the "volt-nuts" list.

You'd like to share your table with us?
Title: Re: Ultra Precision Reference LTZ1000 (table of Vref data)
Post by: JBeale on February 27, 2013, 07:31:59 am
I don't know if I can attach the spreadsheet here...  when I print a PDF it does not look good (long rows are split across too many pages). Anyway you can find my list of voltage reference specs here:  http://www.bealecorner.com/pcb/vr1/Vref-table-Feb2013.xls (http://www.bealecorner.com/pcb/vr1/Vref-table-Feb2013.xls)

I did this just for my own purposes, it may not be accurate, and is surely not complete. But if you do see any mistakes, let me know.
Title: Re: Ultra Precision Reference LTZ1000 (table of Vref data)
Post by: EEVblog on February 27, 2013, 07:43:43 am
I don't know if I can attach the spreadsheet here...  when I print a PDF it does not look good (long rows are split across too many pages). Anyway you can find my list of voltage reference specs here:  http://www.bealecorner.com/pcb/vr1/Vref-table-Feb2013.xls (http://www.bealecorner.com/pcb/vr1/Vref-table-Feb2013.xls)

I did this just for my own purposes, it may not be accurate, and is surely not complete. But if you do see any mistakes, let me know.

Reposted as a handy PDF.
Nice work.

Dave.
Title: Re: Ultra Precision Reference LTZ1000
Post by: JBeale on February 27, 2013, 07:50:27 am
By the way, I just got two of the LTZ1000ACH (direct from Linear Tech- the ebay versions did not look promising) and am trying one out using the simplified +Vref schematic that uses a single LT1006 opamp, and does not use the heater. 

Both this and the more complex heater-stabilized "7V Positive Reference Circuit" schematic on p.6 of the LTZ datasheet use a 1N4148 diode from the opamp output to the Vref out (LTZ pin 3, zener cathode).  Can anyone explain what that diode is for? This is a single-supply opamp circuit so there is no chance of the output going negative. Is it some protection for the opamp in case of a badly-behaved external load, or something about power-up or power-off behavior?

I have tried this circuit with the diode and without, and as far as I can tell, the only effect of the diode is to require the opamp supply voltage to be 0.7 V higher than it would otherwise need to be to prevent a dropout (unloaded Vsupply threshold of 9.1 V instead of 8.4 V , in the case of a LT1013 and my particular LTZ1000A which provides a Vout of 7.2406 V, with the temperature compensating R1 at 25 ohms and Iz = 4.27 mA).

EDIT: I suppose the circuit is (meta-)stable with both inputs and the output sitting at 0.000 V, so there might be a startup problem. If so, does that diode alone actually fix it?  There is a version with an additional 10k pullup on the output (Figure 65 on p.15 of AN42, http://cds.linear.com/docs/en/application-note/an42.pdf (http://cds.linear.com/docs/en/application-note/an42.pdf) ) suggesting that is the issue.

EDIT2: Thanks alm for pointing to the answer; I should have seen those threads before.
Title: Re: Ultra Precision Reference LTZ1000 (table of Vref data)
Post by: BravoV on February 27, 2013, 08:08:01 am
JBeale, thanks for the spreadsheet, really handy.  :-+

Btw, Intersil also carries quite some number of Vref ICs as well.

Just read the link in your sheet -> http://www.febo.com/pipermail/volt-nuts/2010-September/000447.html (http://www.febo.com/pipermail/volt-nuts/2010-September/000447.html) , quite interesting result about MAX6350.

Quote pics :

(http://phk.freebsd.dk/misc/20100911_max6350_2.png)

(http://phk.freebsd.dk/misc/20100911_max6350_1.png)
Title: Re: Ultra Precision Reference LTZ1000
Post by: alm on February 27, 2013, 07:12:14 pm
Both this and the more complex heater-stabilized "7V Positive Reference Circuit" schematic on p.6 of the LTZ datasheet use a 1N4148 diode from the opamp output to the Vref out (LTZ pin 3, zener cathode).  Can anyone explain what that diode is for?

As you already guessed, it is for startup. See this thread on EEVblog (https://www.eevblog.com/forum/projects/does-anyone-know-what-this-diode-is-for/) and this thread on volt-nuts (http://www.febo.com/pipermail/volt-nuts/2012-November/002200.html) for more information.
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on February 27, 2013, 10:50:16 pm
Andreas Jahn determined in his measurements, that references in plastic packages are very sensitive to humidity compared to their brothers in ceramic package. This sensitivity results in additional drift next to tempco. This is also a reason why I was searching for the MAX6350MJA, it's the CERDIP version.
It seems, that I now found a trustable source for that part, but it's even more expensive (16,10€) compared to the LM399AH (9,95€).
The LTC6655 is now available in a ceramic smt package (LS8) with excellent specs, but up to now only available as a 2.5V reference. This could be a worthy alternative to the good old stuff, awaiting the 5V version. And as by a wonder it is again some Linear part.
Title: Re: Ultra Precision Reference LTZ1000
Post by: JBeale on March 13, 2013, 05:10:39 am
Quote
The LT1021-7 also comes to mind-- with fewer internal resistors, this part will show exceptionally low long-term drift.  Again, you can find the hermetic part from various vendors, and you would be wise to ovenize it (along with any circuitry you might add to gain it up to 10V).

Interesting you should mention the LT1021-7. I have three of those (plastic DIP) that I've been running for several weeks. I am seeing several ppm of short-term drift (10-100 sec) but just a few inches away, a pair of MAX6350 parts (also plastic DIP) is stable to less than 1 ppm over the same time period.
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on March 13, 2013, 09:53:06 pm
Quote
As far as long term drift goes, probably the Burr-Brown REF102 (now the TI REF102) is the best available voltage reference that is not ovenized

Well, we need to have a look on the new Linear Technology LS8 packaged voltage references, such as the mentioned LT1236-LS8 or LTC6655-LS8.
It's interesting that manufactors seem to have "unlearned" what was already done in the LTZ1000. I wonder if there will be a heated, 5V output buried zener in LS8 package or similar in near future, supplied with moderate voltages around 5V, a (still) typical value in todays analog circuits.
Title: Re: Ultra Precision Reference LTZ1000
Post by: BravoV on March 14, 2013, 02:05:03 am
At Intersil's voltage reference they're using the term so called "floating gate" technology, is this the same as buried zener ?
Title: Re: Ultra Precision Reference LTZ1000
Post by: amspire on March 14, 2013, 02:32:22 am
At Intersil's voltage reference they're using the term so called "floating gate" technology, is this the same as buried zener ?
No - very different. The floating gate depends on a stored charge as the voltage reference - a bit like a eprom memory cell.
Title: Re: Ultra Precision Reference LTZ1000
Post by: BravoV on March 14, 2013, 02:56:40 am
At Intersil's voltage reference they're using the term so called "floating gate" technology, is this the same as buried zener ?
No - very different. The floating gate depends on a stored charge as the voltage reference - a bit like a eprom memory cell.
Ah thanks, no wonder they're worrying that the trapped charge will somehow "leaked" caused by xray like their appnote -> X-Ray Effects on Intersil FGA References (http://www.intersil.com/data/an/an1533.pdf)

Curious if there are any advantages of this floating gate over other old & proven technology like buried zener ? Cheap to produce maybe ?  :-//

PS : I've been "imagining" to use this xray effect to deliberately fine tune the trapped voltage to get high absolute accuracy, and once finished, sealed it in thick lead box.  ;D
Title: Re: Ultra Precision Reference LTZ1000
Post by: amspire on March 14, 2013, 11:06:55 pm
I have never seen anyone use a floating gate reference in lab gear. I have never used one myself either so I have no experience, but I prefer standards somehow constructed as hardware - like a buried zener - rather then something that sounds as vulnerable as a stored charge that can be affected by radiation and probably by heat as well.

I wonder how much the reference changed when if goes though soldering?

Buried zener references have a good record of maintaining accuracy after 20 years or more. Wouldn't have a clue about the floating gate reference performance over 20 years.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Jay_Diddy_B on March 15, 2013, 12:07:24 am
Hi,
The X-ray doses mention in the Intersil document are pretty low. They are using a machine capable of 10uA to 1000uA

A medical X-ray machine, used in hospitals are typically 30, 50 or 80kW. The 300kW machines can typically do 500mA at 60kV. The 80kW machines can do 1000mA at 80kV.

This is a thousand times higher than the dose mentioned in the Intersil document.

I think the concern here is that if you X-ray the board after soldering, for example to examine BGA soldering, then the reference will drift.

I would also be concerned about X-ray inspection during shipment.

The floating gate is essentially a charged capacitor. I just don't like the idea of assuming the capacitor won't discharge over time.

Jay_Diddy_B

Title: Re: Ultra Precision Reference LTZ1000
Post by: BravoV on March 15, 2013, 02:02:12 am
I just don't like the idea of assuming the capacitor won't discharge over time.
I believe that the 1st thing that pop out at everybody's mind is the word "leak" when seeing the word "trapped" charge.  ;D
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on March 17, 2013, 12:32:09 am
In a don't think, just act manner, I cobbled together something with a chinese ebay LTZ1000A.
Other ingredients are spare PCB space from another project, General resistance econistors, LT1013,Cu-Te lab sockets from Multicontact and a pair of homemade Cu-Te cabling.
The worst  thing about it is the layout which was thrown together without any patience. Star ground right in front of the LTZ, and a king-sized "GUARD" polygon.
I have only a HP3456A, workplace has only a bunch of 34401A multimeters, so there is still a long way to go for telling if it is doing well. During the next weeks I will have a meeting with a local volt-nut who has a bunch of LTZ and more knowledge. If number  one is doing well, I still have 2 or 3 more boards so I could set up a horde of references or give them away for others.

My idea is to use the HP3456A in DC:DC Ratio mode, using the LTZ as one of the DCs to have a low-drift DMM. EZGPIB (or processing cough cough) could turn the RATIO reading into a maybe not overaccurate but hopefully low-drift voltage reading.


Attached is some show-off:
Title: Re: Ultra Precision Reference LTZ1000
Post by: saturation on March 17, 2013, 12:04:31 pm
All you need from a reference is stability over time and high precision, once you know how much it drifts and resolve down to 1uV with the 3456a, 'accuracy' is just offsetting with a trimmmer the reference output or just applying an algebraic correction to obtain the accurate value.  So, now its a waiting game. 
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on March 17, 2013, 01:46:40 pm
I allready found your post on volt-nuts archive yesterday. As far as I understood you are using a LT1013 for A1/A2 (7V Reference Includes A1 Heater Control Amplifier, A2 Zener Current Regulator
and LTZ1000A Zener. AN-86 p.46) and a further LT1013 as output buffer?
Refering to your high resolution but low quality picture (wouldn't it be better to downscale the picture, ~1.6MB of such bad quality doesn't make sense) I found a CERDIP packaged chip in the lower left corner (doesn't look like a LT product)  and a plastic packaged LT part (LT1013?) in the lower right/middle. Both are directly soldered to the pcb. I would have used a precision DIP socket instead.

The next steps are clear, a complete burn-in, also thermal cycling and monitoring the ouput as already mentioned by Andreas Jahn.

Are you sure the LTZ1000A is a genuine Linear Technology product and not a plagiarism? Is it a new one or already been used and desoldered from a pcb? I feel sick to buy such products from chinese sellers, you never know what's inside the package.

BTW: As far as I recognized you are from germany?  So you could get the LT1013ACJ8 from a german distributor:

http://www.electronic-search.de/cgi-bin/electronic.pl?t=temsearch&f= (http://www.electronic-search.de/cgi-bin/electronic.pl?t=temsearch&f=)*BEZEICHNUNG1&start=1&dif=100&c=LT1013ACJ8&submit=Suchen

After all my experience with plastic packaged chips and humidity in conjunction with CDCs for capacitive sensors and after all I have read about humidity concerning voltage references I'm extremly sensitized to that theme and would prefer a CERDIP packaged instead of a plastic type in such a high precision arrangement.
I didn't get why all the metal can packages and most of the CERDIP are obsolete today without giving any hermetically sealed alternative to them. Did we lose the requirements or is it that they are just through-hole parts? So lets hope that manufactors find back to ceramic based hermetically sealed packages, that can be made as smd part too.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on March 17, 2013, 09:08:42 pm
@saturation: I even thought  about replacing the LM399-family reference in the HP with this first but canceled that idea. Keeping it external has some advantages. I need to check drift - a german volt-nut lets me compare mine to his LTZ bunch, that will help, and maybe I somehow get a good voltage reading from a cal lab on a napkin or such. DC:DC ratio, GPIB readout and some calculations are the most convenient way to measure referenced to the LTZ.

@branadic: Sorry, I right now replaced the unneccesary big image with a much smaller one. Yes the LTZ is from china, came to me presoldered and is the single ebay part, Resistors are from Rhopoint, Sockets Multicontact, other parts from Farnell including the LT1013s who shouldnt sell foo. The buffer opamp was a second thought, I first was considering leaving it off and use the mounting holes for a output filter but decided that i drop it in finally.

I am not that afraid of plagiarism, as it performs better than its heated REF102 predecessor and as i try to not be a real volt-nut, i will be satisfied easily. What gives me trust is that the voltage readout changed at most 40 µV according to my meter from the first power-up after soldering thru flux removal with LR, a run in the dishwasher and a 3-hour 60°C (not 80, sorry) cycle in the thermal chamber, all over the course of the days since march 2, with the DMM in a living room instead of a tempered cal lab.

Further thermal cycling will follow.

Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on March 27, 2013, 03:04:42 pm
because I was very buisy lately, I just saw I missed quite a few posts.

But here is my short update:
I still have no board yet, but have most of the main parts (directly from LT) and orderd some sets of Vishay Z-Foil Resistors.
I decided to go for VHP101 and Z201 to be able to compare, if the much higher price of VHP is worth it.
Because the Vishay leadtime is quite long, I hope I will find a board meanwhile.

bye
quarks
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on March 27, 2013, 05:36:50 pm
Short update: I got sick so not a lot of work done, but I found a suitable power supply for the reference which has a linear regulator and is marked as a medical grade TYPE CF supply.

Tomorrow I will buy a outer case and hopefully mount everything next week.

 

Title: Re: Ultra Precision Reference LTZ1000
Post by: free_electron on April 07, 2013, 11:50:45 pm
Allright you voltnuts... i got a piece of real hardcore volt p.rn for you... a teardown of a real standard.... keep your eyes peeled... massaging the pictures right now...
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on April 20, 2013, 03:35:16 pm
Further progress with my reference: Its no longer hidden in a cardboard box and now its really transportable and self-contained due to a power supply fixed to it.

The power supply is a medical TYPE CF grade device (the secondary will only push very low currents in the direction of PE), and the tinned sheet metal case is placed inside a bigger plastic case.
Also, the always necessary required red power LED is added.
 
The additional yellow 4mm socket is a connection to the "guard", consisting of the metal case and a copper pour on the PCB.

What is missing is labeling...



Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on May 04, 2013, 02:19:26 pm
While still waiting for the Vishay Z-Foil resistors, I decided to try (with Diptrace) to make my own board.
Here are the first things I consider what the final board should have:

- dual layer, material FR4 with 18 or 35µm copper, Gold plated 
- choice to place different sized components (https://www.eevblog.com/forum/diptrace/make-your-own-components-with-multiple-pads/ (https://www.eevblog.com/forum/diptrace/make-your-own-components-with-multiple-pads/))
- stress relief cutouts around LTZ1000 (https://www.eevblog.com/forum/diptrace/stress-relief-cutouts-on-pcb/ (https://www.eevblog.com/forum/diptrace/stress-relief-cutouts-on-pcb/))
- Output probably 7.2V plus 10.00000V (and maybe 1.018V)
- thermal isolation housing for LTZ1000
- low thermal output connectors
- guard copper pour on both layers
- star ground
- switchable external PSU to battery/accu powered

Are these valid thoughts so far or are there better suggestions?
What else could be a good idea to consider?
Any hints and suggestions are welcome.

Thanks
Quarks

edit: changed a few things in the schematic (incl. error correction)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Hypernova on May 06, 2013, 01:20:26 am
I have been studying this problem for a very lonnnnnng time...

And that folks, is why Volt-Nuts have "nuts" to their name... In comparison the most accurate references I got are three MAX63xx (one of each voltage) references I got from digikey.
Title: Re: Ultra Precision Reference LTZ1000
Post by: BravoV on May 06, 2013, 01:24:11 am
DiligentMinds, thanks for great post !  :-+

I have been studying this problem for a very lonnnnnng time...

Damn, I'm curious on how long is that ?  ???
Title: Re: Ultra Precision Reference LTZ1000
Post by: chickenHeadKnob on May 06, 2013, 06:45:53 am

I have been working in electronics (professionally) since 1975.  I have been working on this problem for about 3 years now.  I finally decided to go with the 18-bit DAC for trim (as opposed to the PWM method that Datron used in their 4910 voltage standard, which later became the Wavetek (and then Fluke) 7000 series voltage standard.  The 18-bit DAC [0.25 LSB INL] will easily provide for +/-1.3mV of trim, at about 10nV per count.

And yes-- I am a "Volt-Nut" (I own a Fluke 732B), "Resistance-Nut" (I own an ESI SR104 resistor), and a "Time-Nut" (I have a GPS disciplined 10Mhz OCXO and network time standard)-- (well, really, I guess I'm really a "Precision-Nut"... Then...).  Very few people (other than Cal-Labs) actually *need* this kind of precision-- but there are those of us that strive for the ultimate in precision because it's fun, (and because we can)-- it is also an area of electronics that requires great skill and "finesse" to squeeze out that last bit of accuracy...  OR-- maybe we are just sick...


I completely understand the psychological draw of precision nuttery, it activates the same part of the brain as mind teasers, crossword puzzles ect. with the added attraction that you are dealing intimately with the real physical world and not merely some mental construct . I am too pragmatic to spend my time solving crossword puzzles but after looking a bit through the volt-nut mailing list I became consumed with thinking about  high precision analog to digital conversion. Looking back maybe my posts indicated some derision, but really I am making fun of my own captivation.
Title: Re: Ultra Precision Reference LTZ1000
Post by: amspire on May 06, 2013, 06:57:02 am
I have been working in electronics (professionally) since 1975.  I have been working on this problem for about 3 years now.  I finally decided to go with the 18-bit DAC for trim (as opposed to the PWM method that Datron used in their 4910 voltage standard, which later became the Wavetek (and then Fluke) 7000 series voltage standard.  The 18-bit DAC [0.25 LSB INL] will easily provide for +/-1.3mV of trim, at about 10nV per count.
I would imagine that to the Datron/Wavetech/Fluke designers, the PWM solution is very much superior to the DAC solution as the errors and drift for the PWM are fully calculable. The errors for the DAC are not - you have to trust the specs of the DAC manufacturer and that is something metrology people hate to do. It is possible to get much better stability then an 18 bit dac with PWM but as always, when you get to that precision, it doesn't take much to destroy a few ppm accuracy.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on May 06, 2013, 10:19:57 am
Hello DiligentMinds.com,
wow that is a lot of very good and interesting Information. Thank you very much, for sharing your knowledge. That gives me indeed some new ideas.

...consider using an LT1051 instead of the LT1013 ... you could use a quad chopper (LT1053A)
I will check the LTC1051 and LTC1053

The two 70K resistors should be ultra-high quality metal-film type-- accuracy is not that important (they could be 0.1%), but they should have as low a temperature coefficient of resistance [TCR] that you can find-- probably 5ppm.

The 120-ohm, 12K/1K-divider, and 10V-output-divider resistors all need to be a metal-foil resistors from Vishay Precision Group-- their newest "Z1-foil" resistors are the best, and will have the best long-term stability.  These will be about $10 each, if purchased from an authorized distributor.  If you can afford it, the hermetically sealed type will have even better long term stability-- and will reduce variability due to humidity and barometric pressure changes.
I tried to find the best possible resistors and ordered Vishay Z-Foil (the hermetically sealed VHP101 and the lower cost Z201). They have very long lead time (probably 5 more weeks for me to wait). I wonder where your 10$ price point comes from, because I paid up to more than 50$ for one, so please let me know. 
The "LTZ1000(A)" does *NOT* need to be "stress relieved". 
That is good news, because I still have no idea how to do that with diptrace (but I am still interested to know if/how it is doable)

although it should be covered with something (both top and bottom), because any air flow (as in convection currents) will cause tremendous low-frequency noise (due to the thermal EMF's between the Kovar package leads and the copper PC board).  You can use anything for the cover-- you're just trying to stop air-flow-- not make an insulated box.
I have only thought of a top cover so far, but now will think about how to do a bottom one too
Your schematic doesn't appear to have any way to adjust the 10V output.  If I were doing this, I would make the 10V-divider a 3-resistor type (with 3 resistors in series).  The bottom resistor would always be 5K (for 5V) and the next two would total up to 5K, but their values would be governed by the actual voltage of the 7V reference output (after 300-hour burn-in).  When you order metal-foil resistors, you can specify the exact value you need, and they will supply you with that value (at the tolerance that you pay for).  The 10V-divider resistor tolerances should be (at least) 0.01%.  The reason for the 3-resistor divider, is that now you have a 5V node that can be used to adjust the 10V output.  At this 5V node, you would tie 1M-ohms to 5M-ohms of resistance (depending on the sensitivity you need) made up of one or more 5ppm metal-film resistors.  The other end of this large resistor would be tied to either a potentiometer (the Vishay Precision Group 1285G series) or (even better) an unbuffered 16-bit or 18-bit DAC (TI DAC8871 or Analog Devices AD5781).  You would tie the Vref-H directly to 10V-out at the banana jack, and Vref-L directly to ground at the banana jack-- do not use buffers (as their ap-notes say to do) on either the reference inputs or on the output voltage.  Without the buffers, the INL spec will suffer some, but the repeatability and time+temperature drift of the output will be ~0.05ppm.
About the trimming of the 10V, I have considered some options, but not decided yet which to choose. That is why the schematic still shows the AN86 suggestion with fixed Vishay VHD200 ratio set. But I have not orderd this and will probably go for either:

- similar to your suggestion with a precision trimming circuit as you decribed (also already ordered together with the other parts the Z-Foil Trimmers 1260 and 1285)

- with KVD as shown in AN86 (see att.), for this I could use my Fluke 720A and/or my ESI Dekapot
   In the drawing are X= solder copper juctions, how should/can this be done? Just leave a gaps and bridge it with solder or with a via and fill it with solder?

Your DAC suggestion is probably to complicated for me.
And I do not know yet if I even maybe leave it as simple as possible and just go for a chopper buffered nominal 7.2 Volt Output and do anything else externally (if I ever want/need it).

Another thing you could do is place a (fc=100mHz) low pass active filter between the reference output and the 10V amplifier-- something using a very low-noise amp (like the LT1112), and a DC-agnostic topology for the filter (like here):

http://www.millertechinc.com/pdf_files/MTI%20TN094%20Zero%20DC%20Offset%20LPF%20and%20the%20D%20Element.htm (http://www.millertechinc.com/pdf_files/MTI%20TN094%20Zero%20DC%20Offset%20LPF%20and%20the%20D%20Element.htm)

Another thing you might consider (especially if you are using an LT1053 quad chopper), is to use 2 of the chopper amps to "discipline" a low-noise amplifier (like the LT1097 for example)-- this will lower the DC error and 1/f noise of the LT1097, while at the same time, you will enjoy the low voltage noise.
I will check that
And finally, if you can afford it, I would do 2 additional things-- first, think about putting the whole circuit in an oven.  In this case, you would use the lower-cost LTZ1000 (not the 'A' version), and control the outer oven's heater with the LTZ1000's temperature sensing transistor.  In this configuration, the LTZ's internal heater is not used.  Set the oven temperature to keep the LTZ die at ~45-deg-C, for a reasonable compromise between a good environmental temperature range, and low long-term drift-- (higher temperatures cause more drift over time).  The other thing I would do (if you are really serious) is go with the hermetic resistors (as I mentioned above) and also use a hermetic packaged LT1053.  (The other amps and the buffer don't matter-- their drift is controlled by the LT1053, and the LT1112's drift will not affect the DC value of the reference if you use the DC-agnostic filter I showed you in the link).

If you can only afford one hermetic resistor, let it be the 120-ohm Zener current control resistor (or whatever value you use for this)-- as this resistor is the most critical in the circuit.  The other resistors (the temperature control divider and the 10V divider) will tend to drift together, and we are only interested in their ratio (mostly if you discount the error caused by the output trim circuit if the 10V divider drifts).

If you do all of the above, you would probably get around 0.75ppm (7.5uV) of drift per year, which is *VERY* good.  But if you REALLY want to get crazy, forget about the hermetic resistors and LT1053 (just use the epoxy versions) and place the whole circuit on top of a double-stack Peltier device-- all inside of a hermetic package (yes, they make them that big).  Inside of the hermetic package, you would insulate the circuit with silicone foam rubber (which can handle the high heat used to solder the cover on).  The hermetic package would then be fastened to a heat sink that would be fan-cooled.  So now, instead of controlling the LTZ's die temperature to 45-deg-C, you are controlling it to 10-deg-C (or even lower if you filled the package with nitrogen or argon before sealing).  Doing this would get the long-term drift down to less than 0.25ppm (2.5uV) per year-- which is phenomenal, and would far exceed anything currently available at any price (except for a Josephson Junction Array).

How do I know all of this?   I have been studying this problem for a very lonnnnnng time...

I have already bought several LTZ1000ACH, LT1013ACN8 and Z-Foil resistors (and have some Burster) to make two or maybe 4 boards.
The hermetic packaged LTC1051/LTC1053 I have not seen/found yet. Do you know where to get it?

About your really get crazy suggestion, I would like to talk about that in more Detail. 

Thanks again for your great post.

bye
quarks
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on May 06, 2013, 10:39:25 am
...yes-- I am a "Volt-Nut" (I own a Fluke 732B), "Resistance-Nut" (I own an ESI SR104 resistor), and a "Time-Nut" (I have a GPS disciplined 10Mhz OCXO and network time standard)-- (well, really, I guess I'm really a "Precision-Nut"... Then...). 
You are indeed a "Precision-Nut" and I probably have the same "sickness".
I already have build up my own/private small "Cal-Lab" also with ESI SR104 and other goodies. Unfortunately I do not have a Fluke 732B (or equiv.) thats why I hope to be able to make a very good LTZ1000 reference.
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on May 06, 2013, 01:24:58 pm
Only LTC1051 was produced in hermetical sealed package:

LTC1051MJ8
LTC1051CJ8
LTC1051AMJ8
LTC1051ACJ8
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on May 06, 2013, 04:50:23 pm
The building phase of my LTZ1000A reference is over, besides a yet undecided buffer opamp removal which will happen after the next measurement for longterm behaviour after 1 or 3 months. Or not.

It was measured against a HP3458A for 31.75 h statistics: standard deviation 0,09 ppm, measured values spanning a range of 0,55 ppm. 24 h statistics: standard deviation 0,06 ppm, spanning 0,47 ppm.
(Measured in 5s intervals.)

Sure good enough for me. Thermal/Stress relief drills close to the LTZ1000A, stress relief milling for the connectors,
quarks knows the board.

BR

Hendrik
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on May 06, 2013, 06:29:32 pm
...What you are building here, is something close to (or possibly even better than) the Fluke 732B.

Close to a 732B, that would be a great result for my project.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on May 06, 2013, 06:33:23 pm
quarks knows the board.
yes thanks to Hendrik, I already have two of his boards for experimenting purposes
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on May 06, 2013, 06:59:49 pm
@quarks: And I think I am going to visit you when you have finished :)

I am very happy about the outcome of my reference. Way better than its ovenized REF102-based predecessor.

I am currently preparing a webpage about this device with basically the things I already told you and have shown here, where I will put the future measurements too. Just because there seems to be interest.

Having 2 useless HP boatanchors sitting here (HP6632B which spikes to full voltage and shuts output off see other thread here and a HP3455 that has trouble starting up and which 100V range reading of a 10V value is about 0.4% higher than in 10V range, possibly some switching FET died.) there are other things to do. But the LTZ Reference & Dekavider will certainly help with validating the 3455A repair, but still wish me luck and patience :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Rufus on May 06, 2013, 07:19:56 pm
The "X"'s in the ap-note you attached are exactly what you said.  Just create a 2-lead symbol in your schematic editor and a footprint in your PC design package that it just 2 pads very close together-- these are shorted together with solder.  The reason this is done in this circuit is to balance the thermal EMF's of the various connections made with dissimilar metals.

So how is that supposed to work? If you have essentially identical copper->solder and solder->copper junctions in series how is that supposed to generate any emf unless there is a thermal gradient across the blob of solder?
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on May 06, 2013, 07:44:27 pm
Quote
The Z-foil resistors you bought are quite good-- the newer Z1-foil resistors only have a slight improvement over them (mostly in load-life).  Since you will be operating the resistors at very low power levels (in relation to their ratings), there should not be much long term drift anyway.  Make sure you bake them in an oven at 125-deg-C (~257-deg-F) for 1000 hours before you install them-- this will help "season" them, so the initial drift will be very low.

You can also pre-age the resistors by power cycle the resistors near the specified maximum rating with an ac voltage, 1.5h power cycle, half an hour cooling and again...
Something similar is also described in Vishay's app notes.
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on May 06, 2013, 08:03:32 pm
Quote
Yes-- if you have no thermal gradients, you will have no thermal EMF's.  Unfortunately, you can try to get rid of the gradients, but it is impossible to do so completely-- thus the need for other solutions.  I think I mentioned on an earlier post, that using epoxy-glass printed circuit material for this kind of precision circuitry is just *asking* for trouble.  This board material (like most epoxy materials) soaks up the water from the air, and then swells-- creating higher leakage and mechanical stress on the components.  With this kind of circuitry, it is wise to use a higher grade material-- like Rogers "RT/Duroid 6035HTC"-- they won an award for developing this material-- it is made of a mixture of powdered alumina and PTFE (Teflon)-- and so you get excellent electrical properties, with very low thermal impedance and gradients, which is exactly what is needed in this application.

What about using LTCC as substrate? In this range of precision this can't be that expensive and you could spend the second layer for an temperature control circuit such as this:

http://www.kuhne-electronic.de/en/products/crystal-heater/qh-40-a.html (http://www.kuhne-electronic.de/en/products/crystal-heater/qh-40-a.html)

a single sided design.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Rufus on May 06, 2013, 08:22:34 pm
Wall of text omitted

So basically you can't say how it is supposed to work.
Title: Re: Ultra Precision Reference LTZ1000
Post by: c4757p on May 06, 2013, 10:20:41 pm
He's quoting your entire, long previous comment and leaving out the text so as not to fill the reader's screen.
Title: Re: Ultra Precision Reference LTZ1000
Post by: chickenHeadKnob on May 07, 2013, 12:00:12 am
@diligentMinds:

I am still digesting the references you have given, thank-you. One question to clarify: you state that epoxy packaged parts are still useable if the entire unit is in a hermetic (ovenised) package, does that also apply to FR4 ?  For the hobbyist, not having to chase down exotic circuit board material and rare parts would be a great benefit. I am pretty sure I can construct an aluminum can with teflon gaskets and purge/fill with dry nitrogen at a lesser cost.

Edit: sorry I see you already answered this, use the best stuff you can get. I need to do some experiments in a chamber just to confirm to myself how good or bad FR4 is.
Title: Re: Ultra Precision Reference LTZ1000
Post by: chickenHeadKnob on May 07, 2013, 08:45:59 am


Unless you use a package that has true hermetic sealing (borosilicate glass and Kovar steel, with the cover silver-soldered on), I'm not certain that you would be able to keep the humidity out.  Water vapor has a strange way of finding a path through most materials-- including most epoxies and most silicone compounds.  Like this (for example):

http://www.sinclairmfg.com/catalog/hybrid.html (http://www.sinclairmfg.com/catalog/hybrid.html)

The problem with the Kovar/Pyrex hermetic packages is that the Kovar leads make *great* thermocouples when in contact with copper-- this is *not* what we want-- (is there another way that is effective)?

I'll be the first to admit that I am *not* a mechanical engineer-- Is there someone on this forum that can speak to this?  How do we make a high-performance hermetic package using materials a hobbyist can obtain?  We want something that will stop water vapor from entering the package, and (secondarily) we want to mitigate the effects of barometric pressure changes.  (The temperature we will control actively).

I am way behind and trying to play catch-up hockey here, so my ideas are likely something you have already thought of and discarded. As I only possess a simple mind my solutions tend to the brutally  simple. Some ideas I have been toying with tonight:

with all of the above I am also considering an oil bath (degassed,dewatered mineral or silicone) with pressurized nitrogen headspace, connections to outside through nitrogen so that you wouldn't have leaking oil.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Hypernova on May 07, 2013, 12:32:55 pm
Regarding the issue of board thermal conductivity, how about metal core PCBs? Just wondering out loud.
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on May 08, 2013, 07:27:18 pm
Quote
I'll be the first to admit that I am *not* a mechanical engineer-- Is there someone on this forum that can speak to this?  How do we make a high-performance hermetic package using materials a hobbyist can obtain?  We want something that will stop water vapor from entering the package, and (secondarily) we want to mitigate the effects of barometric pressure changes.  (The temperature we will control actively).

Well, there are a few possible ways. You could use second-hand double oven oxco's in the bay and reuse the package, buy hermetic packages from companies like Schott:

http://www.schott.com/epackaging/german/overview/products/gtms/index.html (http://www.schott.com/epackaging/german/overview/products/gtms/index.html)

or construct your own hermetic package using rf-metal cases, copper or brass sheet metal and feedthrough capacitors for the electrical contacts from inside to outside.
Avoid sealing materials such as silicon, this is not hermetic at all, no epoxy glue nor plastic. To reach hermetical sealing prefer soldering, welding and ceramic or glas as they are diffusion resistant, exactly what you want. I again would also recommend to use LTCC, the substrate you will find if you open for example an oscillator. This could be cheaper compared to your mentioned Rogers 6035HTC.
Dry the package and fill it with nitrogen before closing it.
Title: Re: Ultra Precision Reference LTZ1000
Post by: muvideo on May 08, 2013, 08:12:55 pm
Quote
I'll be the first to admit that I am *not* a mechanical engineer-- Is there someone on this forum that can speak to this?  How do we make a high-performance hermetic package using materials a hobbyist can obtain?  We want something that will stop water vapor from entering the package, and (secondarily) we want to mitigate the effects of barometric pressure changes.  (The temperature we will control actively).

Well, there are a few possible ways. You could use second-hand double oven oxco's in the bay and reuse the package, buy hermetic packages from companies like Schott:

http://www.schott.com/epackaging/german/overview/products/gtms/index.html (http://www.schott.com/epackaging/german/overview/products/gtms/index.html)

or construct your own hermetic package using rf-metal cases, copper or brass sheet metal and feedthrough capacitors for the electrical contacts from inside to outside.
Avoid sealing materials such as silicon, this is not hermetic at all, no epoxy glue nor plastic. To reach hermetical sealing prefer soldering, welding and ceramic or glas as they are diffusion resistant, exactly what you want. I again would also recommand to use LTCC, the substrate you will find if you open for example an oscillator. This could be cheaper compared to your mentioned Rogers 6035HTC.
Dry the package and fill it with nitrogen before closing it.

Another option, for prototypes at least, is copper clad pcb board used as enclosure,
35um copper should be enough as moisture barrier.

As for sealing materials,  it's true that most polymers are unable to stop water.
Silicone is uneffective as barrier, it let pass water and also higher molecular weight
vapours,  we learnt the hard way, I had some headaches with white leds blackening,
and then understood that silicone dome is essentially transparent to most vapours.

The only exception I know is parylene, AFAIK it's used as conformal coating, and
is deposited using chemical vapour deposition, not easy to deal with.

I'm still wondering if there is any cheap way of pumping out the water, to keep the
umidity constant.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on May 09, 2013, 03:45:17 pm
I have worked on the schematic a little more (if I go for a experimenting board, where I can change between different OP amps and Output options), but do not yet know if I will go that way.
I have implemented the X=solder-copper-junctions and jumpers to be able to either connect to Zener + Sense directly or after the chopper amp buffer or if I want to, to connect the KVDs (like in AN86 Figure C7).
I also think about guard rings (as descr. in att.) but do not yet know how to exactly do it with DipTrance. 

For the best possible results I will probably make a design without any jumpers/sockets. But I first want to find out what is the best possible solution.

Comments and/or more hints or are welcome
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on May 09, 2013, 04:32:13 pm
I consider a big guard polygon sufficient, there is not a big need to count picoamps here and use guard rings which are seemingly a problem in DT. Compared to real picoampere applications where a leaked electron is gone when it is gone, the ZENER+ supply will provide the next one. I wouldnt put too much sub units (amplifiers) on the reference board; maybe you want the amplifiers behind a KVD, maybe not, and good housing is easier with a smaller PCB.

You can easily use an external buffer/amplifier circuit and measure the voltage difference from the well-known input coming from the LTZ to your output if it is external, an internal buffer hides the reference from you if you dont go for a test point. (My possible-problem, my buffer brings up a unknown value of 35-40 µV offset but hopefully stable.



Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on May 10, 2013, 09:20:04 am
The most sensitive point on this circuit is the base connection to the temperature sensor transistor on pin-6 of the LTZ1000.  You could create a guard-ring (which is just like all other traces in your PC design package) that surrounds all of the connections to this node (on all layers of the PC board).  The guard would be connected to the collector of that same transistor (pin-8 of the LTZ1000).  The rest of the circuitry will work just fine without any guarding. 

Very good, I will try the guard ring around pin-6.
What do you think about the guard copper pour on both layers connected to the case? Will this help if it is also inside the ring?

If you use higher quality PC material (the material I already suggested in a previous post, or the ceramic material already mentioned by others), then this guard ring could be left out-- *however* do *NOT* use water-soluble flux for soldering-- this leaves a residue behind that is very difficult to remove-- and it is conductive enough to cause problems with sensitive circuits such as these.

Does anyone know a PCB maker (pref. in Europe) offering PCBs with this material?
What is the best way and stuff to clean the board? I have tried several (IPA, Aceton, pure bio Ethanol), but I am not yet happy, if I look at it under my microscope after cleaning. Even "my special mix" for camera cleaning looks not quite perfect.

...LT1881, it seems that this part doesn't have sufficient common-mode [Vcm] down to ground on the Inputs...bias the LTZ1000 *must* work down to 0V on the inputs.

thanks I missed that (LT1881 is now off the list, already changed it in the schematic)

The LT1051A has excellent specs for this purpose-- and you can probably find original units in the ceramic package...
I'm guessing Minco-Tech will charge you ~$10-$20 for each one in small quantities.  ...
...I would use an LT1051A, because it has the best long-term (time) drift and temperature drift of all of the possibilities...
If your not going for the highest accuracy possible, then, what's the point?

That is the plan and I will try to do the best I can achieve (within my limits).
I will now first order the easy to get LTC1051CN8 package, so I can at least have a first look at it and compare to the LT1013 I already have.

Thanks again for sharing your knowledge
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on May 10, 2013, 09:56:19 am
Hi Quarks,

The guard ring: should be on both sides of the PCB, and be the first thing the guarded node sees, so no cooper pour inside the guardring. Dont just guard pin 6, but the whole node. I suppose you will not be able to discern if you lose anything when you don't guard but as it is free...

Advanced PCB substrate: Leiton HF provides rogers RF materials, will not be cheap. You might want to get a brick of teflon and press cooper nails in it and build your circuit from nail to nail.

For cleaning:  Run the populated board through the dishwasher gets rid of a lot. Or try to get your hands on dichlormethan (toxic!). I have Tetrahydrofuran as a plastics solvent at work, so not so healthy, if you want to give it a try and report back, i could bring you a sample.

You might want to do a hermetic housing for the reference circuit, suggestions:

Use the same tuner-style sheet metal boxes from otto schubert that I used. If you solder them all around you should have your hermetic seal and the sheet metal will spread heat if you use a double oven - cooler method.
To connect the power supply and extract the stable voltage and test points, use thru-hole capacitors - otto schubert or oppermann have them. Ceramics should block water better than silicone and plastics.

You could heat it up gently for a while - below 70°C for a extended period of time, and when sealing use cold air which contains low humidity and silicagel  to keep water vapour content out of the sealed box.

Include a heating resistor and a NTC inside the box, too, in case you want to double ovenize it one day or at least want to know the inside temperature.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on May 10, 2013, 11:08:40 am
Hello Hendrik,

yes I alredy planned to have the guard ring around all involved connected parts (similar i.e. see att. Datron/Wavetek board). That makes a ring around a "quite large" space. That is why I wonder, if I also should have the copper pour inside connected. 

If I can find a manufacturer with Rogers or similar material, I will hopefully not be shocked by the price. I already invested so much time and money, therefore it will probably not stop me to go for it. But we will see. 

About your dishwasher tip, I would not do that. I do not even like to use my ultrasonic cleaner, because I do not want the whole assembly ducked in the fluid.
About Tetrahydrofuran cleaner, that sounds like asking for cancer and I will stay away from it.

So far I have pre choosen ROSE housing like
http://www.reichelt.de/Rose-Aluform-Gehaeuse/ROSE-04-08-12-08/3/index.html?;ACTION=3;LA=5;ARTICLE=102470;GROUPID=5200;artnr=ROSE+04.08+12+08 (http://www.reichelt.de/Rose-Aluform-Gehaeuse/ROSE-04-08-12-08/3/index.html?;ACTION=3;LA=5;ARTICLE=102470;GROUPID=5200;artnr=ROSE+04.08+12+08)
It is not exactly hermetic housing but at least IP66 sealed.
But I also would like to see your solution, when we meet.

Temp meassurement inside the box is an option I have already thought of, but not yet implemented.

bye
quarks
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on May 11, 2013, 12:37:02 am
Quote
So far I have pre choosen ROSE housing like
http://www.reichelt.de/Rose-Aluform-Gehaeuse/ROSE-04-08-12-08/3/index.html?;ACTION=3;LA=5;ARTICLE=102470;GROUPID=5200;artnr=ROSE+04.08+12+08 (http://www.reichelt.de/Rose-Aluform-Gehaeuse/ROSE-04-08-12-08/3/index.html?;ACTION=3;LA=5;ARTICLE=102470;GROUPID=5200;artnr=ROSE+04.08+12+08)
It is not exactly hermetic housing but at least IP66 sealed.
But I also would like to see your solution, when we meet.

Dear countyman, you better use TEKO 371 / 372 / 373 / 374 / 392 / 393 / 394 / 1680 / 16120 or 16160 instead of your IP66 case.

I clean my pcbs with IPA in a bag in a heated ultrasonic cleaner, that works fine for me.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on May 12, 2013, 01:06:27 pm
Dear countyman, you better use TEKO 371 / 372 / 373 / 374 / 392 / 393 / 394 / 1680 / 16120 or 16160 instead of your IP66 case.

I clean my pcbs with IPA in a bag in a heated ultrasonic cleaner, that works fine for me.

Hello branadic,

thank you for your reply.
The TEKO could be good for inner shielding. But they only seem to be max. 28mm high (that is not enough, when I want to use my Burster resistors). For the outer box I want a decent + sealed one and the ROSE is so far overall the best I could find.

About your cleaning method, please confirm if I got it right.
You put the fully assembled board in a bag and then fill this bag with IPA and then put it in the heated ultrasonic cleaner.
What is your tip/method after that with the cleaned but "wet" assembled board?

bye
quarks
Title: Re: Ultra Precision Reference LTZ1000
Post by: robrenz on May 12, 2013, 01:39:21 pm
Please excuse my butting in.  IMO Ultrasonic cleaning only dislodges the dirt and gets in suspension/disolved in the cleaning solution. You must thouroughly rinse after the ultrasonic with virgin solvent that is at the purity level you are after. And that rinse should be a spray rinse not a dip rinse.
Title: Re: Ultra Precision Reference LTZ1000
Post by: BravoV on May 12, 2013, 01:44:26 pm
And that rinse should be a spray rinse not a dip rinse.

Cause the dissolved contaminant even dilluted, will form an even "coating" throughout the object ? |O
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on May 12, 2013, 01:57:36 pm
There are bigger rf cases available, not at Reichelt but from several other providers:

http://www.ukw-berichte.de/ukw-docs/bau-lit/gehause.html (http://www.ukw-berichte.de/ukw-docs/bau-lit/gehause.html)
http://www.pollin.de/shop/dt/NjI5OTM1OTk-/Bauelemente_Bauteile/Gehaeuse/Gehaeuse_5.html (http://www.pollin.de/shop/dt/NjI5OTM1OTk-/Bauelemente_Bauteile/Gehaeuse/Gehaeuse_5.html)

You got me right, that is exactly the way I clean my circuits. The ultrasonic cleaner is filled with DI-water, sure. Avoid using acetone as not all parts and packages can withstand it. There are a few sorts of plastic that will get etched. I'm not sure how your Burster resistors will behave.
After cleaning I dry the board with compressed air, but you can also put it in an oven for a while.

I don't know a manufactor for LTCC or the mentioned Rogers 6035HTC in germany, but maybe the use of RO4003C is an alternative choice? There is a german guy who manufactures them for a fair budget www.pcb-devboards.de (http://www.pcb-devboards.de) also in 1.6mm. I expect this material has lower moisture absorption compared to FR4 and the Tg is much better Tg>280°C DSC (IPC-TM-650 2.4.24).
Even if it is said to be not nessecary I would nevertheless provide the slots in the substrate. Do what ever you can to get minimal stress to the leads. You will find statements that quote that this package is not sensitive to stress, but I wouldn't count on that if you want the last ppm.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on May 12, 2013, 03:46:57 pm
There are bigger rf cases available...

You got me right, that is exactly the way I clean my circuits...
After cleaning I dry the board with compressed air, but you can also put it in an oven for a while.

I don't know a manufactor for LTCC or the mentioned Rogers 6035HTC in germany, but maybe the use of RO4003C is an alternative choice? There is a german guy who manufactures them for a fair budget www.pcb-devboards.de (http://www.pcb-devboards.de) also in 1.6mm. I expect this material has lower moisture absorption compared to FR4 and the Tg is much better Tg>280°C DSC (IPC-TM-650 2.4.24).
Even if it is said to be not nessecary I would nevertheless provide the slots in the substrate. Do what ever you can to get minimal stress to the leads. You will find statements that quote that this package is not sensitive to stress, but I wouldn't count on that if you want the last ppm.
Thanks again.

the inner shielding would be nice to have, but if it not fits in size, I will have to change my board size, outer housing connector positions and so on. Therefore I think I will start with the ROSE and get a Teko 392 which seems to just fit inside. 

good, I got the cleaning part right. For a naked board this sounds good to me. But when I order my boards, I will hopefully get a perfectly clean surface. And for my soldered fully populated one, I still feel uncomfortable.

I will contact your suggested manufacturer. I also found a note, that my local PCB maker can offer a so called "Teflon" Type board (he descr. it Keramik-Polymer for HF and names Rogers, Taconic). I will try to find out details about it next week.
If I can, I will still try to get Rogers RT/Duroid 6035HTC and also ask for your suggestion RO4003C.

About the slots in the board, I still do not know, how to do it in Diptrace (other than just drill holes around the LTZ like Hendrik did, but still interested to find out). If it is easy doable and does not make problems with the rest of the layout I am considering it.

bye
quarks
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on May 12, 2013, 04:15:50 pm
Quote
And for my soldered fully populated one, I still feel uncomfortabe.

You can do the cleaning procedure twice, but up to know I had absolute no problems. As said I heat the ultrasonic bath too.

Quote
About the slots in the board, I still do not know, how to do it in Diptrace

Just draw the slots in the dimension layer and make them as suggested by the pcb manufactor (normally >2mm for the milling tool), that's it.

(https://www.eevblog.com/forum/index.php?action=dlattach;topic=15982.0;attach=44722;image)

https://www.eevblog.com/forum/projects/lm399-based-10-v-reference/msg219527/#msg219527 (https://www.eevblog.com/forum/projects/lm399-based-10-v-reference/msg219527/#msg219527)

But why not use an "off the shelf" program such as Eagle Light or KiCad?
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on May 12, 2013, 06:31:44 pm
You can do the cleaning procedure twice, but up to know I had absolute no problems. As said I heat the ultrasonic bath too.

with a "cheaper" design I would just give it a try, but here :-//


Just draw the slots in the dimension layer and make them as suggested by the pcb manufactor (normally >2mm for the milling tool), that's it.

(https://www.eevblog.com/forum/index.php?action=dlattach;topic=15982.0;attach=44722;image)

https://www.eevblog.com/forum/projects/lm399-based-10-v-reference/msg219527/#msg219527 (https://www.eevblog.com/forum/projects/lm399-based-10-v-reference/msg219527/#msg219527)
in principal I know what to do, but in Diptrace I just have not yet found out how to really do it.

Have you build the linked board? That really looks great  :-+!!!
What have you used to make it?
How did you make the thermal isolation block, is it Teflon or plastic?

But why not use an "off the shelf" program such as Eagle Light or KiCad?
I have used Eagle many years ago and now tried Diptrace, which I already like better.
KiCad I have not yet tried. Have you compared these programms?

bye
quarks
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on May 12, 2013, 07:12:36 pm
Yes I have build the linked board twice, one for a friend and one for me. It was also cleaned in IPA with ultrasonic. ;)
After first measurements I replaced the LT1001 by the CERDIP version, had the luck to find two of them in the bay.

I made a >300h thermal burn-in at 120° without power supply, it was then running a few weeks. I then assembled a heater on top of the resistors cause I only used 5ppm 0805 SMD resistors and it is now running a few weeks in the lab at 21.4°C and RH=32% with heated resistors. The reference is stable at 10.00179V-10.00180V there, measured with a 34401A the whole time. Still have no own >6.5 digit meter yet.

The board is normal black FR4, manufactured at pcb-devboards. The layout was inspired by the different pictures of boards with the LM399 in web.
The thermal isolation is made off PA6/6, two caps screwed together and filled with non-conductive foam, the slots are still free inside. It is just to keep airflow away. It's not that I have a use for this board as the voltage is also not common these days, but it was nice to see what can be reached with small amount of invest.

I've tried KiCad but decided to use Eagle Light as we have the "professional" version at work, so I can easy transfer bigger and multilayer designs at work and finish the layout there.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on May 12, 2013, 07:23:05 pm
@Quarks: There is a PCB distribution house right in your town, chances are good that you can ask them for Diptrace hints.
I can tell for sure as my wife was working there, spending a lot of time explaining customers (like me) how to do stuff with EAGLE, as that was the CAD system she was responsible for :-DD



Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on May 12, 2013, 07:40:08 pm
Quote
I can tell for sure as my wife was working there, spending a lot of time explaining customers (like me) how to do stuff with EAGLE, as that was the CAD system she was responsible for

There is no need to discuss about the quality of Eagle, it's far away from professional even in the "professional" version, the libs are mostly buggy and wrong in layout, but if you have updated most of the libs it is as easy to use as MS Paint :)
I have made a lot of boards with Eagle up to now, from rf to high resolution pcb based sensors, high precision or simple analog stuff, test equimpent like my DC-500MHz FET probe etc. It is up to the user to be able to calculate impedance matching and things like that, but it's not always a disadvantage to be able to do that. It's good to have the basic layout skills. ;)
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on May 13, 2013, 09:36:13 am
@Quarks: There is a PCB distribution house right in your town, chances are good that you can ask them for Diptrace hints.

I already contacted them. They unfortunately do not seem to know Diptrace at all.

edit: with the kind help of fmaimon here is the solution http://www.diptrace.com/forum/viewtopic.php?f=4&t=8964 (http://www.diptrace.com/forum/viewtopic.php?f=4&t=8964)
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on May 15, 2013, 01:07:07 pm
Lets not start a hate-war-religion-thread about layout packages :-DD

Does any of you gentlemen seeing the need for burn-ins and big magic slots and guardrings and vhp202 resistors have a method to show or measure the improvements of those thingies against a easy "cheap-ass"approach as mine? How much better do you want to be ?

Title: Re: Ultra Precision Reference LTZ1000
Post by: saturation on May 16, 2013, 11:10:41 am
I wouldn't be surprised if your 732B actually is far less than 2ppm/year as its a much aged reference.  The real trick is measuring that because at least either the reference or your DMM will need calibration against a known standard in the calibration cycle to check the rate of drift of both.  From the factory the 732B is 0.8ppm in 3 months.  The real hard part in making a metrology setup is tracking the data over time to insure the stability is true.


It's not totally "easy" to make a voltage transfer standard that holds 2ppm per year-- but is can be done by the average engineer using average (and well known) approaches to the problem.  But-- (short of going out and buying a million-dollar Josephson Junction Array)-- to get below 2ppm per year takes a lot of study, hard work, patience, and help from your friends.

I already own a Fluke 732B, which (according to it's published specifications) will hold 2ppm (or less) per year.  So, I want something better.  I want *less* than 1ppm per year, and my goal is (somehow) to achieve 0.1ppm/year-- a lofty, but worthwhile goal (for me at least).  For most people, the Fluke 732B is "Good Enough"-- but not for me.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on May 16, 2013, 04:59:29 pm
The principle of calibration (how does a reference or DMM agree with a better one?) and the quest to distribute the SI representation is clear to me.

My cheapo-no-magic reference aka "Vbs" ( Volt babysitter) was compared to a volt-nuts 4 other references (including a LTZ in a 3458 which was used for the 24h, 31h, 1 Week and noise experiments) that haven't been calibrated relative to a cal-lab for a while but their behaviour is closely compared to each other regularly. (If one of them wants to go for a ride voltagically speaking, you wouldn't realise if it is alone, but it will visibly differ from its siblings!)

My Vbs  behaves closely like that cluster, how it differs in the long term I will see, got it back yesterdays from the first experiments. Now the Vbs is on its own for say one or three months...

What I meant with my last question is that it is increasingly getting harder to tell who is the reason for a deviation, the noise and drift of the Vbs is no longer easy to discern from that of the 3458A.

Also, in October iirc I will get a 34401A at work back from the Böblingen Cal lab of Agilent. This has not the greatest long-term-stability, but immediately after arriving it will be reasonable close to what they measured against their traceable references. Knowing the instruments error as noted by the lab, taking some readings from the babysitter volt abusing the 34401As gratuitous extra digits coming out of GPIB will give a cloud of datapoints for you know what... the VAgilent vs. Vbs and later Vbs against Vvolt-nut and also Vbs vs VQuarks .  :blah:

But I have to tell, the main reason for the LTZ reference is the predecessor, a REF102-based 10V, did not comply with my minimum requirement: Measure it at work, stuff it in the backpack, take it home and back to work the next day, measure again, delta only at the last digit. This one does so, after travelling much more. Now I can trust it for testing the DMM at home.

Greetings,
Hendrik
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on May 19, 2013, 05:36:15 pm
...any capacitor in the signal path should be high quality metal-film, or NP0 [aka: C0G]...

So far I have planned to use WIMA MKS for all caps. But also I will try to get NPO caps. Is there a recommended brand/type to go for? 
Title: Re: Ultra Precision Reference LTZ1000
Post by: saturation on May 20, 2013, 07:57:19 pm
Several years ago I explored doing a more robust reference but found the old Krohn Hite portable calibrators, like the MV106, incidentally also purchased by Dave about the same time.

Its rated for ~ 2ppm/yr, through a range of operating temps.  Since it was made over 30+ years ago, the references are well aged, and the unit robustly made.  But kept at room temperature over just short a year now, the accuracy is far better, <<= 1ppm.  You can find these on eBay for under $100, with some watchful waiting, or choose variations of that voltage references made by KH from that era; they have a similar look but differ in their dial-a-volt capacities.

(http://www.emeco-sa.com/img/productos/instrumentacion/patrones_laboratorio/krohn-hite_mv106.jpg)

Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on May 22, 2013, 04:07:30 am
@Dilligentminds.com

Looks like a reference diode Kindergarten to me :) Nice to see such a project coming up even if it differs from the topic LTZ1000A...

Does it differ from the 4400, and if, by how much ?

In my opinion its too many resistors for trimming, especially the trimpot. If it is not something special to cancel out individual drifts and tolerances, I would go for one single hi-spec feedback resistor at the op and the resistor to gound would be a slightly-too-low hi-spec value in series with a few lower-specced low-resistance types calculated for slightly too much resistance of the series circuit, and short each of the low-value resistors on the PCB.  This way I can use a knife to "activate" the additional low-resistors one by one - and solder bridge if i activated one too much. 

@Whoever is making a own circuit:
Learn from my fail (no test point for unbuffered voltage) and make a test point for the raw voltage before the amplifier !
This way you can either measure dc:dc ratio or simply the ~2.8V difference (with LTZ, ~3.1V here)  between your trusted Vref and what the amplifier does.

I will either accept what my buffer does to my reference or do a opampekcomy (decide later after watching longer-term drift), may set up a external chopper amp circuit which will be used with a existing KVD to make other voltages.

Still, I am confident that nailing together the AN86 circuit with standard means without black magic (teflon or Al2O3 board, stress relief slots, ultralow temperature, super secret thermal cycling and aging, VishayPG VHx Resistors) is sufficiently close to optimum that you you have a hard time to prove the difference to references made with black magic by usual means like 8.5 digit multimeters. 

Title: Re: Ultra Precision Reference LTZ1000
Post by: grenert on May 23, 2013, 01:29:54 pm
taking some readings from the babysitter volt abusing the 34401As gratuitous extra digits coming out of GPIB
Sorry for the off-topic (but in the spirit of volt-nuttery in this thread):
Does the 34401A put out more than 6.5 digits via GPIB?  I know the 3457A can do this, but had not heard the 34401A does the same.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on May 24, 2013, 04:34:33 pm
taking some readings from the babysitter volt abusing the 34401As gratuitous extra digits coming out of GPIB
Sorry for the off-topic (but in the spirit of volt-nuttery in this thread):
Does the 34401A put out more than 6.5 digits via GPIB?  I know the 3457A can do this, but had not heard the 34401A does the same.

Hello,

yes, definitely, 7 1/2 digits over the bus in each mode, if I remember right..
If you use the internal statistics, additional digits are also available.
I did not check recently, how stable the additional digit is, but have in mind that it's quite useful (for the LTZ1000 and on 4WOhm measurements), at least when additional averaging is applied.

Update: Just've checked by programming:
The GPIB output format is: x.xxxxxxx0E+yy, i.e. 8 digits, the ninth is zero.
The 7th digit is relatively stable.
It's better to average on 8 digits than on 6,5, because the average converges quicker.

The Min/Max function already delivers stable 7,5 digits for the average also, which I have used for measuring with higher resolution without GPIB.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: alm on May 24, 2013, 04:40:18 pm
You can get the extra resolution from averaging anyhow, so if it's down in the noise, you might as well average the 6.5 digit value.
Title: Re: Ultra Precision Reference LTZ1000
Post by: alm on May 24, 2013, 06:21:13 pm
I believe the recent(ish) Keithley 200x meters give more digits over GPIB. No idea how significant they are.
Title: Re: Ultra Precision Reference LTZ1000
Post by: cyr on May 24, 2013, 10:39:35 pm
The 2015 does, not sure what the actual quantization steps are but seems like an extra digit or so of usable resolution. The 2001 on the other hand gives the exact same value as the display, at least by default.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on May 27, 2013, 07:40:20 am
Hello together,

On ordinary Zeners (1N829A) there exists a "zero tempco current" where the tempco is nearly 0 ppm/K.

On 2 samples of LTZ1000A with the standard 120 Ohms resistor, which gives around 4 mA zener current, I have measured a tempco of the zener element of around 50 ppm/K. So to come to the 0.05 ppm/K the heater stability has to be better than 1/1000 K.

Has anyone tried to find out the optimum current/resistor value for the zener of the LTZ1000(A) where the tempco gets zero?
That what I tried is to change the 70K Resistors. Lowering them to 50K will increase the tempco of the reference slightly.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Rufus on May 27, 2013, 08:32:04 pm
On ordinary Zeners (1N829A) there exists a "zero tempco current" where the tempco is nearly 0 ppm/K.

It isn't an ordinary zener it is a zener + forward biased diode combination and the temperature coefficient of the diode forward voltage depends on the diode current. As far as I know zener avalanche breakdown voltage temperature coefficient isn't dependant on avalanche current.

The LTZ1000A nominally 7.2v 'zener' is the combination of the zener and transistor Q1 Vbe so is is the same and you can control Q1 emitter current by changing the collector resistor. If dropping from 70k to 50k made it worse I assume you were going the wrong way.

The LTZ1000 datasheet shows a circuit to trim the temperature coefficient when you don't bother to use the heater, you would think the same approach could also improve the temperature coefficient when you do.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on May 27, 2013, 09:31:24 pm
Quote
I would also encourage you to use (2) LTC2057 op-amps to control your LTZ1000(A)-- these op-amps have almost zero drift over time, and they have much lower wide-band and DC-10Hz noise than the LT1013 used in the original applications note. 
Thats good news for me the LTC2057 is noted on my wish-list. Up to now there are only samples available in LTC web shop. But I think they will be soon available on DigiKey too.

Quote
I strongly suspect that the ~7V buried Zener in the LT1034 is the exact same one that is used in the LTZ1000(A),
I cannot believe this: If I compare the "voltage change over current" diagrams of LTZ1000 and LT1034 at 1.25 mA the change of LT1034 is much larger (100mV) against the LTZ1000 (20mV). The LT1034 is developped as "micropower reference".

Quote
The LTZ1000 datasheet shows a circuit to trim the temperature coefficient when you don't bother to use the heater, you would think the same approach could also improve the temperature coefficient when you do.

Hello Rufus,

my intention is to use the sweet spot by changing the 120 Ohms resistor to either enhance the stability or be able to use cheaper resistors for the temperature setpoint. I know the cirquit from the datasheet which makes only sense if you need a low noise source. The additional resistor adds its own tempco to the cirquit and increases the output voltage by up to 1V.

So is there anyone who tried changing the 120 Ohms resistor and checked tempco before and after?
Or does anyone shurely know the maximum current limit for the LTZ zener without degradation?

with best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Rufus on May 27, 2013, 11:07:00 pm
my intention is to use the sweet spot by changing the 120 Ohms resistor to either enhance the stability or be able to use cheaper resistors for the temperature setpoint. I know the cirquit from the datasheet which makes only sense if you need a low noise source. The additional resistor adds its own tempco to the cirquit and increases the output voltage by up to 1V.

In typical LTZ1000 circuits the zener current is very dependant on temperature (about -16uA/C) combined with the slope resistance of 20 to 60 ohms that gives a relatively huge -ve zener voltage temp co of around -640uv/C at 40 ohms. Yet that temp co + the avalanche temp co + Q1 Vbe temp co add up to almost nothing. That has to be by design. Who knows, they might even laser trim some deliberately added slope resistance to make them sum to nothing with 100uA Q1 emitter current.

So yes it seems adjusting the 120R will adjust the effect of the slope resistance temp co. Adjusting some added slope resistance (like the 200 ohm variable resistor in the datasheet circuit) will also adjust the slope resistance temp co. Adjusting Q1 collector resistor will adjust the Q1 Vbe temp co.

It is strange that the datasheet adjustment circuit seems to indicate you can null the overall temp co just by adding slope resistance without other component changes.

Which is best to adjust I don't know. I would imagine shipped parts are close to their sweet spot with 120R and 70k resistors, by design and/or trimming.

Won't there already be some 'voltnut' talk about stuff like this? I'm just looking at the circuit and making it up.

 
Title: Re: Ultra Precision Reference LTZ1000
Post by: Rufus on May 28, 2013, 02:14:01 am
Snip another wall of text with lots of knowledge and not enough understanding.

We are not trying to adjust an LTZ1000 Zener current to find a 'sweet spot' because there isn't one. Show me a reference that indicates zener avalanche voltage temperature coefficient depends on avalanche current.

We are trying to cancel the temperature coefficient of the zener avalanche voltage with the temperature coefficient of the Q1 transistor Vbe. Q1 Vbe temperature coefficient will drop by about 200uV/C per decade increase of collector current so changing the collector resistance is a valid way to adjust the temperature coefficient matching.

Additionally the recommended circuit configuration of the LTZ1000 gives the zener diode current the same temperature coefficient as the Q1 Vbe. That current temperature coefficient produces a zener voltage temperature coefficient proportional to the zener slope resistance / the 120R. Using the mid range datasheet slope resistance of 40 ohms means the effect of the Q1 Vbe temperature coefficient is multiplied by 1.3.

So yes you can change the balance of temperature coefficients by adjusting the 120R with the likely undesirable side effect of changing the zener current. You can also change the zener slope resistance in one direction by adding some resistance on top of the zener. As I said I would not be surprised if they add and laser trim some resistance on top of the zener to get the temperature coefficient match in the first place (has anyone cut the lid off one to look?).

Lol at sitting for hours with an LTZ1000 in an oven with Q1 collector base shorted to determine a perfect operating point which the circuit doesn't actually use. What winds me up is you are supposed to be the volt nut and I am just an electronics/software engineer who has never even seen an LTZ1000.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Rufus on May 28, 2013, 09:17:01 am
Well, we will just have to disagree.  You keep talking about "avalanche mode" and I am talking about the transition point between "tunneling mode" and "avalanche mode";

I keep talking about avalanche mode because the zener in the LTZ1000 is required to operate with a +ve temperature coefficient of around 2.3mV/C which means the zener is predominantly if not completely in avalanche mode. The transition point (if it has one) is irrelevant it doesn't operate anywhere near the transition point.

We are trying to cancel the temperature coefficient of the zener avalanche voltage with the temperature coefficient of the Q1 transistor Vbe. Q1 Vbe temperature coefficient will drop by about 200uV/C per decade increase of collector current so changing the collector resistance is a valid way to adjust the temperature coefficient matching.

Good luck with that.  It didn't work in my simulations, nor did it work on the bench with any of the LTZ1000 circuits I tried.  But, maybe I did it all wrong-- why don't you show me the Right Way?

Andeas stated a couple of posts ago that changing the 70k to 50k did change the temperature coefficient of the reference slightly. Vbe temperature coefficient is proportional to log of collector current so I am not surprised a 30% change in current only made a slight difference. You might also consider this from the Fluke 732B manual
Quote
Biasing the Refamp for Low Temperature Coefficient 4-8.
As mentioned earlier, the Reference Amplifier contains an NPN transistor and a zener diode in series. The TC (Temperature Coefficient) of the Reference Amplifier is the sum of the TC of the zener voltage and the transistor base-emitter voltage. The zener voltage TC is negative and the transistor TC is positive with a value dependent on its collector current. Each Reference Amplifier is pretested to determine the collector current at which the two TCs cancel out yielding an overall Reference Amplifier TC very close to zero. To generate this same collector current in the standard, a voltage of 2.976V is generated across thin film resistor Z1-R3 on the Reference Hybrid (HR1). This resistor is pretrimmed with a laser to the value that results in the correct collector current.

So yes you can change the balance of temperature coefficients by adjusting the 120R with the likely undesirable side effect of changing the zener current. You can also change the zener slope resistance in one direction by adding some resistance on top of the zener.

Well, as I said, good luck with your experiment.

What experiment? The LTZ1000 datasheet already shows resistance added on top of the zener as a method of adjusting overall temperature coefficient. What do you think the intentionally introduced -ve temperature coefficient of zener current is going to do with the zener slope and any added resistance?

Thanks for the link to the LTZ1000 die photos. I don't see any trimming so I guess they just rely excellent temperature control which probably indicates there is scope for improved temperature coefficient nulling.

Therein lies the difference between you and I-- I have been working in electronics for over 30+ years, and have built my fair share of electronics projects.  And yes-- I have built many voltage references using almost all of the available components.  I have built dozens of circuits based on the LTZ1000, many of them with novel improvements which I cannot go into here due to patent issues.

If you have built so many LTZ1000 based references especially with novel improvements you ought but apparently don't know how they work.
Title: Re: Ultra Precision Reference LTZ1000
Post by: nukie on May 28, 2013, 11:25:34 am
I love how our friends in China are pushing the development of precision DIY references. Having easy access to affordable used/recycled precision parts sure helps alot. Unfortunately, it seems that used LTZ1000 is getting rare lately which drives up the prices. This is v3.0 of the LTZ1000 reference board I got(v2.0) from jj3055, use lots of high precision resistors. v3.0 improvement is low noise suitable for 10V buffer but it is not tested for long term stability. V2.0 is the 'community' standard.

http://item.taobao.com/item.htm?spm=0.0.0.0.jPEYqD&id=22720224575 (http://item.taobao.com/item.htm?spm=0.0.0.0.jPEYqD&id=22720224575)

More expensive version sports the AE metai foil oil filled resistors.

http://item.taobao.com/item.htm?spm=0.0.0.0.jPEYqD&id=17294769547 (http://item.taobao.com/item.htm?spm=0.0.0.0.jPEYqD&id=17294769547)

(https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=49067;image)
Title: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on May 28, 2013, 09:10:13 pm
Dear fellow-nuts & EEVbloggers,

I’ve followed this thread here and in other forums around the LTZ1000 with great interest, especially the discussion about gimmicks (*), additional to the original simple design from the datasheet.

But on the implementation of those gimmicks I am desperately missing well-founded engineering & metrology practices, as:

1) A qualitative explanation, what their purpose is / how they work / root cause of additional instabilities (theoretical model)
2) A quantitative analysis, how much they  mitigate possible instabilities (error analysis calculus)
3) Practical stability measurements on realized LTZ1000 references, especially on the effects of additional gimmicks
4) An estimation, if the additional gimmicks noteworthy improve the LTZ1000s basic circuitry stability (e.g. 1ppm/yr., 0,2ppm/K), or if they contribute in the range of a few tenths ppm only, which would make them useless or exaggerated.

Instead, most designs only copy those gimmicks found in references of Datron, Fluke or HP, obviously without understanding, and forgetting the most obvious aspects.

I have to admit, that me too, have only copied the basic circuitry from the LT datasheet, but the few features I have considered or added, have been analyzed deeply in terms of 1) – 4).
Also, before I started, I had a design goal concerning the stability of my box, which was met.

I also have designed additional circuitry to set up a complete reference system, i.e. to calibrate the complete DCV range on a calibrator like the Fluke 332B, or a HP34401A, from one LTZ source.

I would like to present my design and my findings in the following days, also presenting some ideas, what perhaps is much more important than all those gimmicks (i.e. low hanging fruits)

Anyhow, I’m still interested in a profound discussion of those gimmicks, and would implement them in a future redesign, but only after given explanations 1) – 4).
 
Btw: The Chinese colleagues are doing a great job of showing the interior of the Holy Grails, and also in setting up LTZ boards in numbers, but sorry to emphasize that, I did not yet find any new/improved designs, or explanations there, either. (Hope I haven't overlooked that)

Anyhow, I would greatly appreciate, if some of them also would join the discussion here, or on Volt-Nuts.

And of course, I would also like to personally meet and exchange with Metrology-Nuts here in Germany.. 'babysitter' I already have met  :-+

Frank

(*) slots in the PCB, guarding rings, usage of “A” version, excess temperatures. metal foil resistors, zero offset OpAmps, ovenizing the complete LTZ circuitry, Burn-In on assembled PCB, cleaning in US bath, TempCo trimming .....
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on May 28, 2013, 09:24:42 pm
Which is best to adjust I don't know. I would imagine shipped parts are close to their sweet spot with 120R and 70k resistors, by design and/or trimming.

Won't there already be some 'voltnut' talk about stuff like this? I'm just looking at the circuit and making it up.
Definitely no trimming as you can see on the chip photo. Btw. the transistors Q1 and Q2 are built each out of 4 single transistors in a staggered configuration around the zener. This will shurely help to average out any temperature gradients on the chip.
On voltnut we talked about the additional 200 ohms resistor which is definitely no option for me. The TC of this resistor goes with a 1:7 ratio directly to the output voltage. The other resistors have a much lower influence on the output voltage.

Quote from: DiligentMinds.com
I have built dozens of circuits based on the LTZ1000, many of them with novel improvements which I cannot go into here due to patent issues.
Can you give us the affected patend numbers?

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on May 28, 2013, 09:41:29 pm
Instead, most designs only copy those gimmicks found in references of Datron, Fluke or HP, obviously without understanding, and forgetting the most obvious aspects.

Hello Frank,

one often overlooked gimmic from the datron schematic is the 100nF capacitor from the base of the temperature sensing transistor to the emitter. Especially if there is any connection outside the shielded box (power supply) this capacitor will help to stabilize the output voltage. Otherwise any mains disturbance has a direct influence on the temperature setpoint of the reference and the output voltage will drift.

with best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on May 28, 2013, 09:55:32 pm
That's great, Andreas!

my references suffer from short termed disturbances, as such, obviously. (but that does not affect the more important long term stability)

The temperature control knot, i.e. the base of Q2 obviously is very sensitive to external influences, so far I've explored the circuitry by myself.

The box is currently undergoing a refurbishment, so I will add that.

Thanks.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on May 28, 2013, 10:13:06 pm
This very interesting tread contains a lot of hot air  :-DD but not very many builds (except from the ready built unit shown above).

The tread imo also contains quite heated arguments  :box: arising from different views on what really constitutes necessary elements in actually building a working reference. Some of the things mentioned in this tread are: Enclosure, PCB, Thermal Gradients  and EMF.

It amazes me however that some posters use all this energy on discussing the merits or lack of such for details of building without first checking datasheets (& more) from manufacturer(s). {I might be wrong, but I cannot find any references to the text below}.

Here is a "copy and paste" from  http://cds.linear.com/docs/en/datasheet/1000afd.pdf (http://cds.linear.com/docs/en/datasheet/1000afd.pdf)  :
(Please forgive me the lack of decent formatting - blame "Control-C"   :palm: ).

---

LTZ1000/LTZ1000A
4
1000afd
a
pplica
T
ions
i
n
F
or
M
a
T
ion
LT Z
1000 and
LT Z
1000A are capable of providing ultimate
voltage
reference
performance. Temperature
drifts
of
better
than
0.03
ppm/°C and long-term stability on the order of
1?V per month can be achieved. Noise of about
0.15
ppm
can also be obtained. This performance is at the expense
of circuit complexity, since external influences can easily
cause output voltage shifts of more than 1ppm.
Thermocouple effects are one of the worst problems and
can give apparent drifts of many ppm/°C as well as cause
low frequency noise. The kovar input leads of the TO-5
package form thermocouples when connected to copper
PC boards. These thermocouples generate outputs of
35?V/°C. It is mandatory to keep the zener and transistor
leads at the same temperature, otherwise
1
ppm to
5
ppm
shifts in the output voltage can easily be expected from
these thermocouples.
Air currents blowing across the leads can also cause small
temperature variations, especially since the package is
heated. This will look like
1
ppm to
5
ppm of low frequency
noise occurring over a several minute period. For best
results, the device should be located in an enclosed area
and well shielded from air currents.
Certainly, any temperature gradient externally
generated
,
say
from a power supply, should not appear across the
critical circuitry. The leads to the transistor and zener
should be connected to equal size PC traces to equalize
the heat loss and maintain them at similar temperatures.
The bottom portion of the PC board should be shielded
against air currents as well.
Resistors, as well as having resistance temperature coef-
ficients, can generate thermocouple effects. Some types of
resistors can generate hundreds of microvolts of thermo-
couple voltage. These thermocouple effects in the resistor
can also interfere with the output voltage. Wire wound
resistors usually have the lowest thermocouple voltage,
while tin oxide type resistors have very high thermocouple
voltage. Film resistors, especially Vishay precision film
resistors, can have low thermocouple voltage.
Ordinary breadboarding techniques are not good enough
to give stable output voltage with the
LT Z
1000 family
devices. For breadboarding, it is suggested that a small
printed circuit board be made up using the reference, the
amplifier and wire wound resistors. Care must be taken to
ensure that heater current does not flow through the same
ground lead as the negative side of the reference
(
emitter
of Q1). Current changes in the heater could add to, or
subtract
from, the reference voltage causing errors with
temperature. Single point grounding using low resistance
wiring is suggested.

---
Title: Re: Ultra Precision Reference LTZ1000
Post by: nukie on May 28, 2013, 10:33:04 pm
Hello Frank,
The Chinese community has more shared data and discussion on long term stability. Actually, it's a little hard to find schematics and implementation. You need to dig harder in the Chinese forums to find what you find missing.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on May 28, 2013, 10:46:54 pm

The temperature control knot, i.e. the base of Q2 obviously is very sensitive to external influences, so far I've explored the circuitry by myself.


Copied from the datasheet (link in earlier post):


It is mandatory to keep the zener and transistor leads at the same temperature, otherwise 1 ppm to 5 ppm shifts in the output voltage can easily be expected from these thermocouples.

The leads to the transistor and zener should be connected to equal size PC traces to equalize the heat loss and maintain them at similar temperatures.

..
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on May 29, 2013, 07:00:21 am

The temperature control knot, i.e. the base of Q2 obviously is very sensitive to external influences, so far I've explored the circuitry by myself.


Copied from the datasheet (link in earlier post):


It is mandatory to keep the zener and transistor leads at the same temperature, otherwise 1 ppm to 5 ppm shifts in the output voltage can easily be expected from these thermocouples.

The leads to the transistor and zener should be connected to equal size PC traces to equalize the heat loss and maintain them at similar temperatures.

..

No, I meant electrical / electromagnetic disturbances, those thermoelectrical aspects I have considered in the design and the enclosure sufficiently, I think.

If you simply measure the U(BE) of Q2 with a high impedance DMM, the stabilization circuitry will be easily disturbed and the regulation runs wild.
Accordingly, this is an entry point for EMC disturbancies, explaining perhaps my observed instabilities.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on May 29, 2013, 07:41:33 am
This very interesting tread contains a lot of hot air 

Where is your design?
Can you show us a photo? + Schematics?
Mine is shown on the profile picture.

(http://IMG_4671.JPG)

Instead of copying the whole text. A link to the datasheet would be sufficient.
The most important sentence is missing in your "citate"

Quote from: LTZ1000 datasheet
"The LTZ1000 and LTZ1000A references can provide superior
performance to older devices such as the LM199,
provided that the user implements the heater control and
properly manages the thermal layout."

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on May 29, 2013, 08:17:32 am
That's great, Andreas!

my references suffer from short termed disturbances, as such, obviously. (but that does not affect the more important long term stability)

The temperature control knot, i.e. the base of Q2 obviously is very sensitive to external influences, so far I've explored the circuitry by myself.

The box is currently undergoing a refurbishment, so I will add that.

Thanks.

Frank

Hello Frank,

i have looked up my measurements to this theme:

Without the 100nF I had following measurements on the heater output voltage (Setpoint  is 50 degrees for LTZ1000A):
5,4 V DC  + 6 mVpp with sporadic peaks of up to -40mVpp.

with 100nF at both bases of Q1 + Q2 transistors:
5,47V DC + 3mVpp with sporadic peaks of up to +/-5mVpp (10mVpp maximum).

further capacitors  C13,C14,C15 (10nF,100nF,100nF) around the heater OP-amp.
(Schematics is on Volt-Nuts).
5,49V DC + 0.5mVpp and no sporadic spikes.

My opinion: due to the large tempco of the zener section you should pay attention to the noise on the heater section too.

Theory: 40mV equals around 5.3uW heater power (300 Ohms) which will give 2.1 mK (400K/W) temperature fluctuation and 0.1ppm (50ppm/degree) output voltage variation.

With best regards

Andreas

edit:
damned: of course the heater power calculation is wrong: since the -40 mV are a offset to the 5.4V (minus 1 diode drop of 0.7V) we have to calculate the difference of 4.7V and 4.66V for the heater power which is effectiveliy 1248 uW. Or 0.5 degrees or 25ppm variation. (ok the thermal mass of the die will avoid large jumps).
Title: #1 - Setup - LTZ1000 Reference System
Post by: Dr. Frank on May 30, 2013, 06:30:43 pm
The goal of the design of the reference system was to calibrate a HP34401A on all DCV ranges, and a Fluke 332B, including its linearity to 1ppm of decks A, B; the uncertainty originating from one LTZ1000 reference only .

Here's the setup, as intended, for the case that the 34401A is the DUT.

The 332B simply serves as a high stability volt source (uncalibrated).

Its output of roughly 100V were first divided precisely (<0.5ppm) by 10.000  by a Hammond type divider (analogue to Fluke 752A).
The divided 10V were compared to the LTZ1000 box, which delivers different reference voltages, i.e.
7.1479788V for Ref_1, 7.1762318V for Ref_2, directly from both of the LTZ1000, 7.000000V divided very stable from one of the LTZs.

All three voltages can be very precisely (< 0.2ppm) amplified by 10/7, for the purpose to have usable calibration voltages for the instruments, in this case to 10.00000V.
(The 10,251760V and 10,211398V would also be accepted by the 34401A, and would have lower uncertainty).
 
The 1µV sensitive DVM compares the 10V reference to the divided one from the 332B, so that latter output is tuned to exactly  100.00000V.

Those 100.00000 V are fed into the HP34401A, for calibration.

All Cardinal Points: 1kV, 100V, +/-10V, 1V, and 100mV can be obtained precisely by different set ups of the 10:1 / 100:1 divider together with the LTZ box.


Later on, a HP3458A and a Fluke 5442A were available, so the stability and uncertainty could be checked.

The additional requirement then was, that the LTZ1000 references should be more long term stable than the 3458A. (1ppm/yr. vs. 8ppm/yr)

The 100:1 divider has a lower uncertainty on measuring 1000V (1ppm) than the 3458A (12ppm), because latter one has no compensation for the heat effect of its internal HV divider.

- to be continued -

Frank
Title: Re: #1 - Setup - LTZ1000 Reference System
Post by: Andreas on May 31, 2013, 09:58:30 am

Here's the setup, as intended, for the case that the 34401A is the DUT.

The divided 10V were compared to the LTZ1000 box, which delivers different reference voltages, i.e.
7.1479788V for Ref_1, 7.1762318V for Ref_2, directly from both of the LTZ1000, 7.000000V divided very stable from one of the LTZs.


Hello Frank,

nice gear.
I hope you will describe further how you get (with which parts) the precise division ratios.
By the way: do you know the (traceble) uncertainity of your references against PTB standards?

Another gadet that is missing in the datasheet cirquit of LTZ1000 is the bandwith limitation of the current source amplifier like C2 + R12 in the jj3055 cirquit ver3.gif in the posting above. Without this R/C-Combination the reference voltage output will be unstable with capacitive loads above around 1nF. (The current source will have heavy oscillations).

Also a output buffer is missing. Although introducing additional errors, the advantage is that with a short cirquit to ground at the output the setpoint voltage of the heater is not affected. The heater will go to the maximum possible temperature on a short introducing hysteresis or even ageing effects to the chip.

With best regards

Andreas



Title: #2 - Requirements - LTZ1000 Reference System
Post by: Dr. Frank on May 31, 2013, 10:50:51 pm
I'd like to ask for some patience..  |O practical stuff will be delivered definitely.

But first, here’s the summary of the requirements of the reference system, already with some instructional comments.

1) The Reference System should be capable of calibrating a HP34401A on each DCV range, over years. The HP34401As 24h specification is 15ppm for 10V, 20ppm on 1000V, 30ppm on 100mV.

For Good Metrological Practice, an uncertainty of <2ppm is required for each range of the Reference System.

That means, if the uncertainty (relative to SI) of the LTZ1000 output is known (i.e. calibrated once), all transfers to the different ranges must be done below that limit.
Also, all drift parameters should contribute in total below that limit.

2) The stability over time of a single LTZ1000 reference should reach realistically <= 1ppm/year:
LT specifies for the naked chip typ. 2µV/sqrt(khr.) @65°C, that’s 0.8ppm/yr.

This stability requirement (aging or deterioration) is most important, as it is not reversible, and cannot be mitigated or hardly be compensated (at most by elaborate trend analysis), like other instabilities.

3) The stability over temperature of the LTZ circuitry should reach realistically < 0.2ppm/K.

This instability is reversible, by returning to the nominal temperature, and can be cancelled statistically or even mitigated.
For that purpose, the reference system has to be operated in a stabilized / controlled environment. Temperature changes must stay within a few 1/10°C during 10 min, or over the measuring period (hours), and for long term stability measurements, the room temperature must be reproducible to e.g. +/- 2°C.

Otherwise, drift measurements on sub-ppm level are not possible.

Btw.: All other sources of similar instabilities, e.g. thermo electrical/mechanical force induced, humidity/leakage current, pressure, gravity, and so on, have to be analysed in value, relative to the ageing drift, i.e. if it's worthwhile to cancel them.
Like the temperature coefficient, those drift sources lead to spurious / reversible modulations of the reference output "only".   

4) Simply for convenience, the raw 7,2V output of the LTZ1000 has to be attenuated and trimmed precisely to a plain value of 7,00000V.
This secondary output should be very stable over time and temperature.
This transfer might be done by a 34401A with 2ppm uncertainty (linearity error), or with a 720A (<0.2ppm), or a 3458A (<0.05ppm). 

5) All reference outputs shall be amplified and buffered with low impedance by an exact factor of 10/7 to around 10V, so that the 34401A will accept this for calibration.
This amplification factor may be auto-calibrated at any required time with an uncertainty of < 0.2ppm, so that the stability / uncertainty of the LTZ1000 is maintained in the 10V output.

6) A decade divider with precise 10% steps, 0.1ppm nonlinearity, should provide a means for calibrating the linearity error of a Fluke 332B to 1ppm.

7) The experimental verification of the stability of the LTZ reference and the complete system is required, due to Good Metrological Practice.
A theoretical model about the instabilities is not sufficient on its own.

Therefore, a measurement system is required which is capable to perform sub-ppm comparisons, of the DUT against a more stable standard, or against a group of equivalent stable references.

8.) A Reference Divider should provide precise ratios of 10:1 and 100:1 with uncertainties of < 0.2ppm and < 0.5ppm of output, latter one for a burden of 1kV also (like a Fluke 752A)

- To be continued –  :=\

Frank
Title: #3 - Interior & Schematics - LTZ1000 Reference System
Post by: Dr. Frank on June 01, 2013, 03:25:40 pm
It’s teardown time; we’re on EEVBlog, aren't we?

First picture shows the double LTZ reference board.

It was intended as a Prototype design, to first check its characteristics, and some additional features.
It worked so well, that I left it in this state.

The PCB is single sided, and the LTZ1000 sits on the solder side (pic. 2).
That makes the thermal shielding of all solder junctions and the isolation of the LTZ much easier.
(Remember: Other ref. PCBs contain solder junctions on both sides and they are well exposed to air flow, especially in the 3458A, also in the Datron units)

The solder side is thoroughly cleaned with a strong solvent / PCB cleaner, and afterwards sealed with highly isolating plastic spray.
This method does not harm the other components.

The complete PCB is tightly assembled (with screws) on a polystyrene box, with additional small cavities for the LTZs, and the volume is filled with an additional foam cushion (pic. 3).
This easily fulfills the requirement that no air flow will affect the solder junctions, and also that the LTZ and its junctions are all on the same temperature, heated by its own power dissipation.

I have chosen an LTZ1000 (no A version!), so I could select 45°C as stabilization temperature for lowest drift.
That implies R1 = 1k, R2 = 12k. The rest of the LTZ circuitry is copied from the LT datasheet.
The LTZ1000A, as in ‘babysitters’ design, would require 10°C higher temperature, therefore 12.5k, and therefore 2 times higher drift rate, theoretically.

I have chosen wire wound resistors, because the necessary values were available instantaneously from stock, were reasonably cheap, and have low enough T.C. and specified long term stability of 25ppm/yr, no load. No load usually means, up to 10mW. Even the 120 Ohm resistor has 2.5mW only.

The OP07 simply were in my inventory, there’s no further idea behind using single opamps.

In the current budget schematic, (pic. 4) you see, that there are about 5mA flowing out of the negative side of the LTZ. That would have led to level shift on the negative line, especially as I connected both LTZs to one common GND.
So I added another OP07 to sink the negative current, that’s some sort of current cancellation, which is incomplete (and to be improved), but also used in the Fluke references (732B, 5720A).

That required a negative supply, so I have split the 15V to +12V/-3V, visible on the 2nd PCB.
The LTZ circuitry is effectively running on 12V only.   

The direct LTZ1000 output is divided by a resistive divider trim able precisely to 7,0000V.
This divider is set up by ordinary wire wound resistors also, i.e. T.C. ~ 3ppm/K, drift < 20ppm/yr., but the output is much more stable, i.e. 30 times more stable than those numbers.
That’s a metrological trick, I think, based on the fact, that at a division factor close to 1.0, all drifts will be suppressed strongly. The error calculus will be presented later.

Both LTZ1000 and one 7,000V outputs are fed directly to front jacks – gold plated Cu plugs.

The cables are Teflon isolated, to avoid leakage.

The precision switches allow choosing any of those 3 voltages, for amplification by a ChopAmp 7652 exactly by a factor of 10/7.

This is accomplished by 11 Vishay metal foil Z201 resistors, 49k99, which can be equally trimmed to 50k000 by a Wheatstone bridge, and from the front panel in Cal Mode.
The divider chain can be identified in pic. 1, on the right side, all those green components.
The linearity of this divider is 0.1ppm. This is acting like the first decade of a 720A Kelvin Varley divider. See 5th picture.

Therefore, the theoretical error of this 10/7 ratio transfer is 0.14ppm, which does not spoil the uncertainty/stability of the LTZ1000.

This is the 2nd metrological trick, as all other references (732A/B, 7001, etc) suffer on stability because of their fixed resistive amplification.

This divider delivers decade steps from 1 to 10, with an uncertainty of 0.1ppm of input , i.e. +/- 1µV on each output tap. (See 720A datasheet).
Therefore, 1V output is uncertain to 1ppm.

1V and buffered 10V are directly fed to output jacks, also all voltage steps can be selected by a 2nd switch.

Last, the calibrated 11 step divider can be used isolated for precision ratio measurements.

- To be continued -

Frank
Title: Re: #3 - Interior & Schematics - LTZ1000 Reference System
Post by: fmaimon on June 01, 2013, 05:53:25 pm
In the current budget schematic, (pic. 4) you see, that there are about 5mA flowing out of the negative side of the LTZ. That would have led to level shift on the negative line, especially as I connected both LTZs to one common GND.
So I added another OP07 to sink the negative current, that’s some sort of current cancellation, which is incomplete (and to be improved), but also used in the Fluke references (732B, 5720A).

That required a negative supply, so I have split the 15V to +12V/-3V, visible on the 2nd PCB.
The LTZ circuitry is effectively running on 12V only.   

Can you explain this part of the circuit? From the schematic, I see that you are forcing the bottom of the 120 resistor to 0V, using the opamp, but isn't this same current still going to ground through the opamp?

Maybe I'm missing the point on what's ground in your circuit The ground symbol (upside down triangle) is the negative of your power supply (-3V of the split supply)?

And what is that diode marked with an * doing?

Quote from: Dr. Frank
The direct LTZ1000 output is divided by a resistive divider trim able precisely to 7,0000V.
This divider is set up by ordinary wire wound resistors also, i.e. T.C. ~ 3ppm/K, drift < 20ppm/yr., but the output is much more stable, i.e. 30 times more stable than those numbers.
That’s a metrological trick, I think, based on the fact, that at a division factor close to 1.0, all drifts will be suppressed strongly. The error calculus will be presented later.

I'm looking foward to see this error calculus.


Quote from: Dr. Frank
The precision switches allow choosing any of those 3 voltages, for amplification by a ChopAmp 7652 exactly by a factor of 10/7.

This is accomplished by 11 Vishay metal foil Z201 resistors, 49k99, which can be equally trimmed to 50k000 by a Wheatstone bridge, and from the front panel in Cal Mode.

How do you select the cal mode  50K resistors? How close does they have to be to each other and how (why?) do you need the balance trimpot? As I see it, as long as all resistors (R1..R10) are trimmed to be the same is enough.

Felipe

Title: Re: #3 - Interior & Schematics - LTZ1000 Reference System
Post by: Dr. Frank on June 01, 2013, 10:42:23 pm

Can you explain this part of the circuit? From the schematic, I see that you are forcing the bottom of the 120 resistor to 0V, using the opamp, but isn't this same current still going to ground through the opamp?

Maybe I'm missing the point on what's ground in your circuit The ground symbol (upside down triangle) is the negative of your power supply (-3V of the split supply)?

And what is that diode marked with an * doing?

I'm looking foward to see this error calculus.

How do you select the cal mode  50K resistors? How close does they have to be to each other and how (why?) do you need the balance trimpot? As I see it, as long as all resistors (R1..R10) are trimmed to be the same is enough.

Felipe

Hi Felipe,

yep, the designators are not consistent, sorry for that.

I have +12V / GND / -3V; the triangle is -3V.

GND is the negative reference potential for each LTZ circuit.

My problem was, that I had to connect GND from the 1st and the 2nd LTZ circuit at the negative output jack, to have the opportunity to amplify each by the Chopper.

So, the negative sink currents of ~5mA for each ref. would have flown over those cables, which would have caused a (constant) voltage drop of several ppm, between the LTZ and the negative jack. So I reduced these currents and also the voltage drop by sinking them directly over the additional opamps, and only 300µA will flow to the negative jack.

Still, its not perfect.

Normally, you would sink the  currents directly at the LTZ negative output to the power supply, and would sense the negative potential with a separate tap, currentless.

Even then, you can only connect those negative sense lines without problems, if each LTZ circuit would have its own floating supply.

It was a first attempt, and I would redesign that , deleting the additional opamp, or would add several more precise current cancellation circuits, see 732B manual, or 5720A schematics.


The additional diode prevents reversing the voltage over UBE of Q1, by the additional OpAmp, which would otherwise cause the BE diode to go into Zener mode, which would destroy the chip.


The Wheatstone bridge is nothing special. It's simply a copy of the 720A bridge circuit, in its manual you may find a perhaps better description of the calibration  process.
In brief: The 50k resistors in the right leg are 0.1% wire wound, and both are trimmed by additional fixed resistors to nearly equal values (not drawn in the schematic).
The trim pot between them is the fine balance for exact match of upper and lower 50k.

The complete bridge has to be balanced to zero in each position, i.e. also if the positions of the 50k resistors in the right leg are reversed.
For that, you have to trim both pots, the trim of the DUT , and also the balance pot.
 
Only then, the resistor under test (R1..R10) exactly matches the reference resistor R11.
With 20V bridge excitation, and 0.5µV zero indication, the match is to 0.1ppm

Frank
Title: Re: #3 - Interior & Schematics - LTZ1000 Reference System
Post by: Andreas on June 15, 2013, 02:32:36 pm

The OP07 simply were in my inventory, there’s no further idea behind using single opamps.

Frank

Hello Frank,

When comparing the datasheets of OP07 and LT1013 from the LTZ1000 datasheet the main difference is the large signal amplification.
LT1013 has a order of magnitude more amplification than the OP07.
Especially for the current regulating loop a high amplification will reduce the steady state current error.
I dont know how large the remaining error is. But since the open loop amplification is temperature dependant I would prefer using a OP with high open loop amplification like LT1013, OP177 or something else.

By the way. At the moment I am comparing noise levels of different ADCs against the datasheet value of the HP3458A.
If I understood it right the HP3458A has 10e-8 rms noise with a integration time of 2 seconds (100 NPLC).
So this would be +/-0.3uVpp or 0.6uVpp in the 10V range. Can you confirm this value? What is your experience from practical measurements?

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on June 16, 2013, 08:48:21 am
Since about 2008, Fluke changed the design of the reference board in the 8508A DMM. In early versions were installed an LT1413A and LTZ1000. Now it is AD823A, LT1150 and LTZ1000A.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on June 16, 2013, 09:20:19 am
Dang Fluke, we all did everything wrong by selecting unsuitable chips... :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on June 16, 2013, 02:26:58 pm
Since about 2008, Fluke changed the design of the reference board in the 8508A DMM. In early versions were installed an LT1413A and LTZ1000. Now it is AD823A, LT1150 and LTZ1000A.

Do you have a cirquit diagram or at least a hint where the Chips are used? (heater section, current regulator, output buffer?)

I am guessing that the LTC1150 will be used as current regulator for the zener since he has a very high open loop amplification and very low offset longterm drift. The AD823 will be probably used as current buffer since the LTC1150 cannot deliver the 5mA which are needed.

Nice picture: Is the board under the isolation slotted or without slots?

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on June 16, 2013, 03:56:43 pm
I don't have any information about the new Fluke reference. All pictures taken from Web.

Title: Re: 3458A noise
Post by: Dr. Frank on June 16, 2013, 07:32:20 pm

Hello Frank,

Especially for the current regulating loop a high amplification will reduce the steady state current error.
I dont know how large the remaining error is. But since the open loop amplification is temperature dependant I would prefer using a OP with high open loop amplification like LT1013, OP177 or something else.

By the way. At the moment I am comparing noise levels of different ADCs against the datasheet value of the HP3458A.
If I understood it right the HP3458A has 10e-8 rms noise with a integration time of 2 seconds (100 NPLC).
So this would be +/-0.3uVpp or 0.6uVpp in the 10V range. Can you confirm this value? What is your experience from practical measurements?

With best regards

Andreas

Andreas,

I had to take a time-out, but now, several chapters will follow..

Concerning the noise of the 3458A, compared to different stable DC sources, here's a measurement, up to 1s (NPLC50).
You can see, that those sources come near the theoretical limit of the 3458A, but factor 2 higher.
I simply have calculated the statistics of a set of measurements and identified the variance as RMS noise. Perhaps it's Vpp instead, so a factor of 1/ 2.8 would be possible.

Perhaps a measurement on a stable battery would give better results.. I'll see later today, the box is running currently.


As the temperature stabilization of the LTZ is very sensitive, a high open loop gain may lead to instabilities. I'll see, just ordered 5 EA of LTZ1000, LTC1013 and LTC1052 from LT..

Frank

PS: On the 10V range, I've just measured one Weston cell, and the LTZ refs, NPLC 100, 25 samples each:
 
Mean= 1,0186060V, sigma= 195nV (output is drifting)
Mean=7,1479820V, sigma=146nV
Mean=7,1762512V, sigma=120nV

Therefore, I would judge, that NPLC100 really gives +/-1 digit stability on 8 1/2 length.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on June 16, 2013, 08:14:51 pm
HP 3458A 10 V noise (taken from volt-nuts thread on the bbs.38hot.net)
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on June 17, 2013, 09:09:14 am
Do I spot a certain LTZ1000A trace in Dr. Franks diagram?  ;)

My reference circuit will be shipped to another volt-nut this week (off-band for this forum) for further comparison. Also I will start building a LM399AH-based reference at work, or to be exact, prepare a design to be made by our trainee, will be used for performance tests of our 34401A at work (we have a scheme of buying fresh calibrated 34401A in place of a calibration sometimes and do a in-house comparison as performance/fitness check for the older ones, and some smaller fitness tests inbetween)

BR

Hendrik
Title: #4 – Reference Stability – model, calculation & component selection
Post by: Dr. Frank on June 17, 2013, 12:17:01 pm
The stability of the output voltage over time and temperature is determined mainly by the LTZ1000 chip, and the 5 resistors, R1-R5 (see datasheet).

Let’s first consider ageing (stability over time).

For the LTZ1000 chip and all of the resistor technologies there exists an exponential law over temperature, (Arrhenius), saying that ageing rate doubles every 10°C.

Such dependencies from operating temperature are often encountered in deterioration of material, e.g. the decay of LED luminosity and the life time of light bulbs, which is always driven by the operating temperature. The underlying process is the increase of defects in the crystalline structures, or the acceleration of diffusion processes. Therefore, reducing the operating temperature will reduce the ageing rates.

LT claims typical ageing of 2µV/sqrt(t/1000h) @ 65°C, that gives 0.8ppm/yr.
By lowering the temperature to 45°C, typical values of 0.2ppm/yr. may be achieved.
Therefore, the LTZ1000 version has to be used, with R4=12k, R4=1k. The reference then can be operated at an ambient temperature up to 35°C only.

Spreadbury (1) and also Pickering (2) have confirmed this experimentally for the LTZ1000 reference chip. The typical drift rates really apply in most cases. Intermittent operation of the LTZ1000 also will reduce its annual drift.
In the HP3458A the potential ultra-stability of the LTZ1000A is spoiled, by running it at 95°C, which leads to an 8 times higher ageing rate (continuous operation).

The 5 resistors also have to be operated on lowest possible temperature. This is fulfilled by Tamb. < 35°C and limitation of the self heating effect: Pmax. < 10mW. For those conditions, a Shelf Life ageing parameter is specified for precision resistors. In the LTZ1000 circuitry, R4=12k, 4mW, and R1 = 120, 2.5mW fulfill this requirement.

Additionally, there exists an influence from oxygen (and other reactive gases) and humidity on the crystalline structures, causing a sort of corrosion on resistors. Molded components, and to a lower degree conformally coated components suffer from that. Therefore all high quality / precision resistors as Thin Film, Wire Wound and Metal Foil types have similar specified ageing rates of 20...35 ppm/yr., (typical or maximum).

The LTZ1000 chip is hermetically sealed; therefore oxygen and humidity have no influence.

The resistors influence the reference voltage by changing the temperature set point (R4, R5), or by changing the current through the zener (R1).
The instabilities of R1, R4 and R5 are attenuated by a factor of 100; R2 and R3 by 300 and 500, respectively (see datasheet).
If all five resistors have the same instability values (over time or temperature), the total impact will be 0.035 times their instability.
Resistor ageing of 20ppm/yr. will add 0.7ppm/yr maximum.

Only the hermetically sealed, oil filled types (e.g. VHP202Z) give a big advantage. Their rate is typically 2ppm/6yrs., and therefore will add about 0.02ppm/yr only.
In picture 1 you’ll find long-term stability monitoring of 3 EA of my 5 VHP202Z. After 2 years, they really remain within < 0.5ppm of their initial value, so that is obviously no fake advertisement.
(Remark: The measurement stability was improved also during that time.)
 

Second, the temperature coefficient can be calculated in the same manner.

As the LTZ1000 chip is heated and thermally isolated, there is no further mechanism that would directly change its output voltage by change of ambient temperature.

The resistors will contribute with 0.035 times their own T.C.

Thin film resistors of 10ppm/k would give 0.35ppm/k, wire wound and metal foil have 2..5ppm/K max. T.C., which will yield 0.07... 0.18 ppm/K.

The metal foil resistors have no appreciable advantage over the wire wound types. Even the hermetical sealed VHP202Z, advertised as 2nd level standard and with typical T.C. of < 0.05 ppm/k will not give better results, because this extraordinary low value will not be achieved in reality. Also, the parabolic shape of the T.C. does not exist.

Picture 2 shows a high precision T.C. measurements on one of my 5 VHP202Z.
I measured values of 0.3, 0.6, 0.7, 0.8 and 1ppm/K, which is below the max. value of 2ppm/K, but far beyond the advertised typical 0.05ppm/K.

Therefore, an improvement can be made only by measuring the individual T.C. and selecting the ones with the lowest value. If R4 and R5 are matched concerning their individual T.C.s, this would improve the overall T.C. the most.

Conclusion: Using the LTZ1000, temperature set at 45°C, and using wire wound or metal foil resistors yield around 1ppm/yr. ageing. Using hermetically sealed, oil filled resistors may improve ageing to around 0.3ppm/yr.
Both resistor technologies yield a T.C. of < 0.2ppm/K.

Rem.: Sample variation of the LTZ1000 and other effects will lead to lesser stability figures.

(1) “ The Ultra Zener ... is it a portable replacement for the Weston cell?”,
P J Spreadbury, Meas. Sci. Technol. 1 (1990)
(2) “Setting new standards for DC Voltage Maintenance Systems. A Solid State DC Reference System”, John R Pickering, Metron Designs and Paul Roberts, Wavetek
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on June 17, 2013, 03:37:22 pm
Here is a short update, I received some more parts of my Vishay order, but still wait for the VHP101T. This feels like waiting forever.

Unfortunately (or better luckily) I am also quite busy right now, because I upgraded my lab with some gear that is planned to replace some/most of my actual calibration gear.   

@Mickle T., can you share where you found the Fluke 8508A details?
@Dr.Frank, are you planning to make a new PCB for your new ordered parts?

BTW Very interesting to see what is going on meanwhile.

@Dave + all interested, what do you think of the idea to try to design/make/build a kind of "EEVblog 10V Master Reference" based on LTZ1000 for this community?

bye
quarks
Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on June 17, 2013, 03:59:03 pm
@Mickle T., can you share where you found the Fluke 8508A details?

Thread about the voltage references: http://bbs.38hot.net/forum.php?mod=viewthread&tid=969&extra=page%3D1&page=12 (http://bbs.38hot.net/forum.php?mod=viewthread&tid=969&extra=page%3D1&page=12)
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on June 17, 2013, 06:05:12 pm
Hello Mickle T.,

thanks a lot. I had a look also at some other topics there and found a lot of infos from you. I am really impressed, great work!!!.

Bye
quarks 
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on June 17, 2013, 07:20:24 pm
Quote
@Dave + all interested, what do you think of the idea to try to design/make/build a kind of "EEVblog 10V Master Reference" based on LTZ1000 for this community?

Still I believe 10V are very uncommon today, just usable to check your DVM/DMM in one range but nothing more as most circuits require a reference voltage of 5V or less.
Sure, it's all about stability, but wouldn't it be worth having a LTZ1000 based voltage source with all common voltages of todays need? On the other hand, wouldn't it be worth to have a decade voltage output to verify all voltage ranges on your DVM/DMM instead of only the 10V range? Mh...

BTW: I wonder if there is knowledge in here about that rejustor stuff by Microbridge? Some american guys in here taken their sample offer to verify the improved stability of standard references? How could the LTZ1000 be improved by such a value und tc settable resistor?
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on June 17, 2013, 07:38:37 pm
Hi branadic,

Ich habe mir die rejustors mal im Zusammenhang mit Sensoren für den Aufbau von pga angesehen, ich denke trotz einstellbaren tc sind die letzten ppm Einstellung schwer. Und dass Prog-gerät plus Software eine über 1k Euro Sache.
Title: Re: 3458A noise
Post by: Andreas on June 17, 2013, 07:47:49 pm

As the temperature stabilization of the LTZ is very sensitive, a high open loop gain may lead to instabilities. I'll see, just ordered 5 EA of LTZ1000, LTC1013 and LTC1052 from LT..

Hello Frank,

The instabilities can be handled by frequency compensation without spoiling behaviour at DC.
Why not the brand new LTC2057? I fear that the LTC1052 cannot deliver enough current if the short cirquit current is around 5mA. The LTC2057 is now available at DigiKey.

With best regards
 
Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on June 17, 2013, 07:50:24 pm
Thanks babysitter for that info. One grand for only the programming device plus software is inacceptable to give it a try and to come a cropper in worst case.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Christe4nM on June 17, 2013, 08:18:43 pm
BTW: I wonder if there is knowledge in here about that rejustor stuff by Microbridge? Some american guys in here taken their sample offer to verify the improved stability of standard references? How could the LTZ1000 be improved by such a value und tc settable resistor?
This is the basically the same question I asked in the Keithley 2015 teardown topic (https://www.eevblog.com/forum/reviews/keithley-2015-teardown/msg246725/#msg246725) and was contemplating to ask here as well. Can anyone elaborate on this?

@babysitter, my German isn't that good that I can understand your post completely, but from what I understand you used those Rejustors and you found that even with "adjustable TC" it's hard to get the last ppm's down?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on June 17, 2013, 09:25:15 pm
Can anyone elaborate on this?

@babysitter, my German isn't that good that I can understand your post completely, but from what I understand you used those Rejustors and you found that even with "adjustable TC" it's hard to get the last ppm's down?

The key spec of precision is "long term stability".
On resistors we expect values below 25ppm/kHr or even less.

Mhm... What do you expect from a device which has a long term stability of 0.5%??? 50000ppm???
And when the adjustment range is +/-100 ppm I would not expect that you can get better than around 1/10th of it as stability. So in precision cirquits It might be useful at maximum to adjust the last 0.001% of trimming.

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on June 17, 2013, 10:14:56 pm

Still I believe 10V are very uncommon today, just usable to check your DVM/DMM in one range but nothing more as most circuits require a reference voltage of 5V or less.
Sure, it's all about stability, but wouldn't it be worth having a LTZ1000 based voltage source with all common voltages of todays need? On the other hand, wouldn't it be worth to have a decade voltage output to verify all voltage ranges on your DVM/DMM instead of only the 10V range? Mh...

BTW: I wonder if there is knowledge in here about that rejustor stuff by Microbridge? Some american guys in here taken their sample offer to verify the improved stability of standard references? How could the LTZ1000 be improved by such a value und tc settable resistor?

You're right.. for a complete calibration, you have to transfer the basic voltage reference (e.g. 10.000V) to the desired range voltages, i.e. 1kV, 100V, 10V, 1V, 100mV.

So you need a very precise and linear divider, like the 720A and the 752A, or the HP3458A.
They will aos deliver "uncommon" reference volatges as 5.0000V...
And you need a very stable voltage source, which can be compared against your basic reference by means of the dividers.

That's all.  >:D

Frank
Title: Re: #4 – Reference Stability – model, calculation & component selection
Post by: Andreas on June 17, 2013, 10:16:31 pm

 Therefore all high quality / precision resistors as Thin Film, Wire Wound and Metal Foil types have similar specified ageing rates of 20...35 ppm/yr., (typical or maximum).


That is valid for the first year with decreasing ageing rates in the following years.
And ageing can be further stabilized by some burn in. I am currently ageing my Z201 resistors with around 100mW (1/3rd the precision rating) intermittend 1,5 hours on 0,5 hours off at room temperature to stabilize the resistors.

With best regards

Andreas
Title: Re: #4 – Reference Stability – model, calculation & component selection
Post by: Dr. Frank on June 18, 2013, 07:37:08 am


That is valid for the first year with decreasing ageing rates in the following years.
And ageing can be further stabilized by some burn in. I am currently ageing my Z201 resistors with around 100mW (1/3rd the precision rating) intermittend 1,5 hours on 0,5 hours off at room temperature to stabilize the resistors.

With best regards

Andreas

Hello Andreas,

I do not recommend a burn-In on Z201 resistors, neither does Vishay!

If you have the molded types, ageing is mainly determined by the oxygen and humidity reactions on the resistor active area. That you cannot influence by elevated temperatures.

If you have the hermetically sealed type, you will create a big ageing rate, as specified by Vishay by the parameter: "Load Life stability", that's about 50ppm @ full load, 20ppm @ 100mW (Z203).
If you simply leave those components as they are, drift is 2ppm/6yr only!

Compare those parameters: A burn-in makes no sense, obviously, if you use them in shelf life mode afterwards, i.e. with very low power dissipation.


Additionally, there's another serious problem on metal foil resistors:
They show a very pronounced hysteresis effect, if they are exposed asymmetrically to high temperatures and brought back to room temperature only.

I have not yet discussed 'conditioning of the LTZ ref. circuit'  (~ chapter 6),  but the other two VHP202Z resistors were re-measured by Vishay, @ 125°C and came back with a +5ppm shift. One of them creeped back to its initial value, but that took over one year. The other resistor did not creep back by itself, instead  I made a temperature cycling on it, analogous to degaussing, and was able to "reset" the hysteresis to < 0.2ppm of the initial value.


Burn-In normally is used for early failure detection (accelerated life test) on "ordinary" parts and assemblies, but I think, burn-in on  high precision devices is not always the right way.
That's also true for the LTZ1000, as I will demonstrate later.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on June 18, 2013, 01:44:14 pm
Still I believe 10V are very uncommon today, just usable to check your DVM/DMM in one range but nothing more as most circuits require a reference voltage of 5V or less.
Sure, it's all about stability, but wouldn't it be worth having a LTZ1000 based voltage source with all common voltages of todays need? On the other hand, wouldn't it be worth to have a decade voltage output to verify all voltage ranges on your DVM/DMM instead of only the 10V range? Mh...

With high precision gear (6.5 to 8.5 Digit DMM and Calibrators and especially traceable NIST/PTB calibration) all DC measurements are accuracy wise directly related to a 10V Standard Reference. Also with all gear using artefact calibration it is a must have. So at least for me it is the most common/important DC value to have/know.

All other DC values are delivered through a calibrator or as described by Dr.Frank with KVD/Reference Devider/Ratio/transfer measurement.
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on June 18, 2013, 08:31:35 pm
Wow, a whole bunch of german guys in here :)

Quote
With high precision gear (6.5 to 8.5 Digit DMM and Calibrators and especially traceable NIST/PTB calibration) all DC measurements are accuracy wise directly related to a 10V Standard Reference. Also with all gear using artefact calibration it is a must have. So at least for me it is the most common/important DC value to have/know.

All other DC values are delivered through a calibrator or as described by Dr.Frank with KVD/Reference Devider/Ratio/transfer measurement.

This is like stating "We've done it always this way". As you know the Josehpson standard is everything but 10V, the value is reached by connecting serveral Josephson contacts in series. The definition of a 10V standard therefore is confusing itself. So the question might be is 10V still up to date?
Nothing we can really answer as there are enough people out there in the wrong positions stating the same as you do (We've done it always this way.), but please allow for this question.

BTW: SI unit uses 1 volt per definition, not 10V ;)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on June 18, 2013, 09:39:02 pm
Wow, a whole bunch of german guys in here :)



This is like stating "We've done it always this way". As you know the Josehpson standard is everything but 10V, the value is reached by paralleling serveral Josephson contacts. The definition of a 10V standard therefore is confusing itself. So the question might be is 10V still up to date?
Nothing we can really answer as there are enough people out there in the wrong positions stating the same as you do (We've done it always this way.), but please allow for this question.

BTW: SI unit uses 1 volt per definition, not 10V ;)

Well, Germans (e.g. Wernher-von-Braun) have put the first man on the moon, or not?  ;)

The SI definition currently is indirect, i.e. the VOLT is not one of the basic units.
The SI Volt uncertainty currently is not better than 2e-7 (e.g. volt balance), i.e. an HP3458A and a well designed LTZ 1000 reference are totally sufficient.

I'm waiting desperately for the redefinition of the kg, by a Si sphere, and/or the Watt balance.. then, the Josephson Volt and the von-Klitzing-Ohm will define those units directly and in accordance with SI with much smaller uncertainty.. up to  1e-16, or so.
Then it'll be the time to scrap the analogue DMMs..especially the 3458A.  :palm:

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on June 18, 2013, 09:47:21 pm
Quote
So, to answer your question, "10V" is the more modern cardinal point for this purpose.

Sorry, but this answer doesn't make me happy at all. A 10V based definition of a SI unit that is made by series connection of smaller value voltages can't be accepted by any physicist. It's the same stupid thing as nV/sqrt(Hz), this also makes physically no sense. It would be worth talking in a more physical language, not matematical.
The primary kilogram is what it is 1kg, not 10kg made by 33 parts à 303g. Got me?
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on June 19, 2013, 04:46:46 am
Thank you Dr. Frank for some real live experience and interesting explanations.

Whatever the output voltage is used for, in my eyes this thread is for discussion about care & feeding of LTZs, and together with some reading of the volt-nuts archives and discussion with Dr. Frank gave me an idea how to care and feed mine. Intentionally not going to the end in some directions, like selecting-from-batches the resistors and the LTZ itself due to self-inflicted money limits :) I didn't even try the high-end resistors but settled in the sub-10-eur-each league.

My reference was made for travelling (its predecessor wasn't stable when doing a round-trip from work, where the calibrated instruments are, and home.) And it successfully made it to Frank who inspected it further and has shown a quite good behaviour. For long-term experience it is just too young, but will go back to Frank for that. Also it will go to some other (hello quarks, your time will come soon! Lets have a Cali-barbeque with this weather when I bring the LTZ! :-+)  people just to collect experience and comparisons, and help giving a idea about the SI volt from some of them who are PTB-traceable to others who aren't (yet?). At home it will take some beating, being a transfer standard and part of a calibrator but also supplying sensors and some fooling around with oscillators and how they perform with a good voltage source, but also spending time off-line. So far I am happy with the results.

But sometimes I see men riding on black horses burning down metrology labs when I look at this thread and it goes into a certain direction...  :box: even metrologists know the art of multiplication and statistics, so why not 10V out of 30K Josephson elements? They even are happy with cesium clocks that probe more than a single but a bunch of cesium ions. And they swallow it that this bunch of atoms isn't even standing still but wobbling around in space and temperature range, giving some nasty effects.

I intend to send my reference to somebody off-forum today, then again to Frank and from there it will go to quarks.
Gotta go, our trainee will etch and populate his second PC today, and test... a LM399 Source as described a few posts above ! :)

Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on June 19, 2013, 08:46:19 am
Quote
Anyway, it doesn't matter why 10V is the "de facto" standard cardinal calibration point, it just is, and we will all have to live with it.

Please allow for other opinions instead of yours. If you can accept this concept, fine.

Quote
Oh-- and I still don't see what this has to do with the original subject of this thread.

In this case I quote you: "... it just is, and we will all have to live with it..."

Quote
I intend to send my reference to somebody off-forum today, then again to Frank and from there it will go to quarks.

Can offer you to visit Stuttgart, several calibrated test gear is available Keithley 2002, Prema 5017, 34401A and my private Prema 5000.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on June 19, 2013, 09:08:47 am
Quote
So, to answer your question, "10V" is the more modern cardinal point for this purpose.

Sorry, but this answer doesn't make me happy at all. A 10V based definition of a SI unit that is made by series connection of smaller value voltages can't be accepted by any physicist. It's the same stupid thing as nV/sqrt(Hz), this also makes physically no sense. It would be worth talking in a more physical language, not matematical.
The primary kilogram is what it is 1kg, not 10kg made by 33 parts à 303g. Got me?

I don't accept the kilogram because "kilo" is Greek for 1000 and  don't :wtf: ing want one thousand small  :rant: grams. Give me ONE gram  :-DD
Title: Re: Ultra Precision Reference LTZ1000
Post by: Jay_Diddy_B on June 19, 2013, 11:43:03 am
Hi,
I believe that 10V was chosen as the standard for voltage calibration to minimize the contribution made by thermal emfs.

This is quite different than the SI unit system where the unit of measurement is the volt.

In 1990 the volt was redefined, using Josephson junctions. Prior to this there was about 1.2 ppm difference between the North American Volt and the European Volt.

I have some old HP3455 meters with a green sticker with 1990 inside a diamond. This means the meter was calibrated with the 'new' volt.


Jay_Diddy_B
Title: Re: Ultra Precision Reference LTZ1000
Post by: nukie on June 19, 2013, 01:00:12 pm
10V is easy to calculate on the resistor divider such as the Fluke 720A
Title: Re: SI system, Volt definition & mise en pratique
Post by: Dr. Frank on June 19, 2013, 01:54:00 pm
Dear fellow-nuts,

please don't struggle about the absolute value of volt standards:

there's always an abstract definition of direct and derived SI units, and in most cases a totally different way to practically realize the unit. (called 'mise en pratique')

For the volt, the official SI-definition from 1948 is:

"The (SI) volt is the potential difference between two points on a conductor which carries a constant current of 1 Ampère, if the dissipated power between those two points equals 1 Watt"

You see, that this academical definition really is  "1 Volt" (based on kg, m,s).

(The kilo-gram is the only SI unit which is based on 1000 units of a gram, for practical reasons of its realization also, i.e. the kilogram artifact in Sèvres, France)
 
The realization of the SI Volt is done either by an Hg electrometer, by Clothier at al (1989) basically a balance, measuring the electrical force, uncertainty 0.27ppm, or a design by Funck et. al. (1991), measuring the force on a capacitor plate (0.31ppm uncertainty).
Both experiments are very complicated and clumsy, and might have been performed only once.

The output voltages of those primary standards (SI) can be different from 1V, given by the practical setup.


Then, there is a different representation of the Volt, that is a more easy-to-manage-way to deliver the volt.
Currently this is the Josephson Volt, a few mV in 1972 were amplified by a cryogenic divider to ~ 1V, 1V in 1985 by a Josephson Junction array, and 10V in 1987 by a longer array. (based instead on 2e/h)

Remark: The definition of the Representation of the Volt has been redefined in 1990, but not the definition of the Volt in the SI. The definition from 1948 is still valid!

The uncertainty between two different Josephson experiments can be as low as dV/V = 3e-19 for single JJs and  1.2e-17 for 0.6V arrays if you compare directly on the cryogenic (i.e. quantum) level.

Remember: The uncertainty between the Josephson Volt and the SI is still 0.4ppm!

10V for secondary reference standards has been chosen only for practical reasons in the analogous world (in contrast to the cryogenic world),  reasons are: the typical offset  of several µV, the limitation to measure volt differences to a few nV only, and because the typical cardinal points 1kV, 100V, 1V and 100mV are symmetrically situated around 10V, so that a 100:1 and a 10:1 divider is sufficient to transfer 10V to all of them.

Anyhow, if one selects a different standard value, e.g. 5V or 7,147V, it's also ok, as there  will be always the necessity to make a transfer to other needed calibration points (by KV divider, or by 3458A).

Therefore, the foregoing  discussion, which calibration point is the 'correct' value is simply a lack of knowledge of the concept of SI - 'Definition', - ' Realization' and - 'Representation'.

I recommend the lectures 'école de physique, Les Houches, “Quantum Metrology and Fundamental Constants” ', Blaise Jeanneret, "Volt metrology: The Josephson effect and SIS junction arrays":

http://www.metas.ch/LesHouches/downloads/talks/15_Jeanneret.pdf (http://www.metas.ch/LesHouches/downloads/talks/15_Jeanneret.pdf).

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on June 19, 2013, 06:25:31 pm
And finally ....

10V is the voltage which can be handled by +/-15V supplied OP-Amps without input voltage dividers.
Thus giving a nearly infinite input impedance instead of the 10 Meg in the other ranges.
10V is the normalization voltage of analog calculators.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: SeanB on June 19, 2013, 07:01:35 pm
I don't accept the kilogram because "kilo" is Greek for 1000 and  don't :wtf: ing want one thousand small  :rant: grams. Give me ONE gram  :-DD

I do have a calibrated masspiece traceable to a national standard ( it is only 2 levels away from the standard SA kilogram, which is replica 56 of the International standard kilogram. It is 1.000g, with an error of 0.2 mg on that mass. Has held that over a few calibration cycles, as it is rarely used, only being used on a strain wire massmeter that has a resolution down to 0.1mg. It also has a 24 hour warm up time to be stable to that though, and you need to close the doors and not breathe, as it can detect that, and looking in through the doors will show up from the IR radiation you emit. I use it to check linearity across the measuring range, along with the 20,10 and 50g masspieces in the set. I actually had them all calibrated, even the 0.5mg masspieces.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on June 19, 2013, 07:18:14 pm
Hello,

just to bring up some discussion points to the original theme, I want to say some words to my design.
(see attached cirquit diagram below).

The main idea was to have a portable "transfer standard" which can be shipped hot.
To avoid mains disturbances it is also battery powered during operation.
So main concern is minimum current consumption.

This leads to a design with a LTZ1000A with around 50 degrees temperature setpoint.
Thus setpoint divider has 12.5K to 1K.

As resistors I choosed precision wirewound of type UPW50. Unfortunately the 12.5K and 70K values
where not available and had to be replaced by 10K+2K+0.5K and 50K+20k.

Further restriction: the whole cirquit should fit into a Euro-Card aluminium case.
For the first step it should be a unbuffered 7V output reference.

Cirquit description:

Power supply consists of 12 AA NiMh-cells.
The raw voltage of 17.5V (battery full) downto around 14V (empty) is stabilized with a low drop (0.17V), low noise (20uV), low power consumption (1mA) voltage regulator LT1763 down to 14.0V.

The reference section has some modifications to the datasheet.
R13 enables startup even with negative offset of the LT1013.
T2 removes the reference current from LT1013 lowering the self-heating of the LT1013.
Second effect: more headroom for the current regulation with low battery voltages.
R18 limits inrush current through the zener to about 14mA max during switch on.
R17 enhances stability of the current regulation loop since the load is now missing.

C11 + C12 are adapted from Datron cirquit. The effect is having lower noise on heater +
reference current regulation.
To further calm down heater current noise C13, C14 are added. C14 keeps RF noise away from the
negative input of the heater OP preventing the RF from being demodulated by the input diodes
which would give an offset. But this capacitor leads to loop instabilities which are compensated
by C13.
Similar C15 keeps RF away from current regulator OP. C8 together with the additional resistor

R19 compensate for loop stability.
C9 keeps RF from the output connector away from the reference.
C9 can only be added with R19+C8. The original cirquit from datasheet is not stable with
capacitive loads.

R16 is a NTC near the LTZ. So the temperature within the cirquit can be measured from outside.
The auxiliary connector J6 can be used for several things.
- changing the temperature setpoint (Pickering patent)
- heater monitoring (environment too hot/cold) or influencing
- buffered output (or voltage divider cirquit as calibrator)

J1 is a D-Sub connector which I use as main output. The advantage is that neighboured pins are nearly on the same temperature giving low thermoelectric voltages. The metal shield of the connector further equalizes the temperature of the pins.

J4 + J5 are auxiliary outputs.

Mechanical description will follow...

With best regards

Andreas
Title: #5 –Amplification to 10V
Post by: Dr. Frank on June 19, 2013, 09:28:12 pm
If the LTZ1000 and the other buried zener references SZA263 and LT-FLU potentially are stable to 0.2ppm/yr @ 45°C, including 0.02ppm/yr. by usage of hermetically sealed resistors as in the Datron 4910, why are they all specified no better than 1.5ppm/yr? (See picture 1.)

The main reason is, that 10.000V are generally used as a reference, not the raw 6.6 …7.2V of the reference amplifiers, and that those 10.000V have to be generated by resistive amplification.

The error calculus of such a resistive divider is given in picture 2

=> Output Stability (10.000V) = 0.52 *[(TC(R1)-TC(R2)dt +2*|AC(R)|dT]

A T.C. matching of R1 and R2 is required, so that the T.C. effect is nearly cancelled.

The ageing rate of both resistors cannot be matched, so their individual ageing rates add up instead.

Therefore ultra time stable (statistical resistor network in the 7001) or pre-aged resistors (wire wound, sealed types in 732B) must be used.
They still will contribute several tenths, or up to 1 ppm/yr.
Two hermetically sealed VHP202Z would contribute ~ 0.7ppm/yr.

Therefore it’s clear, that the 10.000V reference output is much less stable than the reference amplifiers itself.

The adjustable decade divider in my design will contribute 0.14ppm uncertainty only, and can be recalibrated at any time; therefore the basic stability of the LTZ circuitry is maintained in the 10.xxx V output.

To get plain 7.000V (and 10.000V) from e.g. 7.176V, an additional attenuation of 0.975 is required (see schematic of my design).

Therefore R2= 52k, R1=1k31=>

Output Stability (7.000V) = 0.0245 *[(TC(R1)-TC(R2)dt +(|AC(R1)| + |AC(R2|)dT]

All instabilities of this divider are attenuated by a factor of 40, i.e. T.C. < 0.05ppm/K without matching and A.C. < 0.02ppm/yr with VHP202Z are possible.


If the ultra linear HP3458A would be used to adjust the 7.147 => 10.000V transfer by a  non-inverting amplifier, an uncertainty of < 0.05ppm of the 10.000V output may be achieved.

(to be continued)
Title: Re: #5 –Amplification to 10V
Post by: Dr. Frank on June 19, 2013, 10:49:36 pm


I *think* the drift specs are *very* conservative figures for the first year-- all of the references listed "calm down" after a year (or a few years), and their real drift rate is far less-- and yes, I agree with you, most of this is due to the surrounding circuitry (especially the resistors) that control and/or condition the Zener device.

There was an interesting article on the Vishay Precision Group website about resistors in a hermetic network-- the gist of it was that the resistors in this case, (if they came from the same production lot, and were trimmed by the same person using the same equipment) will have a tendency to drift together over a long period of time-- for a divider (which we are mainly concerned with the ratio, and not the absolute value), then the long term drift can be quite small-- or at least that's what the article *says*-- you have to consider the source-- they are trying to sell resistors, so of course they will not show any data that make their resistors look bad-- all you see is the good outcomes.  You can find the PDF here:

http://vishaypg.com/doc?63512 (http://vishaypg.com/doc?63512)

I do like the architecture of your design-- I have stolen some of the concepts for my own use-- I hope you don't mind...

Well, thank you for the compliment.

I intended to share for copying - so I don't mind at all ..

But I also wanted to initiate some further reviews about the other "gimmicks", I have summarized in my first post.. for reuse in my own redesign..
(Several LTZ1000 directly from LT arrived just by today.)

The documents of Vishay have to be read very carefully, the "typical" parameters are sometimes very optimistic, but they always hold the maximum boundaries, which are mostly impressing, anyhow.
After all, if you dig in the last few tenths of ppm of your design, uncertainty by design or by specification is rarely possible, instead you always have to monitor your finished reference and select the most stable one as your "golden device".
That's what Fluke et al are doing also.

There was a classical document (1) by Fluke, where they monitored many different 732B over years.. and there was no sign of decrease of the ageing rates. The specifications are also quite realistic..
No, in contrary, for the 7001, even a linear drift prediction was specified, which might improve the uncertainty of this reference.

Only heavily drifting references like the one in the HP3458A may calm down, but only because they are operating far away from an equilibrium state.

Frank

(1) "Predictability of Solid State Zener References", David Deaver, Fluke Corp.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on June 20, 2013, 08:31:47 pm
A important point with those stable references is the enclosure. As shown, mine is sitting inside a tinned steel case (solderable) which is housed in a bigger ABS box. The power supply is simply fed thru a hole in the metal box, the output jacks are going thru their own hole each.

This was my simple, cheap and lazy approach but I know that with a few euros and workshop minutes more might have been possible ;D

If I really want to do it better next time (*) I would think about the following:
Discuss those things with me! I am bored if you dont :)

Use the same kind of solderable tinned sheet metal box again
Buy a batch of feed-thru capacitors
use only the feedthrus and hermetically solder the case
I would try to solder it in a nitrogen or co2 atmosphere, so the oxygen and humidity dont eat our resistors!

to get really esoteric, one could solder in two thin cooper tubes and use them to fill it with silicon oil (available as gear oil for RC cars), remove the air and have a good thermal conductor inside. pinch and solder the tubes to seal.

Also, did anybody consider sodium silicate as a coating to prevent moisture and oxygen reaching the resistors ?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on June 20, 2013, 09:58:30 pm

Buy a batch of feed-thru capacitors
use only the feedthrus and hermetically solder the case

Also, did anybody consider sodium silicate as a coating to prevent moisture and oxygen reaching the resistors ?

Hello babysitter,

if the feed thru C's are really hermetically tight (sealed with glass) the pins probably will be COVAR pins (the only material with the same expansion coefficient as glass) having around 39uV/K against copper.

So you will need at least additional thermal shielding for the feed thru C's.

For me another question is more important.
What is better: having long legs on the LTZ1000 or keep them as short as possible together with a slotted board.
The pins are also of covar (because of the hermetically tight case).
So we are having 2 large thermoelectric junctions on each pin.
One between the bonding wire and covar pin, the other on the pcb between covar and copper.
The junction from the bonding wire to the aluminium mask are also a junction.

And unfortunately the bonding wires are not equally distributed (see chip photo)
http://www.amobbs.com/thread-3593996-1-1.html (http://www.amobbs.com/thread-3593996-1-1.html)
picture ourdev_464495.JPG

The heater pin (Pin 1) has 3 bonding wires (cooling the chip down at this edge) and the next 3 pins 8+7+6 of the temperature sensing transistor are also on the same side of the chip at the next edge.

The idea would be to find a geometry where all thermoelectric junctions (or at least those two of the Zener output) have the same temperature.

My idea would be the legs as short as possible and a slotted board having low thermal mass at the solder junctions. But since I have no thermal camera I cannot prove it.

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: robrenz on June 21, 2013, 03:19:36 am
It seems like the participants here may have some applied knowledge of thermal EMF generation.

Below is my current understanding as a starting point for a discussion on something that I have been curious about. I am not assuming what I state here is correct.

The thermal EMF generated in a conductor is caused by a thermal diferential/gradient from on end to the other. the gradient does not need to be uniform.

Two parallel identical conductors joined at one end with a thermal gradient have thermal EMFs but they are equal and cancel.

By joining two condutors at one end that have different Seebeck coeficients you will get the differential of the two different Thermal emfs generated by the temperature gradient, a thermocouple.

If you drop the thermocouple junction 200mm deep into a heated nonconductive liquid bath there will be negligible thermal gradient in the junction or the leads that are in the bath and all the gradient will occur in the conductors from the bath surface back to the cold junction. The actual junction itself is not generating the emf

The thermocouple law of intermediate metals states a third metal can be introduced in the juction between the two main leads of a thermocouple and as long as all three materials at the junction are the same temperature the thermocouple will give the same ouput.

This implies to me that no thermal emf is generated just by the contact of two dissimilar metals. The Thermal emfs occur only in the thermal gradients of the two materials. The contact is only an electrical connection.

Even though the mathematics is over my head currently on the inter relationships of the Thomson, Seebeck and Peltier effects. It seems that the Peltier effect is not purely a junction effect but has gradient aspects like the Seebeck effect and joule heating also.

I bring all this up because I think the frequently shown Thermal emf charts for paired metal combinations are extremely misleading They imply that the mere contact point of two particular metals will generate a certain µV/deg. As an example copper vs lead tin solder is listed at 5µV/degC.  So if I take a 2" long piece of copper wire and solder it to a 2" long piece of lead tin solder and maintain a 1 degC temperature difference between the end of the copper wire and the end of the solder and magicaly measure the temperature across that without introducing additional thermal emf  junctions I would read 5µV.

But if I make a Thermocouple out of two identical copper wires and use lead tin solder to make the junction according to thermocouple law of intermediate metals the solder will have no effect on the thermocouple output and since the two leads of the thermocouple are identical materials the output will be zero, no thermal emfs generated.

Should we discuss this here or should I cut and paste this a new thread?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Rufus on June 21, 2013, 04:55:22 am
So if I take a 2" long piece of copper wire and solder it to a 2" long piece of lead tin solder and maintain a 1 degC temperature difference between the end of the copper wire and the end of the solder and magicaly measure the temperature across that without introducing additional thermal emf  junctions I would read 5µV.

But if I make a Thermocouple out of two identical copper wires and use lead tin solder to make the junction according to thermocouple law of intermediate metals the solder will have no effect on the thermocouple output and since the two leads of the thermocouple are identical materials the output will be zero, no thermal emfs generated.

If there is no thermal gradient across the solder bit of the connection. 

How about you take a loop of copper and put a blob of solder on one side? I reckon that will make a thermocouple as long as the solder blob experiences some of the temperature gradient and you could argue there isn't any junction.

The solder effectively changes the seebeck coefficient of that part of the circuit to something between copper and solder. Probably in proportion to the relative cross sectional area and electrical resistance.

So easy to test I just spent 5 minutes trying it. Attached is photo of 2 22 swg tinned copper loops one with some 60/40 solder blobbed on one side. Playing a flame on the end of the loop produced more than 20uV at the other end of the one with the solder and less that 1uV (possibly nothing) with the other.
Title: Re: Ultra Precision Reference LTZ1000
Post by: eevblogfan on June 21, 2013, 09:12:11 am
hey


Flame ?  isn't that <700C ? 

at <700C that's rughly 29nV !

witch is 0.0029ppm/C  at 10V

and 0.029ppm/C at 1V

is that so bad ?

oh and another question : does High silver load of soldier helps ?

if so . can you check that somehow ?

PS , 20uV is too low to be sure you measured 100% of that affect , can you investigate further more and confirm us that it was 20uV ?  ( I know that even 5% if accuracy is enough for that test bt I am wondering if those 20uV are fully result of that thermo couple junction )

Thank you
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on June 21, 2013, 09:30:29 am
just to bring up some discussion points to the original theme, I want to say some words to my design.
(see attached cirquit diagram below).
...
Mechanical description will follow...

Hello Andreas,

thanks a lot.
That is exactly the kind of info/input I hoped to see, when I opened this post. 
I hope this will lead to a well thought through optimized design.
 
bye
quarks
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on June 21, 2013, 09:33:28 am
It seems like the participants here may have some applied knowledge of thermal EMF generation.
...
Should we discuss this here or should I cut and paste this a new thread?

I think this discussion fits very well to the subject.
But please feel free to open a new thread.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on June 21, 2013, 11:54:56 am
"The voltage is not generated at the junction of the two metals of the thermocouple but rather along that portion of the length of the two dissimilar metals that is subjected to a temperature gradient. Because both lengths of dissimilar metals experience the same temperature gradient, the end result is a measurement of the difference in temperature between the thermocouple junction and the reference junction."

The "characteristic voltage difference (is) independent of many details (the conductors' size, length do not matter)".

http://en.wikipedia.org/wiki/Thermocouple (http://en.wikipedia.org/wiki/Thermocouple)


You can also choose whatever practically implemented "couple" coupling joint you want as long as it is made of one and the same material and its endpoints are at the same temperature (which means no net temperature gradient).

So 1 inch of thin copper wire soldered to 1 foot of thin iron wire as in the symbol < with temperature t1 to the right (both endpoints at one and the same temperature t1) and temperature t2 at the soldered junction to the left will give the same voltage as 1 yard of copper bar and 1 inch of iron bar interconnected with 2 feet of lead tube provided that the two junctions now created at the left side both are at temperature t2. This holds only in equilibrium, i.e. all connecting points and endpoints have settled.

This is imo implications from the link. Please check for yourself. A search 'thermocouple theory" gives several sources stating similar propositions.
Title: Re: #5 –Amplification to 10V
Post by: Andreas on June 21, 2013, 07:21:58 pm

There was a classical document (1) by Fluke, where they monitored many different 732B over years.. and there was no sign of decrease of the ageing rates. The specifications are also quite realistic..
No, in contrary, for the 7001, even a linear drift prediction was specified, which might improve the uncertainty of this reference.

Only heavily drifting references like the one in the HP3458A may calm down, but only because they are operating far away from an equilibrium state.

Frank

(1) "Predictability of Solid State Zener References", David Deaver, Fluke Corp.

Hello Frank,

perhaps I have overlooked something. But I cannot find any statement for a linear drift in the document (1).

As far as I understand they are simply comparing all references which where calibrated at Fluke during a certain time against the predicted drift. And not monitoring several specific devices over time.
The only monitoring for drift over time is made for 1 single device where the drift is interpolated linear and nonlinear with the result that for the nonlinear drift model the limits are halved against the linear model for this particular reference.

http://www.vishaypg.com/doc?63003 (http://www.vishaypg.com/doc?63003)

Quote from: Dr. Frank
I do not recommend a burn-In on Z201 resistors, neither does Vishay!

I will do it anyway just to be shure having done all what I could do.
Vishay offers PMO services to stabilize "foil resistors"

http://www.vishaypg.com/doc?49789 (http://www.vishaypg.com/doc?49789)

The only recommendation that I see is that PMO cannot be used for wire wound, and film resistors.
Of course it is mentioned especially on the hermetically VHP resistors.

And yes, you are right. The soldering shock after the pre-ageing will shift further the molded resistors with some hysteresis.
But since I am doing at least 1-2 gentle thermal cyclings (15-40 degrees centigrade environment) for measuring the tempco of the whole reference the introduced hysteresis should be mostly removed.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: robrenz on June 22, 2013, 03:26:14 am
@ Rufus
That is an interesting take on the situation that I had not considered. It makes sense that that solder blob has effectively changed the "alloy" therfore the Seebeck coeficient of the wire with the blob is different now.  You have inspired me to do some experiments myself.

@ quantumvolt
So we are both picking the same internet info to believe about thermocouple theory.

@ All interested
I welcome your comments on my current opinions: That a plating on a connector can have no appreciable affect on a connection because it is too thin and intimately connected to the connector base metal to have any significant thermal gradient that would produce a thermal emf.   Likewise that the mere fact of soldering a good mechanicaly coupled copper to copper joint with lead tin solder is not going to create a thermocouple with the charts 5µv/degC thermal emf.  So here are some tests that hopefuly illustrate my point.

Twisted tinned copper wire soldered with thermocouple next to it.
(https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=52424;image)

Loop of copper wire only and then the twisted wire that was soldered. In both cases the heat was on the junction/bend only to heat the junction to as uniform a temp as possible
(https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=52426;image)

Heat on one leg only of the twisted soldered copper to get a thermal gradient across the juntion.  Full scale is 1.78µV delta V for the 85degC rise was 1.424µV = 17nV/degC.  The up down ramp effect of the heated section is from moving closer to and further away from the junction itself on the one leg.
(https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=52428;image)

Then I made this very small area soldered junction from swaged tined copper wire to have minimal solder volume.
(https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=52430;image)

I strapped my thermocouple to my heat gun for a fixed position and set the temp to be stable at 115C. Then I tried the three positions heat on each side of the juntion then centered on the junction itself. 
(https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=52432;image)

Straight line section is ambient temperature then one side of junction heated then the other side of juntion heated then natural cooling to the center then repeat both sides of junction and then heat the juntion itself.  The heated juntion is basicaly zero output. The minor undulations are minor variatons in centering the heat on the junction. Full scale is 3.543µV and the delta V for 115degC rise was 1.5945µV = 14nV/degC.
(https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=52434;image)

Here is one leg of tinned copper sodered to one leg of flux core solder as a thermocouple. The delta for a 55degC rise is 294.8µV which equals 5.36µV/degC.  That is very close to the published 5µV/degC for copper vs lead tin solder.
(https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=52436;image)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Rufus on June 22, 2013, 04:23:13 am
@ Rufus
That is an interesting take on the situation that I had not considered. It makes sense that that solder blob has effectively changed the "alloy" therfore the Seebeck coeficient of the wire with the blob is different now.  You have inspired me to do some experiments myself.

Good work. I don't find any of your results surprising but nice to see them all the same.

On changing 'alloy' I think it more like two conductors shorted together along their length. The emf you get must depend on the output impedance of the seebeck effect. I don't have a clue what that output impedance is, but, I guess it has to be inversely proportional to the conductor cross sectional area. Then the resistance of the conductors doing the shorting might be significant or perhaps not.

If you twisted equal gauge solder and copper together to make one side of a thermocouple do you think you would get about half the 'notional' solder/copper junction emf?
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on June 22, 2013, 08:16:05 am
Here is an easy-to-understand scholarly article that might describe the 'blob'-thing. http://www.msm.cam.ac.uk/utc/thermocouple/pages/Drift.html (http://www.msm.cam.ac.uk/utc/thermocouple/pages/Drift.html)

"Drift (permanent change) occurs because of metallurgical changes of the thermoelements ...."

"... a change in Seebeck coefficient is a necessary condition to have drift, but it is not a sufficient condition for it: the change in Seebeck coefficient needs to occur in a region of temperature gradient ..."

"... changes at the junction of the thermocouple usually does not play any role in drift, as it can be assumed that the junction is at constant temperature ..."

I suggest you take a nice clean piece of wire and shape it as a U (no sharp bends). Heat it locally at the bottom of the U. Versions (apply a thin layer of solder on):

1. Half of the curved part of the U. Thermocouple effect.

2. The bottom part of the U (the entire curved part of the wire).  No thermocouple effect.

3. A small blob at the far end of one of the legs of the U. If the blob is not heated too much, little or no thermocouple effect.


BTW The statement "... a change in Seebeck coefficient is a necessary condition to have drift, but it is not a sufficient condition for it: the change in Seebeck coefficient needs to occur in a region of temperature gradient ..." explains why there not neccesarily is a  thermocouple effect on every contact point of different materials - the connection is at thermal equilibrium (and having no or small current) ...
Title: Re: #5 –Amplification to 10V
Post by: Dr. Frank on June 22, 2013, 01:25:25 pm


Hello Frank,

perhaps I have overlooked something. But I cannot find any statement for a linear drift in the document (1).

As far as I understand they are simply comparing all references which where calibrated at Fluke during a certain time against the predicted drift. And not monitoring several specific devices over time.
The only monitoring for drift over time is made for 1 single device where the drift is interpolated linear and nonlinear with the result that for the nonlinear drift model the limits are halved against the linear model for this particular reference.



I will do it anyway just to be shure having done all what I could do.
Vishay offers PMO services to stabilize "foil resistors"

http://www.vishaypg.com/doc?49789 (http://www.vishaypg.com/doc?49789)

The only recommendation that I see is that PMO cannot be used for wire wound, and film resistors.
Of course it is mentioned especially on the hermetically VHP resistors.

And yes, you are right. The soldering shock after the pre-ageing will shift further the molded resistors with some hysteresis.
But since I am doing at least 1-2 gentle thermal cyclings (15-40 degrees centigrade environment) for measuring the tempco of the whole reference the introduced hysteresis should be mostly removed.

With best regards

Andreas

Hello Andreas,

I have never spoken about linear drift concerning the Fluke study of the 732A/B!

In this document, they analyse the drift prediction of up to 7 measurements on the same device.
The real behavior (either linear or other function over time) is not described in detail, that's right.

But one can see, that the specification of say 3ppm/yr. for the 732B, is not very conservative, as many devices (with SZA263) are on the edge of this spec, and will also not improve noteworthy after some years.

It's only a myth, that such devices get very stable, if one waits long enough.
That might happen on a few devices, but on many others, the ageing rate might increase again.

The study shows that, indirectly, because the drift prediction is quite uncertain.

For the 7000 reference only, Fluke specifies a typical drift rate of -0.7ppm/yr, and also an ucertainty of the prediction.
This parameter is specified as being linear, isn't it?


Vishay offers an PMO for the VHP101, and the details disclose, that this PMO is definitely no Burn-In!

Burn-In means: Storage for an extended period of time at constant high temperature.

PMO includes temperature cycling, in form of temperature shocks.

All metal foil resistors show hysteresis, therefore for real precision applications, a careful "degaussing-like" cycling is necessary.


I have shown my own T.C. measurements over a range of 10...35°C, and there you can hardly see any hysteretic effects.. that's only for greater temperature ranges.. >30°C in one direction, and therefore, 'gentle' cycling won't bring them back to zero.
You'll need trips to -23°C, up to +60°..80°C, whatever you have available in your home..

Anyhow, soldering ultra stable VHP resistors is always done with a heat transfer pincer, so that there is no heating of the resistive element.

Frank



Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on June 22, 2013, 08:17:37 pm
to protect Parts from the thermal solder shock, i use hemostat clamps (the latching clamps for blood vessels) on the wires quite often to dissipate heat away fm the Part.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on June 22, 2013, 09:15:35 pm
Part 2 LTZ1000A of Andreas

Mechanical

All is placed in a aluminium Euro-Card case EG2 of ProMa with dimension 168*103*56 mm.
http://www.reichelt.de/Proma-Gehaeuse/GEH-EG-2/3//index.html?ACTION=3&GROUPID=5199&ARTICLE=50424&SHOW=1&START=0&OFFSET=16& (http://www.reichelt.de/Proma-Gehaeuse/GEH-EG-2/3//index.html?ACTION=3&GROUPID=5199&ARTICLE=50424&SHOW=1&START=0&OFFSET=16&)


(http://LTZ1_IMG4670w.JPG)

Top side from left to right:

D-Sub connector for the main output with neighboured pins for the Zener output.
Next to the D-Sub is the auxiliary connector for a daughter board


Below the D-Sub you can see one of the auxliary 4 mm banana plug connectors (Hirschmann).
Both connectors are placed intentionally on the same height of the reference.
I cannot understand why on most instruments the positive (red) plug is always higher
(and hotter) than the negative (black) plug which is then cooler. Especially when having
a device with a power consumption that makes it necessary to use a fan.

In the middle the reference section with the parts around LTZ1000A. They are all placed into a TEKO 3710 metal shield with top cover.
http://www.reichelt.de/Teko-Stahlblech-Gehaeuse/TEKO-3710/3//index.html?ACTION=3&GROUPID=5202&ARTICLE=34042&SHOW=1&START=0&OFFSET=16& (http://www.reichelt.de/Teko-Stahlblech-Gehaeuse/TEKO-3710/3//index.html?ACTION=3&GROUPID=5202&ARTICLE=34042&SHOW=1&START=0&OFFSET=16&)

The teko shield has the intention to equalize thermal gradients from outside to the reference section.
And calm down air currents.

All resistors (pre-aged before use) are in the upper half of the shield.
The LTZ1000 is thermally shielded with some foam.
Since the PCB is not plated through I could not settle the LTZ1000A directly on the PCB.
So the legs of the LTZ are surrounded by additional foam to keep air currents away.
This can be better seen on the closeup (IMG_4671.JPG) which I posted May 29, 2013 (page 12) in this thread.
The BF245C FET for the current source is above the LT1013.
The NTC for temperature sensing of the board temperature is near the LTZ
(between the FET and the LTZ).

(http://IMG_4671.JPG)

The air gap between the top plate of the TEKO and the rest of the cirquit
is filled with additional polystyrene foam material (plate with 10mm thickness).

The right half of the aluminium case is filled with 12 AA cells in 2 battery holders
which are fed by a simple constant current charger during charging.

Between the TEKO inner shield and the battery holder some parts of the voltage regulator can be seen.


(http://LTZ1_IMG4675w.JPG)

Bottom side:

On the bottom side you can see that the zener voltage is Kelvin sensed at the LTZ1000A
and going directly to the neighboured pins of the D-Sub connector only being connected
to the output filter capacitor.

There are many additional capacitors and some cirquit modifications which where not planned
from the beginning but which are already included in the cirquit diagram.
The base emitter capacitors at the LTZ are connected as close as possible to the LTZ1000 pins.

On the right side lower the low noise 14V power supply around the LT1763 is built.

Unfortunately the Aluminium case has not much place on the side below the pcb.
So there can be no metal plate for the LTZ-Section on the lower side.
There is only room for a 4 mm polystyrene foam sheet for thermal isolation.

By the way: for first firing of the cirquit I did not use the LTZ1000A but replaced the
zener section with a self built "refamp" consisting of a ordinary zener and a transistor.
Just to look whether the current regulation will do it's job properly, especially
when powering up/down.


Shielding and guarding:

ince I use a battery powered design where all components are within the aluminium case,
and no mains line can introduce any common mode noise to the cirquit,
the topic of shielding and guarding can be much simplified:

I have only a guard and dont need a outer shield (connected to eart ground)
All parts are "inguard".
A 2mm banana plug connected directly to the EG2 aluminium case can be used to connect
to the guard pin (or if not available to the negative pin) of the multimeter or calibrator.

Of course I have a large metal plate connected to earth ground on my desk which I use as outer shield
during my measurements. The guard of the LTZ is isolated against shield by some bumpers.

problems observed with the cirquit:

A short ciruit to the (unbuffered) output will set the heater setpoint to a large value.
This shifts the output voltage of the LTZ1000A. (Hysteresis probably due to the die attach).
Fortunately I could remove the hysteresis by simply power cycling the reference for several times.

When measuring immediately after charging Im observing some shift to the output voltage.
Probably this is due to thermal gradients going across the PCB due to the "hot" AA-cells.

todo list will follow ...
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on June 22, 2013, 09:25:24 pm
Part 2 LTZ1000A of Andreas

...

Nice, compact design, Andreas.. like it.

I will later show some LTZ1000 hysteresis measurements, I have encountered similar trips to extreme temperatures and hysteresis on one of my references also, but was able to reset it.

See Pickering patent.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on June 22, 2013, 09:37:55 pm
For a future design, there are feed through capacitors available by the ham guys, that are affordable:

http://server3.gs-shop.de/200/cgi-bin/shop.dll?AnbieterID=9187&Seite=frameset.htm&PKEY=9D07 (http://server3.gs-shop.de/200/cgi-bin/shop.dll?AnbieterID=9187&Seite=frameset.htm&PKEY=9D07)

They are made of a zylindrical ceramic with inner and outer metallization. In the inner on a wire is soldered, the outer side can be directly soldered into a hole of your galvanically tinned metal case.

So you are able to fully encapsulate your bord with your reference, the buffer circuit and the current regulator. You are then also able to fill the case with what ever you want (hydrogene, nitrogene etc.) and hermetically seal it by soldering the lid to the rest of the case.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on June 22, 2013, 09:51:30 pm
Exactly what i talked about. You can solder in tubes for filling and sealing in advance. The feedthru caps are fine. I need to come out as being a ham :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on June 23, 2013, 06:26:16 pm
What about long-term stability of multiple paralleled references? Does it to average decrease? All I could read about is improved noise, sure by sqrt(N), but nothing more. Any experience about that?

We all know that it needs at least three references to find which one drifts in which direction and by what value, so wouldn't it be worth building a 3ref-ADC-DAC system regulating the voltage output after initial calibration?
Title: Re: Ultra Precision Reference LTZ1000
Post by: SeanB on June 23, 2013, 06:29:26 pm
5 devices would be better for long term drift, as you can use it to check which ones have the most variation from the average.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on June 23, 2013, 06:33:23 pm
With heated refs having a bunch might help spreading the thermal aging. Also, averaging groups was described by pease, using 4x4 lm399.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on June 23, 2013, 06:38:42 pm
Inspired by the Datron/Wavetek 4910, I thought 4 would be perfect, that is why I ordered 4 LTZ1000A.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on June 23, 2013, 09:21:12 pm
Inspired by the Datron/Wavetek 4910, I thought 4 would be perfect, that is why I ordered 4 LTZ1000A.

Yes 4 is the minimum for maintaining the "volt". At least 3 are on your site and you can find out a defective one.
The fourth you can send for calibration and import the calibrated volt to the group.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on June 23, 2013, 09:35:21 pm
Quote
Inspired by the Datron/Wavetek 4910, I thought 4 would be perfect, that is why I ordered 4 LTZ1000A.

Are they paralleled or compared against each other?
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on June 24, 2013, 06:34:23 am
Yes 4 is the minimum for maintaining the "volt". At least 3 are on your site and you can find out a defective one.
The fourth you can send for calibration and import the calibrated volt to the group.
@Andreas, for this the 4910 would not be the right choice (but Fluke 732 and 7000 would be see picture)

Are they paralleled or compared against each other?
@branadic, see 4910 Front Panel with 4 individual cells (10V each) plus average (see schematic)
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on June 24, 2013, 09:15:12 am
Sorry I was a bit handy-capped by using only my mobilephone which has no english dictionary, so entering those twitterlike short posts was hard work :)
My bit of thermal management was

* a "starlike" layout at the LTZ footprint, same-thickness, same-length traces on both sides of the PCB, if they are needed or not. Kept the "equalisation" star part free of other traces.

* cooper pour used as GUARD, will help spread out heat to reduce gradients.

* small bottle cap over the LTZ keeping regional airflow off

* Tinned metal sheet (heat-spreading) inside ABS outer enclosure (insulating).

Regarding DIY hermetically housing (close soldered box, feedthru capacitors):

Putting the feedthrus in close proximity will give only small temperature gradient. Symmetrical construction will help cancel out thermal EMFs. Putting all feedthrus on one side would allow using a socket or PCB.

BR
Hendrik

Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on June 24, 2013, 10:17:37 pm

* a "starlike" layout at the LTZ footprint, same-thickness, same-length traces on both sides of the PCB, if they are needed or not. Kept the "equalisation" star part free of other traces.

Regarding DIY hermetically housing (close soldered box, feedthru capacitors):

Putting the feedthrus in close proximity will give only small temperature gradient. Symmetrical construction will help cancel out thermal EMFs. Putting all feedthrus on one side would allow using a socket or PCB.


So if I understand it right: up to now you have no slots or cutouts within the PCB.

The feedthrus from "Geist" have copper leads as inner lead according to data sheet.
So on one side you will have lesser problems with thermoelectric effects. On the other side
I fear that the hermetical tightness is only a question of time.
How do you plan manage the barometric / temperature pressure relief with your close soldered box?

Has anyone a Idea how we can test the quality of a thermal design.
What is better: having long or short legs. Slotted or non slotted PCB.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on June 25, 2013, 06:38:29 am
Quote
Has anyone a Idea how we can test the quality of a thermal design.
What is better: having long or short legs. Slotted or non slotted PCB.

I think that is something you can only answer by measuring the heat spread with a thermal cam at an exemplar and as you expect I have access to one. Maybe one of those questions we can answer at one weekend?

BTW: Does anyone have the "ultimate" LM399 circuit that is worth building? Have build the "10V Buffered Reference" circuit with LT1001ACJ8 shown in the datasheet, but there are a few LM399 left, waiting for something useful.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on June 25, 2013, 09:38:59 pm
I think that is something you can only answer by measuring the heat spread with a thermal cam at an exemplar and as you expect I have access to one. Maybe one of those questions we can answer at one weekend?

BTW: Does anyone have the "ultimate" LM399 circuit that is worth building? Have build the "10V Buffered Reference" circuit with LT1001ACJ8 shown in the datasheet, but there are a few LM399 left, waiting for something useful.

Sounds good. This would be a good possibility to verify the design.

Mhm, why not use some LM399 to verify the thermal design. Principally they have a similar layout. But are much cheaper. And possibly the higher chip temperature gives an advantage in seeing temperature differences.

My ultimate idea (not already a complete design) would be building some kind of "calibrator" from 0-10V with a LM399 as basis. Either with a PWM divider (EPN cirquit idea) (cheap but noisy) or 2 interleaved 16 Bit DACs giving about 28 Bits resulting (more expensive but less noise) resolution. When having 2 independent calibrators of this sort and a 24 bit low noise ADC with some relays or MUX switches you could build a "self calibrating" system which calibrates out the non-linearities below around 1ppm. Similar to Franks 10V/1V divider but with a binary adjusting scheme.

For e.g. Set the first output to 10V the second to 5V compare the output voltages as 0V + 5V and 10V - 5V and re-adjust the 5V output to get the exact PWM/DAC values for half of the range. Store the values and determine + store the ADC INL error for half of the range. Then measure the difference of both outputs and adjust the first output to exact the 5V value of the 2nd output. Repeat the same for 5V and 2.5V and the 7.5V points. And so on.... until the linearity error of the DAC + ADC (e.g. 4-8 ppm) are well below 1ppm. I guess that about 32 - 256 measurement points should be sufficient as a basis.

By the way Frank: How often is it necessary to re-calibrate the 10V/1V divider to maintain the 0.1ppm divider accuracy. Which drift over time did you observe after calibration?

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: tinhead on June 25, 2013, 09:53:21 pm
BTW: Does anyone have the "ultimate" LM399 circuit that is worth building?

no, but i'm thinking about REF102CP with the QH40A crystal heater attached to it.

At 40° there is almost no drift at all, see attached picture (QH40A temp drift is between two blue lines)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on June 25, 2013, 09:57:12 pm



By the way Frank: How often is it necessary to re-calibrate the 10V/1V divider to maintain the 0.1ppm divider accuracy. Which drift over time did you observe after calibration?

With best regards

Andreas


Andreas,

this kind of divider (720A, K.V.) is 0.1ppm of input, i.e. 1ppm of output @ 1V!

This is stable for 3 months or more, I guess.
I did not calibrate it more often.

I monitored the 10/7 output only, and that was accurate to ~ < 0.5ppm for 1 year, or so.

If you need more precise 10:1 division, have a look on the Hammon type divider, it's also much easier to switch and to calibrate!

Also have a look on the Fluke 5440A self calibration scheme, it has a nice 0.1ppm linear DAC ( 2x 1bit) and automatic  range calibration, uncertain to < 0.5ppm, or better.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on June 25, 2013, 10:00:17 pm
At 40° there is almost no drift at all, see attached picture (QH40A temp drift is between two blue lines)

I have measured the tempco of many references. But no one of them had ever seen the "idealized" curve out of the data sheet. In reality you will have to select many references to find one which has a maximum or minimum in the range of 40 degrees. Further the maximum/minimum will probably be shifted by hysteresis effects.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on June 26, 2013, 08:41:43 am
I have not yet calculated the forces due to pressure on the DIY hermetic can, as I dont have decided for a size and know very little about the pressure behaviour of the gas yet. But my boss as a physicist will :) Nitrogen could be a good match as a filling gas, as most of air is it. Pressure exchange without mixing of internal gas and ambient air could be done with a flexible membrane, say, a hole in the close-soldered can, with a brass ring soldered in the inside and a piece of copper foil soldered on the ring. The cooper foil might be shaped to improve movement, cold-forming concentric rings like done with pressure sensor membranes comes to mind.

Man, I didn't want to go for gimmicks... :)

Lets see if our trainee is free for building the LM399 voltage source today...

EDIT: Have a look at those: http://www.servometer.com/products/metal-bellows/standard-bellows/ (http://www.servometer.com/products/metal-bellows/standard-bellows/)
These might be a off-the-shelf practical membrane for pressure relief. Just solder it to the case with one end and solder it close on the other end.



Title: Re: Ultra Precision Reference LTZ1000
Post by: SeanB on June 26, 2013, 06:37:46 pm
If you want a pressure equaliser bellows cheap go get an old mechanical fridge thermostat, there is a nice phosphor bronze capsule inside with a capillary tube attached. Vent the gas and solder the cap tube to the finished case and it will flex to accommodate the expansion of the fill gas or oil.

If I was building one I would fill it with an inert refrigerant gas like R134A,  all the housing has to do is be capable of being pumped down to a vacuum for a short time then filed and sealed with a standard method, either a schraeder valve or a soldered pinch tube.
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on June 26, 2013, 06:51:42 pm
Quote
My ultimate idea (not already a complete design) would be building some kind of "calibrator" from 0-10V with a LM399 as basis. Either with a PWM divider (EPN cirquit idea) (cheap but noisy) or 2 interleaved 16 Bit DACs giving about 28 Bits resulting (more expensive but less noise) resolution. When having 2 independent calibrators of this sort and a 24 bit low noise ADC with some relays or MUX switches you could build a "self calibrating" system which calibrates out the non-linearities below around 1ppm. Similar to Franks 10V/1V divider but with a binary adjusting scheme.

I was more thinking of using three references, several switches, an adc and "a dac."

1. Initial calibration for one of the three references. Therefor Ref1 is connected to the dac and output voltage of a specific digital word is measured, you just need to know its initial output voltage.
2. Ref1 is divided and act as the adc ref, Ref2 and Ref3 are divided to be within the range of the adc and are measured.
3. Ref2is divided and act as the adc ref, Ref1 and Ref3 are divided to be within the range of the adc and are measured.
4. Ref3 is divided and act as the adc ref, Ref1 and Ref2 are divided to be within the range of the adc and are measured.
5. Now the drift and the new digital word for Ref1 connected to the dac is calculated as well.
6. Start again at point 2.

After inital calibration the system needs to stay powered up.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on June 28, 2013, 04:28:59 am

Also have a look on the Fluke 5440A self calibration scheme, it has a nice 0.1ppm linear DAC ( 2x 1bit) and automatic  range calibration, uncertain to < 0.5ppm, or better.

Frank

Hello Frank,

I had a look on the documentation (hope I picked the right one)
http://assets.fluke.com/manuals/5440B_AFsmeng0000.pdf (http://assets.fluke.com/manuals/5440B_AFsmeng0000.pdf)

But with my experiences from a PWM-divider (see EDN cirquit)
http://www.edn.com/design/other/4326640/DC-accurate-32-bit-DAC-achieves-32-bit-resolution (http://www.edn.com/design/other/4326640/DC-accurate-32-bit-DAC-achieves-32-bit-resolution)
I expect that besides the selection of the FETs (1 Ohm) in the 5440 unit a part of the linearity is hidden within the calibration constants.

Otherwise you have too much problems with "voltage dependancy of the switch (High/low)", "charge injection from PWM to output", "different rise times high/low" which makes it hard to get around below 2 ppm linearity even with tweaking all resistors.
See attached pictures. Linearity deviation is in mV referenced to 5000 mV of the reference. So 0.005 mV would correspond to 1 ppm linearity deviation.

There are different switches (CD4053, 74HCT4053 and MAX4053 without and with tweaking of the surrounding resistors) with different results ranging from about 100ppm to 4 ppm maximum deviation.

With best regards

Andreas

Edit: of course the genious trick of the 5440 with cutting out the non-linear range near zero (and full scale) by a offset would help at least for the tweaked cirquit.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on July 21, 2013, 07:25:55 pm
Part 3 LTZ1000A of Andreas (Todo list):

For the next 2 devices I plan the following.

- Using pre-aged Z201 resistors (100mW power cycling).
  the main intention is to have more room within the shielded area for additional improvement measures
  like cut outs on the PCB against the wirewound types.

- perhaps I can find also another suitable case with more room
  I would also like to have larger battery capacity.
  The 12 AA cells are only good for around 36-48 hours with heater on.
  I have already found the "Fischer TUG V 17" aluminium case which would be ideal in 160-220 mm length.
  But unfortunately its so new that there are no dealers who sell them in small quantities.
  The other solution would be the "ProMa EKG 3" aluminium case but the mounting possibilities
  for PCBs are very limited.
  Anyone better ideas for similar cases?

- use the additional room within the shield for some slots around the LTZ to keep thermal gradients
  away from the legs. The base emitter capacitors will remain as 0402/0603 capacitors directly at the LTZ.
  The LTZ will be mounted directly to the PcB (platet through PCB is necessary).
  My prefered solution up to now is using short legs and a slotted board.
  In this case I believe that the critical thermal junctions between PCB
  and LTZ have the lowest temperature differences.
  But if branadic gets other results with his thermal camera I will use the optimum solution.

- perhaps (depending on space used) additional isolation (slots) to the FET and the heater transistor.

- replacing the Hirschmann banana plug connectors with POMONA low emf connectors (equal height for both pins)

- buffered output with 10V probably with a LTC2057 low noise low drift amplifier (recently arrived from DigiKey)
  The resistors for the divider could be the trimmed version of the VSMP series which is offered by DigiKey.
  The fine trimming could be done by a DAC. And all on on a PCB cutout which can be thermally regulated.
  Of cause I will check the crystal oven heater which was recommended by branadic (good idea) as an option.

- I bought some LT1013A-devices in hermetically tight case (CERDIP) some time ago.
  One for the price of a half LTZ1000A. I wanted to use them to see if
  I could get more stability over time against the standard plastic case.
  And all pictures of references that I could see with an LT1013 had usually a TO-99 metal case
  which is now unobtanium.

  But now I have the LT2057 which is more stable over time and temperature,
  has more open loop amplification lesser problems with thermal emf and less noise than LT1013A.
  So at least I will give the LT2057 a try with the option to use the LT1013A as backup.

- The 14V voltage regulator and the battery pack should get at least a slot in the PCB for thermal isolation
  eventually further measures like additional (styrofoam-)wall between power supply and LTZ section.
  With a larger case I will put the batteries above the whole cirquit with a thermal isolation between
  the battery compartment and the rest of the cirquit. So the idea with the slot will only be for the
  local voltage regulator.

- Doing further thermal cycling (-18 degree celsius) after soldering (thanks Frank for the idea).
 
  With a AD586LQ-device I have made the experience that a gentle cyclical stress has a positive effect on
  the ageing rate. Of course every stress will give a new starting point
  (usually with a higher momentary ageing rate) for the device.
  But if you have luck the ageing will stabilize as can be seen on the graph.
  At the moment I repeat the same for a LT1236AILS8-5 device which has a too high ageing rate for my purposes.

  To the picture:
  Day 200 is start of the measurements with ADC13 and AD586LQ reference
  measuring a LTZ1000A by a capacitive LTC1043 2:1 divider (giving around 3600mV).
  The first firing of the reference is day 0.
  Initial ageing rate around 3.4ppm/sqrt(kHr) referenced to day 0.
  Around day 415: begin of cyclical stress during night with 15mA
  load at the output of the reference.
  Ageing slope increases new ageing rate around 2.4ppm/sqrt(kHr) referenced to day 415.
  Around day 460: Ageing stabilises to around 1-2 ppm/year cyclical stress continued.
  Around day 540: Finishing cyclical stress.

Any further Ideas for the LTZ?
How is the pre-ageing for the LTZ1000 done at the manufacturers of metrology equipment?

With best regards

Andreas




Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on July 24, 2013, 07:47:15 pm
Quote
But if branadic gets other results with his thermal camera I will use the optimum solution.

It will take some time, but I'm working on that.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on July 25, 2013, 06:53:54 am
Part 3 LTZ1000A of Andreas (Todo list):

For the next 2 devices I plan the following.

- Using pre-aged Z201 resistors (100mW power cycling).
  the main intention is to have more room within the shielded area for additional improvement measures
  like cut outs on the PCB against the wirewound types.


Once again, I cannot recommend that at all.
This is necessary only for stabilization of the load life stability of the resistors, which is not the case for LTZ references (P< 10mW)
Also, you will get hysteretic resistance value shifts of several ppms, which you might want to remove by thermal cycling (similar to demagnetization)
If you use heremetically sealed VHP202Z, the drift is so low from the fab, that a "stabilization" as you want to do, causes more harm (i.e. several ppm of non reversible value shift) to the resistor than the ultra low natural drift (2ppm/6yrs.).


..
  The LTZ will be mounted directly to the PcB (platet through PCB is necessary).
  My prefered solution up to now is using short legs and a slotted board.
  In this case I believe that the critical thermal junctions between PCB
  and LTZ have the lowest temperature differences.
  But if branadic gets other results with his thermal camera I will use the optimum solution.


Try to use a single sided PCB, so all thermal junctions are on that side and can easily be shieleded thermally by a single layer of foam.
Again, the effects of slots in the board have not yet been explained, neithertheir physical mechanism, nor the quantity.

In contrast, a noisy power supply, or bad shielding cause shifts of up to several ppm, obviously by rectifaction of those AC disturbance signals on the temperature measuring Q1, therebyd shifting the temperature. To mitigate this effect, is much more important.
(I use an external PSU, so the transformer and its magnetic field is outside the case)

- The 14V voltage regulator and the battery pack should get at least a slot in the PCB for thermal isolation
  eventually further measures like additional (styrofoam-)wall between power supply and LTZ section.
  With a larger case I will put the batteries above the whole cirquit with a thermal isolation between
  the battery compartment and the rest of the cirquit. So the idea with the slot will only be for the
  local voltage regulator.


A magnetic shielding and multiple filtering of AC disturbance signals are much more important.


- Doing further thermal cycling (-18 degree celsius) after soldering (thanks Frank for the idea).

You have misunderstood me.
Thermal cycling means, that you apply temperature differences from the stabilization point (e.g. +50°C) cyclicly, with decreasing amplitude.
E.g. you have to apply -18°C (delta T ~ -68K), +100°C  (+50K), -10°C, +80°C, 0°C, +70°C, ... and so forth, until you are below ~ +/- 20K, where all hysteresis is gone. See also the patent of Pickering, realized in the Fluke 7001 reference (but which does not work properly in the 7001, I assume)

  To the picture:
  Day 200 is start of the measurements with ADC13 and AD586LQ reference
  measuring a LTZ1000A by a capacitive LTC1043 2:1 divider (giving around 3600mV).
  The first firing of the reference is day 0.
  Initial ageing rate around 3.4ppm/sqrt(kHr) referenced to day 0.
  Around day 415: begin of cyclical stress during night with 15mA
  load at the output of the reference.
  Ageing slope increases new ageing rate around 2.4ppm/sqrt(kHr) referenced to day 415.
  Around day 460: Ageing stabilises to around 1-2 ppm/year cyclical stress continued.
  Around day 540: Finishing cyclical stress.



I do not understand your diagram correctly, I fear, please help me:.

The drift shown comes from the AD586 only, the difference of both LTZ outputs is constant to ~ 1ppm in 1 year or so, therefore the drift rates of 3.4ppm/sqrt(khr) you have additionally drawn apply to the AD 586, but not to the LTZs.
Those are rock stable and need no further improvement. (I have measured similar behaviour over 4 years)



Any further Ideas for the LTZ?
How is the pre-ageing for the LTZ1000 done at the manufacturers of metrology equipment?


I doubt they make real pre-aging.
The LTZs and the peripheral components are so stable that the total drift mainly depends on the different LTZ samples, i.e. on fabrication variation.
The first might drift 1ppm/yr. (@60°C), the next 0.7ppm/yr. only, and so forth.
So they do a monitoring over 3-6 months only, and select the most stable references.

To my understanding of the physics, at such low drift levels, a pre-aging makes no sense, but causes more harm (higher drifts, hysteresis...) than it would really stabilize the reference even more.

I will publish my own results (measurements) soon, as an indicator for that thesis.

regards Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on July 25, 2013, 08:07:15 pm
Travelling Reference Info: My LTZ1000A cam back a few days ago, will visit Dr. Frank soon and after that it could travel south, to Quarks and Branadic !

Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on July 25, 2013, 09:41:34 pm
It will take some time, but I'm working on that.
Fine we are all curious on the results.

Once again, I cannot recommend that at all.

Try to use a single sided PCB, so all thermal junctions are on that side and can easily be shieleded thermally by a single layer of foam.
Again, the effects of slots in the board have not yet been explained, neithertheir physical mechanism, nor the quantity.

Sorry but my ageing experiment is already running some 1000 hours. So I should have known it last year in February. On the other side: when looking at the derating diagram the 100mW will heat about 17 degrees.
Thats nearly the temperature change between winter and summer of my "lab".  I will see the result in the run-in phase of the LTZ.

If using a single sided board I cannot use slots. So this will be only a option if the slots worse the temperature distribution between the pins. I have now a board without slots so I will most probably try one with slots the next time.

In contrast, a noisy power supply, or bad shielding cause shifts of up to several ppm, obviously by rectifaction of those AC disturbance signals on the temperature measuring Q1, therebyd shifting the temperature. To mitigate this effect, is much more important.
(I use an external PSU, so the transformer and its magnetic field is outside the case)

A magnetic shielding and multiple filtering of AC disturbance signals are much more important.

I will use a AC wall plug adapter only during charging. In operating mode I will use battery power.
The best shielding at line frequency is keeping far away from transformers.

You have misunderstood me.
Thermal cycling means, that you apply temperature differences from the stabilization point (e.g. +50°C) cyclicly, with decreasing amplitude.
E.g. you have to apply -18°C (delta T ~ -68K), +100°C  (+50K), -10°C, +80°C, 0°C, +70°C, ... and so forth, until you are below ~ +/- 20K, where all hysteresis is gone. See also the patent of Pickering, realized in the Fluke 7001 reference (but which does not work properly in the 7001, I assume)
Ok now I got it. But the first point at my side is the 270 degrees soldering temperature when using short legs.


I do not understand your diagram correctly, I fear, please help me:.

The drift shown comes from the AD586 only, the difference of both LTZ outputs is constant to ~ 1ppm in 1 year or so, therefore the drift rates of 3.4ppm/sqrt(khr) you have additionally drawn apply to the AD 586, but not to the LTZs.
Those are rock stable and need no further improvement. (I have measured similar behaviour over 4 years)
Yes you are right.
My test setup are the 3 references, fed trough a multiplexer, then divided by a capacitive 2:1 divider (LTC1043) and then measured by a 24 Bit ADC with a temperature compensated reference AD586LQ.
So if you assume the LTZ1000A as being constant you see the inverse ageing of the AD586. If the reference voltage of AD586 goes down you see the measurement going up in the diagram.
Unfortunately I do not have the absolute ageing rate of my LTZ1000 references up to now.

To my understanding of the physics, at such low drift levels, a pre-aging makes no sense, but causes more harm (higher drifts, hysteresis...) than it would really stabilize the reference even more.

I interpret the diagram above as follows: Every stress on the device will start a new ageing cycle with usual higher ageing rate as before.
It may be accidently in the test above or perhaps can be repeated: (I will know this in a few 1000 hours with a LT1236AILS8). After the stress (training) the reference is falling to its "sweet spot" remaining stable.

My theory:
I blame the hysteresis effects of hermetically tight packages on the "die attach" of the chip.
On AD586 change notes you can see that it is usually a silver filled epoxy compound.
I do not know how they dose the glue on the Kovar plate/lead frame of the housing but probably this
process is the one with the most errors in fabrication. And every manufacturer has its own mixture.
With temperature cycling the erratic connections of the die attach at the edges of the chip might be egalized somewhat giving better results.
The interesting thing with a LT1236AILS is that the hysteresis of the chip seems to have a time constant/delay of about 50 minutes (measured on one device). So the hysteresis curve on this device is drastically changed between a temperature slope of 0.1K/minute against 0.3K/minute. So also this effect could be some creeping effect of the die attach.

With best regards

Andreas


Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on July 26, 2013, 07:38:57 am

Sorry but my ageing experiment is already running some 1000 hours. So I should have known it last year in February. On the other side: when looking at the derating diagram the 100mW will heat about 17 degrees.
Thats nearly the temperature change between winter and summer of my "lab".  I will see the result in the run-in phase of the LTZ.

I think, that effect is so small (e.g. attenuation 1:100), that you won't be able to identify it, or distinguish it from other effects.



I will use a AC wall plug adapter only during charging. In operating mode I will use battery power.
The best shielding at line frequency is keeping far away from transformers.
The battery mode is the best you can do.
Although I use an improved DC - out wall plug adapter also, my LTZ refs still struggle from other AC irraditions.
There is even a small effect from the GPIB cable, if the measurement is running.
Therefore the next LTZ design will include multiple shielding and a double filtered DC supply.
 
Ok now I got it. But the first point at my side is the 270 degrees soldering temperature when using short legs.

I used heat removal pliers, and left the legs long.
If in doubt, whether the LTZ has "seen" this heating, you should do this thermal cycling afterwards.

For the first of my two references I was able to prove, that it did NOT see any hysteresis effect from soldering and only small other heating effects, because after a thermal cycling, its output value went back to exactly the initial value.
That means, that this LTZ reference  drifted less than 0.5ppm in about 3-4 years!!

So if you assume the LTZ1000A as being constant you see the inverse ageing of the AD586. If the reference voltage of AD586 goes down you see the measurement going up in the diagram.
Unfortunately I do not have the absolute ageing rate of my LTZ1000 references up to now.

Simply trust on the datasheet-stability of the LTZs..the AD586 drifts, but not the LTZs..

Well  8), you indeed HAVE absolute ageing rates of your LTZs already!!
Simply create a diagram where you draw the change of the difference in output voltage over time, i.e. LTZ_1 minus LTZ_2 and LTZ_1 minus LM_2 (what's that kind of ref? not the LM1236 ?).
Those two graphs will show, that these 3 refs drift apart not more than 1-2ppm/2years!!

The individual drifts will not be more than those determined values, theoretically half of that for each one.
So, you already own two/three ultra-stable references, based on a design with "simple" wire wound resistors, like "babysitter" and me...

I interpret the diagram above as follows: Every stress on the device will start a new ageing cycle with usual higher ageing rate as before.

That's another argument for NOT doing a pre-ageing stress on the LTZs, as they might behave the same, but on a much smaller scale.


regards Frank

Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on July 26, 2013, 06:30:37 pm
Therefore the next LTZ design will include multiple shielding and a double filtered DC supply.

I have already tried to filter out the interference of a (switchmode) power supply X+Y capacitors + chokes. But with very little effect.
I guess one will have to use a transformer with a shield like on most metrology gear between primary + secondary side to get effective filtering.
So let me know if you have a solution.

Simply create a diagram where you draw the change of the difference in output voltage over time, i.e. LTZ_1 minus LTZ_2 and LTZ_1 minus LM_2 (what's that kind of ref? not the LM1236 ?).

LM #2 is one of my LM399 references now running 24/7 since 2008.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on July 27, 2013, 04:57:47 am
Hello Ken,

thanks for sharing. This will be another point on my todo list: At least providing the space for ferrite beads on PCB at critical points.

Do you have any practical experiences with these parts mentioned in LT AN101?

When having a closer look to AN101 I see that J.W. has not used a real switching supply but has generated the waveforms by a "artificial circuit".
The main difference is that he supplies the spikes in differential mode to the input of the analog regulator (a LT1763 which I also use). The noise from a real switch mode supply usually clocks in as common mode so the noise will be equally on the +supply and the GND pin. At least this is true for a wall wart with 1.5 m cord length on the 24V side.
So instinctively I would use 2 ferrite beads on the input. Preventing that the input capacitor of the analog regulator will feed the noise on the GND line to the input of the regulator after the bead on the +supply again.
But why is he using especially this ferrite bead having only around 50 Ohms at 100 MHz. I would go for a higher Impedance e.g. 300-1000 Ohms at the critical frequency of 100 MHz if the DC voltage drop due to the DC resistance would permit this.
Further I would never use only a electrolytic capacitor alone on input and output of voltage regulator like it is shown in AN101. I always use additional 100nF ceramics to short the RF spikes to the GND pin of the regulator.

With best regards

Andreas
 
Title: #6 Measurement Results: Monitoring, Conditioning & hysteresis effects
Post by: Dr. Frank on July 27, 2013, 02:17:07 pm
Monitoring Setup

If you want to find out how well your LTZ reference performs, you have to compare it to either one much more stable standard, or to several equally good ones.

This much better standard would be a JJ primary standard only.

Equally good ones would be supervised Zener standard banks as Fluke 734B, 7010 or Datron 4901, or an ensemble of at least 3 DYI LTZ1000 references, or similar.

Also, a precise transfer standard is needed, to be able to compare the raw 7.xxx volt precisely against the 10V references.

I use 2 different kind of references, the first one is the internal LTZ1000A of a 3458A, running on 65°C (instead of 95°C), and a Fluke 5442A which is based on the SZA263.

The HP3458A also serves as a very precise transfer standard, due to its linearity.

Both devices are well aged, 13 years for the HP3458A, 23 years for the Fluke 5442A, and both are running intermittently only. During power down times, their drift should be close to zero. Their combined drift over 3 ½ years is obviously 1ppm maximum, see figure 1.

Both of the DIY LTZ1000 references, including the 7,147V => 7,0000V => 10,0000V transfer derived from Ref_2, have been running continuously for 3 ½ years, and were compared also against the HP3458A. Therefore, my reference ensemble consists of 4 equivalent sources.

Long Term stability measurements

During the first 2 years, I have checked only, that all references were stable to within 1ppm of their initial values. Therefore, intermediate data points were missing.

In June 2011, I have moved my complete analogue equipment to the basement, where a stable room temperature of 20.0 … 22.5 °C is available during all seasons.
The room temperature may be constant to +/- 0.2K during several hours and to +/- 1.0K during successive measurement days. 
This measure improved the short term stability of all measurements significantly.
In other words, without a stable environment, sub-ppm stability measurements are not possible at all.

Ref_2 and its 10V output drifted about 1ppm in 3 ½ years. (See figure 2)
The 7,000V output has been calibrated initially only, but the 10/7 transfer was calibrated several times, about 2 times a year.

On Ref_1 I performed some experiments, so it was “mistreated” several times, i.e. the temperature control got out of regulation. Thereby it encountered several temperature trips to 100°C (estimated).
Afterwards, its output voltage then restarted at a lower value, and drifted much more than Ref_2, (see figure 3). As the output voltage of the LTZ1000 is lower at 100°C, the reference @ 45°C obviously memorized its short trip to 100°C, and drifted towards that direction.

Conditioning

The last such accident happened in June 2013, which left a hefty additional shift of -3ppm.

I remembered the patent of Pickering, to remove temperature induced hysteresis effects in the LTZ1000.

So I temperature-cycled the complete box to remove the hysteresis (see figure 4).

To do that cycling right, it is important to have big enough, but decreasing temperature differences, related to the stabilization temperature, in this case compared to +45°C.
This means, +100°C gives +55K, storage at -23°C gives a dT of -68K, +80°C equals dT = +35K and so forth.

I found out (see fig. 4 also), that below a difference of about +/- 20K, there is no big hysteresis effect.
So I really doubt, that the Fluke 7001 really did operate efficiently in removing hysteresis, because at a stabilization temperature of +45°C, the lowest available negative temperature difference at around 22°C room temperature (i.e. -23K) is too small to deliver a sufficient 'reset' effect. (The 7001 has been terminated by Fluke in the meantime.)

During that procedure, Ref_2 also showed a hysteresis effect, but in the end returned to its initial value of 3½ years before. This indicates that the cycling really removed the hysteresis in both references.

Ref_1 ended at about +1ppm, and its future drift behavior will show, if it’s now more stable. If it resides at about +1ppm, possibly the LTZ1000 had been heated too much during soldering, although I used a thermal transfer pincer and soldered very quickly.

Further effects

It is important to have a proper shielding (e.g. case connected to ground) and a quiet power supply.

An external wall plug power supply was used, to avoid magnetic disturbance from the transformer. In first instance, its output  of 18V AC induced disturbances on the LTZ temperature regulation.
The case had to be connected to ground of the LTZ output, and that induced a shift of 0.5ppm of each output. Exactly this same effect could also be demonstrated on another LTZ based reference, designed by 'babysitter'.
When I redesigned the wall plug in for an output of 25V DC, the LTZ references became more short term stable, and the 0.5ppm shift vanished.


Short Term stability measurements

Figures 5 and 6 show short term stabilities of Ref_1 versus HP3458A during 10 minutes and 35h, using an aperture time of 2 seconds, i.e. NPLC100.

The internal temperature of the HP3458A increased continuously from 33.7 to 34.8°C during those 35h. ACAL was performed only once, before the start of the measurement.
 
The 10min measurement shows  fluctuations of +/- 0.05pm, which is very well consistent with the transfer specification of the HP3458A,  +/-  0.1ppm, and especially with the noise specification of the LTZ1000, i.e. 2µVpp (equivalent to +/- 0.15ppm).

The average 35h drift of ca. 0.3ppm is well below the HP3458A 24h specification, i.e. +/- 0.55ppm, or its T.C. of 0.5ppm/K without ACAL.



Conclusion:

The basic LTZ1000 circuitry, output of around 7,2V, running on 45°C and built with wire wound resistors is capable of drifts well below 1ppm/year. Noise and short term stability are below +/- 0.1ppm.

Sophisticated shielding and a low noise DC PSU are very important for obtaining that degree of stability.

The influence of additional ‘gimmicks’ as summarized in part #1, have not been demonstrated yet, but they have to compete in value with the effects demonstrated here.

<end>
Title: Re: #6 Measurement Results: Monitoring, Conditioning & hysteresis effects
Post by: Andreas on July 27, 2013, 03:54:32 pm
To do that cycling right, it is important to have big enough, but decreasing temperature differences, related to the stabilization temperature, in this case compared to +45°C.
This means, +100°C gives +55K, storage at -23°C gives a dT of -68K, +80°C equals dT = +35K and so forth.

After such a "mistreatment" with a output voltage shift (shorting the 7V output by a unpowered ADC) I simply switched off the power of my LTZ#1 for some minutes to cool down and switched on again. The deviation was much smaller so I repeated power cycling on the LTZ until the deviation was small.

With best regards

Andreas
Title: Re: #6 Measurement Results: Monitoring, Conditioning & hysteresis effects
Post by: Dr. Frank on July 27, 2013, 04:09:38 pm

After such a "mistreatment" with a output voltage shift (shorting the 7V output by a unpowered ADC) I simply switched off the power of my LTZ#1 for some minutes to cool down and switched on again. The deviation was much smaller so I repeated power cycling on the LTZ until the deviation was small.

With best regards

Andreas

I did not mention that: Although I unplugged the box for several days after such an event, the Ref_2 output recovered a little bit, but afterwards quickly drifted towards negative values.
Therefore, "resetting" at 21°C did not really help.

Another indicator: Under normal conditions, the outputs recover stably to their initial values, i.e. to within a few tenths of a ppm, if the box is switched off for a longer period of time and then switched on again..

My wife tested that for me when she saw the illumination of  the multiple socket outlet, and switched it off to 'save energy'   :scared: (the box consumes around 2,5W).
3 weeks later, I was very relieved to determine, that absolutely no drift occurred afterwards.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on July 27, 2013, 04:18:49 pm
Hello Frank,

Would be interesting if the LTZ1000A are easier to get to the initial condition than the LTZ1000 or if my accident was simply not so hard as yours.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on July 27, 2013, 04:23:41 pm
Hello Frank,

Would be interesting if the LTZ1000A are easier to get to the initial condition than the LTZ1000 or if my accident was simply not so hard as yours.

With best regards

Andreas

Hello Andreas,

currently I don't want to test that again. No experiments on the revised box any more..
Perhaps I could do that when I assemble those five new LTZ1000..

I assume that the hysteresis depends on sample variations.
And the LTZ1000A should heat up to a higher temperature than the LTZ1000, due to its higher thermal isolation.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on July 27, 2013, 04:43:30 pm
currently I don't want to test that again. No experiments on the revised box any more..
Perhaps I could do that when I assemble those five new LTZ1000..
I can understand this ;-) And I do not know if I wanted to repeat the experiment with only having 7 devices.
Except when having one candidate with a very large drift over time.

I assume that the hysteresis depends on sample variations.
And the LTZ1000A should heat up to a higher temperature than the LTZ1000, due to its higher thermal isolation.
Of course so we would have to test many samples to get a statistical prove.
And dont forget: I have only a voltage regulator in SO-8 which will go easier to current limiting than other voltage regulators.

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on July 28, 2013, 09:28:55 am
Quote
So let me know if you have a solution.

http://www.haufe-uebertrager.de/ (http://www.haufe-uebertrager.de/)

delivers shielded (toroidal core) transformers. If the transformer is not shielded it should be shielded within your case. An important point is the way you arrange the transformer compared to the pcbs. Place it so that coupling by its field into your pcbs is as small as possible. You don't need to use expensive mu metall but galvanically tinned metal sheet / rf sheet metal is a perfect choice.
I would avoid using switching supplies in such an application at all and use linear components instead. But if not avoidable my tip is: There is a good book by Würth Elektronik you might want to have a look to, you will find all the info about ferrite beads and filter strategies using them.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on July 28, 2013, 03:57:59 pm
At 100Mhz, a few pF looks like a low impedance.  This is not an easy problem to fix, and there is no guaranteed "recipe" that will always work in any situation-- theory is not very helpful here-- only experience.

Good Luck!

-Ken

True words.
I worked on EMC optimized PCB layouts for some years. The 3rd truth is that you have to fight against every mil line length producing high impedances for filter capacitors shifting the maximum usable frequencies towards dramatic lower values.

Quote
So let me know if you have a solution.

http://www.haufe-uebertrager.de/ (http://www.haufe-uebertrager.de/)

delivers shielded (toroidal core) transformers. If the transformer is not shielded it should be shielded within your case. An important point is the way you arrange the transformer compared to the pcbs. Place it so that coupling by its field into your pcbs is as small as possible. You don't need to use expensive mu metall but galvanically tinned metal sheet / rf sheet metal is a perfect choice.
I would avoid using switching supplies in such an application at all and use linear components instead. But if not avoidable my tip is: There is a good book by Würth Elektronik you might want to have a look to, you will find all the info about ferrite beads and filter strategies using them.

Hello branadic,

perhaps I have not explained it correctly: I do not want to use a transformer within my device. I am looking for a solution of keeping external mains disturbances like the "green" switchmode supplies away from my cirquit so that I could (optional) charge while measuring. My shielding and guarding concept does not allow a transformer within the device.
For this I would need a outer shield connected to earth ground additional to my existing housing which is a floating guard.

And there is a difference between the magnetic shield (a outer ferromagnetic housing around the transformer) which they offer as option for standard parts and a electrostatic shield (a single ended isolated metal foil connected to earth ground) reducing the coupling capacity between primary and secondary side of the transformer.

So I will most probably try to get some enhancement by some ferrites.
Which of the books did you mean on the Würth homepage?
The "Trilogy of Magnetics" or one of the other books?

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on July 28, 2013, 04:09:23 pm
Quote
Which of the books did you mean on the Würth homepage?
The "Trilogy of Magnetics" or one of the other books?


Yes, this is the title I think to remember to. I have bought this book for work and think it's pretty helpful.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on August 05, 2013, 05:06:21 am
Of course... Pk4ts. There is the Same Part in normal thermal emf available for comparisons, too.

Sent from my mobile
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on August 05, 2013, 05:34:31 am
IT is in their t&m catalog! I dont know a Distributor, price around 12 Euro each. Minimum Order around 60 euro. Br Babysitter
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on August 05, 2013, 07:50:05 am
so far I have not found a source for the SLS410-TS and the SL425-A/TS, if you find them please share
(what I do not like is, both have solder connection only)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 05, 2013, 08:12:13 am
so far I have not found a source for the SLS410-TS and the SL425-A/TS if you find them please share
(what I do not like is, both have solder connection only)


I've ordered the PK-4TS directly @ MC Germany, therefore I assume that you may also get the other parts there.
Perhaps you ask them first, also for pricing.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on August 05, 2013, 08:55:21 am
Price was about 7-8 eur for the plugs iirc.
Minimum order of ~60 euro in germany !

Greetings from the Babysitter
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 05, 2013, 08:09:15 pm
http://www.edn.com/design/other/4326640/DC-accurate-32-bit-DAC-achieves-32-bit-resolution (http://www.edn.com/design/other/4326640/DC-accurate-32-bit-DAC-achieves-32-bit-resolution)

Interesting about that is, that on volt-nuts the mistakes in the articel were never really identified and that W Stephen Woodward, Chapel Hill, NC; Edited by Martin Rowe and Fran Granville never showed a picture of the circuit (pcb-layout or photo of the pcb) or sold ready to use pcbs. Does anybody have the full text article?

@ DiligentMinds.com

You might want to draw your idea into a real world circuit?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on August 06, 2013, 06:15:10 am
Hello together,

there are other articles from Woodward who explain the circuit more in detail:
http://www.edn.com/design/analog/4323340/Fast-settling-synchronous-PWM-DAC-filter-has-almost-no-ripple (http://www.edn.com/design/analog/4323340/Fast-settling-synchronous-PWM-DAC-filter-has-almost-no-ripple)
http://www.edn.com/design/analog/4329365/Combine-two-8-bit-outputs-to-make-one-16-bit-DAC (http://www.edn.com/design/analog/4329365/Combine-two-8-bit-outputs-to-make-one-16-bit-DAC)

formerly it was possible to download the whole article as pdf and not only the schematics.

I have built the 32 Bit DAC cirquit mentioned above with slight variations. And could by far not reach the the values mentioned in the article.

That what I got is around 4 ppm INL (ok maybe only 3 ppm since my ADC is only linearized to around 1 ppm).
But when thinking twice: Woodward does not mention the switching times in his accuracy formulas neither he mentiones how he gets to the 23 bits as the effect of charge injection of the switches.

R7 has to be carefully adjusted to reach the 4 ppm INL. And naturally should be tracked with the chip temperature of the MAX4053 chip. With a simple 5.1 Ohms Resistor INL was around 13 ppm (65uV change) as can be seen on the diagrams of June 28th in this thread.

Also noise with 2uVpp only seems to consider the OP-Noise of the chopper. And not the additional effects of the switches and the error of the integrator producing some kind of "staircase noise" with much larger amplitude (around 15uVpp measured with a 4th order 10 Hz low pass filter) in my case.

So all in all the circuit idea is not bad. The settling time is excellent. But there is much room for enhancements.

So I for my part will rely on a 2 DAC solution with a precision ADC in loopback as in AN86.

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on August 06, 2013, 07:37:36 am
Quote
http://www.edn.com/design/other/4326640/DC-accurate-32-bit-DAC-achieves-32-bit-resolution
This article is like a joke. Even ancient (30-years old) Datron 4000 calibrators have a two PWM DACs and provide 0.03 ppm of internal resolution with 0.1 ppm INL. But schematics is far more complicated, "The devil is in the details."
http://www.ko4bb.com/Manuals/09)_Misc_Test_Equipment/Datron/Datron_D4000A/Datron%204000A%20Ref%20Div%20Text.pdf (http://www.ko4bb.com/Manuals/09)_Misc_Test_Equipment/Datron/Datron_D4000A/Datron%204000A%20Ref%20Div%20Text.pdf)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 06, 2013, 10:03:56 am
Well, I think, that the steering and auto-calibration software for that dual-DAC concept is substantial to get low INL figures.

A very good description is contained in the service manual of the Fluke 5440/5442, which use also this same approach.
These instruments (like its successor, the actual 5720A) have an internal resolution of 27 bit (0.005 ppm), 0.1ppm front panel resolution, and 0.5 (0.3) ppm +/- 1.5µV output linearity.

The trick is, not to use very low PWM rates (<10 units) on the upper DAC, this nonlinear region is covered by the lower DAC.
All offset and gain errors /drifts are measured and compensated first, and then only those ultra linear specifications are possible, but realistically not much below those given linearity figures.

Therefore, claiming 32bit resolution for a DAC sounds great, but is totally nonsense for physical reasons.
To be checked is the characteristics of the much more simply built DYI divider, i.e. if it's possible to to achieve < 1ppm INL or output uncertainty by appropriate steering of both DACs.


I have measured the linearity of my 5442A by means of the 3458A, and present them here in two versions:

The first version is the linearity referred to the input (13V = 2 x SZA263), that should be the same as the INL, and is well below 0.05 ppm.

The second calculation is related to the output, and tells you, what uncertainty of output you really get.
(That's the specified linearity parameter of 0.5ppm + 1.5µV for the 5440A.)
It's clear, that this parameter diverges towards zero output (covered in the spec by '+/- 1.5µV'), but between 1V and 11V, the deviation is less than about 0.3ppm (w/o offset).

This level of uncertainty is really necessary for usage as a precision divider in a LTZ1000 based reference, otherwise a resistive divider would be more precise.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: bingo600 on August 06, 2013, 01:47:38 pm
Hello together,

there are other articles from Woodward who explain the circuit more in detail:
http://www.edn.com/design/analog/4323340/Fast-settling-synchronous-PWM-DAC-filter-has-almost-no-ripple (http://www.edn.com/design/analog/4323340/Fast-settling-synchronous-PWM-DAC-filter-has-almost-no-ripple)
http://www.edn.com/design/analog/4329365/Combine-two-8-bit-outputs-to-make-one-16-bit-DAC (http://www.edn.com/design/analog/4329365/Combine-two-8-bit-outputs-to-make-one-16-bit-DAC)

formerly it was possible to download the whole article as pdf and not only the schematics.


Could be those attached ?

/Bingo
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 06, 2013, 08:40:01 pm

This design can be further improved by having more PWM circuits that divide the ~7.2V to control the oven temperature of the LTZ1000(A), and the temperature of the multi-stage Peltier refrigerator that houses the entire circuit.  If the refrigerator is set at 0-deg-C, then we can run the LTZ1000(A) at 10-deg-C, and (hopefully) it will show very little drift in comparison to current state-of-the-art voltage references.  WIth the microcontroller in control of the LTZ oven and the refrigerator temperatures, the temperature cycling (ala the Pickering patent) can be used to condition the entire circuit if there is ever an extended power failure.

This is about as far as I have come with this design.  I am doing some experiments with an LTZ-based reference to see if I can coax the device to operate in a mode where "very good" temperature compensation is achieved at the chosen die temperature-- (which is not easy, as the data sheet is devoid of much needed information).

A Peltier element, great idea, at least this should make the Pickering patent work, finally..
Have you mentioned those bugs in his patent, i.e. that asymmetrical temperature interval, which will not work theoretically for 'degaussing'? Neither in practise, in the 7001, as 45°C is far too near room temperature.

I still think, the 7001 box was not successful, as the hysteresis effect is so low for a power down situation, or buried under other effects, that this technique does not give real advantages.
Below 20..30K temperature intervals, the hysteresis is very small.

How do you plan to realize the duty cycle programming?
Any calibration features?

Btw: The famous Datron 4910 reference, doesn't this device contain PWMs for the 7.2V => 10.0000V transfer also, instead of a resistive divider?
How's this realized?

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on August 06, 2013, 09:27:13 pm
The 500KHz is used on the final output to provide a gain of 2. 

Really 500 kHz? = 2000ns.
how much will the break/before make time > 15 ns and the difference between rise and fall time (30-40 ns) affect the 2:1 ratio?

One other question: Why do the calibrators use 125 Hz as PWM. It's neither a multiple of 50 nor 60 Hz line frequency and might give a beat frequency with both NPLC integration times.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on August 07, 2013, 10:26:50 am
Quote
Btw: The famous Datron 4910 reference, doesn't this device contain PWMs for the 7.2V => 10.0000V transfer also, instead of a resistive divider?
How's this realized?
No problem. 4910 schematic is well-known.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 07, 2013, 11:17:39 am
No problem. 4910 schematic is well-known.

Mickle,

thank you very much!!
Partly I've seen the schematics, as I remember now..

The content of the Vishay resistor array still is not known yet?

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on August 07, 2013, 11:26:11 am
PIN
1-2:48.7795K  1-3:48.6536K  1-4:48.6543  1-5:47.9947K  1-6:38.9950K  1-7:38.9952K  1-8:39.0165K
2-3:127.422ohm  2-4:127.326ohm  2-5:786.248ohm  2-6:9.78619K  2-7:9.78629K  2-8:9.80740K
3-4:0.3380ohm  3-5:659.267ohm  3-6:9.65917K  3-7:9.65927K  3-8:9.68048K
4-5:659.163ohm  4-6:9.65910K  4-7:9.65920K  4-8:9.68033K
5-6:9.00017K  5-7:9.00028K  5-8:9.02140K 
6-7:0.3355ohm  6-8:21.5500ohm
7-8:21.4520ohm
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on August 07, 2013, 11:28:01 am
The content of the Vishay resistor array still is not known yet?
which one do you mean (part# 315532)?
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on August 07, 2013, 12:02:44 pm
PIN
1-2:48.7795K  1-3:48.6536K  1-4:48.6543  1-5:47.9947K  1-6:38.9950K  1-7:38.9952K  1-8:39.0165K
2-3:127.422ohm  2-4:127.326ohm  2-5:786.248ohm  2-6:9.78619K  2-7:9.78629K  2-8:9.80740K
3-4:0.3380ohm  3-5:659.267ohm  3-6:9.65917K  3-7:9.65927K  3-8:9.68048K
4-5:659.163ohm  4-6:9.65910K  4-7:9.65920K  4-8:9.68033K
5-6:9.00017K  5-7:9.00028K  5-8:9.02140K 
6-7:0.3355ohm  6-8:21.5500ohm
7-8:21.4520ohm

I guess that answers my question.
Your meassurement looks close to what it really is (an array of 8 resistors some in series and some parallel).

If there is interest I can probably deliever all details you like to know, because I have the original datasheet somewhere.
I also was in contact with VPG and they offered me to make my own personal part number, if I want this obsolete part. 
But I stopped there, because I do think this is not as good as you can do today.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 07, 2013, 01:05:56 pm
I guess that answers my question.
Your meassurement looks close to what it really is (an array of 8 resistors some in series and some parallel).

If there is interest I can probably deliever all details you like to know, because I have the original datasheet somewhere.
I also was in contact with VPG and they offered me to make my own personal part number, if I want this obsolete part. 
But I stopped there, because I do think this is not as good as you can do today.

Hi,

you have the original datasheet of that Datron specific part.
Well, should be 5 different resistors only, i.e.
21.2, 127, 39k for the reference, 21.2 for improvement of T.C., 127 for the 4mA Zener current, 39k as collector resistor.

Then, 9k over 659, as divider for the heater control, but regulated additionally from outside the module.

So, the complete schematic may be sketched in a nicer way.
Also, the points which have that guard ring, might be drawn in a schematic... I still would like to understand, what Datron has done there, and which influence this might have.

I agree, todays components have improved a lot, especially the oil filled resistors are superior, and allow a more stable design also without the elaborate heater control in the 4910.
Anyhow, the schematic is interesting, for improving the sensitivity of the regulation part. Datron has included some caps there..

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on August 07, 2013, 01:54:52 pm
you have the original datasheet of that Datron specific part.
Well, should be 5 different resistors only, i.e.
21.2, 127, 39k for the reference, 21.2 for improvement of T.C., 127 for the 4mA Zener current, 39k as collector resistor.

Then, 9k over 659, as divider for the heater control, but which regulated additionally from outside.

So, the complete schematic may be sketched in a nicer way.
Also, the points which have that guard ring, might be drawn in a schematic... I still would like to understand, what Datron has done there, and which influence this might have.

I agree, todays components have improved a lot, especially the oil filled resistors are superior, and allow a more stable design also without the elaborate heater control in the 4910.
Anyhow, the schematic is interesting, for improving the sensitivity of the regulation part. Datron has included some caps there..

Frank

there are 8 resistors in 4 diff. values (5x 64R, 659R, 9k and 39k)
Tolerance of the individual resistors is only 1% and abs. TCR is 3ppm/K.
Tracking is mentioned to be 0.5ppm/K for all 64R and for the others only 4ppm/K
my copy (Rev E from 11/92) is not very good but see the schematic
there is nothing about the guard
Besides the surprise that it is not as percise as I guessed, I did not really understand why it is as it is.
Maybe you or Mickle or anyone else can explain what could be the idea behind it.

bye
quarks

edit: I just saw, I am probably missing pages, because I only have page 1/4+2/4
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 07, 2013, 07:24:14 pm
Wouldn't it be enough to use two 16bit pwm, one for pre-dividing and the second for boosting it to 10V?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on August 08, 2013, 05:04:09 am

The analog switch I am looking at is the Analog Devices ADG1419-- which has 2.1-ohm switches, and will transition in about 200nS.


Did you already do some linearity checks with the ADG1419 for your 2 first stages?
How does it compare against the results (13 ppm INL un-tweaked and 3-4 ppm INL tweaked circuit ) that I got with the MAX4053A? see Diagrams (x-axis digital code y-axis error voltage in mV against 5000 mV full scale) of  June 28th in this thread?

With best regards

Andreas

 
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on August 10, 2013, 02:06:45 pm

I have only done simulations at this point,


Hello,

which simulation program do you use?
and from where do you get the models of the ADG1419?

I am not the simulation expert. And doing simulations mostly with LTSPICE.
My experience with simulations is that the models are only simplified.
Simulation of deviations below the 100 ppm level seem for me to be
very hard to simulate with a spice based simulator.

with best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 10, 2013, 03:54:20 pm
Looks like he used LTSpice. My guess is, that he used a simple Voltage Controlled Switch model behind the ADG1419 since there is no spice model available from Analog Devices.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on August 12, 2013, 10:08:25 am
because of new interesting discussions here, I kind of paused my DIY LTZ1000 project, to wait and see what I can possibly adopt.

But yesterday I saw something totally unplanned but interesting hapen.
It is topic related, because the used gear (a calibrator and two DMMs) has internal LTZ1000 references.

Here is what I did/observed:

- all three units were started at the same time (no warm up time at all)
- all were connected together and setup to measure 10VDC with 6 digit resolution (for fast reading)
- no adjustments (like Zero or offset corrections) were made

Result was stable 10.000 00 V reading from a few minutes after start until around 2 h when I stopped

This morning I repeated it but with 8 digit resolution and this time with Peak to Peak (PKPK average stat) monitoring running.
Start value was almost spot on 10.000 000 0V
difference between the two DMMs readings was most of the time <1µV (=0,1ppm of 10V)
Overall PKPK was around +10µV  (=1ppm of 10V) running >2h
Final value after >2h was around 10.000 010 0V
Ambient 24,3°C to 24,9°C at 57% rel. humidity during measurement

That altogether is an agreement of within 1ppm of all three units from switching on to >2h.
BTW the gear is of different age and origin and the DMMs are not tweaked/adjusted/calibrated to show this behavior.

Besides I am very pleased with that result, I really wonder how this can statistically be.

Is this possibly only by accident/random, although I alredy repeated it?

Or is it really possible that the warmup drift of all three units almost match identical?
If that could be the case, in future I could save a lot of time, because so far (for serious measurement) I always waited the recommended at least 4h to warm up the gear, before I started to measure anything. But with that finding, I would only have to make sure to start at the same time and could expect to be within 1ppm.

bye
quarks
 
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on August 12, 2013, 05:14:39 pm
Can you tell us what equipment you have?

In the test I used Wavetek 4808, 1271 and Fluke 8508A
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 12, 2013, 05:58:18 pm
Can someone please link to the "Pickering Patent", would be great if all mentioned patents would be linked so it's easier to follow.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 12, 2013, 08:48:46 pm
Hello DiligentMind, nice thoughts!


Just turning off the 3458A, and waiting 1 day, then turning it back on (and waiting 4 hours), then doing an auto-cal, I noticed that the reading at 10V can be different by as much as +/-0.2ppm.
I see the same fluctuations of 0.2ppm in the monthly  measurements of 3458A vs, 5442A, as I also switch them off. (Energy is very expensive here in Germany).
That's perhaps not a pure hysteresis effect, as the difference drifts forth and back.
Pure hysteresis would add up or stick to fixed output values, as both oven see two different temperatures only, i.e. RT and 65°C only.

It is much more difficult to get the last 2 digits to behave well, but the next to the last digit can be made to be very stable (long term and through power cycles) if the room temperature is kept at 23C +/-0.5C, and there is at least a 4-hour warm-up period before I auto-cal.  So, with care, 7.5 stable digits are possible-- the last digit is always bobbling around-- and I have an idea that it is DC-10Hz noise from the 732B (combined with the DC-10Hz noise of the DMM).

My measurements (10min stability) of the 5442A and of the DYI LTZ1000 also show jitter on the last digit only, i.e. on the order of 0.01ppm, @NPLC100, const. temp.
I also relate that to the Zener noise only, after I improved the circuitry by better PSU and shielding. The 3458A itself should be more stable / less noisy than that.

That in itself makes me confident that if the DMM manufacturers put a carefully designed LTZ1000A based reference in their 6.5 digit DMM's, then they could be made stable to 1ppm per year (ie., the last digit would change no more than +/-1 per year) on their best DC accuracy figures.  I would pay a lot of extra money (up to $500 extra) to have that in a 6.5 digit DMM.

Well, I prefer the LTZ1000 @ 45°C, and this reference can be built much better than HP by yourself for about 100$.

Indeed, using highly selected components together with the special hand tuned wire-wound resistors in the boost circuit of the 732B are key to the unit's stability.  I'm not certain that these techniques could be applied by the home experimenter-- you need a vast inventory just for the selection process, and this can become very expensive very quickly.  ...

To my opinion and experience, instead of hand made / selected wire wound resistors, those hermetical oil filled VHP201Z will do the same job, perhaps even more stable.


Note that resistors will have double the drift rate for each 10C rise in temperature-- so keeping them cool (but at a constant temperature) is best.  How cool?  I don't know-- but I imagine that there is a point of diminishing returns-- so there is probably an optimum temperature to keep all of the circuitry at. 

Room temperature is sufficient for 2ppm/6yrs for the VHP201Z resistors.
And that level of stability is over sufficient for the LTZ1000 circuitry.
Any heating or cooling will lead to more problems.

Another problem is temperature hysteresis-- and this occurs in the LTZ1000, as well as any op-amps that you are using-- and as Dr. Frank has proven-- in foil resistors too.  (I'm not certain what hysteresis effects there are in wire-wound resistors, but I am willing to bet a premium beer that they at least have some hysteresis effects).

The hysteresis effect of the metal foil resistors is relatively low (5ppm after 125°C), and due to the 100:1 attenuation effect, does not play a big role.
On +/- 15K, there is no appreciable hysteresis, so if you take care, there's no problem at all.
Afaik, the wire wound types have neglect-able hysteresis effect, only the tightness of the winding has to be relaxed first.
Some time, we all should meet for a beer, anyhow.

 
The only way I can see around this is to run the LTZ1000 at cryogenic temperatures-- like in liquid nitrogen (LN2).


Will cause problems due to the thermal shock, and will stress the tightness of the package.
Also, as the internal gas will dilute and create a vaccum, the package might implode, or will get a leak and suck LN2 inside. 


 This would result in an absolutely stable temperature, because the LN2 always boils at a specific temperature--

No, that's not really the case. (I worked with cryogenic liquids for years, and was also responsible for air and nitrogen liquefaction in our institute for some time).

LN2 will quickly soak oxygen from the surrounding air, so the boiling point will rise from 77.2K (or so) to above 80K within a few hours, depending on the amount of liquid you use.

I used LN2 as a reference bath for a low temp thermo couple, and it was important to always use 'fresh' LN2 from the Philips liquefier. 

1 l of LN2 used to cost around 50 Cents, or so, LHe4 was 10-20 times more expensive.
A usual 50l cryostate (Dewar) will cost 500-1k $/€ only,  but a complete JJ  is how much? 100k, I think.
A lot of first class calibrations can be paid from that.  8)

I think, the stability of the LTZ1000 circuitry at RT is fully sufficient, and putting it completely in an oven or a crogenic bath will only cause problems and complications
Chose those ultrastable external components instead. Compared to that LN2 steadily boiling off, it is a one time investment only.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on August 12, 2013, 09:24:31 pm
Can someone please link to the "Pickering Patent", would be great if all mentioned patents would be linked so it's easier to follow.

http://www.google.de/patents/US5369245 (http://www.google.de/patents/US5369245)
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 13, 2013, 07:08:55 am
Quote
http://www.google.de/patents/US5369245

Thanks in advance.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on August 13, 2013, 01:20:17 pm
I propose a volt-nuts beer meeting at the Weinheimer UKW-Tagung ham festival (September 13th-15th). I will be there, Quarks is living less than 20 km from there, affordable trip for branadic and Dr. Frank too... bargain hunting on the flea market is possible, at the camping site a few km away we could sit down for some $BEVERAGE.

Greetings

Hendrik


Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 13, 2013, 05:04:48 pm
Quote
I propose a volt-nuts beer meeting at the Weinheimer UKW-Tagung ham festival (September 13th-15th).

Beer sounds good, but I'm outta town this time.

Quote
I will be there, Quarks is living less than 20 km from there, affordable trip for branadic and Dr. Frank too...

I'm sure for Andreas too.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on August 13, 2013, 07:58:32 pm

I'm sure for Andreas too.

The saturday after flea market in the morning sounds good to me.
But I would prefer some coffee.

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 13, 2013, 11:07:25 pm
Sorry, I hope to travel to Guadalajara exactly at that time.

Another date, and I will fetch my equipment for that meeting.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on August 14, 2013, 04:13:13 am
That is quite some response!  :-+

I suggest a meeting at 10:30 in/at/near the "coffee tent" on the meadow in the center of the open-air part.
PM me for my phone number if you think you need a support hotline.
This should give everyone the possibility to hunt for nice stuff on the flea market early and also add just little delay for those who have to leave early.

Myself will stay a little longer, lingering on the camping site from saturday to sunday too. This will improve my Weinheim look & smell :-DD

Greetings

Hendrik
Title: Slot or Not
Post by: branadic on August 17, 2013, 06:29:50 pm
As announced by Andreas we wanted to find out what is best: "Using a solid pcb or spend some slots around the heat controlled voltage reference?" and furthermore: "Keep the leads as long or as short as possible?".
Therefore Andreas provided two LM399H (National Semiconductor) and me the pcb. The layout, similar to the one you can find in the world wide web for the LTZ1000, contains two equal trace arrangements but one of them has milled slots.
The test starts pretty simple, the two LM399 are soldered into the board keeping the leads as long as possible and with nearly equal lenght for both references. A 7k5 resistor for each reference as given in the datasheet and supplied with 15V from a bench PSU.
The pcb is arranged on a styrofoam with a window inside so that the IR cam can record the bottom of the pcb. To keep other IR sources away that can produce reflection on the tinned copper traces some foam was put around the references and another styrofoam box was put on top. At some places a black tape was necessary, to avoid reflection.
In the following pictures the references were running 30min minimum. To verify the results I always made a picture of the pcbs top side.

I used same temperature scale for all pictures, but as I have the raw data here I can change that if necessary.

So what can we see? With long leads and slots (left side) the temperature of the pcb seems to be higher compared to the version without slots (right side). However, in both versions all 4 pins seem to have the same temperature, no thermal gradient visible.
As expected the pcb gets more hot with short leads...

So let's start the discussion :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on August 17, 2013, 06:53:20 pm
A very good experiment, good to see the IC itself spreads the heat evenly around itself. Thanks to HSG too? 8)

Glass as main ingredient of PCB material is a known bad conductor for heat, the second ingredient epoxy resin is also not that good, and cooper a good known conductor (Yes, there is a rule that eletrical conductivity is often similar to heat conductivity..) , knowledge of those properties allows for some thermal designing. The cooper pour on my PCB was intended to spread the heat evenly.

What we not see, however, are the air currents I am afraid of, slots on the PCB allow them to circulate and I suppose they will be very erratic and turbulent in such an environment. Possibly I will study some liquid-to-liquid-turbulences at some time during the next few months, maybe I can put such a circuit in my test jig then ?

BTW, the Volt-Nut Meetup at Weinheim gains some traction !

Greetings
Hendrik
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 17, 2013, 07:34:05 pm
Thanks and yes, it's great having access to such equipment at weekend, one of the advantages working there. But I'm sure the cam is happy to have a sense and I also hope that you guys are happy that you can profit by this too. ;)

About the turbulences I wouldn't care if the critical parts are put into foam, this can avoid air flow, see picture attached.
Title: Re: Slot or Not
Post by: Andreas on August 17, 2013, 09:30:56 pm

So what can we see? With long leads and slots (left side) the temperature of the pcb seems to be higher compared to the version without slots (right side). However, in both versions all 4 pins seem to have the same temperature, no thermal gradient visible.
As expected the pcb gets more hot with short leads...

So let's start the discussion :)

Ok.

Mhm.

The pictures are really surprising me.

This will change my picture of the thermal world.

My favourite up to now was the short leads with the slotted pcb.
I would have expected all pins and the pcb around the pins at the same temperature.
And the large temperature gradient on the lines till the outer rings.
(Why should Datron have made them if they would not be necessary).
The thermal gradients are much more locally around the reference than I expected.

In my opinion we do not only have to regard wether all pins are equally but also the heat distribution within the solder junction.
The lead of the reference (in the middle of the solder junction) is made of Kovar which has around 40 uV/K against copper (at the edge of the solder junction).
Solder itself has around 3uV/K against copper. So lets assume that Kovar against solder has around 37uV/K.

So from middle to edge of the solder junction it seems for me to be much more critical for the slotted board with short legs than the not slotted board with long legs.

So long legs and large thermal mass (perhaps the PCB material which diligent minds recommends) seem to be the best.
Of course you have to take more care keeping air currents away from long legs.

Other opinions?

@ branadic: with the raw data it should be more easy to evaluate the gradient within the pads than for me having difficulties with the colour gradients of the picture.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 18, 2013, 07:33:28 am
Branadic,

those are great measurements!

2 simple conclusions:

- short leads will increase the temperature of the PCB, long leads will give more thermal isolation between component and PCB => long leads preferred
- slots in the PCB give higher thermal isolation between solder joint area and outer PCB area, thereby reducing cooling of the solder joints through the PCB, and in turn increasing the local heating of the solder joints. Higher solder joint temperatures might cause temperature differences more easily and  will increase thermo voltages.

So I still do not understand the purpose of those slots, especially in the Datron PCB, where the LTZ1000 at a temperature of ~60°C is used.

I'm now very confident that 45°C and a PCB without slots is the most stable solution.

Thanks for sharing.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Jay_Diddy_B on August 18, 2013, 08:55:35 am
Hi,
Just a little observation for you.

I noticed that your thermal camera is pointing at tinned traces on the PCB. These have a very low emissivity compared to the Fibreglass (FR4).

I suggest that you could try painting the traces with liquid paper, type correcting fluid, or give the board a quick spray with black paint (flat or matte).

Jay_Diddy_B
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on August 18, 2013, 09:00:03 am
Very interesting, so long legs and no slots look thermally good.

But what about mechanical stress relief, what I though is the main reason for the slots?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 18, 2013, 09:21:44 am
Very interesting, so long legs and no slots look thermally good.

But what about mechanical stress relief, what I though is the main reason for the slots?

Yeah, what about??

Does "mechanical stress" cause any deviations in output voltage or decrease stability?
How does "mechanical stress" affect those electrical characteristics?
How is "mechanical stress" defined in this situation?

I still cannot see any mechanism, how that buzzword "mechanical stress" would have influence on the electrical output.

Especially, as all of the relevant components are leaded parts, not SMD parts, where you really would have mechanical forces on the solder joints, due to differences in thermal expansion between component and PCB.

Cold versus hot areas on the PCB may lead to bending / distortion of the PCB, but what would be the electrical effect of that, once again, when using leaded components?

Such mechanical effects would be relevant for oven temperatures of 90°C.. down to 60°C perhaps, but at 45°C, that's completely irrelevant.

Does anybody have any other ideas?

Sorry, but I am still searching for solid real physical effects and their explanations.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: eurofox on August 18, 2013, 10:25:08 am
Hi nV freaks,

I'm passionate too with this subject and follow this daily but I'm happy with 6 1/2 digits.

There is "the multimeter" you need on eBay with a failure but I suppose it should not be to bad to fix it.

http://www.benl.ebay.be/itm/261263662099?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2649 (http://www.benl.ebay.be/itm/261263662099?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2649)

eurofox
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 18, 2013, 11:57:54 am
Hi,

as said the emission coefficient of metal is really worse, it's rather an infrared mirror reflecting all infrared sources from the surrounding.
The resolution of the cam is about 0,3K so with the sum of both you can't see a thermal gradient along the leads or the temperature of the solder junctions (they're also reflecting) directly. What you can do is cover the surface with a coating as pointed out by Jay, but I found that this wasn't necessary here.
The FR4 material in this case radiates enough to make things visible although glas is an infrared isolator (you can hide behind a glas but not behind a silicon wafer).
We can assume that the solder junctions are of the same temperature like the pcb material, so one can calculate the temperature gradient along the leads.
The heat conductance of the pcb material is worse but better compared to glas only. Because of the smaller thermal mass with slots the temperature is a bit higher, but not critical. The heated reference over-radiates the pcb temperature at bottom view, so the top view is more significant in this case.

About the stress with or without slots the only answer can give a measurement and the board exists so let's do some measurement and find out, shall we?
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on August 18, 2013, 12:15:35 pm
Yeah, what about??

Does "mechanical stress" cause any deviations in output voltage or decrease stability?
How does "mechanical stress" affect those electrical characteristics?
How is "mechanical stress" defined in this situation?

I still cannot see any mechanism, how that buzzword "mechanical stress" would have influence on the electrical output.

Especially, as all of the relevant components are leaded parts, not SMD parts, where you really would have mechanical forces on the solder joints, due to differences in thermal expansion between component and PCB.

Cold versus hot areas on the PCB may lead to bending / distortion of the PCB, but what would be the electrical effect of that, once again, when using leaded components?

Such mechanical effects would be relevant for oven temperatures of 90°C.. down to 60°C perhaps, but at 45°C, that's completely irrelevant.

Does anybody have any other ideas?

Sorry, but I am still searching for solid real physical effects and their explanations.

Frank

I do not know the answers, but my thoughts (until now) are influenced by the fact that the Datron/Wavetek design is with the slots.
Also the gear that uses this design (like 4808, 4910 and 1281 to name a few) is representing and rated top notch.
So my guess is, there probably is a good reason for slots.

In the LT Application Note AN82
http://cds.linear.com/docs/en/application-note/an82f.pdf (http://cds.linear.com/docs/en/application-note/an82f.pdf)
there is a statement (page 6) that Metal Packages are largely immune to board stress
(DiligendMinds also stated this before, especially for the LTZ1000ACH),
but negative effects are described/shown there (see att.).

Also on http://www.amplifier.cd/Technische_Berichte/Spannungsreferenzen/Spannungsreferenz.html (http://www.amplifier.cd/Technische_Berichte/Spannungsreferenzen/Spannungsreferenz.html)
(sorry forgot the translation to english)
http://translate.google.de/translate?sl=de&tl=en&js=n&prev=_t&hl=de&ie=UTF-8&u=http%3A%2F%2Fwww.amplifier.cd%2FTechnische_Berichte%2FSpannungsreferenzen%2FSpannungsreferenz.html&act=url (http://translate.google.de/translate?sl=de&tl=en&js=n&prev=_t&hl=de&ie=UTF-8&u=http%3A%2F%2Fwww.amplifier.cd%2FTechnische_Berichte%2FSpannungsreferenzen%2FSpannungsreferenz.html&act=url)
there are some findings about mechanical stress.

At least for me that is good evidence to think there probably must be something behind it.
But if it is not worth the effort to implement it, than of course that is good news, because things get a lot easier.

bye
quarks
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on August 18, 2013, 12:24:13 pm
I will have AT our zwick AT work for pushing and pulling on pcbs. sent from mobile.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on August 18, 2013, 12:41:36 pm
But what about mechanical stress relief, what I though is the main reason for the slots?

Ok that would be another thing of consideration.

I still cannot see any mechanism, how that buzzword "mechanical stress" would have influence on the electrical output.

As is stated in many datasheets of precision voltage references mechanical stress applied to the PCB shifts output voltage.
And you can simply measure it.
Usually I do this with any of my "ADCs" and try to keep the influence to below 5uV drift due to mechanical bending of the PCB. With every 5V reference that I tried up to now I got many 100uV shift when all pins where soldered to the PCB. So up to now this ended always in this way that only the GND-Pin is soldered to the PCB and the other PINS are wired with a thin VERO wire. This is a bit tricky for SMD devices but it works.

I was very surprised when testing the VRE3050AS reference which is in a hermetically sealed package with gull wing leads. In this case I thought that the influence of the PCB to the chip would be negligible. But this was not the case. I measured values from -16 to +186 uV against untwisted PCB.
Other references have other values depending on construction and individual make.
A LT1236ACS8 gave around 400uV between minimum and maximum value.

Unfortunately I did not do a test with my LTZ1000 boards. And now I don´t know if I shall risk a new ageing cycle.

On the other side a LTZ1000A should have a better behaviour than a LTZ1000 because of the different die attach.

A similar test like the LTZ1000 could be done with the LM399-Board of branadic.

About the stress with or without slots the only answer can give a measurement and the board exists so let's do some measurement and find out, should we?

Yes of course.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 18, 2013, 12:42:10 pm
We have a "Zwick" too and also a second push/pull machine with temperatur control, but I guess this is breaking a butterfly on a wheel. A simple alternating force an top of the voltage reference is more than enough.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on August 18, 2013, 12:55:44 pm
We have a "Zwick" too and also a second push/pull machine with temperatur control, but I guess this is breaking a butterfly on a wheel. A simple alternating force an top of the voltage reference is more than enough.

I do not put force on the top of my references (because this would also change temperature)
I simply bend the board by applying forces to the edges of the board.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 18, 2013, 01:32:47 pm

I do not know the answers, but my thoughts (until now) are influenced by the fact that the Datron/Wavetek design is with the slots.
Also the gear that uses this design (like 4808, 4910 and 1281 to name a few) is representing and rated top notch.
So my guess is, there probably is a good reason for slots.


Yep, and that's exactly, what's always done during Copy-And-Paste designs ... i.e. copying possible errors or nonsense also.

Many people in this or in the Asian community simply copy the LTZ1000 'A' version, available from China, because it's sitting in the venerable 3458A, including the 95°C operating temperature, without thinking.

Perhaps, Datron did not calculate anything also, and simply introduced those slots by similar hand waving arguments also.
All those instruments you mention, base on the same reference module (afaik), so that's no argument, either.

And there exist also several successful  designs without any slots, I think in the 732B, the ceramic substrate doesn't have those, either.

If there's a physical effect, it should be obvious, or should be measurable in an appropriate experiment.


The cited AN or the experiment in the CD forum deal with a different effect, than directly on the PCB, I think!

It's well known, that molded IC are sensitive to bending, due to their internal construction.
The mold compound might imply pressure on the chip directly, and the base lead frame, where the chip is epoxied onto, is flexible, because it's a thin sheet of copper only.

So, if you bend or distort the legs of such molded IC, you will easily bend the chip itself.

But that is perhaps not the case for the hermetical TO case. There's no mold compound, it has long legs, and the chip carrier is more solid.
And I do not see an advantage of those slots, if you want to avoid flexing of the PCB.
In contrary, slots might intensify bending forces on the legs, because slots can move, but a solid PCB area can not by that amount.
 
Sorry, no, I still can not identify any causality in there.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 18, 2013, 02:01:53 pm
As is stated in many datasheets of precision voltage references mechanical stress applied to the PCB shifts output voltage.
And you can simply measure it.
...
I was very surprised when testing the VRE3050AS reference which is in a hermetically sealed package with gull wing leads.


Andreas, I have read that also, but always in conjunction with molded components only.
And that mechanism I can understand, due to the mechanical construction.

Unfortunately I did not do a test with my LTZ1000 boards.

Well, that would have been very interesting, as the TO case is a totally different mechanical construction.

On the other side a LTZ1000A should have a better behaviour than a LTZ1000 because of the different die attach.


Why? In the LT datasheet, the different die attach is reasoned only for better thermal insulation, not for better mechanical stability.
The only effect is that the required heater power is decreased a lot. (What other mechanical effect should exist there comparing the different die attaches??)
And that is important (only) if you run the LTZ1000 on higher temperatures (95°C), as in the HP3458A, or in the Keithley DMM.

Other instruments at lower temperature (Datron 49xx, Fluke 7001) simply use the LTZ1000 version.

 
A similar test like the LTZ1000 could be done with the LM399-Board of branadic.



With best regards

Andreas


Yes, that's right.
But I could imagine, that there's an effect in the solder joint only, i.e. that thermal voltages are depending on mechanical forces also. (But I could not name or cite anything around such an effect.)

Regards Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on August 18, 2013, 03:02:30 pm

On the other side a LTZ1000A should have a better behaviour than a LTZ1000 because of the different die attach.


Why? In the LT datasheet, the different die attach is reasoned only for better thermal insulation, not for better mechanical stability.
The only effect is that the required heater power is decreased a lot. (What other mechanical effect should exist there comparing the different die attaches??)


The reason is simple:
AFAIK normally the die is directly attached (by some silver filled epoxy resin) to the Kovar bottom plate of the TO-99 package.
(http://cache.ourdev.cn/bbs_upload782111/files_17/ourdev_464495.JPG)

If you do some thermal isolation it is very likely that it will be something that is not so stiff (ideally some kind of foam or maybe low density glass filled epoxy)  than the bottom plate. Thus introducing less forces to the chip.

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 18, 2013, 03:38:07 pm
Okay, okay... we have the Kovar package with gold finish but what are the bond wires made of? Obviously the chip metallization is not gold, so I expect this is not an aluminium bond wire?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 18, 2013, 03:50:35 pm
Andreas, Quarks,

sorry that's all not convincing.

The speculation about the consistency of the isolation material does not bring any new information.
Additionally, I don't see at all, how bending forces on the PCB could propagate into the chip. Definitely not over the bond wires  :--, and hardly via the very stiff chip carrier.
 
In the AN 82 from LT, the robustness of the TO package against mechanical stress, compared to molded components  is pointed out definitely.

The diagram from AN 82, of a slotted vs. a stiff PCB, is made on a molded component (LT1460)  in SO8 package, definitely!
To use this as an indicator, that there possibly might be some appreciable effect on the LTZ1000 also, is more than far-fetched.

Sorry again, up to now, there is no document or experiment yet, supporting this mechanical stress theory on the LTZ1000 package.

The contrary is the case, to my impression.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 18, 2013, 03:59:57 pm
Okay, okay... we have the Kovar package with gold finish but what are the bond wires made of? Obviously the chip metallization is not gold, so I expect this is not an aluminium bond wire?

bond wires used to be pure gold, but now were changed in the whole electronics industry to pure copper.

aluminium is always used for the top chip metallization, and the bond landing pattern .

On that photograph, obviously LT uses aluminium bond wires also.

Afaik, this material is used in power semiconductors, and for low bond temperatures, therefore low resistance may have been the argument here.
http://heraeus-contactmaterials.com/en/products/aldr/productpage_smt_adhesives_3.aspx (http://heraeus-contactmaterials.com/en/products/aldr/productpage_smt_adhesives_3.aspx)

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 18, 2013, 04:11:49 pm
Quote
bond wires used to be pure gold, but now were changed in the whole electronics industry to pure copper.
aluminium is always used for the top chip metallization, and the bond landing pattern .
On that photograph, obviously LT uses aluminium bond wires.
Afaik, this material is used in power semiconductors also, therefore low resistance may have been the argument here.
.

I doubt they used aluminium bond wires unless the chip metallization is known and this is not gold (see the picture), what you would need for an aluminium bond wire.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 18, 2013, 04:18:12 pm

I doubt they used aluminium bond wires unless the chip metallization is known and this is not gold (see the picture), what you would need for an aluminium bond wire.

Excuse me, branadic.. I receive PCNs (Product Change Notification) every day, from all relevant semiconductor suppliers worldwide.

So I can tell for sure, that for the chip metallization, usually and definitely aluminium IS used!

Follow the Heraeus link, available bond wires are made of Au, Cu or Al, and can well be distinguished by their color.
And the most probable material in the LTZ1000 cut-off is aluminium bond wires.

The bonding process  is some sort of creating an alloy; Au alloys very nicely with aluminium.
In short, equal metals are not required for bonding, but aluminium/aluminium bonding is of course possible, why not?

This avoids diffusion problems. 

Anyhow, does that play a role in this actual discussion?

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 18, 2013, 04:28:39 pm
Quote
Excuse me, branadic.. I receive PCNs (Product Change Notification) every day, from all relevant semiconductors suppliers worldwide.

So I can tell for sure, that for the chip metallization, usually and definitely aluminium IS used!

This is only true if the bond wire is gold. You can't bond an aluminium wire to an aluminium surface, sorry but I studied microsystem technology and still work in a field where bonding is one way to connect bare dies to a pcb or mid ;)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 18, 2013, 04:37:52 pm


This is only true if the bond wire is gold. You can't bond an aluminium wire to an aluminium surface, sorry but I studied microsystem technology and still work in a field where bonding is one way to connect bare dies to a pcb or mid ;)

Ok, that I didn't know.. learnt something.

Do you also know the reason for that behavior, it's not obvious for me?

I also read those magazines about power electronics, with many macro photographs of bare die technology, there's often aluminium used for the bond wires, but the metallization also have a silver color.

Do you know anything about that?

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 18, 2013, 06:02:39 pm
Quote
I was very surprised when testing the VRE3050AS reference which is in a hermetically sealed package with gull wing leads. In this case I thought that the influence of the PCB to the chip would be negligible. But this was not the case. I measured values from -16 to +186 uV against untwisted PCB.
Other references have other values depending on construction and individual make.
A LT1236ACS8 gave around 400uV between minimum and maximum value.

If this is true you should have no change using the LS8 package right?

Concerning the die attach, using glue with silver particles could be a hint that the backside of the die is connected to ground/case and not for thermal purpose only. The thickness of the glue in any case should be as low as possible. The transparent brown mass on the picture is so called underfill. This is used because the filled conducting glue has low adhesion and with underfill mechanical stability is increased.
I agree that hermetical packages have improved mechanical stability but they are not stress immune. Stress to the pcb and so to the package (a leg of the package will transfer the force over the bond wire to the chip, even if this is orderes of magnitude lower compared to molded chip packages) will have an influence in any case, the question is "Is it big enough that you can measure it?" I don't now because you can't seperate all the effects from another. Bending the pcb will also change the temperature relations inside the chip package and whatever.

The references are running and I will observe if I can see an influence or difference if bending the board.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 18, 2013, 07:09:25 pm
... The TO-style metal cans are 100% immune to mechanical board stress.  More than likely, the slots in the PC board were to reduce heat-loss, and (in turn) reduce power consumption when running on battery for the 4910 reference.  I am guessing that the engineer at Datron that designed this left the company, and then following engineers simply copied the design without understanding why.  The "A" version of the LTZ1000 did not show up for a while-- so Datron only had the LTZ1000 to work with-- and it is going to use more power for the heater than the "A" version-- thus the need to not only reduce heat loss through the PC material, but also to insulate the LTZ1000 with foam.

You can use slots on your LTZ1000(A) design if you want-- this will reduce heater power a very small amount (and even smaller on the "A" version).  If you intend for your reference to run on battery power for extended periods of time, this may be important to you.  If your reference is line-powered only, then I just don't see any benefit to the slots.

...

Your explanations are really striking!

Me also, would also accept different expertise about those darn slots, if a measurement can demonstrate to have effects above 0.1ppm over whatever kind of parameter.

Up to now, the best naked LTZ1000 stability which can be achieved on an ordinary concept (evaluated by test, model and calculation) is around 0.3ppm/yr and below 0.05ppm/K.

If the influence would be below those ppm-numbers, it would be fully useless to spend any effort in this feature.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 18, 2013, 07:34:08 pm
Okay, I made the test. Setup: Philips PM 2534 and Prema 5000 measuring the voltage difference between the references. Both meters are in 6.5 digit mode. One reference side is fixed in a bench vise the other reference side is bend. The leads of the voltage references are as short as possible from the last thermal test. So if there were stress at the leads the references would get it with the maximum amount.
I can't see any change if the board is bend and I can't see a difference between both layouts. So whatever the slots are good for can only be due to thermal purpose. The result is hard to show, but I hope you trust my statement.
What is true for the LM399 is sure true for the LTZ1000.

So my conclusion is, it's worth making the leads of the reference as short as possible to keep the temperature gradient along the legs small and the solder joints at nearly the same temperature as the reference. This is what Keithley does in its 2002 device with the LTZ1000 and sure in other gear too. Against air flow they provide this little black cap what certainly improves temperature stability.
The slots are neither good nor bad. You have to decide individual on your design, because the reference is followed by an amp. If you pass on slots you can put your amp as close as possible to the reference. This can be helpful or causes trouble.

However, sad that such an investigation was never performed or published before. But with the pictures shown everybody can get an idea of what's best. No space for manipulation or speculation ;)
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 18, 2013, 08:11:02 pm
Quote
And there you have it...  'nuff said...

To quote my boss: "An experiment does not forget any question."

But I agree, art work is pretty cool and it would be worth people would spend some more time in designing their pcbs. Everybody could profit by rf design (pcb based filters etc.). At the end of the day electronics can raise a claim to look art like and stimulate the eye.
I call myself esthete.
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 18, 2013, 08:27:59 pm
If you are assured by your approach go for it and let uns know your results :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 18, 2013, 08:41:49 pm
I just return from the basement, where I also tested mechanical stress on the PCB, where an LTZ1000 is sitting beneath.
See my first tear down photo on p. 13 (reply #180).

I simply pressed hard with a screw driver near the LTZ, to bend the PCB locally.
The bending effect from local heating  caused by the reference itself will for sure be much, much smaller.

The output of 7,147 975 5 V did not change under this disturbance, only the normal fluctuations on the last digit could be observed.

This quick and dirty test is no proof, I have to admit.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: alm on August 18, 2013, 09:06:08 pm
This is kind of interesting.  The 0.05ppm/C figure comes from the original Ap-Note.  So, the temperature sensing transistor will have about -2mV/C response.  Using the original circuit in the Ap-Note, many experimenters have found that the ouptut voltage of the LTZ1000(A) will have ~50ppm/C drift with the oven not used.  This means that to achieve the 0.05ppm/C performance we have to maintain the die temperature +/-0.001C.
You can't assume the tempco to be stable over temperature unless you tested it. Tempco will often vary as a function of temperature, current and other parameters. See the Solartron calibration of the 7081 zener current for example. Or the temperature setting on an OCXO.

The 0.3ppm/year stability figure is for the original circuit-- perhaps running the die at 45C.  If you drastically lower the die temperature (to say, 0-deg-C), then you might expect long-term drift to be far less than that-- perhaps as low as 0.1ppm/year (on selected devices of course-- the "average" device will be slightly higher).  This can be done using a multi-stage Peltier device to chill the case of the LTZ1000(A) to -10C, then run the on-chip oven (which has superior temperature control over the Peltier device controller) at 0C.  The results should be quite striking-- almost "spooky" JJA-like stability...
Maybe, but does anyone have data to support this? You can't make solid predictions based on extrapolation. The stability may very well plateau below a certain temperature. It may also be that the high tempco (see above) at 0°C will mess up this idea. Or moisture (condensation).
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on August 18, 2013, 09:47:13 pm
If this is true you should have no change using the LS8 package right?

The LS8-package (LT1236AILS8-5) I did not test with all pins soldered to the PCB. Because I knew that this would have a significant influence.
In a earlier publication (New Product Catalog August 2012) LT published a humidity coefficient for "Humidity (25% RH Change) < 10ppm" for the LT1236 with the LS8 package. I asked them if this was due to the PCB expansion and if I could reduce the rH dependency by some kind of dead bug mounting. The answer was "yes". And with the next version of the data sheet the rH parameter was removed and replaced by "However, PC Board material may absorb moisture and apply mechanical stress to the LT1236LS8. Proper board materials and layout are essential."

And although only having soldered one of the pins to a wire connected directly to the board I have measured
-2 .. +5uV giving a span of 7uV for the LS8-device on my ADC17.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 18, 2013, 10:12:42 pm
Quote
Most PC board materials exhibit piezoelectric (and/or triboelectric) effects with mechanical stress

Do you have a publication to that statement?
Title: Re: Ultra Precision Reference LTZ1000
Post by: alm on August 18, 2013, 10:14:20 pm
No-- I have seen no study with a statistically large enough population that would indicate to any level of certainty that my assertions are correct.  However the Spreadbury study did produce some interesting results [see figure 4 in the attached file], and if you can believe the theory that the drift of the LTZ1000 chip will be reduced by 1/2 with each 10C lowering of die temperature, then it is worth an experiment.
I agree, fig. 4 looks promising. Still, no way to know without testing. I certainly wouldn't assume better stability than 25°C without evidence.

It is possible that this theory is wrong, or that there are other limiting factors that would prevent such low levels of annual drift-- only experimentation with hundreds of units over several different wafer lots would prove this-- and this would take a great deal of money and a great deal of time-- perhaps at least 3 years--
I'm not convinced that you need hundreds. If the drift of individual samples is close enough and the temperature produces a major improvement, then say 5 at 0°C and 5 at some higher temperature may already be enough to show an improvement. You would need to measure over fairly long time periods because of the small magnitude of the drift, and quantifying long term drift may indeed prove interesting.

The only thing we can do as hobbyists, is build a few of these, and see how they drift against each other-- not as good as having a JJA, but it would tell us something at least.
What if at 55°C the references all have a drift of between -2 and -3 ppm/y, but at 0°C the drift would be between +1 ppm/y and -1 ppm/y. Without an independent observer, the latter wouldn't be any better (similar spread). The Spreadbury paper unfortunately doesn't provide enough data to tell if the spread in drift also decreases with temperature.
Title: Re: Ultra Precision Reference LTZ1000
Post by: robrenz on August 18, 2013, 10:32:26 pm
You need to be more Diligent :-DD
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 19, 2013, 10:34:40 am
Quote
No I do not-- I was reading about it on the Internet a few weeks ago, and silly me, I did not bookmark the page-- and I am too lazy to try to search for it-- but, if *you* find it, post a link and then we can all enjoy it-- (and I will read it again-- my memory is not as good as it used to be when I was younger).

I can't find any hint to such a problem in the web for FR4 substrate. And I don't see any physical background that the glas epoxy mixture can produce piezoelectric voltages. I can only imagine that LTCC have possibly such effects but I don't know for sure.

Maybe you mixed it up with the problem of piezoelectric effect of smd ceramic capacitors on pcb?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on August 19, 2013, 11:12:09 am
This quick and dirty test is no proof, I have to admit.
Hello,

But together with the test of branadic this convinces me that at least the LM399 and the LTZ1000 are insensitive to mechanical stress.

Are you certain that it was package stress causing the shift in output voltage?
 I could believe this with an epoxy package, but not the LS8 (ceramic LLC)-- can you make a movie of this and put it on YouTube so we can all enjoy it?

Yes it is repeatable. And  the LT1236AILS8-5 references have a push pull output with a very low impedance. So the impedance of the meter does not play a role when regarding static electricity.
And no. I will not do Daves (video-) job just to repeat a measurement where I have already the data that I need.
Usually I am doing the mechanical stress test only once at the begin of the life of my ADCs.

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on August 19, 2013, 01:56:14 pm
Our laboratory researches wasn't confirmed the insensitivity of the LTZ1000 to mechanical (thermo-mechanical) stress. Output voltage drift is small, but measurable even in 7.5 digits mode and well explained via strain compatibility conditions of LTZ1000 and PCB's.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on August 19, 2013, 02:18:17 pm
Hello Mickle,

Thanks for your information. As I know, you have done a lot of very impressive projects and research around them. I would like to know what you learned and what you would sugest to implement, if you would build your next LTZ1000 board.

Bye
quarks
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 19, 2013, 04:31:54 pm
Interesting results, but is it really due to stress or is the reason to be searched in the change of thermal distribution?

(http://cache.ourdev.cn/bbs_upload782111/files_18/ourdev_483176.jpg)

This picture in mind I can imagine a donat like heat distribution. If you now bend or tild the board the distributions emphasis will change.
On the other hand the deformation in your simulation is quite big, compared to the board size, isn't it?
Title: Re: Ultra Precision Reference LTZ1000
Post by: robrenz on August 19, 2013, 04:39:29 pm
FEA simulations usually exaggerate displayed deformations so it can be visualized.

What about the strain gage effects (resistance change) of the conductors as being the primary cause? Either circuit board traces, chip leads, or chip die level.
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 19, 2013, 04:55:31 pm
Quote
FEA simulations usually exaggerate displayed deformations so it can be visualized.

Sure, but ±0,5mm is at least 1mm deformation!
Title: Re: Ultra Precision Reference LTZ1000
Post by: robrenz on August 19, 2013, 04:59:25 pm
I see what you mean, that is much more deflection than what would occur naturally even with poor handling.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 19, 2013, 05:06:26 pm
Our laboratory researches wasn't confirmed the insensitivity of the LTZ1000 to mechanical (thermo-mechanical) stress. Output voltage drift is small, but measurable even in 7.5 digits mode and well explained via strain compatibility conditions of LTZ1000 and PCB's.

Hello Mickle,
those are interesting findings!

But I do not understand them yet, sorry, it's always me.

What in fact is your explanation of the offset, what you call strain compatibility LTZ/PCB?
I still do not see, which physical effect causes this shift of voltage, and where this effect is located: in the solder junction, in the package, on the chip?

It's clear, high temperatures on a PCB cause strain between rigid bodys of different thermal expansion coefficient , e.g. SMD ceramic components (R, C) versus the PCB itself.
But that's not valid for leaded components, as the leads will simply follow the PCBs expansion.
Only if you have a torsion of the PCB, I could imagine, that the different leads of the TO8 package see different forces in different directions, as you obviously want to demonstrate by your measurement (what was your measurement setup?) and your finite elements simulation.

I see no reason why the PCB should perform such a torsion , if the PCB simply is heated in the middle of the LTZ.

Then, it's not clear to me, why the PCB should heat up to 65°C, if the reference is at this stabilization temperature, or at much lower values (e.g. if run on 45°C), as there is a thermal gradient between the TO 8 package and the PCB.

So, is the observed/measured shift just an exaggeration of this (still not named) effect?

For me, it would be very interesting, how you construct LTZ references today, i.e. which resistor types from Vishay, LTZ A type or not, which temp?
Well I could first investigate on your 7081 modification, but it would be nice, if you would answer specifically to the open questions here.


Thank you very much!

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 19, 2013, 05:08:36 pm
FEA simulations usually exaggerate displayed deformations so it can be visualized.

What about the strain gauge effects (resistance change) of the conductors as being the primary cause? Either circuit board traces, chip leads, or chip die level.

Which resistors do you mean?

I thought of such an effect in first instance also, but there is no such resistor element, perhaps SMD, as all precision resistors are leaded parts also.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 19, 2013, 05:13:12 pm
Quote
FEA simulations usually exaggerate displayed deformations so it can be visualized.

Sure, but ±0,5mm is at least 1mm deformation!

The graphic exaggeration is not what bothers me..
Such strain by temperature always creates very small dislocations (µm) on the PCB, but the forces this generates on the components may be extremely high nevertheless, but only on rigid bodies, i.e. SMD parts only.


Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: robrenz on August 19, 2013, 05:17:48 pm
FEA simulations usually exaggerate displayed deformations so it can be visualized.

What about the strain gauge effects (resistance change) of the conductors as being the primary cause? Either circuit board traces, chip leads, or chip die level.

Which resistors do you mean?

I thought of such an effect in first instance also, but there is no such resistor element, perhaps SMD, as all precision resistors are leaded parts also.

Frank

What about the circuit board traces that are connecting the precision resistors to other elements of the circuit. Those traces are part of the circuit resistance and will change resistance with strain. The cross sectional area of a typical trace will be much greater than a strain gage but the effect will still be there. It might not be large enough to have any appreciable influence. I am just thinking out loud.  You guys are way past my knowledge on this but I find it interesting anyhow.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 19, 2013, 05:31:35 pm


What about the circuit board traces that are connecting the precision resistors to other elements of the circuit. Those traces are part of the circuit resistance and will change resistance with strain. The cross sectional area of a typical trace will be much greater than a strain gage but the effect will still be there. It might not be large enough to have any appreciable influence. I am just thinking out loud.  You guys are way past my knowledge on this but I find it interesting anyhow.

Hey, that's perfectly okay!

Brainstorming is always a good thing, especially, when nobody else has an idea..

The trace resistance should not make a problem for the output, as the voltage is measured by a high impedance instrument.

The Zener feeding current might be affected, but this has to be calculated. (I feel, that this effect would be too small, but guess what?)

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on August 19, 2013, 06:10:34 pm
Hi there,

there is a lot going on today!

Even as I don't expect a big piezoelectric - even less a triboelectric - effect on FR4 PCBs, I can offer some little experiment:

At work I have little PCBs with a "interdigital" or entwined finger-like electrodes on a single side. I can connect each electrode to a 34401A and give them a really harsh bend or hit, either in the direction of the fingers or perpendicular to it.

Ceramics might be something different, it just has this "smell of piezo". Also, Bob Pease wrote somewhere that he found isolated wires to have a piezoelectric effect, but only certain type of isolating material (piezo-material PVDF, Polyvinylidene fluoride, comes to mind immediately, have to look up the source.) A reason to keep the wires outside the test gear fixed during measurement ?

BR Hendrik
Title: Re: Ultra Precision Reference LTZ1000
Post by: robrenz on August 19, 2013, 06:17:00 pm
The trace resistance should not make a problem for the output, as the voltage is measured by a high impedance instrument.
.....
The Zener feeding current might be affected, but this has to be calculated. (I feel, that this effect would be too small, but guess what?)

Frank

I know right now the thread is focused on the affects of the reference only, but what about the trace resistance variation from strain on the pcb traces connecting the precision resistors when looking at the entire circuit?
Title: Re: Ultra Precision Reference LTZ1000
Post by: robrenz on August 21, 2013, 12:48:32 am
I just happen to have a excellent micro ohm meter that is offset compensated and gives very stable single digit micro ohm readings. It will be easy for me to measure if there is an appreciable resistance change of a trace. Even if it is significant, wouldn't it be a simple matter to increase trace widths or copper thickness to reduce it to insignificant levels?  When I get a chance I will post some values on the delta R for various trace widths, lengths, and deflections.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Robert763 on August 21, 2013, 07:52:33 am
Hi,
This is my first post here. I suggest you research strain gauges. This is a known science (and art). Vishay are a big player in the market. I understand, but have no reference, that the Vishay foil resistors were a side benefit of their research into low TC strain gauge materials. This is another area where "RF like" PCB layout can help. If you have opposite arms of a bridge or divider as identical  tracks opposite each other on two sides of a PCB (like a stripline) the PCB bending induced stress changes cancel out.

Robert.
Semi volt nut, DMM's to 7.5 digits, couple of LTZ1000 references, LM399's, Diff. Voltmeters, JRL VDR-107 KVD.
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 24, 2013, 01:27:19 pm
I have also IR-videos from the heat-up phase with long and short leads, but no webspace to put them on. On the other hand I don't won't to register at youtube. Someone out there with some webspace for the videos?
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on August 24, 2013, 03:11:34 pm
To prevent messing up this server, I suggest my Youtube-Account and some of my Webspace to embed it at.
What camera did you use ?
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on August 24, 2013, 03:45:26 pm
Quote
What camera did you use ?

It's a VarioCam hr:

http://www.infratec.de/de/thermografie/waermebildkameras/variocamr-hr-head-600-serie.html (http://www.infratec.de/de/thermografie/waermebildkameras/variocamr-hr-head-600-serie.html)
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on August 28, 2013, 06:54:20 am
Just throwing Ideas at you right as you requested, not really thought thru ;)

Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on August 28, 2013, 07:06:51 am
But, what if I'm wrong?  Would it be possible to use the regular LTZ1000 (not the 'A' version), and just use the on-chip temperature sensing transistor to control the Peltier device and still maintain +/-0.001K die temperature, or would the control function be too unruly?  Before i run out and spend a small fortune experimenting on this, does anyone have thoughts on this?

Hello,

for the first: I´d never try to cool down something because I fear that the condensing humidity would change my output values. But perhaps you have the perfect rooms with humidity control.

Of course the thermal regulation will have to be much slower with a large thermal mass than only with the chip alone.
Otherwise the controller will oscillate.
For my thermal chamber I use a 2 stage concept to speed up the heat up times:
One sensor is mounted to the middle of the PCB with the references. (this would be the LTZ1000 sensor).
The other sensor is mounted directly to the aluminium heat spreader plate where the heater foil is mounted.

Since there is some self heating of the references the PCB is usually 4 degrees celsius warmer than the heater setpoint.
For a temperature of 50 degrees on the PCB the heater has to be kept around 46 degrees in steady state.

So I have 2 control loops.
A fast P (+D part for heat up) control loop for the heat spreader with a setpoint around 46 degrees
and a slow I part for the PCB with a setpoint of 50 degrees.

The I part is managed by correcting the setpoint (46 degrees) of the outer control loop.
For this the temperature on the pcb is measured. If the pcb temperature is only changing slowly (steady state) then the difference between actual temperature (e.g. 50.3 degrees) and setpoint (50 degrees) is subtracted (with a time constant factor e.g. 0.33/minute) from the setpoint. In this case in the first minute the setpoint is corrected to 49.9 degrees. (0.3 degrees times 0.33/minute).

I don´t know if its possible to reach 0.001 degrees stability with this method. A good thermal management (keeping air currents away from the chip, and stable environment temperature) will be necessary to get a stable regulation loop.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on August 28, 2013, 07:17:13 am
(Advertising for sheet metal boxes and feedthrough capacitors again to realize small spaces with pretty good temp and humidity control, and also excellent for electromagnetic compatibility resons :))
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 28, 2013, 08:37:49 am
OK, I'm looking for some additional ideas on my Peltier-cooling idea.  I originally was thinking that there is no way that the Peltier device could maintain the die temperature of the LTZ1000 at +/-0.001-deg-K (in the same way that the on-chip heater can with proper external circuitry).  So, I thought that I would have to use an LTZ1000A, then chill that down to (say) 10 or 15 degrees-C lower than the chosen die temperature, then use the on-chip heater to heat the die back up to where you want it.  This is going to use a lot more power though-- more power to chill it down further (and to fight the on-chip heater), then add even more power for the on-chip heater.  This is not good for battery operation.

But, what if I'm wrong?  Would it be possible to use the regular LTZ1000 (not the 'A' versio n), and just use the on-chip temperature sensing transistor to control the Peltier device and still maintain +/-0.001K die temperature, or would the control function be too unruly?  Before i run out and spend a small fortune experimenting on this, does anyone have thoughts on this?

Forget all about that:
In an oven assembly, you always need high thermal gain, especially when you strive for 1mK stability.
That's always coupled with a small-as-possible thermal resistance  between heater/cooler and the sensor.

The thermal resistance of the LTZ1000 is about 80K/W, the A version even worse for that purpose, 400K/W, and both much too high to make a stable, well regulated oven.

If you simply use another sensor, outside the LTZ1000 case, then the case itself may be temperature regulated, but not the reference amplifier, again due to the high thermal resistance. The self heating will always create a temperature difference, and te temperature of the reference amplifier will be free to fluctuate strongly.

Only if the reference amplifier is also thermally coupled tightly to the sensor and heater, this might work.

Frank

PS: Best read are the articles of Richard Karlquist, one of the most experienced designers of the HP OCXO oven technology.
eg.: www.karlquist.com/oven.pdf (http://www.karlquist.com/oven.pdf)
Title: Re: Ultra Precision Reference LTZ1000
Post by: cosmos on August 29, 2013, 05:19:23 pm
Just a thought..
Dealing with fast external variations one might have a look at what some heatpumps for houses do (those are the ones I know).
They look at the house as balanced energy system, where the goal is to generate exactly the correct amount of energy on the inside to balance the energy leaking out.
This assumes the heat leakage out of the house is constant (should be true for a oven too).
Tuning it is a matter of selecting a gain variable matching the heat leakage and multiplying that with the temp difference inside to out to get a target energy level.
The leakage trough the walls change near instantly with temperature changes.
Stored energy inside a house creates a delay that messes with the regulation when it is only based on inside temperature.

If the mass inside the oven has limited ability to store heat I guess the above has little effect and that measuring temperature directly is the simplest way.
Using the temp difference to control the gain of the regulation loop might still improve the precision?
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on August 29, 2013, 08:28:14 pm
Well, there are uncounable ways of optimizing, but you get into the "crazy" area quickly:

[li}Several shells of thermal mass for equalisation and thermal insulation, [/li]
[li]Spread Temp Sensors everywhere, log, look for patterns, use programmable heater control to play compensation games like "start decreasing inner temp when you realize the outer ambient is heating up even before the decrease appears outside[/li]
[/list]

Most thoughts in this field are immediately related to your mechanical construction.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on August 29, 2013, 09:47:59 pm

I wonder if there's a hybrid approach-- something with the main temperature sensor close to the temperature driver (heater or chiller)-- and this would control the temperature, but then we mix-in the [conditioned] signal from the LTZ1000's on-chip temperature sensor, in order to steer the final die temperature closer to the ideal.  To me, this seems at least possible, because the internal burden heat from the Zener current of the LTZ1000 would be constant.  Just thinking out loud here...

Have a look on the tear downs of the 732B.. a hybrid is used with the heater and all the aligned circuitry on the ceramic PCB. 
I do not remember, if the LTFLU is used as a bare die, or housed... Bare die would be bad, because it would be prone to humidty/oxygen.

But this design comes near to what you propose..

The Fluke 5720 calibrator also has got a hybrid with two stacked LTFLU.

Anyhow, to my opinion, the stability can be improved, w/o such big effort.

Simply select the most stable external components( e.g. VHP202Z) and chose 45°C for the LTZ1000, and that's it.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: eurofox on August 29, 2013, 09:57:20 pm

I wonder if there's a hybrid approach-- something with the main temperature sensor close to the temperature driver (heater or chiller)-- and this would control the temperature, but then we mix-in the [conditioned] signal from the LTZ1000's on-chip temperature sensor, in order to steer the final die temperature closer to the ideal.  To me, this seems at least possible, because the internal burden heat from the Zener current of the LTZ1000 would be constant.  Just thinking out loud here...

Have a look on the tear downs of the 732B.. a hybrid is used with the heater and all the aligned circuitry on the ceramic PCB. 
I do not remember, if the LTFLU is used as a bare die, or housed... Bare die would be bad, because it would be prone to humidty/oxygen.

But this design comes near to what you propose..

The Fluke 5720 calibrator also has got a hybrid with two stacked LTFLU.

Anyhow, to my opinion, the stability can be improved, w/o such big effort.

Simply select the most stable external components( e.g. VHP202Z) and chose 45°C for the LTZ1000, and that's it.

Frank

If you use 5 LTZ1000 with all perfect conditions specified in the different posts and make the average it will be even better  :-DD

eurofox
Title: Re: Ultra Precision Reference LTZ1000
Post by: eurofox on August 29, 2013, 10:49:15 pm

I wonder if there's a hybrid approach-- something with the main temperature sensor close to the temperature driver (heater or chiller)-- and this would control the temperature, but then we mix-in the [conditioned] signal from the LTZ1000's on-chip temperature sensor, in order to steer the final die temperature closer to the ideal.  To me, this seems at least possible, because the internal burden heat from the Zener current of the LTZ1000 would be constant.  Just thinking out loud here...

Have a look on the tear downs of the 732B.. a hybrid is used with the heater and all the aligned circuitry on the ceramic PCB. 
I do not remember, if the LTFLU is used as a bare die, or housed... Bare die would be bad, because it would be prone to humidty/oxygen.

But this design comes near to what you propose..

The Fluke 5720 calibrator also has got a hybrid with two stacked LTFLU.

Anyhow, to my opinion, the stability can be improved, w/o such big effort.

Simply select the most stable external components( e.g. VHP202Z) and chose 45°C for the LTZ1000, and that's it.

Frank

If you use 5 LTZ1000 with all perfect conditions specified in the different posts and make the average it will be even better  :-DD

eurofox

I didn't mean a "hybrid integrated circuit"-- I meant a hybrid system approach to temperature control (as opposed to an oven inside of a refrigerator).

As far as averaging multiple LTZ1000's-- yes, this will reduce noise by the square-root of the number of references you average (and more than 4 would be cost prohibitive).  The problem with long term drift is that the LTZ1000's all seem to drift in one direction (IIRC, in the "down" direction)-- it's just that some drift more than others.  A better idea would be to select the best references that drift the least, and throw the rest away (or sell them on eBay or something).

What I want to do is get better long term stability than the standard circuit can provide.  Let's say the best you can get with the standard LTZ1000 circuit is 0.3ppm/year-- well I want better than that.  0.1ppm/year would even be better-- and I want to be able to recreate this multiple times without having to make 1000's of references, and throw away all but the few that meet my spec.  That's why I'm looking into cooling the reference as opposed to heating it (to maintain a stable die temperature).

Cooling should be easy with a peltier element and add some control with PID to keep the temperature accurate to 0,1 °C
I suppose aging of the LTZ1000 is worst with high temperature, so by keeping it "cold" it should drift less.
Why not averaging LTZ1000 with another one from another supplier with similar tolerance.

Just another idea but I have no experience in this field just to input maybe new ideas, use precision frequency to voltage controller, based on a rubidium standard you can have an extreme stable frequency, of course your converter will drift as well with aging and temperature.

eurofox
Title: Re: Slot or Not
Post by: Andreas on September 08, 2013, 03:43:56 pm
As announced by Andreas we wanted to find out what is best: "Using a solid pcb or spend some slots around the heat controlled voltage reference?" and furthermore: "Keep the leads as long or as short as possible?".

So let's start the discussion :)

Ok, I have finally evaluated the measurements = .csv tables of Branadic from the LM399 thermograpic measurement.

Long leads without slot:   around 5 degres stray within pad  + 1,7 degrees from pad to pad
Long leads with slots:      around 5-6 degres stray within pad + 2,3 degrees from pad to pad
Short leads without slot: around 9-11 degres stray within pad + 3,1 degrees from pad to pad
short leads with slots:     around 12-15 degres stray within pad + 6,6 degrees from pad to pad

So from the measurement the conclusion is that with long leads the stray (=thermoelectric voltage) within pad is lower than with short legs. And the thermoelectric voltage between different pads is lower without slot.
If there should be mechanical issues the slots should be at least around 15 mm from the reference to give the pads the possibility to equalize the temperature.

2 simple conclusions:

- short leads will increase the temperature of the PCB, long leads will give more thermal isolation between component and PCB => long leads preferred
- slots in the PCB give higher thermal isolation between solder joint area and outer PCB area, thereby reducing cooling of the solder joints through the PCB, and in turn increasing the local heating of the solder joints. Higher solder joint temperatures might cause temperature differences more easily and  will increase thermo voltages.

I'm now very confident that 45°C and a PCB without slots is the most stable solution.

So the conclusions of Frank are now quantified.

Before the measurements I planned to make slots near the reference and short leads. Now I will go for long leads and at least keeping the slots far away from the reference. (e.g. at the edges of my inner metal shield).

@branadic
When regarding the videos (the flimmering of the heat) I had the idea that the result should be also be visible by noise measurements. So its a pity that we did not measure the noise voltages of the references in parallel to the thermographic measurements.

With best regards

Andreas

============================== evaluation in detail ==============================

Long leads without slot
=======================

pin upper left:

min   27,64
max   32,81
avg   29,59356083
stddev   1,144305683
max-min   5,17

pin upper right:

min   28,02
max   33,15
avg   30,25453333
stddev   1,000260225
max-min   5,13

pin lower left:

min   27,8
max   33,18
avg   29,33930796
stddev   1,031510593
max-min   5,38

pin lower right:

min   28,87
max   33,48
avg   31,08652597
stddev   0,855853162
max-min   4,61


all in all: around 5 degres stray within pad
1,7 degrees from pad to pad


Long leads with slots
=====================

pin upper left:

min   30,1
max   35,46
avg   32,78116883
stddev   1,173468631
max-min   5,36

pin upper right:

min   28,44
max   33,54
avg   30,48031128
stddev   1,148254067
max-min   5,1


pin lower left:

min   29,24
max   34,46
avg   31,80304
stddev   1,342351664
max-min   5,22

pin lower right:

min   29,3
max   35,7
avg   31,30611111
stddev   1,183186142
max-min   6,4

all in all: around 5-6 degres stray within pad
2,3 degrees from pad to pad


short leads without slot
========================

pin upper left:

min   36,649
max   47,527
avg   42,58773745
stddev   3,146954606
max-min   10,878

pin upper right:

min   38,599
max   49,453
avg   43,66859498
stddev   2,401555908
max-min   10,854

pin lower left:

min   37,141
max   45,726
avg   41,45009881
stddev   2,115676062
max-min   8,585

pin lower right:

min   38,552
max   48,064
avg   44,64691221
stddev   2,080904881
max-min   9,512

all in all: around 9-11 degres stray within pad
3.1 degrees from pad to pad

short leads with slots
======================

pin upper left:

min   41,186
max   55,277
avg   46,75962041
stddev   3,418151728
max-min   14,091

pin upper right:

min   39,226
max   50,78
avg   44,88190295
stddev   3,051374117
max-min   11,554


pin lower left:

min   42,81
max   58,327
avg   50,25318644
stddev   3,742872955
max-min   15,517


pin lower right:

min   40,915
max   56,608
avg   48,35819758
stddev   4,061728375
max-min   15,693

all in all: around 12-15 degres stray within pad
6,6 degrees from pad to pad



Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on September 10, 2013, 05:57:43 am
I think it will be less stable.

Title: Re: Ultra Precision Reference LTZ1000
Post by: Galaxyrise on September 10, 2013, 06:10:21 am
The problem with long term drift is that the LTZ1000's all seem to drift in one direction (IIRC, in the "down" direction)-- it's just that some drift more than others.
Looks like the LM399 drifts upwards. If the two references were summed with the LTZ1000 given a 4:1 weight, perhaps the net stability would be better than the LTZ1000 by itself?  And it would be cheaper than a second LTZ1000, even with some good resistors at the summing point ;D
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on September 13, 2013, 08:40:13 am
Off to Weinheim. See some of you tomorrow !
Title: Re: Ultra Precision Reference LTZ1000
Post by: BiOzZ on September 13, 2013, 08:42:37 am
i have used these before but i had to buy 20 of them from digikey and that was a real bitch to write off but i can see they have been all but one used up!
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on November 10, 2013, 08:56:59 am

I wonder if there's a hybrid approach-- something with the main temperature sensor close to the temperature driver (heater or chiller)-- and this would control the temperature, but then we mix-in the [conditioned] signal from the LTZ1000's on-chip temperature sensor, in order to steer the final die temperature closer to the ideal.  To me, this seems at least possible, because the internal burden heat from the Zener current of the LTZ1000 would be constant.  Just thinking out loud here...

Have a look on the tear downs of the 732B.. a hybrid is used with the heater and all the aligned circuitry on the ceramic PCB. 
I do not remember, if the LTFLU is used as a bare die, or housed... Bare die would be bad, because it would be prone to humidty/oxygen.

But this design comes near to what you propose..

The Fluke 5720 calibrator also has got a hybrid with two stacked LTFLU.

Anyhow, to my opinion, the stability can be improved, w/o such big effort.

Simply select the most stable external components( e.g. VHP202Z) and chose 45°C for the LTZ1000, and that's it.

Frank

Do you have have schematic for the stacked LTFLUs. I looked in http://assets.fluke.com/manuals/5720a___smeng0200.pdf (http://assets.fluke.com/manuals/5720a___smeng0200.pdf) (576 pages!), but the ref board on p. 522 seems not to have a schematic.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on November 10, 2013, 05:45:27 pm
I have saved a copy before Fluke removed the schematics. June 1996, 39MB.
It's on the site of a voltnuts member Didier Juges, ko4bb... Or so, i just do not have access to the link right now.
There you have to work yourself through the drawings carefully.
Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on November 10, 2013, 05:49:32 pm
Thanks. I will look for it at the kobb-site.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on November 10, 2013, 05:53:08 pm
http://www.ko4bb.com/Manuals/Fluke/Fluke_5700A_5720A_Multi-Function_Calibrator_Service_Manual.June_1996.pdf (http://www.ko4bb.com/Manuals/Fluke/Fluke_5700A_5720A_Multi-Function_Calibrator_Service_Manual.June_1996.pdf)
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on November 10, 2013, 07:16:46 pm
The 5720 is just to be replaced by the brand new 5730.
https://www.eevblog.com/forum/testgear/new-fluke-5730a-calibrator/msg296208/#msg296208 (https://www.eevblog.com/forum/testgear/new-fluke-5730a-calibrator/msg296208/#msg296208)
Both are much too expensive if you do not earn money with them.
Therefore my favorite is the unfortunately discontinued Wavetek 4808, which playes in the same league.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on November 10, 2013, 07:42:54 pm
To come back to the OT. I would like to modify a HP3458A reference board to HFL spec (if possible).

Does anyone have details about the Fluke HFL mod?

So far I found at febo/voltnuts that all the boards are suppose to be the same and only aged and selected. I also know from Dr. Frank about his mod and will do the same as a first step.

Because the HFL has not only better DCV but also better resistor accuracy, there must be more changes than only the lower heater temp.

Any hints are welcome.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on November 11, 2013, 07:14:24 am
Thanks Diligentminds,
that is a good confirmation and summary of all things I already knew and have discussed with Dr. Frank.
But I still wonder, if it is really not more to it, why HP/Agilent never offered or does not sell it anymore. Also they never have reached the resistance specs of the Fluke HFL. Besides the document you attached (which I already had), do you know of anything more documented? The only I know of is the att. selection guide.

About calibration, I have actually bought a 3458 mainly because of the artefact calibration, which I already have done and I like it very much. With my lab gear, also I still do not have a real 10V Reference like 732B (or similar 7000/4910), I can repeat the cal any time I like. So far my experiment LTZ1000 board still is the one from Babysitter (7.xxx Volt Output). 

Bye
quarks
Title: Re: Ultra Precision Reference LTZ1000
Post by: captbilly on November 26, 2013, 08:03:38 am
You can read more about it here
http://home.51.com/jj3055/diary/item/10053954.html (http://home.51.com/jj3055/diary/item/10053954.html)

I am fairly sure the slots are there to keep the pcb from transferring mechanical stress to the LTZ1000.  Linear Technology discusses this problem in an app note, but essentially the issue is that movements of the pcb (from temperature, humidity, or even from the user applying force on the board) transfer stress to the reference IC package, as well as the die itself.  The stress effects the output voltage and, according to Linear Technology, is part of the reason that references drift over time (the stresses tend to slowly decrease over time as the board and package slightly flow).  Linear Technology recommend slotting around the reference so that movements on the rest of the board do not transfer to the reference itself.

Leakage currents could be a problem if there are high impedance paths in the circuit, but that is uncommon in voltage reference circuits.

Schematics here
http://bbs.38hot.net/read-htm-tid-36472.html (http://bbs.38hot.net/read-htm-tid-36472.html)

And finally here how to convert 7V to 10V
http://www.crystalradio.cn/thread-229749-1-1.html (http://www.crystalradio.cn/thread-229749-1-1.html)

Looking at the pcb I can't stop wondering why these slots are milled. The voltage isn't that high that you need the extra creepage distance, and right now it seems to me it's only weakening the pcb's structural integrity. Worst case it introduces unknown offsets as the pcb will bend a bit either when moved, or with temperature changes. Am I overlooking something here?
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on November 26, 2013, 06:27:36 pm
Quote
Board stress has no effect on IC dies packaged in hermetic TO packages-- it only applies to devices in plastic packages, and surface mount packages are more sensitive to this effect than through-hole [plastic DIP] packages are.

Quote
AFAIK, the slotted PC board design for the LTZ1000 came from the original Datron 4910 voltage reference design, and because that unit could run off of batteries, in order to reduce the battery energy depletion rate they needed to minimize heat-loss from the LTZ1000-- the slots in the PC board (as well as added insulation) help with this.  The slots are there for NO OTHER REASON.  The voltage reference design on the Chinese "volt-nut" BBS simply copies this technique without knowing why it was done in the Datron design.  If you're not going to run your device on batteries, then you don't need slots or insulation.

You guys know about the investigation that was done on the LM399 with the question what influence these slots have?

http://dg3hda.primeintrag.org/doku.php?id=lm399_thermographie (http://dg3hda.primeintrag.org/doku.php?id=lm399_thermographie)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on November 26, 2013, 07:08:59 pm

AFAIK, the slotted PC board design for the LTZ1000 came from the original Datron 4910 voltage reference design, and because that unit could run off of batteries, in order to reduce the battery energy depletion rate they needed to minimize heat-loss from the LTZ1000-- the slots in the PC board (as well as added insulation) help with this.  The slots are there for NO OTHER REASON.  The voltage reference design on the Chinese "volt-nut" BBS simply copies this technique without knowing why it was done in the Datron design.  If you're not going to run your device on batteries, then you don't need slots or insulation.

Note that the LTZ1000 (and ideally the support circuitry too) does need some kind of cover to prevent air currents from causing thermal EMF's generated by thermocouples created by the Kovar-leads of the TO package to the copper PC board traces.

Hi,

that's the first reasonable explanation I've read (thermal insulation for saving of battery energy).
If the LTZ1000 would run on 45°C, the circuitry would not need so much energy as in the Datron 4910, running on 60°C.

 I totally agree, that the Asians simply have copied, copied, copied and copied those stupid slots, without thinking, why they are there..
And everybody else copied from the Asians.. the slots and the "A" Version, which is copied from the 3458A and which makes sense on that 95°C temperature. (Violating the LTZ1000 as being an LM399)

Although the temperature distribution on the PCB is influenced, observable only at high stabilization temperatures as with the LM399, up to now I don't see any influence on the stability of the reference circuitry, and that'much moremost important, so those slots can be omitted easily.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on November 26, 2013, 07:11:38 pm
You guys know about the investigation that was done on the LM399 with the question what influence these slots have?

Know that, sure.
But it has not been demonstrated any influence on the stability of ref amps, especially not on the LT1000 @ 45°C.

No influence from mechanical deformations on the Output voltage, as you yourself have found out.

So, really nice IR Pictures and experiments, proving that there is no practical use for the slots.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: codeboy2k on November 26, 2013, 10:14:42 pm
So, really nice IR Pictures and experiments, proving that there is no practical use for the slots.

I read the linked article with the IR pictures , and I thought the conclusion was that the slots actually help to maintain the heat at the junction of the can's leads with the copper foil, thus minimizing any delta K and thus minimizing the thermal junctions formed there (copper/kovar)

This is, in fact, still consistent with why it was done in the Datron 4910 battery based design -- to minimize heat loss giving longer battery life.

However, in non-battery applications there might be some useful reason still. 

I'm not a voltnut, and thus not an advocate for slots or no slots, just commenting on the blog I read with the IR photos.


Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on November 29, 2013, 05:21:54 pm
What do you guys think about my cheap connection ?

Its simple single core cooper wire that I gave a 1-minute dip in gold electrolyte...

Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on November 29, 2013, 08:54:49 pm
Single core copper is the best possible for low EMF connection and with direct gold plating the oxidation problem should also be fixed.

Can you share how you have done the plating?
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on November 30, 2013, 08:31:41 am

I got a 3 ml bottle gold electrolyte but it has been sitting for 10+ years, and was completely dry.
Demineralized water and a night on the heater revived it obviously.

The process is easy - a victim electrode and the wire-to-be-plated in a little container with the liquid, applying about 2V (no I didnt start a high precision meter for this...  :-DD) and 1-2 minutes later - golden goodness ! Obvious process improvements: Get fresh electrolyte, use a gold victim electrode (goldfingers of old PC cards, maybe?)

Now I will carry it in my pocket a few days to see if it degrades, I suppose cooper would turn black when exposed to warm sweat like this.
Title: Re: Ultra Precision Reference LTZ1000
Post by: SeanB on November 30, 2013, 09:05:43 am
If the coating is not thick enough, and if you did not put a nickel plate over the copper to provide a backing then yes it will corrode under the gold layer.  To gold plate first you need a chemically clean oil free surface, then etch it with PCB etchant for a second or two ( chemical etch to roughen surface), rinse well then plate with nickel to give an even matt finish. then rinse again twice in pure water then apply the gold plate. When it is bright then rinse and dry.
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on November 30, 2013, 09:32:53 am
The etching of the copper is to remove the copper oxid, the nickel is a diffusion barrier layer and adhesion promoter for the final gold finish.

What do you want to do with such a soft gold surface? Solder? If you want to use it for probing you better have a hard gold surface.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on November 30, 2013, 07:41:52 pm
My wife was for a while working at a PCB shop, I am well aware of the Ni/Au standard process but am I allowed to play by law! ;) This is cheap and easy enough to prepare the wires right before the experiments and dispose them as soon as they start to degrade. I am already mistreating them and I will figure out how bad they behave compared to Multi-Contact CuTe components living in happy dry storage.

@branadic: Says a guy who I suppose applies gold to plastics when I am thinking about your employer >:D

Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on December 01, 2013, 12:31:48 pm
Quote
@branadic: Says a guy who I suppose applies gold to plastics when I am thinking about your employer >:D

Sure, but the golden surface of the MIDs is only for assembling technologies (soldering, glueing, bonding, FlipChip) not for mechanical use.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on December 01, 2013, 01:06:18 pm
I want to use it between those screw terminals which are made of 4mm jacks like Multicontact PK4TS i used on my reference. Ghetto connection O0
Title: Re: Ultra Precision Reference LTZ1000
Post by: eurofox on January 06, 2014, 01:26:47 pm
"The voltage is not generated at the junction of the two metals of the thermocouple but rather along that portion of the length of the two dissimilar metals that is subjected to a temperature gradient. Because both lengths of dissimilar metals experience the same temperature gradient, the end result is a measurement of the difference in temperature between the thermocouple junction and the reference junction."

The "characteristic voltage difference (is) independent of many details (the conductors' size, length do not matter)".

http://en.wikipedia.org/wiki/Thermocouple (http://en.wikipedia.org/wiki/Thermocouple)


You can also choose whatever practically implemented "couple" coupling joint you want as long as it is made of one and the same material and its endpoints are at the same temperature (which means no net temperature gradient).

So 1 inch of thin copper wire soldered to 1 foot of thin iron wire as in the symbol < with temperature t1 to the right (both endpoints at one and the same temperature t1) and temperature t2 at the soldered junction to the left will give the same voltage as 1 yard of copper bar and 1 inch of iron bar interconnected with 2 feet of lead tube provided that the two junctions now created at the left side both are at temperature t2. This holds only in equilibrium, i.e. all connecting points and endpoints have settled.

This is imo implications from the link. Please check for yourself. A search 'thermocouple theory" gives several sources stating similar propositions.

Right  :-+ :-+

When I have to make a quick thermocouple I just twist the 2 wires without welding and work like a charm.

eurofox
Title: Re: Ultra Precision Reference LTZ1000
Post by: Kjelt on January 06, 2014, 01:33:03 pm
When I have to make a quick thermocouple I just twist the 2 wires without welding and work like a charm.
Yeah for awhile. Welding it is not so difficult, use a big low voltage high amp transformer and just short the wires or use a big elco that can deliver 50 Amps for a short while. Transformer works perfect.
Title: Re: Ultra Precision Reference LTZ1000
Post by: eurofox on January 06, 2014, 01:51:30 pm
When I have to make a quick thermocouple I just twist the 2 wires without welding and work like a charm.
Yeah for awhile. Welding it is not so difficult, use a big low voltage high amp transformer and just short the wires or use a big elco that can deliver 50 Amps for a short while. Transformer works perfect.

When I say "quick" is just for some temperature testing, is used +/-1 hour and removed after the test, I did it many times to test temperature of glass in thin film solar cells production because of <> of temperature on the surface.

eurofox
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on January 16, 2014, 06:58:55 am
Mind if I join this club?  :o

(https://www.eevblog.com/forum/testgear/restoration-glory-of-keithley-2001-dmm/?action=dlattach;attach=76909;image)

Title: Re: Ultra Precision Reference LTZ1000
Post by: eurofox on January 16, 2014, 09:47:46 am
Hello Volt Nuts,

I got my version on the LTZ1000A working on slotted PCB with thermal isolation.
I already put it in a box with a power supply, 24V AC outside the box.
I added a switch on the front panel to be able to select output voltage from the LTZ1000A and output of and AD706 with 2 precision resistors (0,1% 15 PPM), a trimmer to set exact 10 V output.

I still have to work on fine tuning of the trimmer part, still to sensitive to set the 10 VDC with the 10nV exactly.



eurofox
Title: Re: Ultra Precision Reference LTZ1000
Post by: eurofox on January 16, 2014, 03:10:03 pm
Some pictures

Title: Re: Ultra Precision Reference LTZ1000
Post by: eurofox on January 16, 2014, 03:26:03 pm
Hello Volt Nuts,

I would like the add that I did some test with this unit and the Geller Lab that is mounted in the same box with the same power supply.

I use Pomona special cables (Shielded and I think they are twisted as well).

No influence of change on the latest digit (10nV) by touching the cable or moving the reference box, it is immune to noise from outside (of course in the area of my lab but there is a RF phone and a computer near with no influence)

This mean that if you have some instable measurement you have to look to your cabling, grounding, connector but basically it is not that critical.

eurofox
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on January 16, 2014, 04:41:31 pm
Hello Volt Nuts,

..
No influence of change on the latest digit (10nV) by touching the cable or moving the reference box, it is immune to noise from outside (of course in the area of my lab but there is a RF phone and a computer near with no influence)

..
eurofox

Hey, eurofox.

Your LTZ circuit is really extremely insensitive, extremely accurately trimmed (10^-9), and your measurement is extremely high resolution.

Could you please tell me the model and manufacturer of your 9 1/2 digits DMM?  :-//

And at which NPLC setting the DMM measures stable on the last digit?
Thanks!

And the slotted PCB is from China?
At which temperature did you set the LTZ?
(Or which resistor over the 1k did you use?)

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: eurofox on January 16, 2014, 05:17:42 pm
Hello Volt Nuts,

..
No influence of change on the latest digit (10nV) by touching the cable or moving the reference box, it is immune to noise from outside (of course in the area of my lab but there is a RF phone and a computer near with no influence)

..
eurofox



Hey, eurofox.

Your LTZ circuit is really extremely insensitive, extremely accurately trimmed (10^-9), and your measurement is extremely high resolution.

Could you please tell me the model and manufacturer of your 9 1/2 digits DMM?  :-//

And at which NPLC setting the DMM measures stable on the last digit?
Thanks!

And the slotted PCB is from China?
At which temperature did you set the LTZ?
(Or which resistor over the 1k did you use?)

Frank

Hello Frank,

Maybe I did not express myself corectly, I mean that moving the reference box (box with inside the power supply closed), touching the cables, there is no influence on the reading with my LTZ1000A or the Geller lab, and yes of course after stabilisation periode and call of the HP3458A the last digit (10nV) change value but stay in the range of the last digit (usually less than 5 with NPLC 100).

I will post the schematic after cleaning up, it is basically the application note wiith a few changes.

The PCB is comming from Germany (at least I bought it there).

eurofox
Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on January 16, 2014, 05:31:23 pm
HP3458A the last digit is much more than 10 nV  :D
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on January 16, 2014, 05:36:06 pm

Hello Frank,

Maybe I did not express myself corectly, I mean that moving the reference box (box with inside the power supply closed), touching the cables, there is no influence on the reading with my LTZ1000A or the Geller lab, and yes of course after stabilisation periode and call of the HP3458A the last digit (10nV) change value but stay in the range of the last digit (usually less than 5 with NPLC 100).

I will post the schematic after cleaning up, it is basically the application note wiith a few changes.

The PCB is comming from Germany (at least I bought it there).

eurofox

Hello eurofox

(the heck, what's your real 1st name?)

Sorry, I just wanted to tease you a little bit.... >:D

The 3458A in the 10V range resolves 8 digits only, that's 100nV, not 10nV!

Well, 5 digits of fluctuation (500nV) fits to what I have measured @ 100 NPLC on LTZ references from babysitter and my two ones.

See 10min stability diagram here: https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/270/ (https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/270/)

Who designed the PCB? And what temperature did you chose?

Well,

each of those LTZ references is very susceptible to external AC sources.
So I banned a nice new LED lamp from my analogue lab, because its switch mode PSU disturbed the reference up to 1 ppm, inside its shielded box.
So I think, your even less shielded case will show even worse behavior, if treated with the "right" HF noise source .
Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: eurofox on January 16, 2014, 05:50:53 pm
Hello Frank,

Yes you are right on the 10V it is 100nV, I'm confused because when use my Time Electronics calibrator I really go to the 10nV in the 10mV range of the calibrator.
I'm quite new with this instrument and start now to be familliar with it.

eurofox
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on January 27, 2014, 05:54:17 pm
Hello Everyone!

I've taken the last month (or there about) to read though this thread, the posted pdf's, associated forums on the LTZ (volt-nuts, etc), and several scientific papers (IEEE, Metrologia) on modern voltage transfer standards.

Let me just say... Wow!

I'm overwhelmed by the shear amount of high quality "amateur" knowledge that is available on this subject. Thank you all for your contributions!


After reading all this information, I've had a few ideas, but I wanted to check with the experts here before I let my mind go too far...

There seems to be a body of information on the environmental conditions that effect the LTZ's long term stability -- atmospheric pressure, humidity, mechanical stress*, etc etc.

There have been a few suggestions in this forum on how to 'seal' a LTZ and its associated hardware away from most ambient conditions (O2, H2O), but has anyone seen a design that has placed the reference board in a vacuum? If so, can you post a link?

Secondly, has anyone seen a study on the effects of ionizing radiation on the LTZ (or semiconductor references in general)? If so, a link would be helpful.


* metal packaging with long leads aside (Dr. Frank, et al)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on January 27, 2014, 09:54:59 pm
There have been a few suggestions in this forum on how to 'seal' a LTZ and its associated hardware away from most ambient conditions (O2, H2O), but has anyone seen a design that has placed the reference board in a vacuum? If so, can you post a link?

Secondly, has anyone seen a study on the effects of ionizing radiation on the LTZ (or semiconductor references in general)? If so, a link would be helpful.

Hello,

I think placing a reference board in vacuum is usually beyond the capabilities of a amateur.
And further it does not solve all problems since you would have to use kovar leads to get through the glas sealing.

I would never place a metrology grade reference into ionizing radiation.

I can remember that there is a application note from Intersil on the ISL21009 on the output voltage shift of their FGA references. But since the voltage is stored into a EEPROM-like cell the shift is more likely than on other references.

And there are some annotations on the Linear Technology LT1021 in the /883 version.
http://cds.linear.com/docs/en/datasheet/RH10215fe.pdf (http://cds.linear.com/docs/en/datasheet/RH10215fe.pdf)

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on January 27, 2014, 10:30:03 pm
And not to forget to mention the problem of gas emission of the pcb itself. I would prefer a solder process of the case under shielding gas conditions with feedthrough capacitor for electrical connections trough the case. IMO the most promissing approach for homebrew application.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on January 27, 2014, 11:04:38 pm
I think placing a reference board in vacuum is usually beyond the capabilities of a amateur.
And further it does not solve all problems since you would have to use kovar leads to get through the glas sealing.

I would never place a metrology grade reference into ionizing radiation.

I can remember that there is a application note from Intersil on the ISL21009 on the output voltage shift of their FGA references. But since the voltage is stored into a EEPROM-like cell the shift is more likely than on other references.


I've read that note from Intersil. As I remember the gist there is they have a MOS-capacitor that stores a reference charge level. That charge level is set before it leaves the factory -- relying on the near-zero leakage rate of the capacitor to store the same charge level for 10+ years. The worry is that an ionizing particle will whiz though the capacitor, altering the charge level.

The effect that I am thinking of in the case of a zener reference is a bit different (changes to the semiconductor structure).  However, I have to admit its simply that -- my imagination. So I was wondering if anyone had seen an expert talking on that subject.

While I don't know much about the subject, but common sense would tell me not to (intentionally) expose a reference to high energy...  anything. But we're awash in radiation day-to-day.  A cosmic ray probably interacts with the zener cross section at some rate of 1's of strikes per day. Of course, you can't really shield against cosmic rays, but other lower energy (terrestrial) sources can be shielded or avoided -- concrete, ceramics, etc etc.

As far as the vacuum idea is concerned, it really depends on the level of vacuum. Ultra high vacuum? Yeah, forget it you might as well build yourself a JJ array. A vacuum on the order of 1's or 10's of Pascals.... definitely achievable.  Sustainable in the long term? Maybe.  It'd have the advantages of isolating the reference from atmospheric conditions and removing convection as a heat transfer mode. But as you've pointed out, it would add construction and design difficulty. I am considering building one, but before I sit down to design it, I wanted to ask for some feedback from the group here.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on January 27, 2014, 11:12:18 pm
And not to forget to mention the problem of gas emission of the pcb itself. I would prefer a solder process of the case under shielding gas conditions with feedthrough capacitor for electrical connections trough the case. IMO the most promissing approach for homebrew application.

I have a few of these feed-though "gizmos" ( http://www.omega.com/pptst/MFT.html (http://www.omega.com/pptst/MFT.html) ) laying around from another project. Three to be exact.

I considered the de-gassing problem and I have to admit, I have no experience with that. Slow degassing could be handled by several pump-down sessions. But I would be worried about placing a large stress on the components during the (relatively) quick pump-down. Has anyone ever seen an LTZ explode? Sounds fun, but I can think of better ways to spend my money.

I considered back-filling with argon. Which would take care of the oxygen, water vapor, and (most) pressure changes, but it would.... I forget a lot of my heat transfer... increase convection(?).







Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on January 28, 2014, 05:49:27 pm
Almost ready for thermal sim :)

Board - FR4 4-layer 40x80mm

+15VDC input
LTZ1000A schematics from datasheet
LT1112A Opamp
PZT3904 NPN in SOT223
Three MAX6610 temperature sensors.
LT1761ES5 5V LDO for temp sensors
Planed resistors: Vishay Z202

Inner layers right now pretty much solid power ground (opamp GND and heater- GND).

Temperature sensor locations:

Sensor 1: Near Opamp with copper plane under it.
Sensor 2: On wires, glued to top of LTZ1000A in dead-bug position.
Sensor 3: On bottom near LTZ1000A

Will run temperature simulation to try get approximate temperature gradients, and tune accordingly before gerber out.

Any ideas?  :-//
Title: Re: Ultra Precision Reference LTZ1000
Post by: sync on January 28, 2014, 06:09:46 pm
A general question about the LTZ1000 circuits from the data sheet. The output is directly connected to the LTZ1000 (ZENER SENSE). Can it used for a reference with external output (long leads, current injection from the DMM, ...) or is a buffer needed?
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on January 28, 2014, 06:35:12 pm
Almost ready for thermal sim :)

What program do you use for the thermal simulation?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on January 28, 2014, 07:21:46 pm
A general question about the LTZ1000 circuits from the data sheet. The output is directly connected to the LTZ1000 (ZENER SENSE). Can it used for a reference with external output (long leads, current injection from the DMM, ...) or is a buffer needed?

If you short the output (a uncharged capacitor is sufficient) then the temperature setpoint will rise to infinity. -> The heater goes to 100% -> The chip will get a (more or less permantent) voltage shift on the output.
So I do not recommend to use a unbuffered LTZ1000 (like mine) as working standard.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: iTist on January 28, 2014, 08:38:43 pm

Secondly, has anyone seen a study on the effects of ionizing radiation on the LTZ (or semiconductor references in general)? If so, a link would be helpful.
* metal packaging with long leads aside (Dr. Frank, et al)

Hi,

here is some information from the  JPL (jet Propulsion Lab)
http://nepp.nasa.gov/docuploads/6A935FBA-65D1-42CE-827076A3DE0C7384/PrecRef-97.pdf (http://nepp.nasa.gov/docuploads/6A935FBA-65D1-42CE-827076A3DE0C7384/PrecRef-97.pdf)


Greetz

Oliver

Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on January 28, 2014, 10:15:08 pm

Hi,

here is some information from the  JPL (jet Propulsion Lab)
http://nepp.nasa.gov/docuploads/6A935FBA-65D1-42CE-827076A3DE0C7384/PrecRef-97.pdf (http://nepp.nasa.gov/docuploads/6A935FBA-65D1-42CE-827076A3DE0C7384/PrecRef-97.pdf)


Greetz

Oliver

I didn't even think of space electronics. Good find!

It would seem the crux of article would suggest its probably not an issue for anything that's kept on the Earth's surface. If I'm doing my maths right, an equivalent exposure for an Earth-based LTZ1000 would occur over the period of 10,000's of years (for natural radiation ~2000 strikes/m^2/s @ 1GeV). And since the LTZ in the article only shifted by 10's of ppm for that level exposure....

It makes me a little sad to think my DNA is drifting more than the LTZ is.


Title: Re: Ultra Precision Reference LTZ1000
Post by: Galaxyrise on January 29, 2014, 12:19:07 am
If you short the output (a uncharged capacitor is sufficient) then the temperature setpoint will rise to infinity. -> The heater goes to 100% -> The chip will get a (more or less permantent) voltage shift on the output.

The datasheet mentions multiple times about using PWM to drive the heater, and you're implying that's bad?  I've been wondering about that myself... PWM would seem to have a lot of undesirable effects on the rest of the circuit.
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on January 29, 2014, 04:45:12 am
Almost ready for thermal sim :)

What program do you use for the thermal simulation?

I'll try to use Solidworks Flow Simulation, using still air box primitive. Problem is to transfer traces from PCB CAD to Solidworks.
Plan to use LTZ1000 datasheet temperature data to reach as a target first, so will have that as a reference for simulation jig
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on January 31, 2014, 05:16:44 pm
And not to forget to mention the problem of gas emission of the pcb itself. I would prefer a solder process of the case under shielding gas conditions with feedthrough capacitor for electrical connections trough the case. IMO the most promissing approach for homebrew application.

I've been considering both the vacuum and inert gas ideas in the past few days. However I'm stuck on one problem -- trimming the circuit.

My common sense tells me that if there's any merit to either idea, one would want to place the entire circuit (op amps, current source, resistors, divider, etc) in the shielded environment. But if everything is nicely setup inside of a sealed container, how does one access the circuit to adjust it for the first time?

One method might be just to build it, insert it into the sealed container, start it up, and accept whatever value it starts with (7.xxxxxx V) as being the reference point -- valuing the (potential) higher stability above having a nice round number (7.000000 V).

But if there was a way to adjust a resistor (or a set of dip switches, etc?) from outside the container, I'd sure like to use that method.

Any ideas?
Title: Re: Ultra Precision Reference LTZ1000
Post by: iTist on January 31, 2014, 06:10:20 pm
Hi,

you can use a DAC for controlling the see:
Maybe at the FLUKE website or her:

http://www.elcal.ch/files/11749-eng-01-a.pdf (http://www.elcal.ch/files/11749-eng-01-a.pdf)

Page3 Figure 5


Greetz

Oliver
Title: Re: Ultra Precision Reference LTZ1000
Post by: SimonSatCom on February 02, 2014, 01:51:43 pm
This is how far I am with my LTZ1000A project. It is not an easy project...but I have learned a lot in here. :)

http://www.simonthenerd.com/LTZ1000A.htm (http://www.simonthenerd.com/LTZ1000A.htm)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 02, 2014, 03:37:42 pm
Hello,

Nice project, from where do you get the housings for your references?

The temperature setpoint of your LTZ is rather low for a LTZ1000A.
In my lab (up to 32 degrees) I would not go below 12K5 for the temperature setpoint resistor.

Do you have a temperature stabilized lab?

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on February 03, 2014, 10:10:12 am
This is how far I am with my LTZ1000A project. It is not an easy project...but I have learned a lot in here. :)

http://www.simonthenerd.com/LTZ1000A.htm (http://www.simonthenerd.com/LTZ1000A.htm)

...but you also made the mistake to buy the A version and 2nd to set it to a too low stabilization temperature.
A temperature difference of 15-20°C above maximum room temperature is necessary for proper operation.

The LTZ1000A is intended for battery operation and for elevated temperatures.
It is badly abused @ 95°C in the HP3458A and in the Keithley 2002, with mediocre 8 ..10ppm/yr.

Burn-in (storage at high temperatures) is also a fault, due to the pronounced hysteresis, which will in turn create a longterm creeping drift, which would not be present by avoiding any elevated temperatures.

The LTZ1000 (non A) can be set to 45°C with <1ppm/yr, which is easily achievable.

Fluke 7000 and Datron 4910 are the most stable references (<1ppm/yr.) and are based on the LTZ1000 (non-A) with 45/55°C!
 
Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on February 03, 2014, 10:24:21 am
But what would be the best operation point for LTZ1000A then?
Asking in practical reason, as have A-based reference in design as well.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on February 03, 2014, 10:53:38 am
But what would be the best operation point for LTZ1000A then?
Asking in practical reason, as have A-based reference in design as well.

Well, 15-20°C above max. ambient temperature, therefore 50-60°C stabilization temperature (12k5 - 13k over 1k).
If you can guarantee metrology grade environmental conditions, i.e. 18-28°C max. all the time,  45°C,  equivalent to 12k over 1k, is tightly sufficcient. But pay attention to sample variation of the LTZ, as +/-5°C can happen easily.

For my references (non A version), running on a nominal  45°C +/-5°C, at 23°C max. RT, the regulation headroom is >20°C, which is safe under all conditions.


Perhaps it is necessary to ship the reference for comparison at a fellow volt-nut, then the expected temperature there has to be taken into account.

Btw.: I cannot understand why people still buy the A-version, even directly from LT.
It is much more expensive (12$) and less suited for high stability.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on February 03, 2014, 12:11:41 pm
Ok, thanks.
I plan to have 40°C as max ambient, so this gives 55°C as reference temperature.

I bought A, because of experience lack, and A-version opamps from LT usually better specs (per datasheets), so thought same regarding zener, lol.
It's still first zener I got, so $12 not a big deal for knowledge.

And actually i still did not decided if I'll have it running battery-backed either. Need check proper battery size/weight to keep reference hot during shipping (I'd take 2 weeks for it, as average).
Title: Re: Ultra Precision Reference LTZ1000
Post by: iTist on February 03, 2014, 12:32:19 pm
Hi,

has someone experience with the technique described in this Patent:

http://www.google.de/patents/US5369245 (http://www.google.de/patents/US5369245)

Greetz

Oliver
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on February 03, 2014, 01:30:46 pm
Ok, thanks.
I plan to have 40°C as max ambient, so this gives 55°C as reference temperature.

....

And actually i still did not decided if I'll have it running battery-backed either. Need check proper battery size/weight to keep reference hot during shipping (I'd take 2 weeks for it, as average).

For 40°C ambient, better use 13k/1k, i.e. 65°C.
I pimped my HP3458As reference to that value, because a dirty fan filter will rise the interior from 34°C to 40°C, max. (at 23°C rooom temperature.
This still would give 1ppm/yr. as default.

...

Batteries will give less output noise / disturbance compared to AC mains supply.
The LTZ itself does not need permanent operation; stability is <1ppm with intermittent operation.

Possibly, resistance based dividers/amplifiers (to 10.0000V) may need constant powering for achieving <  1ppm stability.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on February 03, 2014, 01:54:04 pm
Hi,

has someone experience with the technique described in this Patent:

http://www.google.de/patents/US5369245 (http://www.google.de/patents/US5369245)

Greetz

Oliver

That's the well known Pickering patent used in the Wavetronik/Fluke 7000 reference, for conditioning the LTZ1000 after power down.

I have collected some experience: I did several tests concerning temperature cycling (refrigerators / heater), but not using that schematic.

This patent at first indicates, that temperature excursions of the LTZ induce hysteresis, and if this hysteresis is not removed, by symmetric cycling, it will  in turn induce creeping drift.

From that it directly follows, that any "burn-In" process is bad for the LTZs stability!

I have seen hysteresis of several ppm on both of my LTZ references, but only if the temperature deviation from the nominal value is > +/-25°C. (I accidentially heated one of the LTZs to >90°C.)

If the temp. excursion is less, there is no noteworthy hysteresis, perhaps a few tenths of ppm only.

That means, at 45°C stabilization temperature, as in the Fluke 7000, the conditioning does not improve uncertainty that much.. The reference at this temperature is already very stable without further measures, also in intermittent operation.

I measure that on the HP3458A, each time I power it on again, and two times already in the last 4 years, when I removed my constantly powered LTZ references from the mains for several weeks. Each of those 3 LTZs return to their default value to < 1ppm deviation, every time. Exactly the same goes for the Fluke 5442A calibrator, which uses 2 SZA263.

Only if very high (>70°C) or very low (<15°C) ambient temperature excursions  are encountered, this feature might theoretically have some effect...

But then, as the lower conditioning temperature is limited to room temperature, i.e. 20°C, the complete or symmetric conditioning circle, as described, can not be passed through. 0°C would be necessary, to reset a trip to 90°C, as shown in the  patent in fig. 2.

Therefore, this patent is nice for theory, but did obviously NOT work in the Fluke 7000.
Perhaps this was one of several reasons, why Fluke terminated that product.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: SimonSatCom on February 03, 2014, 02:46:52 pm
Hi all

It is a nice little project but it has not been easy. Information about both the LTZ1000 and the LTZ1000A is pretty
hard to come by. I do think that Linear Tech could do better on they datasheet. :)

First about the temperature set point. Again...I have not been able to find any formula so that I could calculate the
exact set point. A collegue of mine figured out that my setpoint is about 46.5C. I did not have a 13k000 precision
resistor...but I did have a 12k100 resistor. In the article "The Ultra-Zener...is it a portable replacement for the
Weston cell?" I was able to see what happened if the value of R4 is changed. I understand now that this is the LTZ1000
(not the A-version) but since no information was available I decided to try it out. In the document it is claimed that
the aging (ppm/year) will be lower with a lower temperature setting (which makes sense). This leads me to the next
question about the temperature in my lab.

My lab is located in the basement of my building. The temperature in the basement varies from 15C to 20C and the plan
with this reference project is to ovenized it the entire thing. The idea is to set the temperature to 35C since I can
not see the temperature rise to more then that (its always cold in Denmark). In the LTZ1000 and LTZ1000A datasheet I
did not find that much help only that the A-version is set 10C higher then the "not A-version".

And Dr. Frank thinks that I made a mistake by using the A-version...could you specify why that is? I got the A-version
as a sample from Linear Tech and thought the A-version was an improvement over the LTZ1000. But you don't seem to
think so...why is that?! :)

Do any of you have the formulas used to calculate your LTZ1000 projects? It seems there is a lot of discussions about
the correct settings for all the precision values. How did you (Dr. Frank) reach the value for your project? :) And do
you have a lot of experience with the LTZ1000?

Best regards

Simon
Title: Re: Ultra Precision Reference LTZ1000
Post by: Galaxyrise on February 03, 2014, 04:30:04 pm
Do any of you have the formulas used to calculate your LTZ1000 projects?

From page 5 of the datasheet:
"With the values given in the applications, temperature is normally 60°C. This provides 15°C of margin above a maximum ambient of 45°C, for example. Production variations in emitter-base voltage will typically cause about ±10°C variation. Since
the emitter-base voltage changes about 2mV/°C and is very predictable, other temperatures are easily set."


The values given in the datasheet for R4:R5 are 13k:1k across the ~7 output of the reference.  You then have to know that Vbe has a negative temperature coefficient.  From that, I determine that T = 310 - 7*500*R5/(R4+R5)

Quote
And Dr. Frank thinks that I made a mistake by using the A-version...could you specify why that is? I got the A-version
as a sample from Linear Tech and thought the A-version was an improvement over the LTZ1000. But you don't seem to
think so...why is that?!

From the electrical characteristics section of the datasheet, we see the only difference between the two is the thermal resistance.  80C/W for the LTZ1000 and 400C/W for the LTZ1000A. And on page 5,
"Because higher temperatures accelerate aging and decrease long-term stability, the lowest temperature consistent with the operating environment should be used. The LTZ1000A should be set about 10°C higher than the LTZ1000. This is because normal operating power dissipation in the LTZ1000A causes a temperature rise of about 10°C."

So the LTZ1000A runs hotter in most applications making it less stable.  So why have the LTZ1000A at all? The datasheet says "To simplify thermal insulation".  In other words, so you don't have to put it in a temperature controlled box, and it will stay warm with very little power.  If you put a LTZ1000 in an oven which is 10C above ambient, then it will run at the same temperature as an LTZ1000A sitting outside the oven, with potentially similar performance.  If you're going to go through the trouble of ovenizing, then the non-A is preferable.
Title: Re: Ultra Precision Reference LTZ1000
Post by: SimonSatCom on February 03, 2014, 05:05:18 pm
Ahh...okay...now I get it. So I should consider getting the non-A version.

The choice of 12k100 works fine at the moment...but would it make sense to increase it to 14k000 (which I have in the resistor box)...?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on February 03, 2014, 06:12:05 pm
Ahh...okay...now I get it. So I should consider getting the non-A version.

The choice of 12k100 works fine at the moment...but would it make sense to increase it to 14k000 (which I have in the resistor box)...?

Definitely not 14k! That would give 75-80°C, and higher annual drift
12,1k is ok, depending on your envirnonmental condition. Simply calculate.
Perhaps you got another 400Ohm resistor to add for 50°C.
Afaik, you got low TC resistors, wirewound, n'est pas?
And perhaps, you might heat the whole basement to 20-21°C during winter (as I do),

As you have received the LTZ1000A for free, it's definitely no fault, but you have to live with it.

It gives a fine reference also, totally sufficient for ones needs.

Frank

PS: I think I have some experience, as I published the theoretical background and made solid measurements on nearly all of them.
Title: Re: Ultra Precision Reference LTZ1000
Post by: SimonSatCom on February 03, 2014, 07:06:12 pm
Hi

Okay, I will see what I can do. But I think that I will try to get the LTZ1000 version instead of the A-version. I will of course stay away from the 14k000. One of the reasons for ovenizing the whole circuit is because the precision resistors I use are "only" 2ppm resistors. So I figured that keeping the entire circuit (LTZ reference and 7V-to-10V amplifier circuit) at a constant temperature would be a good idea.

If I switch to the non-A-version I guess it would not be a problem with the 12k100 when the complete circuit would be inside a 35C environment. Or what do you think?

Thanks for clearing things up. I had a hard time finding the information I really needed.

Btw...do you have a homepage with more LTZ info? :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on February 03, 2014, 07:51:05 pm
Hi

.. One of the reasons for ovenizing the whole circuit is because the precision resistors I use are "only" 2ppm resistors. So I figured that keeping the entire circuit (LTZ reference and 7V-to-10V amplifier circuit) at a constant temperature would be a good idea.

If I switch to the non-A-version I guess it would not be a problem with the 12k100 when the complete circuit would be inside a 35C environment. Or what do you think?

Thanks for clearing things up. I had a hard time finding the information I really needed.

Btw...do you have a homepage with more LTZ info? :)

Well, you just should start READING (RTFM).

It's all inside the datasheet, or within in this blog. No, I don't have a personal HP.

12k or 12k1 would be perfect for an LTZ1000.

If you avoid that external oven @35°C, and keep the reference below 30°C, 12k1 is also fine for the LTZ1000A.

What the heck do you mean with resistors, 2ppm???

2ppm of tolerance or 2ppm/yr. or - if assume right - you mean 2ppm/K wirewound resistors?

That is just perfect! That's top notch, not beaten even by those miraculous Vishay metal foil resistors.
I use 3ppm/K econistors, and the circuit is rock stable.

Why?

Once again, please simply read that f****d specification of LT!!!

All instabilities or variations are attenuated by a factor of 100 at least!!

A 2ppm/K drift on any resistor will translate into 0.02ppm/K drift of the reference, only.
A 20ppm/year (which is typical for such wirewounds) will translate into 0.2ppm/year for the reference.

Both are phantastic values!!

Therefore, an additional oven is not necessary, if you feel ok with that level of stability.


But somehow, I just feel if I repeat what I already have stated earlier in this thread...

Please, begin to READ this thread thoroughly.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Galaxyrise on February 03, 2014, 08:32:14 pm
For my references (non A version), running on a nominal  45°C +/-5°C, at 23°C max. RT, the regulation headroom is >20°C, which is safe under all conditions.

While we're on the subject, how'd you get the +/-10°C mentioned in the datasheet down to +/-5°C?  I still haven't decided how I'm ultimately going to thermally stabilize the reference, but most of my ideas involved measuring Vbe in situ and computing a resistance from that rather than choosing a resistance to target a temperature.  That +/-10°C was just too big a spread.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on February 03, 2014, 08:39:12 pm
Maybe we should split this thread into a section with fairy dust and one without :)

best regards,
the double babysitter
(cured from ltz-esotherics)  :-DD
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 03, 2014, 08:50:56 pm

the double babysitter


[off topic]
congratulations! and my best wishes for your growing family.
[/off topic]

best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on February 03, 2014, 08:51:39 pm
Thank you Andreas !  :-+
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on February 04, 2014, 06:32:47 am


While we're on the subject, how'd you get the +/-10°C mentioned in the datasheet down to +/-5°C?  I still haven't decided how I'm ultimately going to thermally stabilize the reference, but most of my ideas involved measuring Vbe in situ and computing a resistance from that rather than choosing a resistance to target a temperature.  That +/-10°C was just too big a spread.

Well yes, I didn't remember the max. variation in the datasheet correctly..

I also measured the VBE at room temperature carefully, i.e. with small current and without heating, and then estimated the stabilization temperature. It was very close to those 45°C with 12k/1k.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on February 04, 2014, 06:05:34 pm
Dr. Frank: Could you detect the AfE tower demolition in the output of your LTZ?   :-BROKE
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on February 06, 2014, 10:41:07 am
@Dilligent Minds: The fluffy alternative to foaming enclosures!  O0 but I would never be scared to start fires with such filling - I wouldnt even expect melting temperatures for polyolefines in such low power circuits, without Tantal Capacitors there is just a low risk of flames forming and (ceramics, ok, different story) and in a sleeping bag filling I would expect some flame retardants.

Also, I am the "tuner tin can guy", dont forget !  ^-^ this will contain the fire if it breaks out I guess, without having analyzed the risks yet.

Hollow fiber insulation doesn't belong to the fairy dust class, as it is effective: On the northern hemisphere you can try this with a corresponding sleeping bag.

At work I just got a sample of a Lackwerke Peters Polyurethane potting material which is quite nice to work with. Good if you are confident that you never need to repair the embedded circuit, but I like the thermal mass and thermal conduction properties. No chance for even forced airflows below unpleasant levels :phew:
Title: Re: Ultra Precision Reference LTZ1000
Post by: Rerouter on February 06, 2014, 10:41:42 am
you could always do a low voltage spot weld to the copper if you wanted to get fancy (where you have the weld tips in contact before applying current from a super-cap or single turn output transformer) that reduces it to what ever fun the copper and the lead material cook up,

it just comes down to if this is a better or worse thermocouple
Title: Re: Ultra Precision Reference LTZ1000
Post by: Rerouter on February 06, 2014, 11:18:55 am
I would think as you want minimal heat loss from the reference and power planes anywhere near it would soak up way too much, where by tiny little copper traces on essentially an insulator are almost invisible to the heat-source (now mounting that thing in a shielded box might be an option if you where that concerned with noise coupling, but you would still risk it coming in on your leads,

equally star grounding and ground planes can be very bad, it really comes down to what your doing, the best way to lay out references like this is to treat every trace with resistance and every connection as a thermocouple, and pick where you connect your traces wisely, for instance the 2 input pins of your chopper amp, if there is a small temperature gradient across the board from the references heat source then you could have a few nV-uV more generated in the input pins solder joint closest to the reference and offset your reading,
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on February 06, 2014, 01:27:17 pm
Hello folks,
                 attached the top and bottomview of the voltage reference circuit of the ultimate HP/Agilent 3458A - 8 1/2 digit DMM. What struck me observing the approach of the PCBs is the lackof a solid ground plane, short - wide power rails, wide power tracks, star grounding - any thoughts about improving the overall quality of these PCBs - would a ML-4 or ML-6 layer approach contribute to this ?

Thanks !

This reference is one of the weak components of the 3458A.

HP ignored several design rules to make it more stable.
One of those rules (see LT datasheet for the LTZ1000), is the thermal shielding of all solder junctions, another one - having supreme negative impact - is keeping the stabilization temperature at  65°C or lower.

Some simple improvments on this reference, for metrological use:

- Reduce Tstab. from 95°C to < 65°C. Simply replace the 15k metal foil by 13k (or parallel it with 100k)
- Replace LTZ1000A by LTZ1000 and set Tstab to 45-50°C (12k-12.5k)
- Replace both 70k resistors by metal foil or wirewound types.
- Use instrument at Tamb < 30°C, i.e. at 23°C +/- 5°C only and keep fan always clean => max. internal temperature of < 38°C

=> will give <= 1ppm/yr. DCV stability.

- Put complete reference PCB into a closed box to avoid any air draught from the rear fan.

=> willl reduce medium term random noise

Any further measures are not necessary!
The original LT circuitry, as it is also realized on this PCB, is stable enough to achieve an ultra stable reference.

The reference is prone to external AC noise, which might also cause random unstabilities.
But obviously this is no problem inside the HP3458A, as the PSU and shielding measures around the DC conditioning PCB is extremely good. Therefore, any additional layers or star points, or whatsoever is also not necessary.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on February 06, 2014, 05:58:45 pm
Two of the high-end digital multimeters - Prema 6047 and 6048 was designed in the 1988. Both was build on the Prema's ASIC ADC chip (but not a well-known PRI5610). Model 6047 have a LM399 voltage reference, 6048 - LTZ1000, but in DIP8! Is it possible?  8)
Title: Re: Ultra Precision Reference LTZ1000
Post by: JBeale on February 06, 2014, 07:54:49 pm
Well, it is possible to bend the 8 leads of a TO-5 or TO-99 can to fit into the through-hole locations of a DIP-8 footprint...
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 06, 2014, 07:59:36 pm

Any further measures are not necessary!


Is there no calibration after changes necessary?
My LTZ1000A have around 50ppm/K drift when changing the temperature setpoint.

- LTZ1000, but in DIP8! Is it possible?

I do not know any datasheet which shows a DIP8 package.
At that time many manufacturers used plastic spacers which spreaded the TO-99 pins into a DIP8-pattern on the PCB side.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Galaxyrise on February 06, 2014, 09:32:12 pm
Last year, Linear Tech introduced the LTC2057(HV), which is a zero-drift op-amp that also has very low noise-- even lower than the LT1013.  There is simply no excuse anyone can make for using anything other than two LTC2057's in this circuit if it were redesigned today.
It's nigh impossible to find new parts in hermetic packages any more.  I can find some CERDIP 1013s on ebay, though.  Are these opamps humidity sensitive at all?  "For humid environments, surface coating may be necessary to provide a moisture barrier." is the only datasheet reference I can find to it (in the 2057 datasheet.)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 06, 2014, 10:49:03 pm
It's nigh impossible to find new parts in hermetic packages any more.  I can find some CERDIP 1013s on ebay, though.  Are these opamps humidity sensitive at all?  "For humid environments, surface coating may be necessary to provide a moisture barrier." is the only datasheet reference I can find to it (in the 2057 datasheet.)

You can also find CERDIP LT1013A on RS-Components. (from TI)
If the chip is sensitive to forces any humidity will change the characteristics. Either the PDIP package swelling or from PCB changes which are creating forces to the chip through the PINs of the package.

Even a hermetically packaged voltage reference like VRE3050A or AD586LQ or LT1236AILS8-5 is sensitive to forces through the PCB (like humidity changes) if not carefully decoupled from the board.

A chopper like the LTC2057 does not need to be in a hermetically package. Due to the self adjustment the effects of humidity will be canceled out by the chopper.

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: mrflibble on February 06, 2014, 10:58:52 pm
It's nigh impossible to find new parts in hermetic packages any more.
I came across these recently:
http://www.digikey.com/product-highlights/en/ls8-reference-family/51750 (http://www.digikey.com/product-highlights/en/ls8-reference-family/51750)
Title: Re: Ultra Precision Reference LTZ1000
Post by: dannyf on February 07, 2014, 01:21:49 am
Quote
if I were designing a replacement board for the 3458A,

Wouldn't that depend on why you are redesigning the board?

Many times, we don't design for the highest performing products.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Galaxyrise on February 07, 2014, 05:30:28 am
You can also find CERDIP LT1013A on RS-Components. (from TI)
Obsolete packaging according to the datasheet.  rs-components in the US, aka alliedelec, does not have it.  I went to the german site, and there indeed was the AMJ... but boy do you pay a lot for the M version! 

Quote
If the chip is sensitive to forces any humidity will change the characteristics.
...
A chopper like the LTC2057 does not need to be in a hermetically package. Due to the self adjustment the effects of humidity will be canceled out by the chopper.
I contacted Linear Tech, and asked about this.  The answer is that the LTC2057 will self-compensate for any temperature and/or humidity caused drift-- this is caused by the die being "stressed" when the epoxy package changes dimensions, and since it is "inside the zero-drift control loop", it is compensated for.
Makes sense, and confirmed by Linear, even!  Thank you both :)  The 2057HV is surprisingly affordable, too.

Quote
(such as ... Peltier coolers, etc.).
This is what I've been playing with, and part of why I'm very concerned about humidity.  So far, I can get my, uh, "oven" to Ambient-10C with 1W and Ambient-18C with 4W; still a lot of things for me to learn and play with.  Temperature controlled boxes have been fascinating so far.  I'm aiming to eventually implement your suggestion from much earlier in this thread.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 07, 2014, 07:03:21 am
Obsolete packaging according to the datasheet.  rs-components in the US, aka alliedelec, does not have it.  I went to the german site, and there indeed was the AMJ... but boy do you pay a lot for the M version! 


Hello,

you looked at the wrong datasheet from LT. On Texas Instruments the package is still active.
The chip is identical to the LT chip according to a interview of a LT representative in a paper.

And yes: precision is not cheap if not using low noise choppers.

With best regards

Andreas
 
Title: Re: Ultra Precision Reference LTZ1000
Post by: Blackart on February 07, 2014, 07:21:21 am
Gentlemen first post here.

I recently acquired a Datron 4700 and I plan to "have a go at calibrating it" so I need a 10V ref. I would then use the 4700 etc to keep my 34401A and bank of old 3400A/B/400E/F/Gs etc in line. I have been eavesdropping here and on volt-nuts for a week or so trying to get the threads/approaches clear in my mind.

I think Ive decided to go down the LTZ1000 path with ;
approx 45C ref temp
vishay S102 resistors plus a couple of others I have in stock ( better if the $ are kind when I ask texasinst for a quote )
LTC2057 opamp
unbuffered ref out
10V buffered ref out
self design PCB
powered by a double regulated low noise supply Ive already done.
lots of thermal & magnetic/RF shielding

If your listening, I was wondering if a couple of you could clear up a couple if things I can't work out from my couple of readings of the blog.

Dr Frank
You made a ref with 2 off LTZs, it look like one has the ref and heater supplies buffered by a transistor and the other is driven as per the LT data sheet. Why ? was this to compare the performance of the two approaches ? If so what was the outcome ?
OR
Was the above idea dropped and you were you seeking to combine the references as per the LT data sheet to improve stability and noise etc if so how did this work out ?

Andreas
You also added the ( FET in this case ) buffer i.e. a BF245. How do you think this performed ? as I thought that any self heating in the driver opamp would be nulled out in the FB in steady state. Or is the ref noise/drift such that the opamp has to work hard to maintain a constant current ?

regards Lucas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on February 07, 2014, 09:24:20 am
Dr Frank
You made a ref with 2 off LTZs, it look like one has the ref and heater supplies buffered by a transistor and the other is driven as per the LT data sheet. Why ? was this to compare the performance of the two approaches ? If so what was the outcome ?
OR
Was the above idea dropped and you were you seeking to combine the references as per the LT data sheet to improve stability and noise etc if so how did this work out ?

regards Lucas

Well, I really tried both designs.
The transistor gives more current capability at the output jack, as the OP07 is limited to a few mA only.
On the other hand, it gives more gain, so that at last, I skipped it again, because I feared instability of the circuit due to oscillation.

The whole circuit is quite sensitive to external AC noise, which may lead to disturbance and even unlatching of the temperature regulation.

The transistor is really not needed at that point, and the additional resistor from V+ to the reference high side unloads the Op07 also.

My advise: keep the circuit as simple as possible.

I would also not use a chopper OpAmp, as they create more noise than regular ones.

The LT1013 do not influence the stability so much, so they are fully sufficient.
ank
Title: Re: Ultra Precision Reference LTZ1000
Post by: jd on February 07, 2014, 06:08:35 pm
I would also not use a chopper OpAmp, as they create more noise than regular ones.

With the exception of the LTC2057(HV).  This part exceeds the LT1013 in almost every spec, including noise [and it is better in *all* of the important specs for this circuit].  Note that the LTC2057 is a completely new architecture than most chopper amps [and Linear Tech is not revealing *exactly* how they did it].  For other chopper amps, I would agree with Dr. Frank.

MAX44246 looks quite useful for DC precision. It looks like it might be lower noise than even LTC2057. Also, is chopper amplifier noise not of a different character, being free of 1/f behaviour? So longer integration times continue to reduce fluctuations. Perhaps for use with a null detector/comparison circuit or 7.2:10V booster.

(But I think Franks point was that in the actual LTZ1000 reference circuit the LT1013 opamp errors are not significant in any case).
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 07, 2014, 08:06:10 pm

Andreas
You also added the ( FET in this case ) buffer i.e. a BF245. How do you think this performed ? as I thought that any self heating in the driver opamp would be nulled out in the FB in steady state. Or is the ref noise/drift such that the opamp has to work hard to maintain a constant current ?


Hello,

there were 2 reasons for the FET:

1. to keep away heating from the OP-Amp.
    5mA * 7V = 35mW @ 130K/W gives around 5K self heating.
    Offset drift is up to 2.5uV/K = 12.5uV
    that sounds not much but:
    12.5uV/2mV/K Heater setpoint = 6mK Temperature setpoint drift.
    together with the 50ppm/K drift of the zener there is up to 0.3ppm drift of the LTZ.

2. Battery powered design with only 14V regulated voltage.
    I simply feared that with 1V less voltage against the 15V datasheet design
    there would be not enough headroom to fully stabilize the zener current loop.
    The negative pinch off voltage of the FET compensates for the lower power supply voltage.

Other effects are that with lower load current and with lower excitation of the output the open loop gain (= current regulation precision) increases.

So in a design with LTC2057 only the second point would still have some meaning.

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 07, 2014, 08:32:21 pm

(But I think Franks point was that in the actual LTZ1000 reference circuit the LT1013 opamp errors are not significant in any case).

Note: In the more serious (long term stable) desings (AN86 + Datron reference board) they used a LT1013 in the now obsolete hermetically TO-99 case.
Ok this would also be a possible enhancement for the HP-reference board.

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 08, 2014, 05:23:26 am

As far as the LT1013 is concerned, the note on page 5 of the data sheet for the LTZ1000 says:
"BOTH A1 AND A2 CONTRIBUTE LESS THAN 2uV OF OUTPUT DRIFT OVER A 50°C RANGE"

Hello,

With my calculation above this is only possible with the "typical" drift spec of a LT1013AC = 0.3uV/K.
With the maximum spec of a non "A" device of 2.5uV/K the 2uV drift is already eaten up by the self heating of 5K.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: eurofox on February 08, 2014, 01:21:21 pm
Hello Dr. Frank, Andreas, all VoltNuts,
                                                      Would it be possible to send me your most stable implementation containing all modifications (minor - major) as schematic for review - so that we can reach common sensus - I am willing to put effort in it - in order to create a nice schematic in Altium out of it and upload schematics as (*.schdoc ) and as(*.PDF) on EEVBLOG so that people on the forum have a reference and do not get lost in the big forest ... - as a next step we can make up a list of best engineering practices to implement the LTZ1000(A) as it should be done with and without extra temperature controlled mini-oven ...
I do not really need a nice CAD drawing, something hand-written (scanned) to start with is fine, as long as the paper contains all major & minor details ... it is OK for me.

Thank You !
ps.: You can mail it to forum or send me a PM.
                                     

Good idea because now information is scattered.

I already have a word file with a lot of information that I collected and I have myself a LTZ1000 with a slotted PCB and isolation.
I use the extra 70k resistor to lower the working temperature of the LTZ1000.


eurofox
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 08, 2014, 01:34:36 pm

Good idea because now information is scattered.


I think all information is within this thread.

And: there is no single truth. Every cirquit built here has its special targets.
It makes a difference if you want to have a "primary standard" a "working standard" or a "transfer standard".

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on February 08, 2014, 02:24:09 pm
And collecting related data will be thru personal preference prism, some might think about items which are not important to others, and vise versa.
But in case such work to be done - do it public, so others can benefit.

I don't have any experience yet, and collecting ideas and data, so following this thread as well. Just calibrated my EDC MV106 according to my calibrated 2001, which is only 4 days after cal in Tektronix/Keithley lab.
My first reference will be LTZ1000A, with direct output. MV106 was around 120ppm off (http://dev.xdevs.com/issues/955). I have it constant powered on, so just another reference for future tests.

Also curious, why nobody tried implementing reference with resistor networks? Datron, Fluke in their 8508A (http://dev.xdevs.com/projects/rnd/repository/entry/Fluke/8508A/3_15_3e391ce1b9395fb%20(1).jpg) are using networks, which can be explained by matched tempco. After all, on output it's important
to keep constant resistor divider ratios, not actual resistance values of individual resistors. And networks fit this job best, as their elements are fitted on same substrate, and drift same or very close,
reducing mismatch significantly.
Am I missing something?

Maybe better option would be order needed network from VPG? I'm thinking of doing so, but again, i have zero hours of practice with LTZ, only diode resting in a bag waiting for PCB.

P.S. Fluke 8508A used AD823A and LTC1150 in their reference + custom R network.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 08, 2014, 02:43:53 pm

Am I missing something?


Hello,

for first: usually you will have to order large quantities of those "specialized" networks. (so no hobbyist will afford it).
second: the tempco will only cancel out if the resistors are of similar kind. (resistor film/foil thickness).

If you pair a large (70K) + a small (120R) on the same network. The network will either use a very large area (costs) or the tempco will not cancel out.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on February 08, 2014, 07:56:19 pm

Also curious, why nobody tried implementing reference with resistor networks? Datron, Fluke in their 8508A (http://dev.xdevs.com/projects/rnd/repository/entry/Fluke/8508A/3_15_3e391ce1b9395fb%20(1).jpg) are using networks, which can be explained by matched tempco. After all, on output it's important
to keep constant resistor divider ratios, not actual resistance values of individual resistors. And networks fit this job best, as their elements are fitted on same substrate, and drift same or very close,
reducing mismatch significantly.
Am I missing something?

Maybe better option would be order needed network from VPG? I'm thinking of doing so, but again, i have zero hours of practice with LTZ, only diode resting in a bag waiting for PCB.

P.S. Fluke 8508A used AD823A and LTC1150 in their reference + custom R network.

The answer is obvious:
- very high cost for low volume production
- useful for the temperature regulator only, i.e. the 12k over 1k divider

If you study the calculations in the datasheet, or what I have calculated in this thread, you would recognize, that especially this divider ratio of 1:13 should be very stable, by matching the T.C. of both resistors.
Vishay offers such matched dividers with 0.2ppm/K, and these should also be oil filled for stability of ~4ppm/6yrs.  Resulting impact on the LTZ circuit would be 0.002ppm/K and 0.007ppm/yr.

The 70k resistors don't need to be that stable, as all their drifts are attenuated by a factor of 500.

The 120 Ohm resistor should have low drift values, i.e. low T.C. and low long term drift, i.e an oil filled VHP202Z should be used. The 100:1 attenuation of its drifts would give an impact of 0.01ppm/K and 0.004ppm/yr. Useless to match with the others; and also there is no matching counterpart left.

If an amplifier to 10V is added, this should also be based on a divider in one case. But this would of course add 100 times higher instabilities to the 10V output compared to the naked 7.2 V output of the LTZ1000.
 
So you easily see, that the array of the Datron designs is not necessary, and used perhaps only for convenience of the PCB design.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Blackart on February 09, 2014, 04:44:05 am
Thanks for the responses.

I was worried about the chopper noise and since Ive never used a chopper amps, I have no real experience on the actual spectral makeup ( if any ) of the left over chopper noise. Also I looked at the Maxim MAX44246 data and compared it to the LTC. My worry is that the maxim device doesn't seem to like driving capacitive loads, unless buffered by a resistor c100Ohm. Which just adds more tempo issues to deal with. Since Dr Frank raised concerns about the ref circuit susceptibility to RF instabilities I think the LTC might be the one to try.

Given that others on the forum have successfully tried the LTC1013 I think Ill try the LTC2057 If only to add some more experience the thread.

I was wondering about the 400k tempo resistor required for the LTC1000A implementation. I have not found much discussion on the threads about it apart from it being miss applied in a couple of instances. Since the data sheet notes it is for "TC compensation" - OF WHAT ? the opamps or the LTC1000 ref. i.e. should it be located anyware special i.e. near the REF can or near the opamps.. any thoughts ?

regards Lucas

Oh and with reference to Gazelles question and I agree with the others; both forums ( inc VOLT-NUTS ) are worth the read. Not only for the REF discussions but the insights into the quest.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 09, 2014, 08:45:57 am

I was wondering about the 400k tempo resistor required for the LTC1000A implementation.


Hello,

According to the data sheet the 400k resistor is only necessary for the LTZ1000 and not for the LTZ1000A.
The feed back is with heater power (i.e. the LTZ serves as temperature sensor).

On my LTZ1000A I have tested the TC without the resistor.
over a 10-42 °C range there where around 1.7ppm shift (including noise and hysteresis) which corresponds well with the 0.05 ppm/K from the data sheet.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on February 09, 2014, 10:14:34 am
Quote
If you can live with 30K instead of 70K for the two collector resistors, you can get 5 resistors in a 16-pin ceramic LCC.  60K resistors are twice as wide, so you would have to move up to a 24-pin ceramic LCC or a 14-pin side-brazed ceramic DIP.

Have you ever tried this possibility of individual VSM85, 86, 87, 88, 89 network from Vishay? Can you tell something about the prices for a single piece? I'm looking for 5x 5kohms or 5k/20k (20k made of 4x 5k internally) network, for a gain setting application.

Edit: No need to be a network, can be an array too.
Title: Re: Ultra Precision Reference LTZ1000
Post by: jd on February 09, 2014, 12:30:27 pm

I was wondering about the 400k tempo resistor required for the LTC1000A implementation.


Hello,

According to the data sheet the 400k resistor is only necessary for the LTZ1000 and not for the LTZ1000A.
The feed back is with heater power (i.e. the LTZ serves as temperature sensor).

On my LTZ1000A I have tested the TC without the resistor.
over a 10-42 °C range there where around 1.7ppm shift (including noise and hysteresis) which corresponds well with the 0.05 ppm/K from the data sheet.

With best regards

Andreas

Hi Andreas,

I found that you can use the resistor to fine-tune the temperature coefficient in the LTZ1000 circuit, so start with 400 K and put the circuit in a temperature controlled environment (I used a beer fridge that could heat/cool  :)) Then you might change it to e.g. 470k to reduce the TC below even 0.05ppm/K.

This will also take care of any concerns about opamp TC.

The resistor itself does not have to be ultra-stable as far as I can see.



Title: Re: Ultra Precision Reference LTZ1000
Post by: jd on February 09, 2014, 02:30:58 pm
The answer is obvious:
- very high cost for low volume production
- useful for the temperature regulator only, i.e. the 12k over 1k divider

Hi Frank,

Have you looked at LT5400? http://www.linear.com/product/LT5400 (http://www.linear.com/product/LT5400) Networks of 4 resistors,  2ppm / 2k hours long term ratio stability. And not that expensive really.

A unique part as far as I have seen. On paper they look very interesting for making ratios, provided you can get lucky with the values available. But look, you can make the temperature controller divider using R4=(10+1) in series and R5=(10+1) in parallel. = 11k and 0.9090909k, equivalent to 12.1k + 1k. Perfect!

John
Title: Re: Ultra Precision Reference LTZ1000
Post by: jd on February 09, 2014, 03:37:04 pm
Hello John - the two 70k resistors can be build with LT5400-2 in parallel and LT5400-1 in series, in theory at least but need to be tested in the field. [100k||100k]+[10k+10k] but their 8ppm/K is NOK.
Hello gazelle,

No no no, I was thinking for ratio use only! :) As you mention the absolute TC is mediocre and the absolute value long term stability is completely unknown (I asked them). But for ratio use it ought to be very good.

Quote
The R4:R5 ratio is little bit more tricky - you want to have a matched tracking ratio, all resistors need to be part of same device to maintain matched tracking ratio - meaning to take LT5400-1 and LT5400-4 to build your 12.1:1 ratio - that would offer 0.2ppm/K matched tracking.

Yes sorry, from memory I thought there was a single device with 2x10k and 2x1k but it is 2x100k+2x10k, absolute values too high I think.

I don't think your two-device solution gives matched tracking unfortunately.

In fact I have been working with the -6 which has 2x5k and 2x1k. This gives a ratio of 11k:1k which still works but you have to reduce the 70k collector resistor to 10k say. However my results are not so great, I got about 3ppm drift in the first 6 months or so. But there could be other causes which I am still looking into...

One possibility is the absolute values change too much, since the divider is loaded slightly by the base current this would shift the operating point. Or it could be some other blunder.

Quote

So for the four resistors R2 ... R5 in LTZ1000 ref. design can be replaced with LT5400-x network, and R1=120 ohm should remain a high quality type Vishay PG preferable with Kelvin Varley connections.

:-)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on February 09, 2014, 05:42:35 pm
The answer is obvious:
- very high cost for low volume production
- useful for the temperature regulator only, i.e. the 12k over 1k divider

Hi Frank,

Have you looked at LT5400? http://www.linear.com/product/LT5400 (http://www.linear.com/product/LT5400) Networks of 4 resistors,  2ppm / 2k hours long term ratio stability. And not that expensive really.

A unique part as far as I have seen. On paper they look very interesting for making ratios, provided you can get lucky with the values available. But look, you can make the temperature controller divider using R4=(10+1) in series and R5=(10+1) in parallel. = 11k and 0.9090909k, equivalent to 12.1k + 1k. Perfect!

John

What for?
My design goal was < 1ppm/yr., and I achieved that without any schmuck devices or exotic PCB slots.
Use ordinary precision wirewound resistors and solid thermal management. That's it.

Stability is now < 1ppm / 4yrs.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on February 09, 2014, 06:10:21 pm
Quote
My design goal was < 1ppm/yr., and I achieved that without any schmuck devices or exotic PCB slots.
Use ordinary precision wirewound resistors and solid thermal management. That's it.

Maybe you have one of those tiny little beasts that tend to have less drift compared to the average? You can't say for sure that it is just the way your circuit is build up, because you can't make a statistical announcement to that, right? So maybe you had also a portion of luck with your reference?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on February 09, 2014, 06:50:45 pm
Quote
My design goal was < 1ppm/yr., and I achieved that without any schmuck devices or exotic PCB slots.
Use ordinary precision wirewound resistors and solid thermal management. That's it.

Maybe you have one of those tiny little beasts that tend to have less drift compared to the average? You can't say for sure that it is just the way your circuit is build up, because you can't make a statistical announcement to that, right? So maybe you had also a portion of luck with your reference?

Maybe no scientifically correct statistics.... but I have 2 working references, really existing, and very stable, at least.
The third one is the modified reference in my 3458A, which also is very stable, without any Schickimicky gadgets...
And the fourth one is at "babysitter", a 1: copy of this design, which shows drift < 0.3ppm after 1/2 year.
All those really existing devices all fit within the theoretical calculations and estimations, i have done.

Everything else, I see here on the net, is theoretical stuff only, most of it even without any solid background. Only nice gadgets, without any hint or even proof, that those are useful.

Up to now, I haven't seen any real stability measurements from somebody else.. again.. I have to stress..only speculations all around.

So, you better show something practical,i.e. real circuits with real improvements over my stuff, instead of bringing up such weak arguments.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: jd on February 09, 2014, 07:38:31 pm
Hi Frank,

Have you looked at LT5400? http://www.linear.com/product/LT5400 (http://www.linear.com/product/LT5400) Networks of 4 resistors,  2ppm / 2k hours long term ratio stability. And not that expensive really.

A unique part as far as I have seen. On paper they look very interesting for making ratios, provided you can get lucky with the values available. But look, you can make the temperature controller divider using R4=(10+1) in series and R5=(10+1) in parallel. = 11k and 0.9090909k, equivalent to 12.1k + 1k. Perfect!

John
What for?
My design goal was < 1ppm/yr., and I achieved that without any schmuck devices or exotic PCB slots.

Use ordinary precision wirewound resistors and solid thermal management. That's it.
I had assumed that the slots are for thermal management. They should prevent any external thermal gradient from appearing across the zener terminals. On the Datron reference they also have wide copper tracks around the periphery of the square, which would tend to "short out" any thermal gradient. (Provide a "thermally grounded enclosure") Also the long thin PCB segments will tend to further isolate the LTZ1000 thermally, i.e. the temperature of the zener lead junctions will be nearly that of the LTZ1000 rather than the rest of the board.

Quote
Stability is now < 1ppm / 4yrs.
Frank
My original interest was for use in the 7.2:10V step up, but I think a PWM approach will be better for that. But in fact it ought to be a good solution for the temperature controller divider too, cheaper than two wirewounds, and much more compact of course. But in fact for my next reference I am using wirewounds for the precision resistors (as you suggest). And no slots either :)

John
Title: Re: Ultra Precision Reference LTZ1000
Post by: jd on February 09, 2014, 07:58:02 pm
Hello all volt nuts - has anyone already experience using the LT5400-6 and LT5400-7 as matching resistor network [0.2ppm/K matching temperature drift] ... - one can easily fabric the 12.5k:1k0 and 12k0:1k0 ratios 12k5 = 5k0 + 5k0 + 1k25 + 1k25  or 12k = 5k0 + 5k0 + 1k0 + 1k0. The LT5400-x networks are low cost.

I don't think either of those get you the ratio, you need to do both parts of the divider in a single device as far as I can see. I.e. the 1k resistor has to be part of the same device as the 12.5k or 12k. The best I could find is (5+5+1)k : 1k, or 11:1, which only works if you use a much smaller R3 ~10k.

Quote
Another idea I got in mind - has anyone already considered to replace the Op Amp controlling the heater element with a low power (galvanic isolated) microcontroller that is executing a PID algorithm ? How close could we get in delta mK - would a sensor based on a thermo-couplebe accurate enough ?

Well I have considered it. There is some kind of LTZ heater PWM going on in the Datron reference circuit, but I have not figured it out exactly yet; not sure if it is only used for transportation mode for example.

I don't think a thermocouple would be accurate, you are probably not going to improve on the on-die sensor. If using an external sensor a thermistor is best for non-extreme temperature measurement.

Really, the more I study the canonical datasheet circuit, the more perfect it looks already.

Title: Re: Ultra Precision Reference LTZ1000
Post by: EEVblog on February 09, 2014, 10:48:52 pm
NOTE: Several silly argument posts removed.
Come on guys, talk about voltage references instead...
Title: Re: Ultra Precision Reference LTZ1000
Post by: Blackart on February 11, 2014, 02:42:13 am
Thanks for you responses re the 400k Ill play with this in the prototype unit. Ive order all the bits for my ref and now have to wait for them. Then the fun begins.

As for checking the drift I don't have the luxury of a 3458A/1082 8 digit meter. But I do have a datron 4700. I was thinking I could weakly calibrate this to one of the one of the LTZ1000 REFs and then null the others of against it. At least the intrinsic drift go the 4700 is pretty low to start with.

Does any body have a better idea ? Oh Ive bought 5 off LTZ1000 to play with.

Lucas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 12, 2014, 05:48:08 am

I found that you can use the resistor to fine-tune the temperature coefficient in the LTZ1000 circuit, so start with 400 K and put the circuit in a temperature controlled environment (I used a beer fridge that could heat/cool  :)) Then you might change it to e.g. 470k to reduce the TC below even 0.05ppm/K.

This will also take care of any concerns about opamp TC.

Hello,

when looking up to my TC-measurements (LTZ1000A) that I did, I cannot see how the TC could be bettered by such a measure. (especially above 20 °C environment temperature).
Most of the TC above 20 degrees is related to some kind of hysteresis. There is no linear relation to the environment temperature which could be compensated in my case. The hysteresis might come either from the temperature gradient, from PCB, the LT1013 in plastic case or some effect on the LTZ1000A.

The best measure would be to heat the whole reference to a constant temperature in my case. But with battery operated device this will not be possible.

The diagrams show output voltage (divided by 2) over ambient temperature of the reference and over the internal temperature sensor near the LTZ1000A on the PCB.
Temperature gradient is 0.1K / minute.

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on February 12, 2014, 07:37:01 am
Hello,

when looking up to my TC-measurements (LTZ1000A) that I did, I cannot see how the TC could be bettered by such a measure. (especially above 20 °C environment temperature).
Most of the TC above 20 degrees is related to some kind of hysteresis. There is no linear relation to the environment temperature which could be compensated in my case. The hysteresis might come either from the temperature gradient, from PCB, the LT1013 in plastic case or some effect on the LTZ1000A.

The best measure would be to heat the whole reference to a constant temperature in my case. But with battery operated device this will not be possible.

The diagrams show output voltage (divided by 2) over ambient temperature of the reference and over the internal temperature sensor near the LTZ1000A on the PCB.
Temperature gradient is 0.1K / minute.

With best regards

Andreas

Hello Andreas,

best candidate for this kind of hysteresis is the LTZ itself.

This has already been pointed out by Pickering, but without any hint of the magnitude order of this effect.

I also have seen hysteresis, but to a much higher degree, as I changed the LTZ temperature from -18  to +80°C, i.e. -18° for the whole circuit, +80°C for the LTZ alone. That rules out hysteresis effects from the other components.

Effects from the PCB and the LT1013 case also have not yet been demonstrated yet.

You also use wirewound resistors, normally they do not show hysteresis at all, in constrast to metal foil types.


What I do not understand: The absolute change in Uref is - 0.08ppm/K only, although it should be -50ppm/K.

Have you really compensated the T.C.?

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: jd on February 12, 2014, 10:27:41 am

Hello,

when looking up to my TC-measurements (LTZ1000A) that I did, I cannot see how the TC could be bettered by such a measure. (especially above 20 °C environment temperature).
Most of the TC above 20 degrees is related to some kind of hysteresis. There is no linear relation to the environment temperature which could be compensated in my case. The hysteresis might come either from the temperature gradient, from PCB, the LT1013 in plastic case or some effect on the LTZ1000A.

The best measure would be to heat the whole reference to a constant temperature in my case. But with battery operated device this will not be possible.

The diagrams show output voltage (divided by 2) over ambient temperature of the reference and over the internal temperature sensor near the LTZ1000A on the PCB.
Temperature gradient is 0.1K / minute.

With best regards

Andreas

Hi Andreas,

I have not looked properly at hysteresis so I may have missed it. I did not notice any but it could be there, I will check more next time.

Here are some plots I made with two different TC resistors, the datasheet 400k and 330k. With my particular layout there was a clear TC with 400k which could be made to vanish with 330k.

The red line shows the temperature of the LTZ "module"; it is self-regulating to an extent so the excursion is a lot less than the module "ambient" (fridge) temperature it is in. The black line is the 3458a internal temperature which tracks that of the room.



Title: Re: Ultra Precision Reference LTZ1000
Post by: jd on February 12, 2014, 04:48:08 pm


JD, are you using the LTZ1000 or the LTZ1000A?

Hi,

I am using LTZ1000.

Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 12, 2014, 08:38:02 pm

What I do not understand: The absolute change in Uref is - 0.08ppm/K only, although it should be -50ppm/K.

Have you really compensated the T.C.?


Hello Frank,

perhaps I did not explain it clearly enough:

The tempco of the LTZ1000(A) without heating (or by changeing the temperature setpoint) is around +50ppm/K.
In the measurement above the internal heater of the LTZ1000A was active. So the datasheet value of 0.05ppm/K is the target.
So the TC is not compensated but the chip temperature is held more or less constant.

TC compensation is done only on my ADCs (LTC2400) which I use for measuring the LTZ output values.

I plan on using a 16-bit PWM instead of a divider resistor for the temperature control set-point--   

If the Zener is making about 7.2V, then a 16-bit PWM should give me about a 0.05-deg-C step for each count-- thus allowing very fine grained control.

This 0.05 K step will produce about 2.5ppm/step (50ppm/K) output voltage change.
The interesting question here is how the remaining ripple of the PWM after filtering is minimized.
In order not to increase the noise of the LTZ (1.2uVpp) the remaining ripple should be well below 0.003K (or 7uVpp).


I have not looked properly at hysteresis so I may have missed it.

The red line shows the temperature of the LTZ "module"; it is self-regulating to an extent so the excursion is a lot less than the module "ambient" (fridge) temperature it is in. The black line is the 3458a internal temperature which tracks that of the room.


In your case the temperature change near the LTZ is only around 8 degrees against 25 degrees of my PCB temperature change. Since the hysteresis squares (approximately) with the excursion of the temperature it should be negligible in your case.
Btw. what is the exact type of beer cooler that you are using?
I use a standard thermoelectric cooler (car box).
But I have to use additional ice packs in styrofoam + some fan controller to reach the +10 degree Celsius.


What I am wondering, is which device has less thermal hysteresis-- the LTZ1000, or the LTZ1000A?  Does anyone have data on this?

Interesting question.

According to my measurements on several AD586LQ references the hysteresis shows large differences between individual devices.

Further I have the suspect that the hysteresis is larger on newer devices. And AD: has changed the type of die attach for the hermetically devices according to a change note on their web side. (probably due to ROHS?)

So you would have to test several LTZ1000 + LTZ1000A from around the same date code to answer the question.

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: Galaxyrise on February 14, 2014, 06:04:25 pm
Of course there are the other bits that need to be addressed as well-- I want my 10V reference to have battery backup for an extended period of time (for shipping the device "hot" to/from a cal-lab that has a JJA)....  I am leaning towards the CALB 40Ah if I finally decide to go with the Peltier-cooler method...
In my experimenting with Peltier cooling so far, cooling performance of the hot side is very important for performance of the cold side.  This would seem to further complicate mailing it "hot", where the reference will be inside a bunch of packing material.  Did you have a plan for mailing a peltier-cooled reference or is not mailing it a reason you don't care the CALB isn't flight qualified?
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on February 15, 2014, 12:06:10 am

Yes-- the Peltier system will need to breath outside air in order to function correctly.  My plan is to build a special carrier with inlet/outlet vents for airflow from the fan that cools the hot-side of the Peltier device.  The carrier will act as the "shipping container".


You could pipe the heat to phase-change sink of some sort. Dry ice (maybe too cold)? Regular ice? One of those semi-solid freezing gel packs? etc
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on February 15, 2014, 03:46:41 am
Hmmmm... That's a good thought-- but are those things allowed on cargo planes-- (I know that some of those you mentioned are not allowed in the cabin of a passenger airliner)?

Air flow might be questionable:
(http://p3.img.cctvpic.com/program/bizasia/20111213/images/1323758629567_1323758629567_r.jpg)

But I'm sure you can find something that'd be allowed. Kerosene?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 15, 2014, 08:14:37 am

Maybe the easiest thing to do is restrict the time of year to ship the reference off for calibration to the more temperate seasons-- (avoiding winter and summer-- perhaps in the fall when it is dryer)...  This would only happen once a year anyway...

I do not know how the humidity behaves in your region.
Here in my region the smallest change in humidity is in spring after winter.
So this is the best time to calibrate all your gear if you want to import the "volt" into your lab with the smallest changes during transport.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on February 15, 2014, 09:56:11 am
What is the ideal value/range regarding humidity in your opinion?

My very best gear uses LTZ1000 and I now have a Fluke 732A. So far I have not seen an effect with changes from 45% up to 55% on this gear.

Right now I have around 45% and made very interesting meassurements with my TEK 4050. As I already posted several times, the Fluke 8846A / TEK DMM 4050 are great meters. In my opinion the very best you can buy. I only wonder how it is possible with "only" a LM399 reference inside, because  my 4050 even challenges my gear with LTZ1000 reference.

Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 15, 2014, 11:23:10 am
Hello,

There is no best value. In best case it is constant.
Most instrument datssheets specify < 80% rH non condensing.
The effect is 0.5 ppm/%rH with buried zener voltage references in plastic case. (LT1027CN)
Something similar has been shown in early datasheets from LT1236AILS8 in hermetic case mounted on pcb material.
In this case the changes of the pcb create forces on the chip.

With references (LM399) that have good decoupling from pcb the effect is lesser.
I see around 1-2 ppm over 1 year (cycle) when comparing a well decoupled (from pcb) AD586LQ against a LTZ1000A.
So with a 6.5 digit multimeter you might probably have difficulties to see it.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Galaxyrise on February 15, 2014, 09:45:22 pm
I don't know how radical I need to get in cooling the LTZ1000--- maybe just running it at 25C die temperature (+/- 0.001C) will be sufficient, and further chilling might be wasted because of no further decrease in drift rate.  All of that remains to be determined after construction and testing.  My plan is for a die temperature of 0C, but that may be "overkill", and if 25C *is* sufficient, then the Peltier device will have much less work to do, and the power needs from the battery will be more reasonable.
I'm thinking of trying a three layer approach, where the die temperature and the support circuitry are both held at 10C.  I'm not sure if I'd ever know I succeeded in reducing thermal gradients on the leads, but it seemed like a neat idea.  I'm also guessing 10C should be better for the stability of all the precision parts, so long as they're hermetically sealed.  My lab tends to be 15-20C, 25-65% humidity; so 10C is above the highest dew point and relatively low power to maintain, simplifying the build.  Still many Saturdays and parts orders to go before I know if it's within my abilities to realize this 3-layer design, hehe.

It will take some things out of their tempco sweet spot, but hopefully the temperature regulation will be tight enough to compensate. I will need some good power filtering, too, since having an inner peltier cooler means bringing that bucked voltage into the shielding box. Thanks for posting your PWM filter :)

As this is going to be the most stable thing I have by several orders of magnitude, I may never know if I chose well.  If I ever go for calibration against a primary standard, my "plan" is to have a road trip ;) 
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on February 16, 2014, 12:34:12 am

 I am going to contact them to see if they would give us "Volt-Nuts" a break on calibrations-- (no certificate of compliance, just a "before" and "after" if an adjustment is involved). 

You had better put your game-face on. Calibrations at NIST start around $10k. And all they do is hook your machine up, hit the go button, and tell you how far you are off. Forget having time to tweak it.

I'm reminded of the book "Foundations of Mechanical Accuracy" in which the story of replicating the meter standard in the 1970's was told. The Moore Tool Co would send a rep to France with a briefcase that contained what they though was some fraction of a meter. The guy hops on a transatlantic flight, sets up the standard next to the one in Sèvres, waits how ever many hours/days you need to make sure everything's at the right temperature, and finally shines a laser at the ends of each to figure out how much they are off. The guy gets the report, flies back home, sets up the standard in a room inside a room in the sub-basement of the Moore Tool Co. and they take like a month to file off their best guess at another nanometer. All this just to fly back to France and try again and again until they've gotten "close enough" or they've gone too far.


Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on February 16, 2014, 01:36:35 am

.. Well, after 2 weeks of this torture, the readings in the 3458A suddenly dropped 0.25ppm (when it had been solid, with no discernible drift, for the first part of the year). ...

 and the 3458A dropped another 0.25ppm!  [The 3458A has never recovered-- even though the humidity has been around 8% for months now, it is still low by 0.5ppm-- so I think it was permanently affected].  ...

 My Fluke 732B did not suffer at all through this (as far as I can tell-- I don't yet own a JJA)-- the important resistors in the 732B are all hermetically sealed.



Hi,

If you don't have an JJ, how did you distinguish, or decide, that the 3458A was drifting absolutely and not your 732B?

I assume you noticed the drift by comparing the 3458A against the 732B?

You know, what I mean, the "man with the 1 clock, 2 clocks, or 3 clocks" problem.

Anyhow, if it was really the 3458A, there are 2 possibilities of a permanent shift.

First, the sudden high humidity caused a sudden shift , i.e. an irreversible ageing, of one or two of the bulk metal foil resistors, about 25ppm in total.

Second, the humidity caused the temperature stabilization to fail, heating the LTZ even more than 95°C, where it is supposed to be normally.
Maybe you did not log that event.

This might have caused a reversible but permanent  hysteresis, resulting in a smaller reference value, so the 3458A reading should go up afterwards.

Would be interesting what  you have observed, and how you argue about the 3458A/732B comparison.

And I am interested, how you basically monitor the 3458As stability , against that single 732B only?
Do you leave the 3458A always on?
What if you switch it off, does it return to its former value?

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on February 16, 2014, 01:44:53 am

I don't know where in the country you are, but Sandia Labs has a Cal-Lab that is humidity controlled (one of the few in the USA), and their uncertainty for Zener reference calibrations is 0.017ppm !!! 

No big deal, if they own a JJA and a decent nV voltmeter. Comparisons to the level of <= 0.001 ppm should be possible.

But which Zener based references have that level of stability?

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: robrenz on February 16, 2014, 02:43:39 am
I'm reminded of the book "Foundations of Mechanical Accuracy"

One of the most treasured books in my library. It inspired me to buy (3) 18" square surface plates and take them to 15µinch flatness using the 3 plate method back when I was about 22. I studied that book like the bible and the skills I learned from it served me well thru my mechanical career. I ended up buying a Moore #3 measuring machine where I worked and many trips to the Moore special tool co.
Title: Re: Ultra Precision Reference LTZ1000
Post by: SeanB on February 16, 2014, 06:48:36 am
If you are going to have very precise references you will note the accuracy and cal specs require them to be in a temperature and humidity stable location at all times, just for the ageing. Every cal lab will have all of the precision equipment in a room with a hermetic seal on all the doors and no windows aside from a small one in the door, with airconditioning just for the room and a heat exchanger on the room ventilation. This will be to keep the standards at constant temperature and humidity. The standards lab near me has the mass standards in glass cases with felt linings, all in a room with thick brick walls inside the middle of the building. There is a balance in there as well to check the transfer standards against them. Those then go into the room next door where they can bring the equipment in that they are calibrating.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on February 16, 2014, 07:40:35 pm

I did not think about the hysteresis problem-- is there any way of determining if that is the cause, and if so, is there a way to fix it (put the reference board in the freezer or something)?


I just had an image of a kegerator-style DMM flash into my mind.  Mmmm... 8 1/2 digits of frothy precision.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on February 17, 2014, 12:56:18 am
I have an idea to fix this calibration cost problem for us "Volt-Nuts".  If it turns out to be feasible, I will announce the solution here and on the "Volt-Nuts" list.

I've been thinking that rather than calling and asking nicely, we (the collective group here) should publish a paper on our experiences. If we could get it into Metrologia, I'm sure Sandia (or whomever) would be much more receptive to returning your calls.
Title: Re: Ultra Precision Reference LTZ1000
Post by: hgg on February 17, 2014, 02:48:41 pm

Do you know what is the initial %accuracy of the LTZ1000?
I cannot find any info in the data sheet.  Only its stability of 0.05ppm/C is emphasized. 

I've bought a MAX6325 1ppm/C 2.5V reference with a 0.02% initial accuracy.
How does it compare with the LTZ1000 in terms of initial accuracy?
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on February 17, 2014, 05:09:27 pm
It's first item in electrical characteristics on page 3. Anything from 7.0 to 7.5V with Iz 5mA, or from 6.9 to 7.45 with Iz = 1mA.
Title: Re: Ultra Precision Reference LTZ1000
Post by: hgg on February 17, 2014, 05:21:27 pm
Excuse my ignorance, but is the LTZ1000 a reference that you have to calibrate in the beginning,
but after that it will keep its calibration for a long time?

In the datasheet of the MAX6325, it lists the output voltage to be between 2.499 & 2.501
Title: Re: Ultra Precision Reference LTZ1000
Post by: hgg on February 17, 2014, 06:39:44 pm
Hello Ken,

Very detailed explanation.   Thank you!

Title: Re: Ultra Precision Reference LTZ1000
Post by: casinada on February 17, 2014, 06:40:50 pm
Doug @ http://www.voltagestandard.com/ (http://www.voltagestandard.com/) still sell some nice products. with 2 year free calibration
Title: Re: Ultra Precision Reference LTZ1000
Post by: hgg on February 17, 2014, 06:47:57 pm
Geller Voltage Reference : better than +/- .0005% absolute (+/- 5 ppm)    Wow, that's accurate!
Pitty he stopped producing them.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on February 18, 2014, 01:32:26 am

Do you know what is the initial %accuracy of the LTZ1000?
I cannot find any info in the data sheet.  Only its stability of 0.05ppm/C is emphasized. 

I've bought a MAX6325 1ppm/C 2.5V reference with a 0.02% initial accuracy.
How does it compare with the LTZ1000 in terms of initial accuracy?

Table 6.7 in "The Art of Electronics" lists the LTZ1000's accuracy at 4%.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on February 18, 2014, 07:37:13 pm
Just to expose what got me started on this subject: A REF194AG transportable standard.
Unfortunately, it was not sucessful to export the Volt from my workplace DMM to my home DMM.

With the LTZ it is...



Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on February 18, 2014, 08:21:04 pm
All three are available - this is the one I lend to people with low-digit meters saying it is ok for 4.500 V +/- 5 mV,
the LTZ is doing very very well and circulates between me and frank,
otherwise it is really used, often with a kvd to null out voltages or generate low voltages and stuff, i almost think it is the only one used for stuff here. :)

A battery powered LM399 reference was born a few weeks ago. Not really figured out how well i did it yet, but also ok for backpack transfer.

Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on February 18, 2014, 10:33:20 pm
I found out the reason that most <= 6.5 digit DMM manufacturers still use the LM399 (as opposed to the provably more stable LTZ1000) is that the LM399 exhibits remarkably low hysteresis between power cycles-- and the LTZ1000 may actually have more (without some kind of [proprietary] "pre-conditioning" process).  So, for a bench meter that gets used day-to-day (and sometimes in "non-laboratory" environments), the LM399 is a better choice.

The LM399 is absolutely fabulous for short-term work, but as you probably already know, it takes over 10 years of operation to reach ~1ppm/year stability-- so for very long-term stability, the LTZ1000 is the preferred device.  From what I understand, the DMM manufacturers have racks full of LM399's that they "burn in" for a very long time before they are used in a final assembly.  Some manufacturers (like Fluke) burn them in much longer than others (Rigol, Keithley-- with Agilent/Keysight somewhere in the middle).  That's also why very old DMM's (like the HP3457A) can be scary-stable-- their reference (and the cheap metal film resistors they used in the design) have had decades to become very stable.

The LM399 might also be used because of a pricing decision, not only on the part itself but by the required support circuitry and mechanics (dont forget the mold for a injection molded cap which is already shipped with and mounted at the 399 but strongly sugessted for serious LTZ application), while matching to what you usually want from a 6.5 digit device. I am quite happy with my HP3456A at home and the 34401As at work.

What I dislike about the LM399 it is the FLUKEing very hot temperature setpoint - of course required to use it all over the world even in crammed racks without aircon, but here in mild climate of germany I wouldnt mind if it is anything above 40°C. the fridge with ice packs is close, for those rare very hot days.

The REF194AG reference was made because parts laying around, easily good enough for 3 digit meters but frustrating for transfer from work to home - the backpack trip of 20 minutes has thrown it off for more than 1 mV in any direction. But it is very small and handy as you see.

The LTZ1000A was selected because part available on ebay and known to be a instant near-perfect device without any hassle. I tried to keep out most voodo which doesn't appear inside the datasheet but at the omniscient landfill aka the internet. It works well, it travelled from home to work to hobby room and back, to Dr. Frank and another guy. My cheap and lazy open source design is good enough as shown by Frank that it attracted other people already; selling leftover PCBs helped mitigate the part costs a bit :) And in the short time since its birth, it did very well.

The LTZ reference even gets some non-meter-and-references-comparing exercise, e.g. I do internal calibration of some 34401A and (painful) 34405A at work; when one of the main meters come back from calibration, I use those fresh ones to check some lower-class meters which we need only in certain ranges; which I can do with a Dekavider. Also, during testing sensor front-ends i like to use the LTZ and the dekavider. Either to inject a voltage via a buffer, or null out some voltages.

The LM399 was more or less an afterthought as I felt that I skipped an important step. This is now a battery-powered reference; only the standard application again, a battery undervoltage monitor and again cheap and lazy.

When the babies allow it, I will compare the buried zener devices against each other sometimes, and against the workplace DMMs, and Dr. Frank had my LTZ two times for now. All my refs are allowed to travel :)


Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on February 19, 2014, 05:59:24 am
Check out TiN's awesome teardown of a Keithley 2002 complete with several choice pictures of the LTZ.

https://www.eevblog.com/forum/testgear/keithley-2002-8-5-digit-dmm-review-and-teardown/ (https://www.eevblog.com/forum/testgear/keithley-2002-8-5-digit-dmm-review-and-teardown/)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 19, 2014, 09:03:57 pm
Hello Ken,

why do you use the LTC1144 instead of a LTC1043 for voltage division?

Ok it is faster in settling time due to the higher frequency.

But since it is designed for power supply applications the charge transfer of the switches is not specified.
On the other side I can remember that a standard charge pump has only about 10ppm stability against 1ppm of the LTC1043.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on February 19, 2014, 10:34:59 pm
Quote
Check out TiN's awesome teardown of a Keithley 2002 complete with several choice pictures of the LTZ.

https://www.eevblog.com/forum/testgear/keithley-2002-8-5-digit-dmm-review-and-teardown/ (https://www.eevblog.com/forum/testgear/keithley-2002-8-5-digit-dmm-review-and-teardown/)

Interesting pictures, especially the LTC1043 section. They used 680nF Mylar caps by Cornell Dubilier with 5mm grid.
Also interesting is the way the did the shielding. On the lower LTC1043 you can see a guard trace around pin5 and pin18 that is running all the way to the noninverting input of LTC1050. On the upper LTC1034 there is no such a guard trace on any pin.
The bottom view of the pcb shows another interesting point. They surrounded the cap with the signal of pin3 around pin2 on both LTC1043 with the traces coated, but upper and lower layout are different in that particular case.
Would be interesting to know in what configuration they use the switched capacitor building blocks.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 20, 2014, 05:34:33 am
Honestly, I don't know which is better in this application-- the LTC1043 or the LTC1144. 

When looking at the datasheet: the LTC1144 operates only at voltages > 4V in this application.
I.e. if it is sufficient for you that the output voltage range is limited to 4..10V you can use the LTC1144 too.
The LTC1043 has a seperate power supply pin and is not restricted with input voltage.
But when you want to use full accuracy of the LTC1043 you will have to increase the integrator time constant. (R1/C4).
According to my tests you should not deviate from the around 450 Hz switching frequency which are set by the 10nF capacitor on the LTC1043.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Galaxyrise on February 25, 2014, 05:22:27 am
Certainly a nice reduction in the number of primo resistors! You're really close to the output current limit of the 2057, aren't you? Were you able to divine the heater's temp co from the datasheet, or did you experiment?

I presume you're going with the LTZ1000A so that its temperature changes less as the room temperature changes?  Any idea how much it will vary with 1C room temp change? 

My amateur attempts at a stable TCE have only yielded 0.1C regulation thus far (with ambient changing by 4C), so perhaps I'll be able to learn more on that from this thread, too :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on February 25, 2014, 05:23:39 am
This might seem counter-productive to have a heater inside of a refrigerator, but it is the best way to give the LTZ1000A the best chance at long-term stability.  The only way I can see around this is to run the LTZ1000 at cryogenic temperatures-- like in liquid nitrogen (LN2).  This would result in an absolutely stable temperature, because the LN2 always boils at a specific temperature-- and so the LTZ1000 would be kept at this temperature.  You are not going to get much colder than LN2 (well, unless you can afford liquid helium, and if you can afford that-- then you can afford a JJA), and so I would expect the drift rate of the LTZ1000 to be at it's minimum under these conditions.  But-- who can afford LN2 to "play" with?

It's six months past said, but here it is: my first experiment with LN2...

https://www.eevblog.com/forum/projects/the-cryogenic-p-n-junction/ (https://www.eevblog.com/forum/projects/the-cryogenic-p-n-junction/)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Galaxyrise on February 25, 2014, 08:27:08 am
No, the LTC2057 will output quite a bit of current-- see the data-sheet.

I'm seeing 19mA min, 30mA typ for short circuit current with a 30V supply...  Oh, derp: I misread your schematic.  Sorry.

Quote
Note what I said in the message-- this is intended for a TCE.
Since the TCE is going to be stabilizing the temperature of some thermocouple or something, and not the temperature of the LTZ1000, then the die temperature of the LTZ1000 will vary as the TCE varies the Q it's moving.  The change in temperature of the LTZ1000 will be less than the change in room temperature; how much less depends on more factors than I know, so I was asking about it ;)  You're clearly allowing for it to vary some, or you wouldn't bother with the temperature feedback in the zener current.

It's a neat idea using the heater as a temperature sensor instead of a heater.
Title: Re: Ultra Precision Reference LTZ1000
Post by: fmaimon on February 25, 2014, 09:54:06 pm
Can't you use Jim Williams circuit from Linear AN45 page 7 to find out the Vbe x temperature relationship? Won't be as accurate as your method but will get you in the right ballpark.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on February 26, 2014, 09:08:08 pm
Hello Ken,

wow, your cirquit proposal is far away from being usual.
I am very eager to hear from measurement results when you have built your samples.

Just to look if I have understood it right.
The pull up R2 + the heater resistor are in parallel and give around 100 Ohms. They are in series with the 7.2V zener so this will give around 0.5V more output voltage than the usual 7.2V (i.e. a total of around 7.7V).

R2 + heater are now directly adding to the output voltage with a rejection of around 15:1 against the >100:1 relation of the datasheet circuit for the resistors. So any drift of the both resistors will add significantly to the output stability. The temperature drift will be compensated. (as per design). So the ageing drift and the hysteresis of the 2 resistors has to be tracked carefully. A typical "shelf life" drift of a Z201 resistor of 25ppm/year would shift the output voltage by 1.67 ppm/year. So this resistor has shurely to be significantly better.
What kind of resistor did you plan for R2?

Further some questions arise:
- Do you have any ageing data for the silicon resistor of the heater?
  As heater it is negligible since it is part of a regulation loop in the datasheet circuit.
  As compensation resistor any change will affect output voltage by around 1:50 of the heater resistor drift.

- Is there any known hysteresis for the silicon resistor?
  Since it spans the largest area on the chip without any compensation patterns a hysteresis might be likely.
  (Just in case your battery power gets lost during transport).

- how is the output impedance affected by the series resistor?
  the dynamic impedance of the datasheet cirquit is "specced" with around 5 Ohms for the refamp cirquit.
  If the impedance is higher leakage currents of the buffer amplifiers + pcb material have to be observed.

- How will the diode connected transistors perform?
  In the datasheet the only specification is for a collector current of around 100uA. Nothing is given for 5mA.
  Will the long term stability still be the same for a 50 fold current?

I have still not understood the current stabilisation. (There is no direct measurement of the zener current.) Only that the relation R2+R3 has to do something with it. I hope that there will be no thermal run away. And also hope that there is no current overshoot when switching on/off of the reference.

What function has the cirquit R1+R4+D1? Is it only to ensure start up of the circuit?

So I think you are right: there is much to learn + explore if leaving the usual path.

with best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: fmaimon on February 26, 2014, 11:39:03 pm
In AN45-p7, JW is using the delta-current method, which is great if you want to use any random NPN transistor as a sensor, and you only need +/-1C accuracy.  The problem is that *IF* you are going to use the JW LTZ circuit "as is", then you need to know what voltage to set the divider to, so that you can get the die temperature you want-- and this requires knowing the slope and offset of the Q2 temp-sensor transistor inside the LTZ.  My method will yield (typically) 0.25C accuracy (because the transistor response is not exactly linear).  Knowing the absolute value for Q2 Vbe at the die temperature you want, plus the actual Zener output voltage (after TempCo compensation) is the only way I know of accurately selecting the divider values.  As I wrote before, if you don't want to do all of this precision work for *each* LTZ you are going to use-- then just set the divider to 13K:1K, which will set the LTZ die temperature to 10C higher than it needs to be-- but will guarantee that the heater control circuit has enough "headroom" for proper control [up to 35C ambient] just in case the particular LTZ you are using is 10C lower than the average one.  If you do this, you will have to accept [for the average LTZ] about *double* the long-term drift rate than you otherwise would if you want to do the actual work.

You can still use JW circuit. Read the temperature and vbe with the heater turned off. Turn on the heater slightly, wait for the temperature to stabilize and read another data point. Now you have you two points to calibrate your slope.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Blackart on March 01, 2014, 12:10:44 am
Thanks DilligentMinds

The LTC2997 is interesting , sounds like a ideal way of conditioning the LTC1000s they are around $5 at farewell/element etc. The data sheet says the accuracy is based on the "Ideality Factor" of the junction being tested. They are designed for a generic 3904 NPN but sound like they would only be a few degrees C off, if the factor for the specific device is NOT known ( good enough for a burn in bench I would think ). I found a link here to measuring it http://www.bentongue.com/xtalset/16MeaDio/16MeaDio.html. (http://www.bentongue.com/xtalset/16MeaDio/16MeaDio.html.) Im not sure Ill go that far for burning in, but this would be better than my previous idea of just strapping a temp sensor to the LTC100 case. I might get a few while I await my refs to arrive.

Has anyone out there measured the "Ideality Factor" of the junctions in the LTC1000s already ?

Lucas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on March 01, 2014, 08:23:55 am
Hello Ken,

yes I have some questions.

The LT1013 has up to 2mV influence over a 50 degree temperature span on the output voltage of the LTZ1000.
Additional the LT1013 is no longer available in a hermetically case (at least from LT).
It would be interesting if Bob agrees that replacing the LT1013 by the LTC2057 (lower offset drift, lower ageing, higher gain) will improve performance of the cirquit.

Is there any "zero TC current" on the LTZ1000 as it can be observed with 1N829A Zeners?

The best chip of LT after the LM399 is the LT1027. When will it be sold in a hermetically package again? (or will I still have to use the AD586 chips?)

With best regards

Andreas




Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on March 01, 2014, 08:38:35 am
Hello Ken,

my questions to LT:

- do they think, that their TO package is hermetically tight, regarding oxygen and humidity, and did they fill it with an inert gas?
- What was the purpose of the A version: lower heating loss for application at higher temperatures (> 60°C) only?
- What do they know / have measured concerning drifts to possible thermally induced stress as torsion of the package, and possible strain effects on the solder junctions or the internal junctions. Is there any advantage of the A version over the non A version in this regard?
- Have they made some stability tests (over time) at lower temperatures, i.e between  40 an 60°C, as implied by Spreadbury and Pickering?
- Are the LTZ1000 and the LTFLU similar chips concerning Si-structure of the buried zener and transistor, so that they both have similar stability figures? (I know that's an indiscrete question)
- Is the LTFLU available from LT on demand?

THX Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 02, 2014, 07:01:20 pm

- do they think, that their TO package is hermetically tight, regarding oxygen and humidity, and did they fill it with an inert gas?



+1
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on March 02, 2014, 11:46:41 pm
Proposed question to Bob Dobkin:

LTC has hundreds of demo boards  http://www.linear.com/demo (http://www.linear.com/demo). Many of them use LT1236 and LTC6655.

I suggest a dedicated LTZ1000 10/5/2.5/1.0 V Voltage Reference Board. LTC has the chip and op amps as well as the LTC1043 and the LT5400. With a few components sourced elsewhere, ageing and calibration - this board will eliminate the need of finding very expensive boards from HP3458A.

Combined with an educational user manual / litterature reference / application note in the spirit of the late Jim Williams, this board will imo add great independent value to the metrology hobbyist / volt-nut community.

(Edit: LT1043 to LTC1043)
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on March 03, 2014, 04:33:45 am
Such board would be still few hundreds USD, as usually expensive part is aging and calibration, as it requires huge time investments.
And most of volt-nuts built their own boards for specific needs anyway.
Title: Re: Ultra Precision Reference LTZ1000
Post by: BravoV on March 03, 2014, 04:43:40 am
Ken, please try to invite Bob to join this forum, and show him this particular thread (> 600 posts now  :o), hopefully he will do that once he noticed the huge discussion only for this special chip alone.

Its worth a shot, imo.
Title: Re: Ultra Precision Reference LTZ1000
Post by: chickenHeadKnob on March 08, 2014, 10:20:30 pm

  Ken, thank you for doing this. I don't think I can add to  the list of questions generated by you and Dr. Frank, save for one area. What does LT recommend with respect to circuit board mounting methods and device characterization before and after. What I am getting at is that both LT and subsequent "major customers" are testing and aging devices in sockets (presumably) and then usually solder mounting with the exception of the recent Agilent 34461-lm399. What do they know that they are not telling us? Agilent might want to keep trade secrets but LT should be able to share their statistical knowledge. If a device receives a thermal shock from soldering what does it do to the long-term aging?

  I am contemplating using gas tight gold-plated sockets of my own design instead of solder simply because I don't know  what the resultant effects of  shock/reset will be, and I don't have the resources to buy large sample lots and perform  the necessary tests.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 09, 2014, 12:22:52 am

Most of the manufacturers that are using the LTZ1000A are building it onto a module, and then "burning-in" the whole module-- not just the LTZ. 

Do you know if the modules are powered during this burn-in time?
Title: Re: Ultra Precision Reference LTZ1000
Post by: fmaimon on March 09, 2014, 01:17:04 am
According to the "rumors"-- yes.  And their outputs are monitored as well.  The module for the 3458A has a cheap resistor on it that doesn't appear to be used for anything [R419, 2K67]-- not even on the main board that you plug the reference board into.  The resistor is connected from -15V to ground-- and nothing on the board uses -15V.  It's my theory that they are "watching" the drift of this cheap resistor [in the "burn-in" fixture] to know how far they have come in the burn-in process.  I could be wrong, but it's the only thing that makes sense.

Maybe this resistor is "nulling" the gnd pin current from the main board to the module. 15V @ 2K67 is about 5.6 mA, about the right value considering 5 mA for the  zener curent, considering that the heater goes to another power pin...
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on March 09, 2014, 09:49:14 am
Have you ever seen a board with solder stop mask that was driven to  temperatures up to 125°C or even 150°C over and over again for hours, weeks or months? Its color changes, the color of the solder joints, the board itself and several parts on the board too.
I've done a high temperature storage at 120°C on my LM399 reference board for more than 300h and the color has changed clearly. I  can't see such an evidence on the pictures of the reference boards or even the pcb material on the 3458A reference boards.
Whatever burn-in means for several manufactors, power the circuit up or power the circuit up driving a temperatur profil, most things discussed help keeping a myth alive.
I appreciate an offical publication that do away with all that fairy tales.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on March 09, 2014, 11:06:04 am

I've done a high temperature storage at 120°C on my LM399 reference board for more than 300h and the color has changed clearly. I  can't see such an evidence on the pictures of the reference boards or even the pcb material on the 3458A reference boards.
Whatever burn-in means for several manufactors, power the circuit up or power the circuit up driving a temperatur profil, most things discussed help keeping a myth alive.
I appreciate an offical publication that do away with all that fairy tales.

leaded solder gets soft from 105°C onwards, leadfree a little bit higher.
That softening (euthetic phase) will harm the solder junction.

Therefore, any storage or cycling of soldered PCBs above 110°C will definitely destroy the solder junctions, or at least deteriorate the reliability greatly.
It's possible only to make such  high temperature storage of non assembled components. Here, the max. die temperature of about 150°C is a limit.
Getting close to that temperature will also harm the silicon structure already, due to Arrhenius law.

Burn-In usually is done on components, which have a big drift rate intrinsically, and the purpose of the burn-in is to accelerate those drifts to a state, which otherwise would be reached only years later.

Another technique is to burn-in a completely assembled  PCB (at 90°C max.) to detect early failures and so to get more reliable PCBs, especially under rough conditions as spacecraft, military and automotive applications.
But latter goal is not ultimately required for volt references.
 
So I stick to my opinion, that a real burn-in on ultra precision components, i.e. on components which have low drift rates by design already, will do more harm than really improve the stability further.
 Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on March 09, 2014, 11:13:47 am

The highest performance DMM's on the market today are the HP/Agilent/Keysight 3458A and the Fluke 8508A.  They both use the LTZ1000A, and they use "reference modules" that are burned-in separately from the rest of the instrument.  They are *not* doing this "because it seemed to be the right idea"-- they are doing it because they have spent a ton of money and manpower on this problem, and they both reached the same conclusion.



Hi Ken,

what do you know about the interior of the Fluke 8508A?

I also assumed, this instrument is very similar to the Datron 1281 (very similar manual), an dwould have the same LTZ1000 reference module(s) as in the Datron 4910, but quarks has sent me a photo indicating, that the 8508A is very differently built.
The reference has a resistor ceramic hybrid on it, which resembles more the Fluke 732B assembly inside the oven part.

I'm still collecting all reference types (LTZ/A, SZA, LTFLU) of the instruments, and their different stabilization temperatures.

Fluke 8508A and the Keithley 2002 are still lacking.

PS: The engineers who designed the 3458A reference board have ignored many rules for getting a stable reference, as too high stabilization temperature, using the A version, and exposing the solder junctions to airflow. That's the only reason, why they are always struggling with the stability, and why they have to make such a big effort in selecting the references for 8 / 4 / 2 ppm/yr.

Thanks Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on March 09, 2014, 01:01:41 pm
Quote
I'm still collecting all reference types (LTZ/A, SZA, LTFLU) of the instruments, and their different stabilization temperatures.

By the way, LTFLU is currently available on ebay eBay auction: #111063925120.

Quote
Fluke 8508A and the Keithley 2002 are still lacking.

The pictures by TiN didn't help?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on March 09, 2014, 01:17:55 pm
Quote
I'm still collecting all reference types (LTZ/A, SZA, LTFLU) of the instruments, and their different stabilization temperatures.

The pictures by TiN didn't help?

Nope. Also not the BOM of the circuitry, which I've found on the Keithley site..
I've expected precision divider resistors in the range between 1k:12k and 1k:15k, but there are no resistor relations like this, neither multiple of those.

So I assume, Keithley uses a different scheme to set the temperature. perhaps some PWM or DAC, or so.
It would be useful, if somebody would determine the temperature by measuring the UBE voltage directly.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on March 09, 2014, 02:58:30 pm
I'll do all that later, when recovering schematics.
I just don't want to disassemble meter every time, for this or that, it's still likely in cal.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on March 09, 2014, 08:48:03 pm
Somewhat off topic but since the LTZ needs precision resistors too:

perhaps this fits to Franks results for the TC of the resistors.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: casinada on March 10, 2014, 04:37:52 pm
Is anybody familiar with this unit?
VALHALLA 2720GS
http://www.adret-electronique.fr/valhalla_2720/adret_valhalla_2720.html (http://www.adret-electronique.fr/valhalla_2720/adret_valhalla_2720.html)

It has 0.01 ppm and suppose to be able to calibrate 8 1/2 digit DMMs :)
10.0000000V :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on March 10, 2014, 04:56:11 pm
I am a big fan of Valhalla gear and saw this unit before.
But so far I have never seen a real one and only have seen very litte details. 
see att. Prices and HSR specs
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 10, 2014, 04:58:30 pm
"2 Year Spare Parts" ?

Does it require regular oil changes?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on March 10, 2014, 05:05:28 pm
Valhalla 2720GS = Solartron 7081 (PWM ADC) + Datron 4000A (8x 1N829 zeners and PWM DAC)  ;D

2720GS has a very old and noisy PWM type ADC for a such measurements. 0.01 ppm resolution is absolutely useless.
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on March 11, 2014, 12:34:21 pm
There is dead one on ebay for sale for a while for 1K.

Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on March 11, 2014, 07:44:15 pm
Freshly calibrated 34401A@work arrived from cal today, will measure my LTZ source soon :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on March 12, 2014, 05:41:58 am
Freshly calibrated 34401A@work arrived from cal today, will measure my LTZ source soon :)

Too late: you are already outside the 24hrs calibration window.
For the 90 days tolerances the accuracy is +/- 190uV for a 7V source.

And did you measure the 34401a before shipping?
Or how can you tell that the device did not drift during transport back from calibration?

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on March 12, 2014, 08:26:23 am
Hi Andreas,

its too late to use the 24h-window, yes.
But its not too late to do exactly what I am able to do, although it will be one more day I am afraid.

I have a second 34401A sitting here which was compared to the cal'd one right before shipping, should be able to give a hint from it how the difference between departure and (late) arrival is.

However, Agilent screwed up a bit this time, the meter was out for about one month, they requested more and more order confirmations totalling at 3 (upfront at InfoLine, on paper with signature in the box and after their 3rd request as email again), cal was on 6th and arrived here on 11th.

It only gives a bigger window of where the actual voltage might be. But I dont expect the LTZ to have wandered too far.

Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on March 12, 2014, 09:41:00 pm
Thanks for sharing all this information.

Quote
7) Note that for LM399-based designs, the slots in the PC board [plus a lot of insulation top and bottom] make sense-- Bob said that the less power the heater requires, then the more stable the output voltage will be.  So, in this case, the slots [plus insulation] are helping with this.  Oh-- and he also said that the LM399 should be run at about 1mA of Zener current for best stability.  The more stable you can make the Zener current, the more stable will be the output voltage.  He said that there is about 1uV of voltage change for 1uA of current change.

9) For the LM399 and both of the LTZ parts, he said a good burn-in routine would be to operate them at 125C [in an oven] for 2 weeks.  After that, you can cycle the power on and off 10 to 15 times at normal operating temperature, and this will get them to settle down.  This process should remove most of the initial drift that these devices exhibit.  [So, my initial guess for a burn-in cycle was pretty good-- there is nothing wrong with a burn-in 10 times longer than this.]


That confirms the way I realized my first LM399 reference, a slotted pcb design with a thermal encapsulation on both sides and a thermal burn-in of the board for more than 300h, but sure without powering the circuit, as I had no adequate cables at hand. The reference seems to be very stable and that makes me believe that the humidity depended drift I can observe is the drift of the 34401As reference itself, because all the mentioned actions are not realized in this DMM.

Once again, thanks for sharing.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Vgkid on March 12, 2014, 10:51:39 pm
Thnks for sharing Dilligentminds
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on March 12, 2014, 11:34:34 pm
Thank you very much to Bob Dobkin and to you DiligentMinds.com.

I have been working a lot with the SZA263-type AmpRef/RefAmp circuit, and have found that HP used the circuit/component already in 1963, while Fluke seems to use it first time around 1971. The circuits are so similar that it is imo unreasonable to argue for any originality in the Fluke 731A.

So may be it could be an idea for US based volt-nuts to contact Fluke and ask them to "free" the LTFLU-1.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 13, 2014, 12:02:36 am
Did you happen to get an address that we could send a thank you card to?
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on March 13, 2014, 12:54:25 am
 :-+ great information
Thanks a lot for sharing
Title: Re: Ultra Precision Reference LTZ1000
Post by: robrenz on March 13, 2014, 01:41:22 am
Thanks for the great information :-+

That gives me confidence to continue my practice of only turning on my 8846A when using it.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on March 13, 2014, 02:24:49 am
Thank you very much to Bob Dobkin and to you DiligentMinds.com.

I have been working a lot with the SZA263-type AmpRef/RefAmp circuit, and have found that HP used the circuit/component already in 1963, while Fluke seems to use it first time around 1971. The circuits are so similar that it is imo unreasonable to argue for any originality in the Fluke 731A.

So may be it could be an idea for US based volt-nuts to contact Fluke and ask them to "free" the LTFLU-1.

Good luck with that!  Fluke was purchased by the evil Danaher corporation (along with Keithley and Tektronix)-- they pretty much fire everyone over the age of 30 [thus losing the people that actually know *why* things were done in the past], then they cancel anything that isn't making huge profits, and then milk their "captive customers" for everything that they are worth.  I seriously doubt if they are going to budge even one inch in giving us access to *anything*.  But you can *try*... Let us know how that goes!

I sure will  >:D  Nothing to lose...

I do however not expect much. After seeing the 6-in-1 launch timer/clock running negative overtime far to long after count-down, I sent an email to Tek's Quality Dept. The timer was taken away a few hours later. But they didn't thank me - they are now repeatedly filling my email inbox with US located seats for some post-launch sessions.

But I have seen a video on YT with Fluke's Chief Corporate Metrologist Mr. Jeff C. Gust. If it is not all circus for the masses, I suppose he will at least answer my mail.

I'll be back in due time  :-BROKE
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 13, 2014, 02:45:38 am
Did you happen to get an address that we could send a thank you card to?

No, I did not, but you can probably send it to the main office in Milpitas, CA and it will get to him.

I'm going to send him a thank you card from the [Volt-Nuts].

I think it'd be nice if he could see the international cross section of people that have benefited from his advice. PM me if you want your name or handle in the signed-by section. Include what you want to be called and a city and country if you don't mind.

I'll post a picture of the card before I send it.
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on March 13, 2014, 04:59:16 am
Great, thanks!

Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 13, 2014, 05:39:19 am

No need for the private message-- you could find out who I am anyway just from the domain name registration for DiligentMinds.com ...


The judge said I have to stop cyber-stalking people ... its part of my parole.  ;)

Your name will be the first on the list.
Title: Re: Ultra Precision Reference LTZ1000
Post by: macfly on March 13, 2014, 11:27:15 am
@ DiligentMinds.com

Thanks a lot for sharing your informations  :-+
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on March 13, 2014, 08:09:51 pm
Quote
19) The Zener in the LTFLU-1 is the exact same one as is in the LTZ1000.

Really? So what is different to LMx99?
I expected the LTFLU is completely similar to the LMx99, a 4 pin TO package with heater and zener diode.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on March 13, 2014, 09:36:45 pm
Quote
19) The Zener in the LTFLU-1 is the exact same one as is in the LTZ1000.

Really? So what is different to LMx99?
I expected the LTFLU is completely similar to the LMx99, a 4 pin TO package with heater and zener diode.

The LTFLU-1 and the earlier SZA263 have no heater. They are zener (avalanche) diodes nominally 6.2 V accompanied with a transistor for temperature compensation and reference voltage amplification.

The pins are Collector, Base, Emitter and Anode (Kathode is common with Emitter).
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on March 13, 2014, 10:05:53 pm
Hello Ken,

thanks for sharing the information.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on March 13, 2014, 10:19:44 pm
Really? So what is different to LMx99?
I expected the LTFLU is completely similar to the LMx99, a 4 pin TO package with heater and zener diode.

Hello branadic,

a refamp comes usually as refamp-set with (perfectly) adjusted resistors.
See e.g. fluke 8840A handbook.
So a LTFLU without the adjusted resistor set will be only half of the truth.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on March 13, 2014, 10:40:35 pm
Quote
The LTFLU-1 and the earlier SZA263 have no heater. They are zener (avalanche) diodes nominally 6.2 V accompanied with a transistor for temperature compensation and reference voltage amplification.

The pins are Collector, Base, Emitter and Anode (Kathode is common with Emitter).

Thanks for that, couldn't find such info yet and there is nothing mentioned about LTFLU in the "Current  Sources & Voltage References" by Linden T. Harrison.

Quote
Hello branadic,

a refamp comes usually as refamp-set with (perfectly) adjusted resistors.
See e.g. fluke 8840A handbook.
So a LTFLU without the adjusted resistor set will be only half of the truth.

Thanks, so it's no good idea to go for LTFLU unless you want to spend much time and money for the perfect resistors.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on March 13, 2014, 10:52:37 pm
Really? So what is different to LMx99?
I expected the LTFLU is completely similar to the LMx99, a 4 pin TO package with heater and zener diode.

Hello branadic,

a refamp comes usually as refamp-set with (perfectly) adjusted resistors.
See e.g. fluke 8840A handbook.
So a LTFLU without the adjusted resistor set will be only half of the truth.

With best regards

Andreas

I doubt that one single 'refamp'-type IC has been sold from any chip maker with any resistors. It is the instrument makers that sell matched sets in order to keep their boxes within tempo spec after repair. This is nothing special for the refamp. Even the EDC/Krohn Hite volt boxes (there is an EEVBlog video showing the reference) with a simple 10 USD 1N829A requires a very precise current setting.

There is no magic here. Anyone with good enough instruments to detect changes in net tempco for small changes in current can select resistor(s).

This technology is old and only based on the appr. +- 2 mV tempco of the avalanche diode and the PN-junction respectively. It was used by Hewlett Packard with the Motorola part SZA263 long before Fluke or Linear Technology came into the picture.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on March 14, 2014, 05:38:12 am

There is no magic here. Anyone with good enough instruments to detect changes in net tempco for small changes in current can select resistor(s).


According to 8840A handbook there are 3 resistors in the ref amp set.
It seems that 2 resistors adjust the output voltage.
So the other resistor seems to adjust the "Zero TC" current.
Further there are the magical -50mV in the circuit diagram.

Since I cannot imagine that all the resistors do not influence each other:
Does anybody know a "adjustment specification" for the resistors with minimal effort for trimming?

With best regards

Andreas
 
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on March 14, 2014, 07:10:06 am
Hello fellow forum users,

I just finished with a telephone conversation with Bob Dobkin, and he was short on time, so I was not able to get all of the answers to every question I [and we] had, but here are the ones I was able to get:


3) The LTZ1000A has a different die attach than the LTZ1000, but they are both isolated quite a bit from external lead stress.  Both of them will respond a small amount to lead stress, but the 'A' version if far less susceptible to this.  Bob said that this can show up as a drift in output voltage with changes in barometric pressure.

8: All voltage references *do* age faster at higher temperatures.  So, since the LTZ1000 can be run about 10-deg-C lower than the 'A' version, it will have lower [about 1/2] long-term drift than the LTZ1000A under otherwise identical conditions.  [But, the 'A' version has *other* differences that are positive-- my words.  There are other sources of drift that the 'A' version is less susceptible to-- barometric pressure changes is one of them].

9) For the LM399 and both of the LTZ parts, he said a good burn-in routine would be to operate them at 125C [in an oven] for 2 weeks.  After that, you can cycle the power on and off 10 to 15 times at normal operating temperature, and this will get them to settle down.  This process should remove most of the initial drift that these devices exhibit.  [So, my initial guess for a burn-in cycle was pretty good-- there is nothing wrong with a burn-in 10 times longer than this.]

15) The TO packages are filled with *DRY AIR*-- not even nitrogen-only [so you would expect some degradation from this-- my words]-- but, Bob says that they don't have any parts that are degraded by this.

16) The LTZ1000 will use a lot more power than the LTZ1000A, so for battery operated circuits, it's best to go with the LTZ1000A, and insulate it to save power.

17) There is probably not a scenario where you can get zero-TC out of the LTZ-- he said you probably would have to operate Q1 at around 1uA, and that is not practical.

18) He said that it is not necessary to use the LTC2057, as the drift and noise of the Zener contribute far more than the LT1013.  Bob said that you could use almost any precision op-amp, and the drift would not be affected by much even by the less precise amps.  [I'm still going to use the LTC2057 anyway-- they are as cheap as popcorn, and have very little noise.  Since I *do* need a zero-drift amp in the follow-on boost circuitry, and it's more economical to keep the number of unique parts on the BOM as low as possible.]

19) The Zener in the LTFLU-1 is the exact same one as is in the LTZ1000.  He said that even though the exclusivity portion of the contract with Fluke is no longer in effect, they are not going sell these to the public because they don't want to anger a customer that buys millions of dollars in other parts from them.  Even if they did want to sell it to others, it has not been characterized and there is no data sheet-- so there would be a lot of work just to build that.  It's just not going to happen...

Regards,
Ken

Hello Ken,

thanks that you made the interview, sharing with us.

Actually, there were several myths being busted around the LTZ1000.

Concerning "Burn-In", he missed the point of strong hysteresis after a 125°C trip, which will definitely NOT be removed by simply switching off 10-15 times.

I feel confirmed, to keep the LTZ1000 PCB as simple as possible, at 45°C, with good thermal management/insulation.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on March 14, 2014, 07:45:41 am
I hope that this is not too far off-topic, but [I *think*] Hewlett-Packard make a model 735A "Transfer Standard"; and the Fluke 731B looks an awful lot like it.  What happened there-- did HP sell it's 735A technology to Fluke, or did Fluke make a "copycat" version of it?  Anybody know?

Sorry, but the HP735As schematic  does not ressemble the Fluke 731A at all. At least in the manuals, which are hosted on agilent site.

In the 735A, an ovenized, simple zener diode is used, no sign of a RefAmp device.

Anybody (Quantumvolt??) who can refer correctly to a HP device with a RefAmp, from the 60ties?

Did not find that, not in the 745A, and not in the 740A. All were based on a simple zener diode.


I've read ("Hewlett Packard, the early years") that  Packard and Fluke were close friends. They worked for GE and later both  joined Navy .

Later they made an arrangement to not interfere on certain businesses, so that Fluke made all those analog standards, but kept out of building DMMs and counters, I think. 

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on March 14, 2014, 09:21:34 am
I have also read stories about the persons behind HP & Fluke, but I do not know enough to comment on that.


The HP735A contains an ovenized 1N829A. Very high tech for 1966. And documentation in a class of its own:

http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1966-03.pdf (http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1966-03.pdf)
 
http://www.hpmemory.org/an/pdf/an_70.pdf (http://www.hpmemory.org/an/pdf/an_70.pdf)


The HP3440A is described in 1963:

http://www.hpmemory.org/timeline/dave_cochran/hpj_nov63.htm (http://www.hpmemory.org/timeline/dave_cochran/hpj_nov63.htm).

The manual I have found on the web contains the "refamp" - which as HP part was called "Amp., Ref.". This edition contains pages that are revised in May 1970:

http://www.pa.msu.edu/~edmunds/DVM_HP_3440a/hp_3440a_dvm_manual.pdf (http://www.pa.msu.edu/~edmunds/DVM_HP_3440a/hp_3440a_dvm_manual.pdf)


The HP3050A has a reference supply that is very similar to Fluke 731A, except that it uses a 'discrete' op amp. This manual also contains pages that are revised in May 1970. If I am not wrong, the Fluke 731A is from 1971.

http://cp.literature.agilent.com/litweb/pdf/03450-90007.pdf (http://cp.literature.agilent.com/litweb/pdf/03450-90007.pdf)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on March 14, 2014, 10:38:11 am
Thanks for the hints!

The HP3420A (1971) also uses a RefAmp. Origin not determinable.

The HP3440A uses a 4JX19A519 from G.E.


Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on March 14, 2014, 11:36:22 am

There is no magic here. Anyone with good enough instruments to detect changes in net tempco for small changes in current can select resistor(s).


According to 8840A handbook there are 3 resistors in the ref amp set.
It seems that 2 resistors adjust the output voltage.
So the other resistor seems to adjust the "Zero TC" current.
Further there are the magical -50mV in the circuit diagram.

Since I cannot imagine that all the resistors do not influence each other:
Does anybody know a "adjustment specification" for the resistors with minimal effort for trimming?

With best regards

Andreas

I am referring to http://assets.fluke.com/manuals/8840A___imeng0300.pdf (http://assets.fluke.com/manuals/8840A___imeng0300.pdf) Figure 5-6 p. 5.8.

Assume U702A is ideal. Then both inputs are at 0 volt. Hence the collector of U701 is at 0 volt. To avoid possible reverse bias C-B the base of U702 is biased at -0.05 volt (which gives 6.95 volt headroom for the REFAMP zener appr. 6.5 volt + Vbe appr. 0.45 volt).

Assuming first that the bias current of U702 U701 can be ignored, the bias divider Z701 is fixed in resistor ratio. Choose resistors that give a reasonable current (a few hundred uA or 1 mA or ? - it doesn't matter at the first try) and give appr. the wanted -0.05 V and -7 V.

Now adjust R701 (it will imo only affect bias for U702A) for minimum tempco in the -7 volt.

If TP701 has changed away from -7 volt, adjust U701 base voltage by changing the Z701 ratio marginally, and repeat. If necessary, choose a different Z701 current and start over.

---

I might be totally wrong  :-\. But I could always ask on the forum for resistor values from a real box and start from there ...
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on March 14, 2014, 04:50:03 pm
It would have been helpful to ask Bob what is his opinion... does really make sense to preselect references before solder and age them? Would have been interesting if he could have shared any experience to that.
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on March 16, 2014, 06:53:24 am
Quote
For the LM399, I'm guessing the soldering process doesn't hurt it as much as other references, because Fluke has the most stable 6.5-digit DMM, and they solder the LM399 *after* burn-in...

Thanks Ken for your response, but I doubt that the soldering process is without influence. At least one pin of the TO package is directly connected with the case and will heat up the die attach while soldering.
Title: Re: Ultra Precision Reference LTZ1000
Post by: macfly on March 16, 2014, 09:57:57 am
[Clever!]

Yes, it is!!

Many manufacturers offer a "super grade" version of their voltage references with guaranted initial tolerance and temperature drift. They are measured on 100% of chips. But how?

There's sometimes an advised "TEMP" output on voltage references (With a kind of linear dependance with temperature). This internal circuit is used to test chips after packaging (Internal die temperature is monitored through "TEMP" pin, and the chip is heated i.e. by injecting current through an ESD protection diode. An external control system can provide precise temperature regulation).

In this paper, such a system is diverted from its initial use to provide nearly 0ppm/K temperature drift, with a common plastic-case voltage reference chip :

http://electronicdesign.com/analog/internal-oven-provides-voltage-reference-less-1-ppmdegc-drift (http://electronicdesign.com/analog/internal-oven-provides-voltage-reference-less-1-ppmdegc-drift)

How does this circuit work  :-// ? IMO is could NOT!

1. There is only a positive supply voltage, so the output voltage of the opamp's can only be positive.
2. IC2B is connected as inverting amplifier. It gets a positive voltage from pin 1 of the reference, buffered by IC2A.
3. This should result in a negative output voltage of IC2B, but it could not, due to the lack of negative supply voltage.

Or I am a completely wrong ?

Regards

macfly



Title: Re: Ultra Precision Reference LTZ1000
Post by: fmaimon on March 16, 2014, 11:04:06 am
How does this circuit work  :-// ? IMO is could NOT!

1. There is only a positive supply voltage, so the output voltage of the opamp's can only be positive.
2. IC2B is connected as inverting amplifier. It gets a positive voltage from pin 1 of the reference, buffered by IC2A.
3. This should result in a negative output voltage of IC2B, but it could not, due to the lack of negative supply voltage.

Or I am a completely wrong ?

Although it only have one supply, the voltage reference IC is being fed by a 5V zener, so the applied 12V, through the 10 ohm resistor is always going to foward bias the internal esd diodes.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on March 16, 2014, 12:09:15 pm
The comparator is referenced to VREF = 2.5 V. Nothing goes negative relative to the potential at the GND level.

The sensor voltage at pin 1 falls as the chip cools. When it is 0.53 volt LOWER than VREF (which means the chip has cooled below 75 degrees), the buffer/inverter will have converted the marginal fall to a rise on the noninverting comparator input which then goes positive relative to VREF, and a new heating impulse is initiated.

Since VREF is the only voltage stable enough to use for comparison - the main task for this circuit is to convert the passing of the sensor pin below VREF-0.53 volt to a trig signal for the VREF-referenced comparator. In effect it is an AC-signal at a few hundred hertz.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on March 16, 2014, 12:27:15 pm
Quote
For the LM399, I'm guessing the soldering process doesn't hurt it as much as other references, because Fluke has the most stable 6.5-digit DMM, and they solder the LM399 *after* burn-in...

Thanks Ken for your response, but I doubt that the soldering process is without influence. At least one pin of the TO package is directly connected with the case and will heat up the die attach while soldering.

The pictures of HP34401A reference that I find in web show a LM399 within a socket (horror).   :o |O
I do not know if this is a unmodified unit but with my bad experiences of socket related voltage shifts I would not do that.

(http://www.mikrocontroller.net/attachment/91696/20101105205754.jpg)

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: rf-design on March 16, 2014, 04:10:30 pm
4) It's been 20 years, so he didn't know the exact numbers, but Q1 is only rated to a few hundred micro-amps of collector current.  [This means my single-resistor idea will *not* work! -- back to the original circuit from the ap-note!]  Q2 will have a max rating of around 1mA, and the Zener can take quite a bit more.

5) They *could* make a model for LTspice, but the temperature effects are very difficult to simulate, and the market need is not large enough to support the development effort [my words].  He said the best way is to build the circuit and test it out in real life.

17) There is probably not a scenario where you can get zero-TC out of the LTZ-- he said you probably would have to operate Q1 at around 1uA, and that is not practical.

Refering to the chip photo posted early in this thread I could see 6 heater ring resistors, 4 equal sized diffusion isolated npns, 2 equal sized diffusion resistors and 2 possible lateral pnps or diodes. All in a very old single metal bipolar process. Is this one still in production? Or was it all time before shutting down the process?

Did anyone check the corrospondence between the schematic in the appnote and the circuit seen in the photo?

Based on the layout dimensions operating current ratios of mA to uA for optimum temperature stabilisation seams unlikely. The main target for the circuit operation would be the die temperature. For better understanding it would be very helpful to have a full schematic for modelling purposes which must include thermal coupling of the devices. Did anyone tried such a model?

One aspect of the thermal regulator is that if the interlayer between the die bottom and package is not even the thermal surface plot would get a gradient which is not foreseen in the bipolar layout placement and configuration. To be immune it should circular interdigitized which is not. Further the 4 bipolars are not crosscoupled. So a thermal conductivity gradient in the axis connecting the two electrical groups would give also a gradient between the bipolars vbe. So I see this as a design flaw. But back to the model that could be integrated in thermal coupling resistors as a mounting imperfection.

If the temperature cycle amplitude is less than 20K the hysteris will be closed. So there is no aging but another unidentified process. Is there any idea?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on March 17, 2014, 07:05:22 am
4 equal sized diffusion isolated npns, 2 equal sized diffusion resistors and 2 possible lateral pnps or diodes.

If the temperature cycle amplitude is less than 20K the hysteris will be closed. So there is no aging but another unidentified process. Is there any idea?

Hello,

https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg279145/#msg279145 (https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg279145/#msg279145)

Sorry I see 4 large transistors (horizontal and vertical) connected all to each other building Q1.
And 4 smaller transistors (45 degrees interleaved) building Q2.
The emitters on the outside of the transistors are all connected together.

Hysteresis is usually related to the package of the device.
I.e. the die attach consisting usually of silver filled epoxy resin.
(and perhaps hollow glass filled (low density) resin for the A-device)

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: macfly on March 17, 2014, 11:30:09 am
The comparator is referenced to VREF = 2.5 V. Nothing goes negative relative to the potential at the GND level.

No!
I refer to figure 1 of the article.
The complete OP amp's supply are referenced to ground. The supply of the reference also.
The voltage on R1C can only be positive above ground. But a positive voltage feed
into RN2A (buffered by IC2A) would  (with a negative supply voltage for the OP) result in a negative output voltage of IC2B.
But without negative supply, the output voltage will always stay near zero volts or an erroneous phase-reversal occurs.
Title: Re: Ultra Precision Reference LTZ1000
Post by: muvideo on March 17, 2014, 12:42:56 pm
The comparator is referenced to VREF = 2.5 V. Nothing goes negative relative to the potential at the GND level.

No!
I refer to figure 1 of the article.
The complete OP amp's supply are referenced to ground. The supply of the reference also.
The voltage on R1C can only be positive above ground. But a positive voltage feed
into RN2A (buffered by IC2A) would  (with a negative supply voltage for the OP) result in a negative output voltage of IC2B.
But without negative supply, the output voltage will always stay near zero volts or an erroneous phase-reversal occurs.

From a rapid eye to the circuit, I'm with you on this.
If I understand correctly when the die cools down the
voltage at IC1 pin 1 goes up, in order to begin a new
heating cycle IC2C pin 10 must go above pin 9.
So IC2A/B must buffer and amplify the signal in
non-inverting configuration, so I dont understand IC2A/B
in this circuit. Also IC2D could not work as intended if
opamps input offset voltage is on the "wrong" side  :-//
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on March 17, 2014, 01:45:40 pm
Well ... I guess the schematic needs some adjustments.

The last line in the article reads: "Ensure that the ?5-V supply is on whenever the 12-V supply is on. With the ?5-V supply off and the 12-V supply on, the controller loop applies the heat current continuously."

I am sure the circuit (which is nothing but a meta-thermostat) will work after debugging...
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 18, 2014, 01:09:46 am

I just finished with a telephone conversation with Bob Dobkin, and he was short on time, so I was not able to get all of the answers to every question I [and we] had, but here are the ones I was able to get:


Did Bob give you any indication as to the spread of drifts of the LTZ (or 399) after burn-in? Or the percentage that are rejected even before they are sold to someone like HP?

 
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 18, 2014, 04:27:22 pm
No one wanted their name on the thank you card so I just ended up writing a simple note on some plain 'Thank You' stationary:



Dear Bob,

Thank you for taking time out of your schedule to answer our questions on the LTZ1000. That level of support from you and Linear means a lot to us!

Sincerely,
The [Volt-Nuts]


Went out in today's mail.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on March 18, 2014, 10:49:53 pm
Did Bob give you any indication as to the spread of drifts of the LTZ (or 399) after burn-in? Or the percentage that are rejected even before they are sold to someone like HP?

The more interesting thing would be how the criteria for selections are.
For a instrument with 35ppm/year the drift of the reference should be below 18ppm (the other ppm for resistor + tempco).
To guarantee (3 sigma) that the measured drift of the reference alone should be even below 6 ppm/year or 2ppm/kHr (assuming sqrt (9khrs) = a factor of 3 difference between 1kHr and 1year).

From my measurements about 50% of LM399 are below 1-2ppm/kHr after 2000 hours ageing under power (15 hrs on/7 hrs off per day) without previous burn in.
Now after 4000 hrs further devices seem to stabilize below this limit.

But probably you will have further tests (0.1Hz - 10Hz) noise or even lower frequency noise to filter out the bad references.

I do not know if it is a really good idea to do a burn in at 125 degrees C. Since the heater is off above 90 degrees the aging mechanism will probably change. And changes to the heater will create relative large changes to the output voltage. E.g. changing the heater voltage supply will create several ppm/V output change especially around low (10V) heater voltage.
Although the temperature and so the heater power are regulated on chip.
Of cause it could be that all aging is mainly not related to the chip itself but related to the die attach (epoxy) between chip and package. In this case a burn in at higher temperatures would  make sense.

Good aged LM399 based instruments have about 1-2 ppm drift per year.

with best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: rf-design on March 19, 2014, 10:48:21 am
Did Bob give you any indication as to the spread of drifts of the LTZ (or 399) after burn-in? Or the percentage that are rejected even before they are sold to someone like HP?

The more interesting thing would be how the criteria for selections are.
For a instrument with 35ppm/year the drift of the reference should be below 18ppm (the other ppm for resistor + tempco).
To guarantee (3 sigma) that the measured drift of the reference alone should be even below 6 ppm/year or 2ppm/kHr (assuming sqrt (9khrs) = a factor of 3 difference between 1kHr and 1year).

From my measurements about 50% of LM399 are below 1-2ppm/kHr after 2000 hours ageing under power (15 hrs on/7 hrs off per day) without previous burn in.
Now after 4000 hrs further devices seem to stabilize below this limit.

But probably you will have further tests (0.1Hz - 10Hz) noise or even lower frequency noise to filter out the bad references.

I do not know if it is a really good idea to do a burn in at 125 degrees C. Since the heater is off above 90 degrees the aging mechanism will probably change. And changes to the heater will create relative large changes to the output voltage. E.g. changing the heater voltage supply will create several ppm/V output change especially around low (10V) heater voltage.
Although the temperature and so the heater power are regulated on chip.
Of cause it could be that all aging is mainly not related to the chip itself but related to the die attach (epoxy) between chip and package. In this case a burn in at higher temperatures would  make sense.

Good aged LM399 based instruments have about 1-2 ppm drift per year.

with best regards

Andreas

(http://www.rf-design.de/project/usref/lm399_schematic.jpg)

The LM399 shows two possible entrys for a temperature change related mechanical stress effect. The first and most reasonable is that the regulation temperature change with mechanical stress. The second would be that the buried zener voltage change because of the stress. The second argument is not reported up to now but for the first argument there is much material.

(http://www.rf-design.de/project/usref/mechanical_stress_vbe_shift.jpg)

from:

Package Shift in Plastic-
Packaged Bandgap References
Vishal Gupta
Prof. Gabriel A. Rincón-Mora

So the hysteresis effect is reported by LT as be a square law dependence on the temperature amplitude.

(http://www.rf-design.de/project/usref/ltc6655_note9.jpg)

To my understanding the aging as well as the temperature hysteresis depend on the mechanical stress change of the die attach epoxy over temperature and time. If the temperature change for instance at warmup or at the start of the burn in mechanical stress will built up but will realease after some time to lower level. This effect will repeat but with memory from previous cycles.

(http://www.rf-design.de/project/usref/zymet_zero_stress.jpg)

Zymet Zero-Stress Adhesive

There are alternates for bonding. Interesting is the reduced stress at higher temperatures for silver-filled expoxy. But the zero-stress point is above 150°C.

So the best burn in will be at target operation. A burn in at higher temperature will possible leave a greater stress at lower operating temperature. The effect will either change the regulation temperature or direct the bandgap voltage. So it will impact the obsolete LTZ1000 and LM399 buried zener as well as newer bandgap based designs.

Does anyone know about a IC level temperature regulated bandgap?

Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on March 19, 2014, 09:47:44 pm
Does anyone know about a IC level temperature regulated bandgap?

The LT1019 has a 400 ohms heater on board. (Pin7 to GND).
The info is deleted from newer data sheets. But the heater is still there. (see AN42 p 15 figure 66)
The disadvantage is that the heater current has no separate GND return path, so it affects the output voltage.

And unfortunately the metal can case is obsolete.

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: rf-design on March 20, 2014, 01:46:36 am
Does anyone know about a IC level temperature regulated bandgap?

The LT1019 has a 400 ohms heater on board. (Pin7 to GND).
The info is deleted from newer data sheets. But the heater is still there. (see AN42 p 15 figure 66)
The disadvantage is that the heater current has no separate GND return path, so it affects the output voltage.

And unfortunately the metal can case is obsolete.

With best regards

Andreas

I guess that the heater is for curvature trimming on wafer w/o complete wafer chuck heating. It is possible be faster or simpler. I think it is not for normal operation because otherwise there should be some temperature control. There are 3 DNC pins which are missing in the schematic. Do you know if the third DNC is connected somewhere?

BR

Reiner

"The LT®1019 is a third generation bandgap voltage reference
utilizing thin film technology and a greatly improved
curvature correction technique. Wafer level trimming of
both reference and output voltage combines to produce
very low TC and tight initial output voltage tolerance."
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on March 22, 2014, 08:54:08 am
Somewhat off topic, but does anyone know what kind of oil is used in Vishay resistors VHP100, VHP101, VHP102 & VHP103 series (Ultra High Precision Hermetically Sealed Resistor with Almost Zero TCR) or similar products with hermetical sealing?

I know that in some pressure sensor application silicone oil is filled in, but silicone is unwelcome.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on March 22, 2014, 11:18:33 am
The discussion on Volt-Nuts speaks of "white mineral oil" (purified oil) similar to "baby oil" (without flavor)

with best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on March 22, 2014, 01:49:52 pm
Quote
The discussion on Volt-Nuts speaks of "white mineral oil" (purified oil) similar to "baby oil" (without flavor)

Thanks, I found that pure mineral oil is available in drugstores, I will give that a try.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 22, 2014, 04:18:56 pm
Since we've been talking about burn-in methods, I thought some of you might enjoy this paper I'm reading now:

http://link.springer.com/article/10.1007%2FBF02504039 (http://link.springer.com/article/10.1007%2FBF02504039)

It talks about Datron's burn-in method for its 4000 calibrator and a proposed 'accelerated' method to get the job done more quickly using "soft thermal shocks".


edit: I'm realizing now that many of you might not have access. PM me if you want the paper.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 22, 2014, 04:54:09 pm
Does anyone have experience with this kind of 'oil': http://en.wikipedia.org/wiki/Krytox (http://en.wikipedia.org/wiki/Krytox)

In some of my reading I've come across the use of this stuff in precision references as a thermal transfer fluid. Its Fluorocarbon nature made me wonder if it was oxygen or water vapor resistant?
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on March 22, 2014, 07:31:21 pm
As I had trouble with the spoiling of plant oil and mineral oil (you smell it..) i went at work to a silicon oil sold in small quantities as modelcar differential oil. It is in a makeshift thermal bath (not cal lab grade). Have a good feeling about that stuff. Vacuum and heat extraction sounds resonable.

Title: Re: Ultra Precision Reference LTZ1000
Post by: SArepairman on March 22, 2014, 08:05:51 pm
Somewhat off topic, but does anyone know what kind of oil is used in Vishay resistors VHP100, VHP101, VHP102 & VHP103 series (Ultra High Precision Hermetically Sealed Resistor with Almost Zero TCR) or similar products with hermetical sealing?

I know that in some pressure sensor application silicone oil is filled in, but silicone is unwelcome.

Vishay Precision Group uses silicone oil when filling their hermetic packages with oil.  How do I know?  I asked them.

Mineral oil breaks down over time and becomes acidic.  I would not use mineral oil for a permanently sealed package.  OTHO-- silicone oil will dissolve silicone rubber compounds, so it may not be compatible, and you may be forced to use mineral oil-- just make sure you leave a way to exchange it.

All of these oils absorb both water and gases.  If you don't want these in the oil, you have to extract them first.  Silicone oil absorbs less than mineral oil, but this may still need addressing, depending on your application.

Vishay Precision Group uses a proprietary method of extracting water and gases from the silicone oil before sealing it in a hermetic package.  [Probably a combination of heat and vacuum].

they may also have more elaborate gimics for gas separation, I imagine that making it "rain" oil in a vacuum environment would help degassing, essentially you want to increase the surface area of what you are degassing. Or maybe just a flat dish..
Title: Re: Ultra Precision Reference LTZ1000
Post by: Blackart on March 24, 2014, 01:06:44 am
Hi Guys Ive been away for a while, just caught up with the thread.

Thanks Dillegent minds for organising the chat with Bob Dobkin... great stuff, to hear from a legend in the field. I contacted Bob Pease a few years ago as he mentioned he still used HP 3400As for noise measurement. I was having trouble with mine as the chopper neons were almost dead. He replied immediately and said he'd discuss it with Jim Williams. A few weeks later I got several hand written notes from Jim ( on graph paper ) outlining mods etc to the 3400A. Its terrific that even blokes at there level can take the time to disseminate their knowledge down the chain to us.

I was interested in the thermal fluid discussion as my refs have arrived ( pron pics attached ) so Im onto the design of the burn in rig. I got some nude vishay foils which I thought I might glue to each other for thermal tracking so maybe a oil bath would also improve matters. I downloaded the MSDS for the Krytox and was a little concerned about the "flu like symptoms" which result from burning it - What happens if the ref developed a shorts etc and fills the room with some horrible toxic smoke film etc. Other alternatives Silicone and white mineral oil seem much better in this respect with Silicon having a hight flash point than the mineral oil. I found the MDDS for the mineral oil on the Vishay site incidentally labelled MM ( the ones that make strain gauges etc ). So I might play with some designs using the oils.

Interesting to hear Bobs idea on the LM399s as I got 10 of them and was planning something like the Pease app note. It will be interesting to compare with the LTZ1000 design. Im way behind a lot of you blokes but plan to get the refs all burning soon. So I can get on with the design of the final units.

Lucas

attachments : Pics of Refs as packed from Linear tech - good and easy to deal with, took about 4 weeks to arrive in Oz.
 
Title: Re: Ultra Precision Reference LTZ1000
Post by: robrenz on March 24, 2014, 01:37:50 am
... I contacted Bob Pease a few years ago as he mentioned he still used HP 3400As for noise measurement. I was having trouble with mine as the chopper neons were almost dead. He replied immediately and said he'd discuss it with Jim Williams. A few weeks later I got several hand written notes from Jim ( on graph paper ) outlining mods etc to the 3400A. Its terrific that even blokes at there level can take the time to disseminate their knowledge down the chain to us.

Hello blackart, any chance of sharing that information with the forum?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Blackart on March 24, 2014, 08:09:01 am
Robrenz

Sure, I was originally planning on putting it all on a webpage anyway along with other 3400A stuff I've learnt while keeping mine going. However, I had to move house and the mentioned letters from Jim and Bob are still safely stored for upload. Unfortunately both of them died before I could ask them re permission but I would assume its ok now. Ill post to the eevblog forums under another more suitable thread. ( but it may take a while ). PM me if you want my recollections of them sooner. ( I've got 9 out of which 3 are in spec, the rest we sacrificed for the 3 )

Lucas
Title: Re: Ultra Precision Reference LTZ1000
Post by: robrenz on March 24, 2014, 11:29:24 am
Thanks Blackart,  I am in no hurry I don't even have a 3400 yet. I do have a 3410 that I am going to restore.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 24, 2014, 03:18:07 pm

Unfortunately both of them died before I could ask them re permission but I would assume its ok now.


I think you'd be doing the world a service by posting it.

Which reminds me... does anyone know if they donated their "professional papers" to a library or other organization? (e.g. http://spec.lib.vt.edu/scitech/ (http://spec.lib.vt.edu/scitech/) )
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on March 25, 2014, 10:43:42 am
As I said I would do in an earlier post, I contacted Fluke with a letter to Mr. Jeff Gust, Chief Corporate Metrologist, Fluke Calibration.

I got a very kind and helpful answer, and it seems that the Fluke management probably is not willing to actively do anything to facilitate the LTFLU-1 being a product for unrestricted sale. I am not willing to reproduce the email here due to privacy reasons. If anyone wants to read the full email, I will have to seek permission first. But the general information given below is really all there is to know.

Mr. Gust gave the advice to work with LTZ1000 and to pick up used LTFLU-1 devices. He wrote "... there is very little difference between the LTZ1000 and the LTFLU-1.", which is more or less the same as Mr. Dobkin from LT said. Furthermore - and this is all he said:"What makes the LTFLU-1 special is what we do to the product after we purchase it from LT."

Everyone's guess ... I guess. Whatever it is ... - a big "Thank You" to Mr. Gust for a swift and kind answer.
Title: Re: Ultra Precision Reference LTZ1000
Post by: bdivi on March 26, 2014, 11:27:13 am
... there is very little difference between the LTZ1000 and the LTFLU-1.

I cannot understand how LTFLU-1 is similar to LTZ1000.

LTFLU-1 is a 4 legged design which is thought to be a remake of SZA263 from the 70s. These are often specified as Reference Amplifiers and are not ovenized - just a zenner and compensation emitter junction.

Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on March 26, 2014, 11:56:36 am
I do not want to bother Mr. Gust anymore. He was just about to leave for a trip abroad.

I just sent an email to the "Contact us" address on Fluke's website marked "Attention Mr. Jeff Gust, Chief Corporate Metrologist". Anyone can send their questions this way. But as everyone knows - Fluke is not LT ...

To the post over there is nothing to say but: The LTZ1000 is just a RefAmp (even though the configuration of the 6.2 V diode and the tempcompensating transistor is turned "upside down"). But instead of putting the RefAmp in an oven with a thermistor, LT put a tempsensor transistor and a heater element in the same can as the RefAmp.

If the LTZ1000 was in any way far superior - no Fluke box would ever be used to check/calibrate any HP3458A ...

Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 26, 2014, 01:32:04 pm
If the LTZ1000 was in any way far superior - no Fluke box would ever be used to check/calibrate any HP3458A ...

All beers are superior to Bud Lite. But still people drink it. 
Title: Re: Ultra Precision Reference LTZ1000
Post by: Rigby on March 26, 2014, 01:57:21 pm
All beers are superior to Bud Lite. But still people drink it.

I've seen people get their asses whipped and hospitalized because they WEREN'T drinking Bud Light.  The very one-sided fight was preceded immediately by "Check out that fag and his fag beer!"

Bud Light people could fall off the Earth and I think the world would be at least twice as awesome as it is now.

edit: speeling
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 26, 2014, 02:01:26 pm
Oh-- and he also said that the LM399 should be run at about 1mA of Zener current for best stability.  The more stable you can make the Zener current, the more stable will be the output voltage.  He said that there is about 1uV of voltage change for 1uA of current change.


This is something that's been on my mind since you had your conversation with Bob. We talk a lot on this forum about the zener side of the circuit, but I haven't read a great deal about the current sources for these devices.

I picked up a copy of "Current Sources and Voltage References" (a pdf of which can be found though an easy google search). I'm only half way though it now, but its clear the authors want the reader to understand that (from a circuit design stand point) voltage and current standards are really two sides of the same coin -- its a coupled problem. In fact it might even be proper to say that a Fluke 732 is more a thermodynamic standard than an electrical standard (as the circuit in its totality is comparing concentrations, diffusions, drifts, etc).

I bring the topic up because I am planing a long term drift experiment where a number of zeners (40'ish) are going to be energized for several months -- their voltages being compared every few hours.  And the current source in my design is still an unsolved problem.

Does anyone have any thoughts on this? Do any of the LTZ1000 based references stand out in terms of their current sources?

Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on March 26, 2014, 02:25:19 pm
Oh-- and he also said that the LM399 should be run at about 1mA of Zener current for best stability.  The more stable you can make the Zener current, the more stable will be the output voltage.  He said that there is about 1uV of voltage change for 1uA of current change.


This is something that's been on my mind since you had your conversation with Bob. We talk a lot on this forum about the zener side of the circuit, but I haven't read a great deal about the current sources for these devices.

I picked up a copy of "Current Sources and Voltage References" (a pdf of which can be found though an easy google search). I'm only half way though it now, but its clear the authors want the reader to understand that (from a circuit design stand point) voltage and current standards are really two sides of the same coin -- its a coupled problem. In fact it might even be proper to say that a Fluke 732 is more a thermodynamic standard than an electrical standard (as the circuit in its totality is comparing concentrations, diffusions, drifts, etc).

I bring the topic up because I am planing a long term drift experiment where a number of zeners (40'ish) are going to be energized for several months -- their voltages being compared every few hours.  And the current source in my design is still an unsolved problem.

Does anyone have any thoughts on this? Do any of the LTZ1000 based references stand out in terms of their current sources?

If you supply the zéner current for the LM399 from its own amplified output (e.g. 10V), by a stable resistor (i.e. 3k, ww., <5ppm/K), this already is a stable current source.

The LTZ1000 circuitry also contains two such intrinsic current sources, one for the zener diode, set up by the stable UBE voltage and the 120 Ohm resistor, and the other one for the collector current, built by the zener voltage and the 70k resistor.
The stability of those sources is already included in the stability calculation in the datasheet, stating 1/100 .. 1/500 of influence from the stability of the resistors.
Similar calculation can easily be done for the above described current source for the LM399H.

So everything is fine, it is not required to use external current sources.

Simply build your circuit that way, there are other, more prominent drift mechanisms than those currents.

If you try to monitor your references, question arises, against which other superior or equally stable source you want to measure?

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 26, 2014, 02:37:24 pm


Simply build your circuit that way, there are other, more prominent drift mechanisms than those currents.

If you try to monitor your references, questions comes up, against which other superior or equally stable source you want to measure?



In my case I'm using the LM329 which is just a buried zener. Effects of temperature is what I'm after (77K, 200k, 273k, and 333k). So I'm going to compare them to themselves -- relative drift wrt temp.

My initial plan was to wire them in series using a single current source (~1mA). But that's a fairly non-standard source (280V @ 1mA).
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on March 27, 2014, 10:21:54 am
Time to join club, first LTZ online.

More details and bunch of photos in my thread. (https://www.eevblog.com/forum/projects/project-kx-diy-calibrator-reference-sourcemeter/msg414092/#msg414092)

Title: Re: Ultra Precision Reference LTZ1000
Post by: bingo600 on March 27, 2014, 07:04:59 pm
I'd like to join the LTZ club , but would need some finished PCB's (2..3)
Does any posters have some surplus boards ie. Branadic or ... ?

TIA
Bingo
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on March 27, 2014, 07:55:10 pm
I might have a leftover possibly, if I didnt give everything to Quarks (It was stuffing a half-used pcb panel and had some overproduction)

EDIT: No, sorry, no leftovers.
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on March 27, 2014, 08:56:02 pm
I'm not in the LTZ but in the LMx99 club, that's fine for me up to know.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 27, 2014, 10:57:25 pm
I want to see the LTZ design that uses hot glue, bailing wire, and tar paper. 
Title: Re: Ultra Precision Reference LTZ1000
Post by: bingo600 on March 28, 2014, 06:26:21 am
I might have a leftover possibly, if I didnt give everything to Quarks (It was stuffing a half-used pcb panel and had some overproduction)

EDIT: No, sorry, no leftovers.

@babysitter
Damm  :'(

Any other ?

/Bingo
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 28, 2014, 07:41:46 pm
Anyone see anything fishy here:

http://r.ebay.com/FY1E33 (http://r.ebay.com/FY1E33)

http://r.ebay.com/mZh1JL (http://r.ebay.com/mZh1JL)


I feel bad for the guy that bought the second unit.
Title: Re: Ultra Precision Reference LTZ1000
Post by: codeboy2k on March 28, 2014, 08:34:32 pm
Anyone see anything fishy here:

http://r.ebay.com/FY1E33 (http://r.ebay.com/FY1E33)

http://r.ebay.com/mZh1JL (http://r.ebay.com/mZh1JL)


I feel bad for the guy that bought the second unit.

Batteries dead.  No In-Cal light ...
an expensive recalibration in the future :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 28, 2014, 09:19:40 pm
Anyone see anything fishy here:

http://r.ebay.com/FY1E33 (http://r.ebay.com/FY1E33)

http://r.ebay.com/mZh1JL (http://r.ebay.com/mZh1JL)


I feel bad for the guy that bought the second unit.

Batteries dead.  No In-Cal light ...
an expensive recalibration in the future :)


I'm talking about the fact its the exact same auction: copy and pasted. One seller is a surpluser, the other is a brand new ebay member with zero feedback. 

Note to self: make thousands running fake ebay sales of precision references.
Title: Re: Ultra Precision Reference LTZ1000
Post by: codeboy2k on March 28, 2014, 09:35:41 pm

I'm talking about the fact its the exact same auction: copy and pasted. One seller is a surpluser, the other is a brand new ebay member with zero feedback. 

Note to self: make thousands running fake ebay sales of precision references.

yes, that does seem rather fishy..  Although I didn't notice it at first as you did.

I think those final prices are a little high for an out of cal unit, though. More like I want to pay just $300. :)



Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 29, 2014, 02:02:44 am
  The calibrations of this thing are very overpriced by Fluke-- so much so that I can't afford it, and am looking for a hobbyist friendly way to get it done.


Besides Fluke uses the "Canadian Volt"... which I think is 2/3rds English and 1/3rd French?
Title: Re: Ultra Precision Reference LTZ1000
Post by: ManateeMafia on March 29, 2014, 06:31:50 pm
Anyone see anything fishy here:

http://r.ebay.com/FY1E33 (http://r.ebay.com/FY1E33)

http://r.ebay.com/mZh1JL (http://r.ebay.com/mZh1JL)



I feel bad for the guy that bought the second unit.

Batteries dead.  No In-Cal light ...
an expensive recalibration in the future :)


I'm talking about the fact its the exact same auction: copy and pasted. One seller is a surpluser, the other is a brand new ebay member with zero feedback. 

Note to self: make thousands running fake ebay sales of precision references.

It's possible that eBay [or PayPal] security will stop this transaction before any money changes hands-- since they are liable to restore the buyer's losses [well, most of them anyway].

If something seems too good to be true, it probably is.  I bought my 732B from a dealer.  The calibrations of this thing are very overpriced by Fluke-- so much so that I can't afford it, and am looking for a hobbyist friendly way to get it done.

Good luck to the buyer. I noticed it the day it showed up and reported it to eBay as a potential fraud.
eBay does not care and they will not allow you to state why it is a potential fraud. The fact they do not reply and ask for further information proves they only care about making their quick buck.

Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on March 29, 2014, 06:40:29 pm

Good luck to the buyer. I noticed it the day it showed up and reported it to eBay as a potential fraud.
eBay does not care and they will not allow you to state why it is a potential fraud. The fact they do not reply and ask for further information proves they only care about making their quick buck.

I did the same thing. As you point out... a lot of good it did us.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Rigby on March 29, 2014, 08:59:40 pm
Caveat emptor.

Also if you're paying with PayPal you have some protection there, if you're quick enough.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 02, 2014, 03:07:56 pm
http://www.sciencedirect.com/science/article/pii/S0026271401002517 (http://www.sciencedirect.com/science/article/pii/S0026271401002517)

Quote
In this paper, an accelerated life test and noise measurements during the test for subsurface Zener diodes are carried out. The correlation between the reference voltage degradation and the 1/f noise in the devices is analyzed on the test data, and the physical mechanism leading to the correlation is discussed. In addition, a reliability screening approach using 1/f noise measurement is proposed.

PM if you're interested.
Title: Re: Ultra Precision Reference LTZ1000
Post by: JBeale on April 02, 2014, 04:14:38 pm
Right now there are 4 sellers of LTZ1000 on eBay. Two of them, Polida and yankee_electronic claim new and use the same generic pic from web. Polida is just bulk part seller, I've seen few comments that people got fake parts but can't say for sure, I'd say both of them are questionable as they don't even have the real photo. Other two, hifi-szjxic and bbshonic, sell used and they claim 100% tested. bbshonic even "Gurantee exchange if it is fault" and have some history of selling them.
FWIW, I purchased four used 0.01% 10k resistors from hifi-szjxic in 2012 and as far as I can tell, they are as claimed. The values are consistent but I don't have an absolute reference standard to compare with. I assembled the four into a full bridge and with 10 V excitation, measured an imbalance of 0.1 mV so the ratios of those two pairs are mismatched by 0.001% or 10 ppm.

(edit: oops- confused about the date; didn't realize that post was more than a year old)
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 02, 2014, 04:26:07 pm
Right now there are 4 sellers of LTZ1000 on eBay. Two of them, Polida and yankee_electronic claim new and use the same generic pic from web. Polida is just bulk part seller, I've seen few comments that people got fake parts but can't say for sure, I'd say both of them are questionable as they don't even have the real photo. Other two, hifi-szjxic and bbshonic, sell used and they claim 100% tested. bbshonic even "Gurantee exchange if it is fault" and have some history of selling them.
FWIW, I purchased four used 0.01% 10k resistors from hifi-szjxic in 2012 and as far as I can tell, they are as claimed. The values are consistent but I don't have an absolute reference standard to compare with.

(edit: oops- confused about the date; didn't realize that post was more than a year old)

Got a link?
Title: Re: Ultra Precision Reference LTZ1000
Post by: cyr on April 02, 2014, 04:40:13 pm
http://stores.ebay.com/HIFI-AUDIO-IC (http://stores.ebay.com/HIFI-AUDIO-IC)

I bought some used 0.01% vishay resistors from the same seller a year or so ago as an experiment. I mounted them om dual banana plugs and measured them on the 8508A at work:

1k - 1.000101k
10k - 9.99986k
100k - 100.0074k

Title: Re: Ultra Precision Reference LTZ1000
Post by: Blackart on April 04, 2014, 04:37:35 am
Thermocouple accuracy
After reading that Datron ageing paper ( thanks CaptYellowShirt ) Ive embarked on a oven to thermally work my refs & resistors for use ( and final ref boards ). To that end I've decided to build a automatic oven to do the c110 C ageing. The first step was to get some reliable temperature readings. I had 5 off new type K thermocouple made with "nude" i.e. welded and unprotected ends. Then hooked them up to a HP 34970A mix data acquisition unit. Then mounted the thermocouples in contact with a large block of copper. The first surprise was just how temperature sensitive they were even walking around workshop would show up as temperature disturbances. So I wrapped up all the sensors in a sealed plastic bag and further insulated them. The result is a on the graphs below called RAW data. Yo can see 100 readings at 30s intervals. Note almost 0.3C variance of the thermocouple all off the same real of with made in the same batch. I then applied calibration factors and re ran the test. As you can see in the next graph they are all pretty much in aggreance. I might further tweak TC#5 down abit. NB I have not calibrated these absolutely just to each other. I wanted to be sure they all drift at a similar rate.

Note the rise at 550s is me closing the workshop door by about 200mm ! Allowing the room to heat a bit.

I think I/we can be fairly confident that the thermocouples are good enough to use in the oven and for further a characterising of the Refs after I build them. I what to have consistency in the equipment through this process.

Lucas
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 04, 2014, 09:40:11 pm
Does anyone have experience with this kind of 'oil': http://en.wikipedia.org/wiki/Krytox (http://en.wikipedia.org/wiki/Krytox)

In some of my reading I've come across the use of this stuff in precision references as a thermal transfer fluid. Its Fluorocarbon nature made me wonder if it was oxygen or water vapor resistant?

Hey, good find!  If I had to hazard a guess, I would have to say that it is at least water-vapor resistant.  The fluorocarbon family of compounds is the only thing I have found that is 100% water-vapor proof, but I don't know if that extends to Krytox.  It's always best to go right to the source-- ask your Dupont FAE directly, [and post the answer here please!]

They didn't have data on water vapor, but they did have it for O2 and N2 solubility at 20degC....

O2: 218 ppm
N2: 123 ppm

The rep said hydrocarbon oils are typically in the range of 1000-2000ppm for O2.

No permittivity data was available.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on April 05, 2014, 02:21:31 pm
Thermocouple accuracy
After reading that Datron ageing paper ( thanks CaptYellowShirt ) Ive embarked on a oven to thermally work my refs & resistors for use ( and final ref boards ).

...

Lucas

Thanks to Brandon, I also read this interesting paper.

Anyhow, I still disagree to try to pre-age the LTZ1000, or hermetically sealed, oil filled metal foil resistors, if you use them for the reference.

The paper describes the fast ageing of 1N829 zener diodes, which are used in the Datron 4000, but definitely NOT the LTZ1000, which sits in the Datron 4910!

The big difference is, that the LTZ1000 already has such a small typical drift (1ppm/yr.) from the beginning, that further ageing makes no sense.

The 1N829 have typically 20 ppm/yr, and may be stabilized by that method to  a drift level where the LTZ begins.

The LTZ on the other hand shows strong hysteresis of several ppm, if its chip is heated to high temperatures as 110°C, which may lead to a strong creeping drift behaviour in the following years.

Therefore, a heat shock leads to a less stable reference.

Same goes for metal foil resistors, the oil filled types have such low annual drifts, (and also hysteresis) that any heating will have negative influence on stability.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on April 05, 2014, 03:23:32 pm
Excuse me for being not a expert isn't comparing those physically quite different devices a bit unfair, subsurface Zener Diode in the LTZ vs. 1N829 which I consider a stacked-cylinder-Design with surface effects ?
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on April 05, 2014, 04:04:08 pm
Hm, I have a questions to ones who have LTZ's running.
Any one used "bad" resistors with tempco 25-50ppm/C ?

Since I'm still waiting on proper resistors, I built one LTZ ref to try, and seeing
big voltage/temperature drifts over time.

Like in attached log.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on April 05, 2014, 04:22:01 pm
All of my old LTZ references were based on the "bad" wirewound MRH resistors with 15 ppm/C tempco (max) and metal-film S2-29V with up to 50-100 ppm/C (max). All have a typical datasheets specs without any problem.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 05, 2014, 04:22:55 pm
What are the two temperature columns?
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on April 05, 2014, 04:32:44 pm
There was Honeywell Pt1000 RTD HEL-705 and MAX6610 in deadbug position superglued to top of LTZ can.
So 111 is RTD temp and 902 is calculated MAX6610 temp (T = Vout - 0.75V / 10mV).
Whole board was put into small cardboard package box and wrapped with 2 layers of packaging bubble-wrap.

(http://xdevs.com/kb/kx/vref_a01/small/hel_zener.jpg)

Also what is current consumption of your references?

Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 05, 2014, 05:16:31 pm
Ah I see now. Thanks.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on April 05, 2014, 06:22:15 pm
Since I'm still waiting on proper resistors, I built one LTZ ref to try, and seeing
big voltage/temperature drifts over time.

Since the resistor tempco is divided down by a factor of around 100 to the 7V output it is evident that you should have at maximum around  some 10uV change.
Do you use a switchmode power supply?
Try to use batteries.

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on April 05, 2014, 08:53:13 pm
I used KI 2400, which is kinda switchmode.
Need to find battery which can hold 50mA 15V for quite a time first...
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on April 05, 2014, 10:07:38 pm
I used KI 2400, which is kinda switchmode.

Does the output voltage change when you put your hand over/on the reference or
the power supply or the DMM?

Perhaps 2*100nF capacitors between base + emitter (short!) of the LTZ-Transistors will give a little improvement.

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 05, 2014, 11:47:14 pm


6) After telling him that I wanted a voltage transfer device, he said a better way might be to use [at least 6] LM399's in parallel [like the Bob Pease idea].  He said that the burn-in procedure would be to operate these in an oven set to 125C for 2 weeks, which would be equivalent to 1000's of hours of normal operation.  Any LM399's that are drifting too much after that can be replaced [i.e., you burn-in more than you need, and select the best units for the array].  The LM399 is much more sensitive to board stress than the LTZ [because the LTZ has a special mechanical arrangement in the die mount]-- so the LM399 should be mounted off of the PCB a little bit to allow for this.  The long term stability of the array of LM399's will be directly related to the power required to run the heater-- and this can be minimized with insulation-- the more the better!  The LM399's do not age when they are turned off, and have almost no hysteresis-- so keeping the reference *off* until a few hours before you need to use it [and/or calibrate it] is the best way to keep the long-term drift minimized.

7) Note that for LM399-based designs, the slots in the PC board [plus a lot of insulation top and bottom] make sense-- Bob said that the less power the heater requires, then the more stable the output voltage will be.  So, in this case, the slots [plus insulation] are helping with this.  Oh-- and he also said that the LM399 should be run at about 1mA of Zener current for best stability.  The more stable you can make the Zener current, the more stable will be the output voltage.  He said that there is about 1uV of voltage change for 1uA of current change.


I'm running a small experiment to test these two ideas in another thread. I'd love to get some peer review on my thoughts (especially the math section).

https://www.eevblog.com/forum/projects/long-term-lm399-stability/msg419452 (https://www.eevblog.com/forum/projects/long-term-lm399-stability/msg419452)
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 13, 2014, 07:32:26 pm
A few weeks ago there was a discussion on this thread about the effects of humidity on the Fluke 732a/b.

I'm reading a paper (http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=377817 (http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=377817)) that discusses these humidity effects on a set of 732a's. The paper claims that humidity effects the 1.018 v output -- around  - 1uV /  + 10% change in relative humidity and time constant in the 20-30 day range.

However, the paper claims there is no detectable change on the 10 v output.

The authors seem to imply that the change could be a product of the 1.018 v voltage divider or internal leakage paths.

For those of you with a 732 (or similar), what can you say about this? Seems weird to me that one would change and the other wouldnt?
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 13, 2014, 07:52:46 pm
The cited paper on the physics of moisture effects is here:


"Moisture solubility and diffusion in epoxy and epoxy-glass composites"
http://domino.research.ibm.com/tchjr/journalindex.nsf/0b9bc46ed06cbac1852565e6006fe1a0/0521bda5a94455f085256bfa0067f5f7 (http://domino.research.ibm.com/tchjr/journalindex.nsf/0b9bc46ed06cbac1852565e6006fe1a0/0521bda5a94455f085256bfa0067f5f7)!OpenDocument

 
Anybody here work for IBM?
Title: Re: Ultra Precision Reference LTZ1000
Post by: acbern on April 13, 2014, 07:59:47 pm
the problem is that although I have a 732a and a 4910, I cannot really tell which one is drifting when, unless i would correlate to an external standard. too expensive.
the only other way to test would really be humidity cycling, one against teh other. have no chamber either.

however, I have done some general tests on precision resistors (z-foil, highest grade, not hermetic) agains an hermetic one (said to drift by less than 0.5ppm/a by vishay). within a year, a 1k drifted by 10ppm, a 100k drifted by 3.4ppm (assuming a stable 10k hermetic). both reduced their resistance values. that also is in line what vishay states and mostly related to humidity influence, hence the hermetic versions. oxigen seems oi be of minor importance. vishay also states the effect is reversible.
not having disassembled my 732a, maybe the anser is to check the different resistors used. now probably they are not vishay, but may be a first indication.
Title: Re: Ultra Precision Reference LTZ1000
Post by: ManateeMafia on April 13, 2014, 09:58:02 pm
I have a couple of pics of the reference board inside one of my newer rev 732A. I replaced the 10V pot and thought I should take some pics. These were taken a year ago.

I apologize for keeping these to myself for so long.

These were taken as a visual aid for reassembly of the oven and to note some of the design details not shown in the schematics.

The second picture shows the four resistors used in the 1V and 1.018V dividers. I believe these were a rev to the original design and I don't know what prompted the change.
Perhaps the sensitivity to humidity may have been a factor?

Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on April 14, 2014, 09:25:20 pm
I want to refresh the discussion about the substrate. I already mentioned that LTCC could be interesting, but is that really the case? I reviewed a few datasheets. As always, units seem to be a real problem for some guys :)

Isola IS410
Thermal Conductivity: 0.5 W/mK
Coefficient of Thermal Expansion X, Y: 13 ppm/°C (Post-TG)
Coefficient of Thermal Expansion Z: 250 ppm/°C

RO4003C
Thermal Conductivity: 0.71 W/m/°K @ 80°C
Coefficient of Thermal Expansion X: 11 ppm/°C (-55 to 288°C)
Coefficient of Thermal Expansion Y: 14 ppm/°C
Coefficient of Thermal Expansion Z: 46 ppm/°C

9K7 GreenTape
Thermal Conductivity: 4.6 W/m-K
TCE, (23° - 300°C): 4.4

951 GreenTape
Thermal Conductivity: 3.3 W/mK
TCE, (25° - 300°C), ppm/°C: 5.8

A6M
Thermal Conductivity: 2 W/mK
Thermal Coefficient of Expansion: 7 ppm/°C

L8
Thermal Conductivity: >3 W/mK
Thermal Coefficient of Expansion (25-300°C): 6ppm/°C

So for what I see, the use of more available RO4003C instead of expensive LTCC could be worth a try. It has better thermal conductivity (good for all pins on same temperature) and smaller TCE. LTCC would be quite better, but on what price?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on April 15, 2014, 05:10:47 am
Hello branadic,

kovar has a TCE of around 5.9 if I found it right.

with best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on April 15, 2014, 07:11:08 am
 I've been following along here on the LTZ1000 reference discussion for a few months - this is a really great forum and the best discussion I've seen on LTZ1000a!

I've built several hundred of these over the years for specialize production test equipment on laser diode/detector fab lines - and so far we've never had an issue with using just wirewound resistors (3ppm units typical, some have 1~2ppm parts for the 120 ohm resistor), no slots or gimmicks, lots of copper on a small board and put the ref board in a can, and use a linear pre-regulator.  I promise you that the units in the field for 20 yrs have gotten very, very stable, and drift rates of less than 1ppm / yr are really pretty typical. LTZ1000a devices, and we did some LM199 / 399 also (those are very good also) .  No voodoo. 12.5k over 1k WW for heater circuit, and everything basically as in app notes.   The only boards we had trouble with were when we bought the expensive hermetic magical Vishay resistors - Dr Franks discussion on the hysterisis effect has been our experience also, if the temp gets too warm especially - they will take quite a while to recover.    The Vishay's worked OK but were never worth the extra expense (at least for us). The regular wirewounds - and heck the 5/ 10 /20 ppm surface mounts work fine also if you have reasonable temperature control.  Unless you're running the ref outside, you should have good results with even moderately good resistors.  We've tested the expensive resistor-parts units, and after a 5 & 10 & 15 year tests we're reminded how optimistic Vishay's datasheets are.  LT1013 for the op-amp, we've never had trouble with - plastic package or metal TO in the early days. Humidity has never been an issue for us since our boards are running in a humidity-controlled environment anyway in the clean rooms.  We've never really had any of these refs fail outright, and the older they get the better they are, and its really amazing how well they hold up over time.  Kind of like me (I wish)  :-DD

And then tonight I stumbled across this -   We've been doing it  all wrong all these years...

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=261407717248 (http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=261407717248)

What could go wrong?  I asked the seller what the drift rate per year is, and he says you don't need to keep it on - its always good and it never ages... But you don't want to keep it on a lot...  He says he had an Electrical Engineer with 40 yrs experience look at it and was astounded at the results!  I see he's sold 4, as of tonight. 

Thanks again Dave & Co. for the great forum and discussion!


Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 15, 2014, 03:35:12 pm
He says he had an Electrical Engineer with 40 yrs experience look at it and was astounded at the results!


Hmm... only 40 years of experience? For my money, his imaginary friend would need at least 41 years.
Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on April 15, 2014, 05:24:07 pm
I would be astounded too, if it works.  I was almost going to buy one for fun, then I realized I had better use for the time & money.  I also asked what the load regulation was, and no answer from seller yet.

I think I'll stick to stuff I know works, and works well for a long, long time.  Like Buried Zener refs.
Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on April 15, 2014, 10:24:08 pm
This might not exactly belong in this thread with LTZ1000a's (except to do a direct comparison about vrefs for general knowledge), but this is the seller's response when I asked about drift rates on his  ebay device.  I'm not even sure I can understand it.  It might be a pot attached to an LM317 for all I know. I think he's convinced himself its better than a 732a:

Quote
We do a burn-in to minimize drift. These units do not need to be run continuously like a Fluke 732A/B. So, the drift over time is not coherent. These units have a specified drift per first 1,000 hours of use (~10ppm, and much less for the second and subsequent 1,000 hours). They require roughly five minutes to settle to maximum accuracy. As long as the unit is not powered on for extremely long periods (250 hours or more), it should hold the standard indefinitely.

The stability is greater than the maximum error of the 732A. In the null setup photo, the unit drifted from +3uV to -4uV. -4uV was the maximum error in relation to the 732A. The 732A has a maximum error of 0.6ppm, which @ 10VDC is 6uV. So, the maximum error of the unit using the 732A as a reference is 10uV. We spec the unit @ +-75uV, because reference standards require integrity and reliability. We may change that spec when we have more experience with the units.

We have an electrical engineer with forty years experience, conducting R&D on special mock-ups of this device, and the results have been astounding. But for now I'm sticking with the original spec of +-75uV. But the temperature at calibration is marked on the each unit. If this temperature is observed, the accuracy is in excess of 10x the official spec. Our testing has shown that the units are unaffected by repeated power ups. The drift is in relation to cumulative hours of power-up
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 15, 2014, 10:28:31 pm
There's nothing to understand. Its just double speak and audiophoolery.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 15, 2014, 11:26:21 pm
Maybe we should offer an independent test? Intercompare it with our standards?
Title: Re: Ultra Precision Reference LTZ1000
Post by: jd on April 16, 2014, 12:11:00 pm

3) What is the voltage reference inside the box?  Is he not saying because it would be embarrassing?  Is it a voltage reference with a non-hermetic package?  What is the temperature coefficient [TempCo]?  Is the TempCo compensated?  What is the hysteresis of the part if it experiences radical temperature excursions during shipping?

Hi,
The listing "item specifics" says that it is a LT1021-10. So one of those in a little plastic box I guess.

JD
Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on April 17, 2014, 02:04:19 pm
...or keep the plastic Vref package at constant temp, and give it a jacket of Tfe coating, then its impervious to the local weather.  That helps for just about any Vref.  Most of the stuff on eBay just isn't built that carefully.

The plastic package absolute Vref can sometimes be used for precision work to -some- degree -  IF its really the same die inside (as metal version), and you have good thermal management & humidity control- which normally we do anyway.  We do that on all Vrefs to give them the best chance, and when we build a box with multiples for averaging calibrators - say 35 vrefs in one temperature-controlled box - each Vref circuit or Vref group is replaceable as its own module.  You can spot the drifters that way.  But you have to keep an eye on them - if you have a wonky drifter in the group that's not playing well with the other Vrefs to contribute to a low-noise average, he has to go.

The reason we have to use plastic package Vrefs sometimes is the availability on the Metal cans means kind of a long wait for production, and sometimes we can't do that.   Sometimes a customer can't wait, and we're forced to use plastic - and take all the precautions.

Regardless of plastic or metal package, the big trouble with any absolute ref on a die is they tend to drift -much- more over time than a well-built, stable LTZ1000a / LM399.  The LTC6655 does pretty well (for an absolute ref) , although we don't have any to test for 10yr drift - yet.

Sorry for the thread interruption - back to the good LTZ1000a's...




 
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 17, 2014, 02:14:30 pm
Dr. Frank has discussed the effects of heater set-point temperature on the LTZ1000's drift rate. A lower temperature results in a lower drift rate, however I am wondering about the long-term total drift between a 'hot' and 'cold' LTZ. Will they end up with the similar amounts of total drift over the course of many years -- temperature simply affecting the rate at which they asymptotically approach that point? Or will a higher temperature tend to diverge in total drift when compared to a lower temp reference?
Title: Re: Ultra Precision Reference LTZ1000
Post by: acbern on April 17, 2014, 02:40:26 pm
I was wondering the same, and have nowhere seen any comments/tests. In general, higher temp means faster aging, a simple law of physics (arrhenius law). aging means drift. that is in line with the temp setting observations of the 3458a ltz1000. of course, always, there are additional effects. the ltz, according to the data sheet, needs some time to settle and decrease in drift rate. and of course, there are more and less stable devices. it is beyond me why a 3458a opt. 2 may or may not be powered and still meet its accuracy while all precision references must be on. also, this is a little bit like saying if you have a precision test equipment, keeping it on is better. in my experience, the opposite is the case. of, course, not the exact same thing, and not as simple as that, but anyway.
it woud be worth while doing a test with a set of ltz1000, keeping them on initially until stabilized per data sheet, determining their drift rates, and then switching some off. i bet there is not much difference, maybe the ones in off-state are even more stable, given their initial drift is already over.
Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on April 17, 2014, 05:50:54 pm
Digilant makes some good points, but there other factors too.  The drift story actually begins much earlier in the manufacturing process.

One of the huge mechanisms affecting initial drift rates happens when the die is separated from the wafer - the more energy and stress that's put into dicing the wafer into individual chips - this can cause larger fluctuations in the inertial crystal lattice strain, especially near the chip edges.

Once the wafer is diced, each chip now has to distribute the newly created edge strain around each newly formed faces on four sides - and whether this happens from the wafer saw, diamond scribe or laser scribe process will affect the crystal lattice - and thus electron migration properties.  The crystal lattice can not be the same near an edge as it is in the main body of the die.  So just the fact that the chip is separated from the original wafer starts the stress drift process from the exact moment it is born.

If the die has rough, dusty edges that means it has a lot of stress raisers that will affect all electrical properties as it heats.  Some of this we can't control, and is just the physics of the substrate itself.  Some die will work better because they were handled better or the original substrate batch was easier purified.  At every step the variations are minimized, but in the end each substrate batch is slightly different, and therefore so is each die.  It is just the nature of the beast.

For instance:  You can measure a diffused resistor in a circuit while the die is in company of all its neighbors on a wafer, while its all one piece.  Now dice the wafer into chips by whatever process, and then temperature cycle to help re-distrubute stress.  Those resistors (especially on die that were originally near the wafer edges) will now have different properties - because the substrate they sit on has now been altered.  The die near the edges of the wafer will change differently than the die near the center, and so on.  Also, how these chips re-distribute the stress created by making lots of new faces is also time variant - so some die will relax faster than others.  Wherever there is a change of crystal lattice, you have strain - and strain is everything.  All properties of the lattice - both electrical and mechanical - are changing in areas where the substrate lattice is stressed.

On some substrate materials, like those used for laser diodes (say InPh. GaAS, etc), we can cleave the edge onto a near perfect lattice plane, and that in itself produces a crystal edge with fairly low stress and strain - and is fairly stable as it heats and goes through billions of high-range temperature cycles.  But these substrates do not lend themselves to making particularly good buried zeners.

Then there is the die attach and wire bonding process:  Each of these can put an additional strain on the substrate crystal lattice - some of which can be controlled, and some that can't. 

If you try to make an absolute reference and use a laser to trim resistors on a chip - you also just changed the lattice strain again from the laser heat.  And guess what:  Now the device will behave and drift just a bit differently than every other chip in the batch.  That's one of the reasons absolute refs drift much, much faster than buried zeners. 

Realize that when a wafer is separated and during the wire bonding process, there are regions of the substrate lattice that are liquified for a very brief  instant, and the freeze again into a crystal or can even change into an amorphic region in some cases.  This event can form a new crystal plane depending on the cooling rate; and if its not in perfect alignment with the main substrate, you have a new area of stress.  You can't get away from it.

All of the above effects can be measure acoustically and with Atomic Force microscopes.  You can measure the crystal planes and internal stress pretty accurately, but you can't really cure lattice plane troubles once its in the chip.  You have to minimize these effects during manufacture - and that's the hard part.

What you have to realize when you're building a good Voltage Reference:  You aren't building a voltage reference at all.  You want to build a very in-sensitive thermometer out of something that at atomic scales is really as unstable as "Jell-o" when you look at it close enough.  Getting a steady, non-drifting voltage out of the device is just a by-product of how much of a "bad thermometer" it is.  In other words - everything in the die is related to how electrons move through the lattice, which is all related to heat and stress.


 
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 17, 2014, 07:25:10 pm
... how these chips re-distribute the stress created by making lots of new faces is also time variant - so some die will relax faster than others.  Wherever there is a change of crystal lattice, you have strain - and strain is everything.  All properties of the lattice - both electrical and mechanical - are changing in areas where the substrate lattice is stressed.


This is the conclusion I'm starting to come to though my academic reading.

I'm surprised that there isn't more written on the topic. I have found tons of material on the reliability of semiconductor technology (including zeners) with respect to conditions that lead to complete failure. However,  I've found very little on the subject of drift. The literature shows me: 1) that it does happens, 2) when it happens, 3) how to predict it happening, and 4) how to correct for it. But I'd like to know more about the underlying causes of it.

I'm familiar with the relationship between band structure and strain. Are there any books or papers that deal with the 'art' of semiconductor production -- specifically dealing with stress, creep, and band structure interaction?
Title: Re: Ultra Precision Reference LTZ1000
Post by: rf-design on April 17, 2014, 07:48:26 pm
If all aging is because of stress change, either from die internal stress or from die external stress from the adhesive die attach what would be the impact on the final reference voltage?

To my knowledge the stress changes the bandgap voltage. That would impact the final temperature point because the regulator is based on Vbe. But up to now I do not know a direct effect of stress on the zener voltage.

Is there a significant effect on the buried zener?
Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on April 17, 2014, 08:29:02 pm
Capn': At the company I work for we build some of the equipment used to dice wafers into individual chips, and especially laser diodes, and we solve problems like these for manufacturers.  What we tend to see are the guys on the production line that notice these drift effects on analog (and sometimes digital) circuits, but sometimes the design engineers laying out the circuit masks are half a world away and don't really get to see the hands-on interaction of what's -really- happening at chip level production.  And the Fab line won't talk to Engineering, &  vice versa. And the bean counters just want good sales, not necessarily good parts, so good engineering investigations (that take TIME) don't get rewarded very well. Not all the time but we see it a lot - just how big corporations tend to work.  Sometimes people want to just come to work, push some buttons, get paid, and go home. 

Granted, when you're working at the wafer / chip level you can't even see what you're working on without a microscope - but you -can- study these stress & drift effects.  Its like working with PCB's except much, much smaller.  With a wafer prober, an acoustic microscope (these generate say 1Mhz mechanical waves that we inject sideways into the wafer to watch for reflections - i.e. stress - in the whole wafer) and generally an Atomic Force microscope, where we scan the wafer surface and look for out-of-place crystal planes buried inside the wafer - again this translates to strain & stress in the crystal.  Almost always this will turn up later as a drifting analog circuit later on.

I been at a facility, training people on a new piece of equipment, and they will put on a wafer to dice up into chips - mind you nobody knows what it is, because Engineering didn't tell them.  But I can look at the chip and tell them "Uhh these resistors / transistors / diodes that are over here right on the edge of the die are probably going to be trouble. Just sayin'".... and sure enough in a few weeks after they get 98% rejection during test the production circuit mask gets changed.  That's happened more than once - but it shouldn't happen very often.

What's frightening is the new designers just starting out that lay out masks - and blindly trust what the CAD software produced.  You have to really look at the whole circuit / wafer / die as a whole system, then build some, then test & tweak - no CAD software is a substitute for hand's on experience.  Sorry, Mentor Graphics. 

And strangely enough, you can't Google the answer to some of these questions - you have to go in the lab and study, test, and re-design sometimes until it works.  And the bean-counters really, really hate that... <Laughing!>
Title: Re: Ultra Precision Reference LTZ1000
Post by: quantumvolt on April 17, 2014, 09:40:18 pm
@MisterDiodes

Allow me to say that I really like your posts. They have a certain flavour of theoretical knowledge mixed with hands on experience.

There is a picture of the internals of LTZ1000 in this thread (and on the web). Could you and/or others who feel competent, give us an introduction on how to 'read' it?
Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on April 18, 2014, 12:35:49 am
My favorite sign on my office door - someone gave it to me and its floating around on the web - and I think this is a good description if the this whole thread - and in particular ultra precision voltage refs:

"Where Theory Ends and Reality Begins"

About looking at Die Photos - very,very short story - :
Usually the manufacturers get really upset if you publish a highly detailed images and description of their complete mask pattern - and Linear Technology, Intersil, Analog Devices, etc. are our friends, so its probably best to not go into high detail here. That is their IP.

For Fun: Generally when you're looking at an analog die in the microscope (much easier with your eyes, not a video or photo), anything gold or pink-ish or borwn-ish gold is a conductor.  It depends on what metal was deposited on the die for conductors, and there are myriad combinations.  Sometimes you see where conductors cross and you can make out the insulator (or resistor) between the conductors.  On RF devices it is not uncommon to see square coil structures or waveguides, and sometimes these will have a delicate Air Wire crossover - where you literally have conductor lift up and over another conductor like a freeway overpass - and there is nothing but air in between - a matter of a few microns or less sometimes.  At high frequencies that crossover is also acting as a capacitor or virtual inductor too.  Terminology varies by manufacturer.  But typically you won't see these on Vrefs.

Remember also - when you're looking at a die, you're only looking at the top layer.  If you're doing a real investigation, you can etch away the various deposition materials, and go down into the die layer by layer.  But you can usually get a general idea by looking at the top.

Resistors can be darker brown / black, or will appear yellow or dull grey (for deposited foil).   Its good to use vertical illumination on the microscope so you can really see the contrast (generally not a ring light here) and detail, and you want adjustable zoom.   Follow the path from the wire bond pad, have the schematic in hand and start tracing - and don't forget the substrate is part of the action also - or not.  Depending on how the device is made the substrate can be a 100% insulator, or if its been doped it can be a diode junction to ground, or what they generally call "backside metalization", which would be metal (usually sputtered aluminum / gold but can vary) on the bottom of the device.  Its easier to tell when you're looking through a microscope directly because you can change the light / focus and get an idea of what areas are taller / shorter.  You can usually see larger fets and BJ transistors - some can be pretty hard to see without going to high magnification.  Out at the very edge of the die you can see what's called the "Scribe Street" or "Wafer Channel" and this is usually where you can see the surface of the substrate itself, before it had the circuits masked on more near the middle of the die.

If you have a stereo scope with a Nomarski filter / illuminator system, you can see flat surfaces in sharp detail and high contrast - for instance if you're looking at the edge of the die, you can tell how it was diced apart from the main wafer.   Its in the Channel where the device was separated from its neighbors, and how rough or chipped out that side face is tells you how well the dicing process went that day (think like your looking at a bullet's rifling pattern or the edge of a sawn wooden board)) - and the Nomarski will really bring that detail out.  If we see chips that had a lot of trauma on the edges - something was either wrong or the channel had some sort of grey nitride passivation applied, which is a real challenge to separate - because its almost as hard as diamond.

The heater resistors will probably a pattern of a zig-zag repeating pattern back and forth, and judging by the simplified schematic on LTZ1000a's these are probably diffused into the substrate itself, without an insulating layer -  that's why there is a "diode" junction circuit between them and down into the substrate proper.  That is just an educated guess though.



Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 18, 2014, 01:41:16 am
For those of you who haven't seen the pictures...

We can thank our Russian and Chinese counterparts for posting them.
Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on April 18, 2014, 04:00:30 pm
The structure in the center is the Zener, where it should be, surrounded by the heaters.  Of even more interest to drift studies: In the larger monochrome photo you can see the die edges.  They are fairly rough, and characteristic of what happens when silicon is sawn.  This isn't too bad, but realize at each of the those chipped out places is a spot where the die will be -very- highly stressed during heating / cooling.  Normally this isn't a huge issue for most circuits, but when we're shooting for sub ppm..."everything" makes a difference.  Again, the crystal lattice can not be the same at the edges as it is on the interior of the device, so the more edges there are, the more stress in that region of an edge (this wafer has ~millions of small edges on each side face).

Short Story: If we were to measure that die acoustically, say transmitting at some Mhz from the left edge to the right edge (without any circuit placed yet) - we would see an echo ping that would measure say maybe 3/4's the way across the device.  The sound wave going left to right would reflect off the edge stress discontinuity region in the crystal lattice, not the actual right edge - and then travel back to the left edge - and by timing the arrival of the reflected pulse we can tell how much of the die is actually acting as single crystal.  The speed of sound through the material changes at a discontinuity of the stress region, and we can use that to determine how good the substrate is after its diced into a chip.  It works just like if you were measure a cable with a Time Domain Reflectometer, or shining a laser at a piece of glass - you get a reflection wherever the wave front changes speed.   Simply:  When you measure the die size acoustically, and the closer your acoustic measurement correlates to the actual die size, the better edges you have, because that means there are no big large discontinuities inside the crystal.

The above is a simplification, but you get the basic idea.  There are more details, but I'm kind of distilling it down to the bare basics.

On some materials, like GaAs or InP, there are vertical crystal planes normal to the top surface, so we can cleave a -perfect- mirror edge with a diamond scribe process.  Silicon substrates like these will have a crystal plane running 45° to the top surface (which means a cleaved crystal plane  would leave an angled edge), and so these devices shown in the photo are typically sawn or laser scribed.  These processes leave vertical edges, but rough - because the crystal has no natural plane in that direction.  If you look at the side face you will see small short faces following the crystal plane, never a smooth surface.  And therefore it is will have much higher strain levels near the edges.  If you had an amorphic material like glass, you could achieve a smooth face if you polished it out, but not really with a single crystal if the edge is not lined up to an atomic plane.

So maybe on the die you're looking at, the middle 50% area of the die is probably low-stress, and the internal strain increases as you get closer to the edges - with highest stresses typically in the corners.  That's one of the reasons we try to avoid the corners as much as possible for critical circuits, or use those for non-critical or digital I/O.  These designers did very well - the chip is large compared to the circuit, and the business end of the device is in the middle - where stress changes are smallest. 

Still: If you were to measure the Zener at a known current while these die were in a whole 4" wafer (or whatever size they make these on - 2" thru 6" diameter are common) - and then measure the exact same device after dicing, the device characteristics will be -slightly- different.  Yes, even though the chip-outs and edges are relatively far from the zener.  We're talking ppm here.

It is my theory that this - and other lattice stresses that are induced during manufacturing -  is what contributes to initial drift, and then as time moves on the drift is more dominated by basic electron migration across junctions.  Which is a slow process, but will still happen faster at higher temperatures.  Electrons at higher energy will statistically have a better chance at crossing a PN junction.  Heat the die hot enough and the junctions will become useless.

I think one of the main reasons these buried Zeners work better than anything else is that they are -not- laser trimmed.  Laser trimming gets a desired output voltage, but with damage to the underlying substrate - and that damaged lattice certainly contributes to long term drift.  If that effect can be minimized, the better the device will be.  The Intersil Vref uses a floating gate to acheive a ref voltage, but that has a whole other set of problems.

I think in the end, to be practical there is only so much that can be done with external devices to correct for drift with buried Zener references - that's why when we built these with Wirewound or expensive Vishays for load resistors, the results we had 10 years later the results are not that much different - the Wirewounds stood up even better over time, and are one tenth the cost.  You can control drift up to a point working "outside" the package, and some of the drift is built into the device itself.

So that's why we use other approaches to build a stable calibrator box:  Don't use just one Vref, build a box with 4 or 10 or 25 or 250 refs inside, and average the best performers.  At the very least you get a calibrator standard where you know the whole box is drifting slowly one direction <Grin!> and you still have to calibrate against other sources over time.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 18, 2014, 11:50:54 pm
So that's why we use other approaches to build a stable calibrator box:  Don't use just one Vref, build a box with 4 or 10 or 25 or 250 refs inside, and average the best performers.  At the very least you get a calibrator standard where you know the whole box is drifting slowly one direction <Grin!> and you still have to calibrate against other sources over time.

I love this idea of averaging an array of references. I've been thinking about it a bit on and off recently.

Personally, I see a set of zeners wired in series. If we assume that the fluctuations any one of them experiences w.r.t. time is uncorrelated then the errors add as the root of the square of the sums of their voltages (better than a simple linear sum). Here I'm talking about  error sources like crystal defects and the associated current constriction in the chip but not things like room temperature or atmospheric pressure. The downside here is if one of the zeners goes belly-up, you loose the entire set.

MisterDiodes: do you have any thoughts on these crystal defects? Its something I've only read about. What cause them? Processing speed? Bad design? Just random things noone can control?
Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on April 19, 2014, 05:18:39 am
CapnYellow: Regarding averagers for Vrefs:
This is a good way to tell if you have a drift problem, and the commercial averager standards will typically have 4 or 5 Vrefs.  I say if you've spent all this time losing sleep thinking about one Vref, coming up with good thermal management, getting good wirewound resistors and parts, etc - why stop at building just one?  Anything worth doing is worth doing to excess!  The building part is the least time you've spent and the cheapest if your time is worth anything, so I suggest you make up a batch of 5 or 10 or 20 Vref boards or whatever.  Burn them in and pick the best performers.  Keep a few spares, & give away the rest of the batch to your buddies to check their DVM's.  That's what I do.  You will be very popular.  Or just keep the extras  burning in longer, like a year or two longer, and sometimes they become useable over time.

People have to get used to much longer time scales when working with good Vrefs - when I say "burn in" that means at least -several- weeks - but for a good Vref let it burn in until it stops drifting too much - say 6 weeks ~ 6 months or maybe a year.  So another reason you might as well make up a batch of these instead of just one.

Put the good ones in a box and average them out electrically or computationally - measure each Vref's circuit deviation from the average, and if one Vref is mis-behaving you will know right away.  The averaging can be done with a master output op-amp, or you can use a CPU or FPGA to do a computational average to set an output DAC or whatever is fun for you.  All of these have been done.  It works.  In the end the simpler methods really work the best, though.

The "perfect world" theory says you'll get a reduction of noise by the square root of N Vrefs - so using 4 Vrefs should get you a noise reduction of about 1/2.  Sorta.  In my experience, if you actually do this you'll find you need more like 5 Vrefs to get a solid, honest noise reduction of 50% from where you started out.  Use 17 or 18 Vrefs to reduce the noise by 4, etc.  This is one of those times where you have number theory meeting up with reality - and reality wins every time. 

If it were me, I like to put the Vrefs on their own module board so swap-outs are easy.  But that's me.

In terms of reliability - Like I said we really haven't had much in they way of failures because the circuit has such a low component count, and the currents are low in the Vref itself.

BUT when you work with chip-scale circuits, and do forensic analysis on failures - you will understand why you want to keep your important equipment powered on - in general.  On those die edges, wherever there is a small chunk missing along the edge (a chipout) - that is a potential stress raiser where a crack can form when the die is under maximum stress - and that's when it is heating or cooling at the maximum rate.  i.e. when you turn the power on or off.  If the built-up strain is great enough to start the crystal lattice bonds breaking, then there is enough energy to drive a crack straight across the die.  Sometimes you get lucky where a crack can start and then turn around and run back to the edge, and in this case all that happens if you have another piece of substrate dust floating around in the can.

The other failure mode is at the wire bond pads - They are pretty forgiving, but given enough thermal cycles these can be another weak link in the chain.

Of course there are myriad other things to go wrong - an ESD zap, power supply failure, etc.

The worst is when you zap a die with static that has enough energy to just START a crack in the substrate, but it doesn't drive it far enough in to affect the circuit "today", but it will fail in 20 minutes or 20 years.  That's what we call a "very expensive service call crack".

The rule of thumb for bathtub mortality rates on simple circuits:  If the device works for at least a few weeks/months of torture-test thermal cycling & burn-in, it'll probably be fine for the next 40 or 80 years.  The above failure modes are a very good reason to do a very thorough burn-in before you put your Voltage Ref averager system into dependable service:  You want to break it right away if possible - and get the whole system as stress-relieved as you can.






Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 19, 2014, 03:15:57 pm
Fascinating. Again, thanks for the great info.

If you flip back several pages ( https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg404500/#msg404500 (https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg404500/#msg404500) ) you'll see DiligentMind's conversation with LT and Bob Dobkin.

In this, he says the LTZ packages are back filled with dry air before they're sealed up.  What do you think about that in terms of stability? Is that a common practice?
Title: Re: Ultra Precision Reference LTZ1000
Post by: rf-design on April 20, 2014, 12:31:39 pm
I guess that the passivation is not up to the level of modern CMOS process which require inert gas filling. It is a tribut on having a monolithic buried zener with bipolar based temp control.

Some posts ago I had taken a look at the avaible foundry processes beween 0.5u BiCMOS and 0.13u SiGeBiCMOS and there is no deep n+ to deep p+ implant or buried layer diodes avaible which could make the buried zener. So it seems that this old 1-metal, diffused isolated bipolar process is the only way to go for buried zener with bipolar temp regulator.

Some foundries offering zener but I guess there are all extend to surface because they are lateral built. So they they suffering from less stability because of the surface trap effect.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on April 20, 2014, 03:07:25 pm
So it seems that this old 1-metal, diffused isolated bipolar process is the only way to go for buried zener with bipolar temp regulator.

Have you found any source for a simple buried zener -- no compensation circuity included on the IC?
Title: Re: Ultra Precision Reference LTZ1000
Post by: janaf on May 02, 2014, 12:24:13 am
Lots of interesting reading in the LTZ1000 thread :-)

This is one of my first posts here.

Now a comment and question on the LTZ1000 circuit.

I made a breadboard with the same schematics as in the datasheet. The R2 and R3 values in the datasheet are given as 70K and R4/R5 as 13K/1K. I have made some measurements by making small changes (0.1%) up and down, to temperature setting nominal 1:13, while changing the value of R3 in steps from 40K (edit , extended from 60K) up to 130K. The result; lines straight as a ruler for the output voltage, with a constant 75uV per 0.1% change in R4/R5. This is about 1ppm/100ppm change of output, pretty close to what is specified in the datasheet.

The output also changes by a very constant 765uV per 3K change in the value of R3, over the whole tested R3 range 40K  to 130K, i.e. 0.185ppm change in output voltage per 100ppm change of resistor value at 70K, as given in the datasheet. This is slightly better than the 0.2ppm / 100ppm given in the datasheet.

The interesting part is that it seems one could use just about any value for R3, at least between 60K and 130K. A value like 100K would seem very practical. High values would be preferred as the sensitivity to resistance errors will be lower, but one may run into stability problems if the resistor value is too high / currents get to low. I used the 2N3904 for the NPN.

Any thoughts on this? Anyone made similar measurements?

I also made some measurements at 1:12 (1K:12K) for R4/R5 but the stability is not as good as for the higher temperature setting of 1:13. This is especially true for R3 values above 80K or so.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on May 02, 2014, 08:13:42 am

Any thoughts on this? Anyone made similar measurements?


Hello,

I have made tempco measurements over change of the 70K resistors.
In my design there are a 50K + a 20K in series.
So I wanted to save the 20K.

But with 50K instead of 70K the tempco of the reference slightly increases when I change the temperature setpoint from 50 to 60 degrees.

Btw.: 12K+1K for temperature setpoint is much too low for a LTZ1000A.
12.4-12.5K is the limit for the A-Device.

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: janaf on May 02, 2014, 09:37:29 am
Turning the heater fully on-off, I got a tempco minimum (near zero) at 80K for R3 too. That matches what is indicated the datasheet and what is used for example in the 3458A DMM.

But making small temperature changes, I got the same tempco regardless of R3 value (range 40K to 130K).

So either

If my measurements are right then there is no minimum for the R3=>tempco, but still just below the 0.2ppmV/100ppmR in the datasheet. The good news would be that "any" resistor value could be used for R3 and most likely for R4 too.

That's why I ask if someone else made measurements with small temperature steps, not the on-off method, and what the result was?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on May 02, 2014, 10:54:11 am

Measuring the R3-to-tempco with the on-off method gives other results than the small temperature step method, i.e. the correlation is non-linear.


Hello,

I think we have to be more specific.
- Which device LTZ1000 / LTZ1000A ?
  My results are with 2 LTZ1000A references.
  With nominal setpoint 12K5 + 1K = 50.5 degrees C

- Kelvin sensing of the output voltage?
  my output voltage is directly

- Switchmode power supply?
  my setup is fully supplied from batteries all on a metal ground plane.
  switchmode noise gives a large influence on the temperature setpoint pin

On #1 I get 20.3mV output voltage change between heated / unheated. (22 to 50 degrees?)
With additional 22K resistor in parallel with the 1K (Setpoint = 61.25 degrees)
I get 3.7mV change or 48ppm/K

Changeing R2 from 70K -> 50K gives 4mV output voltage change for 50/61 deg C = 52 ppm/K.

On #2 I did only the test with 70K: with around 54ppm/K change on output voltage for a 10K step.

Since the tempco of the zener is around 50ppm/K I cannot imagine that you get a zero tempco with just changeing R3.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on May 02, 2014, 12:02:20 pm

Unfortunately the video is in French ... : 


Hello,

OMG:
that was the only protection that they could not copy the reference perfectly.
Now with the 2 hints they can.  :palm:

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on May 02, 2014, 02:00:36 pm
Buy from Linear and you will not have a problem :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Galaxyrise on May 07, 2014, 05:25:37 am
I have been trying to characterize my ltz1000.  I've got Q2 set up similarly to how Q1 is in the datasheet so I can measure it's Vbe, Q1 Vbe, Vz, and I have ammeters on pin 3, pin 5, and pin 8.  All three currents have been trimmed to 100uA. After making a bunch of measurements, I started to move towards a heated configuration.  However, it seems like applying any voltage at all to pins 1&2 is affecting all my other measurements, even if pin 1&2 are at the same voltage.  (I either have the heater at 1-10V or I leave it floating.) My guess is that I'm looking at some leakage current through those "substrate devices, do not forward bias", but it's causing millivolts of difference!  Is something wrong with my test setup, or is there something that mitigates this effect in practice?

Title: Re: Ultra Precision Reference LTZ1000
Post by: janaf on May 07, 2014, 08:05:33 am
Some characterizing I have done on the LTZ1000AHC

- Two ICs, two different boards, nominally:
- R1: 120R
- R2 & R3 68K
- R4 / R5 12.5K and 1K

Measured ranges:
- R1 120R, varied by +1%
- R2 & R3: 40K to 130K in steps of 3K and steps of 9K
- R4/R5: 12, 12.5 and 13, varied by +1%

Results

From a couple of hours of measuring, in total about 300 different data points

Output change versus resistor change i.e. ppmVolt/ppmOhm


Some comments

- The sensitivity to R1 variation was surprisingly low, 92uV / 1R1, averaged over 25 step changes, all within 89-95uV / 1R1)

- Also the sensitivity to R3 was lower than indicated in the datasheet

- The R4/R5 sensitivity does not change at all with R3, but decreases slightly with increasing R2 (0.0125 to 0.0093 over R2 values from 40K to 130K)

- Output sensitivity versus R4/R5 ratio decreases, but marginally, over increased R4/R5 (0.0129 at 12.4, 0.0118 at 13.2)

- Stability for low R4/R5 at low values: stable at 12.4 at room temperature, not stable at 12.0, unless cooled to 18C. Stability increases with lower values of R2 and R3.

- Output versus load current: no measurable change (<1uV) to output with current sourced / drained between +5mA and -9mA. The output is very DC stable. (sensitivity to ripple voltage on the supply was not measured). When changing load, the voltage changed by up to 10uV at 0-5mA load

- Output change versus supply voltage: no measurable change to output (<1uV). This is as expected as the power supply is only to the opamps and the heater transistor. The circuit is very tolerant to input voltage level.

- Output change versus low supply voltage, near dropout: unmeasurable, <1uV, down to dropout at
7.5V supply. This is a characteristic of the opamp only.

- The LTZ1000A output voltage was measured directly on the IC pins
- The power came from a linear bench power supply and there was a AS2954 LDO regulator on the LTZ1000AHC board, set to 11V
- The measurements where made with a NI 4021, 7.5 digit DMM (LTZ1000-based)
- The LTZ1000AHC was tested on a PCB made specifically for testing, with breakout jumpers for all components
- The board was populated with thin film 25ppm resistors. The resistors that where varied, where wired from a separate breadboard, with 1% metal film 100ppm resistors.

If of interest I can post photos, data and diagrams...

PS: ignore the results in my previous posts, they where the result of regulation collapsing, using R4/R5 ratio of 12...
Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on May 07, 2014, 08:41:14 am
Measurement results from lymex/BG2VO:
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on May 07, 2014, 09:12:49 am
Quote
If of interest I can post photos, data and diagrams...

Please do :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: janaf on May 07, 2014, 11:04:10 am
As I got some requests, I will post more, but give me a couple of days.

Thanks for posting the Lymex data. If we multiply my data by 100, for 100ppm resistor changes we get:

-DatasheetLymexJanaf
R110.14-0.14
R20.30.4-0.4
R3 0.20.03-0.07
R4/R5 1.00.95+1.2

A couple of conclusions:
- It seems the R1 and R3 values are not as critical as given in the datasheet. Or both Lymex and I are wrong.
- Put your money on R4/R5 and R2 while R1 and R3 are less critical.

In the end, the errors of resistors are uncertain :-DD, it is not meaningful to dive too deep into the decimals.
EDIT: changed signs in the table.
Title: Re: Ultra Precision Reference LTZ1000
Post by: janaf on May 07, 2014, 11:17:40 am
Galaxyrise, could you have damaged your LTZ1000? I's rated 0.1V max forward bias on the heater pins relative to pin 4 which would usually be at around 0.5V, ie pin 2 and 3 must always be higher than 0.5V
Title: Re: Ultra Precision Reference LTZ1000
Post by: Galaxyrise on May 07, 2014, 09:48:39 pm
Galaxyrise, could you have damaged your LTZ1000? I's rated 0.1V max forward bias on the heater pins relative to pin 4 which would usually be at around 0.5V, ie pin 2 and 3 must always be higher than 0.5V

It's certainly possible, which is part of why I'm asking here.  But my heater has only ever been floating, or at least at 1V.  Is floating bad?
Title: Re: Ultra Precision Reference LTZ1000
Post by: janaf on May 08, 2014, 05:42:40 am
I do not think floating could do damage. What about at power on / off, with non-floating heater?
Title: Re: Ultra Precision Reference LTZ1000
Post by: janaf on May 08, 2014, 05:55:36 am
A plot of Impedance of the LTZ1000 zener only, at room temperature, no temperature stuff. Possibly some self heating but the measurements where in mS long bursts only. The impedance was approximately (edit) 33 ohm (0.2V / 6mA) for two samples. Measured with a NI 4132 precision SMU.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on May 08, 2014, 06:13:21 am
A plot of Impedance of the LTZ1000 zener only, at room temperature, no temperature stuff. Possibly some self heating but the measurements where in mS long bursts only. The impedance was approximately 0.03 ohm (6mA / 0.2V) for two samples. Measured with a NI 4132 precision SMU.

Sorry, but differential resistance R = dU/dI, not the other way round.
Therefore R= 0.2/0.006 ~ 33 Ohm, which is in accordance with the datasheet (20..60 Ohm)

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: janaf on May 08, 2014, 06:14:37 am
 :palm: :palm: :palm: how embarrassing, but who cares about Ohms law anyway..... File updated.
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on May 08, 2014, 06:12:06 pm
While updating the 8.5 Digit DMM thread, I had the idea to show a comparision of the LTZ1000 board designs used in these DMMs.
So here is first collection. If anyone has more/other/better pics please share.

(https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=92893;image)
Title: Re: Ultra Precision Reference LTZ1000
Post by: quarks on May 08, 2014, 08:27:12 pm
So, now I am curious... Why is there an LM199A [LM299A/LM399A] in the Datron/Wavetek 1271 [as well as an LTZ1000]?

1271 using LTZ1000 + LM399
1281 using 2x LTZ1000
That is the main design difference and the reason why the 1271 specs are not quite as good as the 1281.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on May 10, 2014, 05:20:48 am
I've been wondering about the 'under power' aspect of the burn-in process. Does it matter if the device is powered at all during this period? Annealing would seem to be dominated by temperature effects. And only if one gets into high current densities does electromigration take effect?
Title: Re: Ultra Precision Reference LTZ1000
Post by: ManateeMafia on May 20, 2014, 12:57:34 am
Ken,

Are you thinking of AN42? Page 4 shows 4ma for the LTZ1000.

I also came across this link http://indico.cern.ch/event/162546/contribution/3/material/slides/1.pdf (http://indico.cern.ch/event/162546/contribution/3/material/slides/1.pdf) while searching. The circuit is listed on page 25.
I have not seen it before and I was wondering if this was from another publication or from an actual design.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Vgkid on May 20, 2014, 09:33:36 pm
How much do the econistors run?
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on May 21, 2014, 04:48:39 pm
I today received this little board, okay it didn't receive I had to collect it from the toll. It was named to be a LTZ1000 reference. The curious fact is, that the IC that used to be the reference has absolut no marking on it, but the circuit indicates a LTZ like style schematic.
Does anybody know something about such a board? It seems to be part of some test gear, beside the Linear Technology label and the date code 10/84 there is a mirrored "Superman" icon on top of the pcb. On the backside is another date code 46-84 and an icon that seems to be a growing sun?

I guess the first thing to do is to reverse engineer the schematic before powering up the beast.
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on May 21, 2014, 05:05:08 pm
So that was you who snatched it off my ebay bid list.
Great job  :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on May 21, 2014, 05:41:19 pm
Very interesting... maybe someone at Linear-Tech is taking stuff out of the garbage bin and selling it on fleaBay...

Bob Dobkin's retirement plan? :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on May 21, 2014, 05:48:36 pm
Quote
So that was you who snatched it off my ebay bid list.
Great job  :)

Me, what?  >:D

Quote
That appears to be a similar board to the one used in Linear-Tech ap-note #86.

You are damn right. Wow, I didn't expect that.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on May 21, 2014, 06:03:05 pm
Is it a LM7872? Is it a LM399? No it is a LTZ1000!

I prefer resistors to lay flat but besides that... beautiful !
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on May 21, 2014, 06:29:38 pm
Those prices really close to Z202's from VPG. And yes, I'm still waiting for my order.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Vgkid on May 21, 2014, 06:47:07 pm
Those are pricey, i will need to see about precisionresistor.com as well.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on May 21, 2014, 09:29:41 pm
I today received this little board,

Who grabbed it from J.W. s desk at the museum?

I priced out a 10K resistor [standard 0.01%], and here are the prices I got:
10K, 0.01% p/n "8G16A10K"

You will only need the 0.1% types they should be available at around half the price.

RS-Components has some resistors out of the UPW50 + UPW25 series which are similar.
I have them in my design. see:

https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=49180;image (https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=49180;image)

With best regards

Andreas


Title: Re: Ultra Precision Reference LTZ1000
Post by: ManateeMafia on May 22, 2014, 03:33:39 am

Those are pricey, i will need to see about precisionresistor.com as well.

I'm *know* that "Econistors" have an excellent track record over many years of use in military, aerospace, and automotive circuitry.  The resistors from Precision Resistor *may* be just as good [or even better!], but I have no historical information about them, and so I am hesitant to use them...  If anyone has long experience successfully using their products, please post about your experiences here!


I have purchased three of their MC-7 kits, one standard and two with values of 2.5 and 5. They were extremely helpful and quoted me accurate lead times of 10 weeks for the two non-standard sets. I also had them make a 10M 2.5W resistor that rounded out my original MC-7 kit. The prices were not cheap, but the kits were made using 1W non-inductively wire wound parts. I used them since I frequently travel near their business.

If you price out similar resistors, you will find that the 1W 5ppm resistors can cost much more. I have not priced smaller wattage units, but you can expect a quick reply from them if you send them your specs. I would imagine that the pricing would be competitive to what you will find elsewhere.


I do not have the capability to accurately verify the tempco but they measure well within tolerance on my 3458A. The quality seems to be excellent but I do not have much historical information on their performance.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on May 22, 2014, 06:33:04 am
Anyway, what I said earlier still stands-- I think one could build up an LTZ1000(A) based voltage reference using these resistors, and the circuit would be just as stable as one built with foil resistors...  Only one resistor needs to be "special order"-- the 12K5 for the LTZ1000A, [or 12K for the LTZ1000].

Regards,
Ken


As described here: https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/180/ (https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/180/), we already DID it! (baysitter an me)

For the LTZ1000, the 5 needed ECONISTOR resistors of 120, 1k, 12k, 2x70k, all 0.1% are available from stock, between 6 and 10 € today, I would guess.

My two references are still stable / in accordance against HP3458A and Fluke 5442A to around 1ppm after > 5 years.

babysitters LTZ1000A reference did not move more than 0.2ppm after 1/2 year or so.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on May 22, 2014, 10:46:52 pm
I've analyzed the circuit board I got, there are a few little different values for the resistors used, here is what I found:

R1: 125R Vishay KS24248L4 0.1% (datasheet value 120R)
R2: 79k7 0.1% (datasheet value 70k)
R3: 79k7 0.1% (datasheet value 70k)
R4: 11k91 0.25% (datasheet value 13k)
R5: 970R 5ppm (datasheet value 1k)

I will post the complete schematic and circuit board if completed.
Title: Re: Ultra Precision Reference LTZ1000
Post by: max666 on May 23, 2014, 12:38:09 am
I've analyzed the circuit board I got, there are a few little different values for the resistors used, here is what I found:

R1: 125R Vishay KS24248L4 0.1% (datasheet value 120R)
R2: 79k7 0.1% (datasheet value 70k)
R3: 79k7 0.1% (datasheet value 70k)
R4: 11k91 0.25% (datasheet value 13k)
R5: 970R 5ppm (datasheet value 1k)

I will post the complete schematic and circuit board if completed.

Yikes!  Those resistors seem to have drifted a little!

I don't think branadic measured these values. I think he just read the labels on the resistors   ;)
Title: Re: Ultra Precision Reference LTZ1000
Post by: retrolefty on May 23, 2014, 12:48:08 am
I've analyzed the circuit board I got, there are a few little different values for the resistors used, here is what I found:

R1: 125R Vishay KS24248L4 0.1% (datasheet value 120R)
R2: 79k7 0.1% (datasheet value 70k)
R3: 79k7 0.1% (datasheet value 70k)
R4: 11k91 0.25% (datasheet value 13k)
R5: 970R 5ppm (datasheet value 1k)

I will post the complete schematic and circuit board if completed.

Yikes!  Those resistors seem to have drifted a little!

I don't think branadic measured these values. I think he just read the labels on the resistors   ;)

 In that case we can't blame drift.  ;)
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on May 23, 2014, 07:22:37 am
Quote
I don't think branadic measured these values. I think he just read the labels on the resistors

Correct, this are just the printed values on the parts, I didn't measure them!
Title: Re: Ultra Precision Reference LTZ1000
Post by: max666 on May 23, 2014, 03:28:01 pm
In that case we can't blame drift.  ;)

We can blame the engineer drifting from the recommended datasheet values   ;D
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on May 23, 2014, 05:47:41 pm
Here is the reverse engineered schematic and board layout.
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on May 23, 2014, 07:39:02 pm
Hello Branadic,

where is the 1 Meg resistor in series to C1 from the datasheet?

R1,R2,R3 are not critical in value. They need only stability.
The R4/R5 value is 12.2 so it would be ok for a LTZ1000 (but not for the A-type).
But I think the 12K resistor is no precision device in your case. (low TC).
The ratio is very critical.

with best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on May 23, 2014, 07:52:00 pm
Andreas,

compared to the datasheet C1 is not 100nF, but some film cap instead (the upper right cap), with now unkown value.
Why? When the board received the solder junction were full with solder flux, so I had to clean it with IPA. The damn thing about that is, that this way I loosed the printing on the caps. WTF!
There is no 1meg resistor in series, as pictured in the datasheet. Also the diode CR2 between heater and ground was added later and not planed in the original board design.
The 12k (11k91 in my case) equals R5 in the photo and I guess it is some precision type too, even though I don't know the manufactor of the resistors.
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on May 24, 2014, 05:32:51 pm
  You can read about it here:

http://www.edn.com/design/analog/4427151/The-last-half-century--Wirewound-resistors-Part-one

http://www.edn.com/design/analog/4427940/The-last-half-century--Wirewound-resistors-Part-two


From Part Two:

"Film resistor users should note that most precision film resistor’s specifications calls for operation at a small fraction of their stated power capacity.  This is indicative of the film resistor’s inherent flaws; they cannot operate with any kind of power applied to them and maintain any semblance of stability.  Their TCR curves literally go out the window with power.  Our TCR’s are linear and flat over their operating range, no hyperbolic curves to correct for, no worries about what applying power to them will do; it has no effect upon them.  There are no applied voltage coefficients either and long term stability under power is better than film resistors sitting on a shelf."
Title: Re: Ultra Precision Reference LTZ1000
Post by: CaptnYellowShirt on June 06, 2014, 01:09:52 am
I have some ideas on the LTZ drift but it will cost $100k to test and I can't justify that.


Volt-Nuts bake sale?
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on June 06, 2014, 05:43:17 am
No news at all..

That drift mainly in the negative direction of the LT references had been investigated by Fluke, comparing the 732B drifts of the Motorola against the LTFLU:
"predictability of solid state zener references", D. Deaver, Fluke, ca. 2001

An asymmetrical drift of (-0.8 +/- 0.7) ppm/yr. is already in the datasheet of the Pickering reference, the 7000 reference module, now terminated by Fluke.

I think, that is caused by the physics of the LTZ silicon, degenerative processes, and hardly to be mitigated by any odd "burn in " process. (still no description about that, from Bob Dopkins, I think?)

So the goal of building ultra references is to bring all external drifts (i.e. from the Rs) to near zero and then to compare, and select the reference with the smallest drift.

It will take 1 year of monitoring for 0.5ppm/yr, and 2years for 0.3ppm/yr predictability, typically.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on June 06, 2014, 06:52:19 am
If anyone is interested, here are some of the Fluke 7000 voltage reference pictures, posted by lymex on the 38hot.

Title: Re: Ultra Precision Reference LTZ1000
Post by: Galaxyrise on June 07, 2014, 03:38:14 am
Nice copper ring, though... soldermasked and not connected to anything... just some thermal mass?  LT1413 instead of LT1013, too, nice little bit of "modernization."
Title: Re: Ultra Precision Reference LTZ1000
Post by: Mickle T. on June 07, 2014, 05:32:50 am
LT1413 opamp for LTZ1000CH was introduced in the Fluke 8508A nearly at the same time (2002) as in the 7000 series. Somewhat later (~2008) Fluke 8508A received a new "modded" reference with LTZ1000A and AD823A/LTC1150 instead of LT1413.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on June 09, 2014, 11:33:53 am
Nice copper ring, though... soldermasked and not connected to anything... just some thermal mass?  LT1413 instead of LT1013, too, nice little bit of "modernization."

Maybe the same reason as the guard cooper pour on my board besides the electrical: helping the thermal coupling to reduce temperature difference between LTZ1000 pins as advised by the datasheet already on page 4:

Quote
It is mandatory to keep the zener and transistor leads at the same temperature, otherwise 1 ppm to 5 ppm shifts in the output voltage can easily be expected from these thermocouples.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on July 04, 2014, 11:29:30 am
I just got some ceramic hollow feedthrus for soldering.

Great to embed a voltage reference hermetically in my beloved tuner boxes and having one massive copper wire going uninterrupted from the inside to the outside.

i.d. 2.5 mm, length 12mm, thin part od 5mm, thick part od 7mm.

Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on July 04, 2014, 03:43:16 pm
No Part number, coming from a ham surplus dealer.
The manufacturer was Stettner, which has been first renamed to Stelco and is now
Sumida

http://www.sumida-components.com/en/welcome-to-sumida-components-gmbh/?no_cache=1 (http://www.sumida-components.com/en/welcome-to-sumida-components-gmbh/?no_cache=1)

They offer pinless feedthru capacitors, thats how you can see those.

Title: Re: Ultra Precision Reference LTZ1000
Post by: branadic on July 04, 2014, 04:43:05 pm
Quote
No Part number, coming from a ham surplus dealer.

And can you tell us who is the ham surplus dealer? Up to now I only found those feedthrough caps in a leaded version. The one in your picture looks somewhat different.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on July 04, 2014, 05:56:15 pm
Oh, sorry, of course! Didn't want to hide my great personal source to sell 65 cent parts to you for 5 euro/each... only if you really want :)

http://www.oppermann-electronic.de/ (http://www.oppermann-electronic.de/)

Its in the ceramic section. Sorry, german page for the foreigners, but I have some spare parts :)

Title: Re: Ultra Precision Reference LTZ1000
Post by: ltz2000 on July 04, 2014, 08:08:35 pm
Great to embed a voltage reference hermetically in my beloved tuner boxes and having one massive copper wire going uninterrupted from the inside to the outside.

Certainly a major improvement, but don't expect to get hermetic seal comparable to glass-metal. Any pressure difference will slowly equalize.

It is possible to get extremely low leakage (similar to glass-metal) with metallized ceramic feedthrough by carefully selecting the ceramic material and metallization for that purpose. But the material it is attached to (the box, wire and solder) affects the most. Difference in thermal expansion coefficients will cause the plating to peel off when soldered and creates a microscopic route between the inside and the outside world.
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on July 05, 2014, 10:33:07 am
@ltz2000

We discussed pressure relief before; see [url]https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg252548/#msg252548]https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg252548/#msg252548] [url]https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg252548/#msg252548 (http://[url=https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/msg252548/#msg252548)[/url] and SeanB's remark.

Thermal expansion is still there...
Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on July 12, 2014, 11:46:49 pm
Well, I think I've waited long enough for someone else to say something about this.  The 70xx series was designed long after the Datron 1281 or the 4910.

!!! L@@K !!! NO SLOTS !!!!

I hope this ends [forever] the debate on slots vs. no-slots...

--Ken

(https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=96661)

<Laughing>  We built some LTZ1000 vrefs almost 20 years with slots, and some in the same batch without.  Tried all sorts of voodoo - even oil-immersed resistors.  Absolutely No difference in the end.  I wish I had a time machine so that my 20-year later self could go back and tell me to quit wasting time and money.

What helped the most out of everything?  Let the Vrefs run about 10 ~15 years or more and quite worrying about yearly drift.  See if they do well over 3~6 months.  They just get better over time, at least any of the ones I've tracked.    Using -good- wirewound resistors worked just as well as any other expensive magical Voodoo resistor in the end.

Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on July 13, 2014, 05:38:57 pm
Finally i got recovered schematics of KI2002's LTZ1000 VREF.

Enjoy (https://www.eevblog.com/forum/testgear/keithley-2002-8-5-digit-dmm-review-and-teardown/15/).

No voodoo magical resistors, just PTF56 68.1K, wirewound 120ohm and precision 2R network. No slots either, just a plastic cup on top and bottom. :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on July 13, 2014, 05:54:48 pm
Hello,
 
can you verify R268?
The value between Pin 1 + Pin 2 makes no sense. It should be more in the 1K - Range (perhaps 1.1K)

So the resistor divider seems to be the most "valuable" resistor pair of the whole cirquit.

With best regards

Andreas
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on July 13, 2014, 06:12:28 pm
KI's repair manual (http://dev.xdevs.com/projects/kei2002/repository/entry/service_manual/2002_902_01B.pdf) (page 138) lists R268 as RES NET, 13.1K, 13.3K, 0.1%, 100MW, THINFILM.
You think it's a typo there? and it's 1.1K/13.3K ? :)
Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on July 14, 2014, 04:21:52 am
Also the inclusion of R251 at 200k can be a problem.  That was included for LTZ1000, but for the A version it is not used.  Some people use it anyway, we found it caused more problems with the LTZ1000a over time.  Linear tells you right in the datasheet to not use it for '1000a.  They are correct in most applications.

Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on July 14, 2014, 05:41:04 pm
Just a note:  The 12k / 12.5k advice is good for applications where ambient temperature is low, but does not apply in every case.  The instrument makers go on the high side of the ratio, say 13k ~15K : 1k or more to raise the temp of the LTZ1000(a).  If the ratio is too low for a warmer ambient climate, then the LTZ1000 zener Vref becomes a thermometer and starts tracking ambient temp.  Its been my experience that if you know you'll be operating at below 85°F ambient, then the lower ratio is good. (Which is probably most of the time).  If you know your device will be above 90°F a lot of the time, then a higher ratio will give you better drift performance.  For instance, if we knew a a Vref would be used on a rack full of hot equipment (Ambient say 105°F to 115°) , the heater ratio would usually be set for 13k or even the standard 15k to 1k.  In other words you need to have the Vref Zener temp. higher than the changing ambient.  The more ambient is fluctuating, you will tend to want a higher ratio.

The other technique that works well is to just forget the on-chip temp transistor and just control the temperature of the space around the LTZ1000 directly with good thermal mass.  It isn't as fast for warm up time, but you'll get as good or better steady Vref performance.   You basically just make yourself an ovenized / potted Vref,  and that can cost a lot less for production than magical, expensive Whoop-Dee-Doo resistors.   Over a long time frame we found this technique is probably one of the better ways to make a solid Vref - IF you can stand a longer warm up, say an hour or two. 

Most of the time the stuff we make is never turned off, so warm-up time is usually not an issue.  That is good advice for any serious PPM-range Vref.

It just depends on application. 

 
Title: Re: Ultra Precision Reference LTZ1000
Post by: ltz2000 on July 17, 2014, 07:41:10 am
For the 3458A, it was also designed for use in up to 55C environments [don't ask me why-- I have no clue]

Business? The ratio of instruments sold must be something like metrology labs 1 - automated test systems 100.

Title: Re: Ultra Precision Reference LTZ1000
Post by: HighVoltage on July 19, 2014, 04:56:45 pm

I'm reminded of the book "Foundations of Mechanical Accuracy" in which the story of replicating the meter standard in the 1970's was told. The Moore Tool Co would send a rep to France with a briefcase that contained what they though was some fraction of a meter. The guy hops on a transatlantic flight, sets up the standard next to the one in Sèvres, waits how ever many hours/days you need to make sure everything's at the right temperature, and finally shines a laser at the ends of each to figure out how much they are off. The guy gets the report, flies back home, sets up the standard in a room inside a room in the sub-basement of the Moore Tool Co. and they take like a month to file off their best guess at another nanometer. All this just to fly back to France and try again and again until they've gotten "close enough" or they've gone too far.

Sounds like an interesting book, that I never heard of before.
 .... but it is $ 186 on Amazon

http://www.amazon.com/Foundations-mechanical-accuracy-Wayne-Moore/dp/B0006CAKT8/ref=sr_1_1?ie=UTF8&qid=1405788804&sr=8-1&keywords=Foundations+of+Mechanical+Accuracy (http://www.amazon.com/Foundations-mechanical-accuracy-Wayne-Moore/dp/B0006CAKT8/ref=sr_1_1?ie=UTF8&qid=1405788804&sr=8-1&keywords=Foundations+of+Mechanical+Accuracy)

May be I can find a PDF of it.

Title: Re: Ultra Precision Reference LTZ1000
Post by: jpb on July 19, 2014, 07:38:40 pm
Sounds like an interesting book, that I never heard of before.
 .... but it is $ 186 on Amazon

http://www.amazon.com/Foundations-mechanical-accuracy-Wayne-Moore/dp/B0006CAKT8/ref=sr_1_1?ie=UTF8&qid=1405788804&sr=8-1&keywords=Foundations+of+Mechanical+Accuracy (http://www.amazon.com/Foundations-mechanical-accuracy-Wayne-Moore/dp/B0006CAKT8/ref=sr_1_1?ie=UTF8&qid=1405788804&sr=8-1&keywords=Foundations+of+Mechanical+Accuracy)

Out of curiosity I had a check on Abe books and they are even more expensive....
http://www.abebooks.co.uk/servlet/SearchResults?an=Wayne+R+Moore&sts=t&tn=Foundations+of+mechanical+accuracy (http://www.abebooks.co.uk/servlet/SearchResults?an=Wayne+R+Moore&sts=t&tn=Foundations+of+mechanical+accuracy)

Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on July 20, 2014, 05:27:07 am
I would expect to see a voltage reference standard or 8 1/2 high end multimeter based on LTZ1000A installed in a climatized lab room between 20°C ... 24°C [68°...74.2°F] and not being stacked on a pile of hot T&M equipment - at least that is how I would do ...
The 12k5:1k00 is a compromise between max. expected temp. in lab room - minimizing long term stability - I my case I opt to minimize the long term drift.
It is hard for me to imagine what the lab room conditions are if you would recommend the use of a 15k0:1k00 ratio - at least on planet Earth.

That's a lot easier to imagine than you think, and this is what the Pickering paper failed to take into account:  On a production line situation, you can't always control the environment.  Its not really the top-end heat setting over ambient, its how much its changing throughout the course of a day, or how much wind is blowing by / or not blowing by.  For instance, the modules we make had to fit behind and under a stack of test equipment, and it was either hotter/ colder than we wanted in the cabinet, OR the best place in the clean room that was convenient for the rest of the workflow meant the cabinet had to be next to the air-lock door, with people coming and going countless times per day.  Every time that door opens there is extra breeze blowing across that part of the room that we have no control over, and the cabinet cooling system couldn't keep a perfectly stable interior temp with the automatic door nearby.  Even though the clean room temp is relatively constant (within a degree or two), the air flow over the control cabinet is not.

The other problem is if the equipment gets hotter or cooler depending on what type of product is running through the machine (the motorized motion slides are running longer or shorter distances), and about a zillion other "gotcha's" that pop up.

If you place an LTZ1000a in this location, you'll find that the system Vref becomes a tracking thermometer in a hurry if the die temp is too low.  We did indeed have to build several versions with a 15k/1k ratio in order to keep the die temp from responding to local rapidly changing local atmosphere conditions that we have no control over.  Generally we found that in real life, about 13k ~ 14k/1k gave the best performance, but we would go to 15k or higher as required.  And yes, the Vref is covered, but you'd be surprised how much the entire enclosure heats or cools - that's a lot of surface area, and airflow has a pretty large heat conduction effect on the enclosure as a whole system.

You actually have to do this in real life to see what I'm talking about - if you use a 12.5k/1k ratio, your Vref will be more wobbly than you want in a real world production situation.  Granted, if you're in a lab you can get away with the lower ratios, but sometimes you have to use these Vref's in non-lab conditions.

I can guarantee you - if you have a rack- stack of 3 or 5or 10 HP3458a' (which I have seen and not uncommon in a semicon QC area) they will NOT be even close to the same temp. inside, and if you run the 12.5k/1k ratio, your boxes will NOT have great stability, even over an hour.  Now if if its a solo 3458a by itself in a cal lab, then you will probably have it work pretty well at the lower ratios. But from experience, when you have 10 high-end DMM's in a cluster rack, you have to have the higher die temp on the Vref - otherwise you have a very large, expensive, sensitive  thermometer.

And then of course there was the situation where you get the system installed and running, and then the room gets re-arranged, and now the cabinet is acting much different than before; you have a new hot piece of equipment covering what you thought was an air vent, etc.  Sometimes, no matter how hard we try, the customer always finds a way to circumvent the resistor selection.  That's why we design these to run at a little higher die temp than book-theory dictates.

Also, a lot of times these are used where you really don't care what the drift is over the course of a year.  Sometimes you want good drift performance over 30 ~ 90 days, and what you're after is an extremely low noise Vref that you know is good over the next -several weeks-.   On a semicon production line that is doing critical testing with wafer probers, the Vrefs can be swapped out as often as needed and calibrated against the master transfer standards back in the cal room.  When you're in a main assembly room where say 10,000 parts are produced per hour, 24/7, and each device is worth $250 ~ $500, then nobody is going to chance that the Vref they are using hasn't been calibrated within the last month or so.  The devices are just too valuable - so the Vrefs tend to get swapped out fairly often - and you aren't really after 365-day drift stability numbers.  Its something good to shoot for, but not really always cost effective in all applications.

So, yes, sometimes you really have to go for the higher resistance ratios on the heater circuit, depending on the application.

Title: Re: Ultra Precision Reference LTZ1000
Post by: eurofox on July 20, 2014, 10:38:45 am
If someone is interested on ebay France, the reference module for HP3458A

I'm not involve in it :)

http://www.benl.ebay.be/itm/161371505611?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2649 (http://www.benl.ebay.be/itm/161371505611?ssPageName=STRK:MESINDXX:IT&_trksid=p3984.m1436.l2649)
Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on July 20, 2014, 03:14:53 pm
Well, 15k/1k how 3458a's come supplied, last I checked.  Do they supply a different ratio on newest units?

"Modifying" the DMM  would be to drop the heater resistor ratio down to 12.5k/1k, and that works also, as long as the environment is compatible.  We've built Vrefs that way sometimes.  For a solo unit by itself in a temperature-controlled room , that can work.  When you run a cluster stack, or are running in a really fluctuating environment its been my long experience that you'll want the higher ratio.  For a large cluster of DMM's (don't care what brand) You will find that the boxes in the top / middle / bottom of the stack will have much different temps inside than what you thought.  In a clean room where floor real estate is at a premium, the DMM's had better run to spec when sitting in a tall stack.

Or in the case of what I design, the Vref sitting in machinery sharing space with a motion control system / processor board / probe board inside of a control cabinet.  You need to have the die temp -well above- whatever the crazy changes in ambient temp are happening around the Vref, even though it might be covered from direct airflow.  Especially the rate of change of temperature.  Calculating for a steady-state ambient temperature is one thing, but when you move the design into reality, then you'll see that sometimes you need a higher ratio for best overall performance - and that includes those Un-controllable environmental issues.   Also known as "Real World":  It'll get you every time <Grin!>   

Nobody on a production line that I know of will make any modification to their 8.5 digit DMM though.  They want them to run in spec as supplied from the manufacturer, and nobody I know if will be changing out heater resistors if its a company-owned DMM.  I have never seen that at least.

You just select the resistors for intended application, that's all.  This isn't a one-size-fits-all design.  You test it out and make adjustments as you go until you get the performance you need.  If the 12.5k/1k heater resistor combo works for your situation, then that is "perfect" for -your- application.  It is not perfect for -every- application though.  A for-profit business may not be at all interested in 365-day drift numbers, they just want their products shipped out and running, and they will calibrate their test systems as often as needed.
Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on July 20, 2014, 07:17:30 pm
Gaz,
Most of the high-end DMMs on the production line will be 3458's, as far as I can see.  Sometimes a Fluke or Kiethly, but usually its mostly 3458's at least in my circles.  In fact, in more than one plant they will have a guy that works for Agilent/Keysight with his own office down the hallway.  These DMM's are the units in battle zone.  Down in the cal room where its quiet, really temperature controlled and you don't have 200 people stomping by every hour, that's where the expensive Fluke calibrator / transfer standards / precision resistors / etc will be found.

Re: Dr. Frank's observations - I agree mostly with those:  Yes, if you were to ask me to build you an LTZ1000a circuit, you would get good wirewound resistors on a copper-filled board; if I didn't know where the unit was going I'd give you 13k/1k heater resistors to start out, and you would never see any sort of expensive Vishay magical foil (and even more magical datasheet) resistor anywhere on the board.  If you're paying more than $5 or $7 per resistor, you're doing something wrong - I least from what my experience tells me.  The Op-amp would be sealed, and you could run the thing under water if you wanted to. No slots, goofy copper layouts or other gimmicks.

The designs I use have lots of hours on them, they run a long, long time - over decades time spans.  The older they get the better they run.  I figure it takes at least 2 or 5 years for the crystal lattice strain to level out on the die, and then you should be getting much better drift numbers after 5 or 6 years.

If I knew you were going to baby it in a very climate-controlled room, I'd give you a 12.5k/1k heater resistor setup.  If it has to work in the real world the ratio would be more like 13.5~14k/1k and if it had to work really hard  for a living in any hard hot/cold situation it would be 14.5k~15k/1k heater resistors.

I would probably not take the time to swap out an LTZ1000a for an LTZ1000 because for me, there is not a lot of long-term benefit in that.  The LTZ1000a's were invented to be less sensitive to surrounding environment changes, and nobody I know in a profitable business worries about 3ppm per year drift vs. 1ppm year drift.   I'm not saying its a bad thing to do, I'm saying that 5, 10 or 20 years down the road - something I have experience with - the '1000a and '1000 will probably drifting at just about the same rate, all else being equal.  If you did do this swap, down the road you would have wasted a good LTZ1000a part, spent money on a LTZ1000 and used up 4 to 8 weeks burn in time.  I dunno.  Maybe if the Vref board you had had a falied LTZ1000a, maybe it would be worth it - but that might be rare.

Outside of the Volt-Nuts crowd - very nice people by the way - I don't know of a lot of people really -needing- a 1ppm /yr  drift circuit.  I know the hobbyists enjoy chasing after low drift numbers for fun and entertainment - and I get it.   If you're a cal lab or on a production line, you'll be doing calibrations MUCH more often than once a year, and you'll probably be using a Fluke calibrator anyway, not some hacked-up board with a Dixie cup on top. <I say that tongue in cheek because I have one of those on my desk now>. :-DD

Its all fun, I understand - and there is certainly more than one way to build the circuit.  Use what works best for you! 

Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on July 20, 2014, 08:29:59 pm
Dear Mister Diodes,

the discussion about the LTZ1000A circuitry began years ago, when the mediocre annual stability of the 3458A compared to other 8.5 (!!) DMMs, partly with the very same reference element, got obvious.

To remind you, besides some nice, harmless Volt-Nuts, the 3458A is also often used in metrology labs, and there, a 8ppm/a stability is really undesirable in many cases.
Yep, if this instrument features 8.5 digits AND 0.02ppm linearity (of input), then 8ppm/a really does not fit to this class of instruments.
No, 8ppm/a make 8.5 digits nearly worthless..


HP could have assembled equally an LM199 inside the 3458A, if they let the LTZ run on 90°C.

Maybe the discussion in this thread is quite academic, but for the design (from amateurs) of a separate LTZ1000 reference, the 3458A instrument and its reference implementation was very instructional. And that was the core of this thread, the idea to pimp the 3458A was more or less a by-product.

Also, the 1989 commercial flyer promoted the 3458A to be a metrological class instrument...
(reprint here: http://cp.literature.agilent.com/litweb/pdf/5965-4971E.pdf (http://cp.literature.agilent.com/litweb/pdf/5965-4971E.pdf))
It would have been easy at that time to achieve a lot more of (metrological) stability by some simple means.
Instead, HP offered specially selected 4ppm/a references for a fortune.


By the way, for none of these 8.5 digits instruments it is specified, that/if their annual stability applies on intermittent or non-intermittent operation.

For the 3458A it's quite clear, from Service Note 18A, that the 90°C reference may show strong hysteresis, if it's NOT operated continuously.
That may be up to 15ppm!

And HP once again did not specify clearly, if that has been a temporary problem of certain S/N, like the SN18A implies by its title description, or if ALL LTZ references suffer from that problem.

Latter one I assume to be the truth, and that additionally would be a reason to reduce the LTZ temperature.. as such strong hysteresis obviously is only present on big excursions over room temperature.
Otherwise, if a spare instrument in a fab site is retrieved from stock, that would cause a big problem... also the high energy prices here in Germany may foil continuous operation.


Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on July 20, 2014, 10:54:25 pm
DR Frank:

I agree with you more than you think, and if my previous message didn't covey that then I apologize.  Let me try again:

Your observations are correct, but I am talking about production line operation here, not hobby use - and a production line is always running 24/7, so that the power-up hysteresis effect is not as much as an issue.  Generally when a new DMM comes online, it will be running in warm-up mode for several hours - at least one shift - before it becomes operational.  Then it is running in parallel with another unit to double check that the go- no-go tests are working right, and then a few shifts later the oldest unit on the stack is powered down and sent to the cal room for testing.  Also what we're looking for at least some of the time is relative differences between two voltage measurements.  The absolute accuracy needs to be good, but relative measurements are more important sometimes.

I understand that in some countries, you can't afford the power cost (especially in a home lab), so of course you would be more aware of power-on hysteresis.  But even in Germany the production clean rooms are "on" 24/7, correct?  It is very expensive to power down a clean room / semicon production facility, so in most cases its better to leave the critical equipment on.  I know here in the States I don't know of any production facility that shuts down completely even for holidays. - the costs would be enormous to get everything turned on and warmed up again.

OF COURSE for a Volt-Nut hobbiest that making the resistor modifications would be great idea if you need those 8,5 digits, and your power cost is so high you can't afford to leave the critical equipment on all the time.  I agree!

How much does it cost to keep an '3458 powered up 24hrs per day where you are??  I know it really adds up over a year I think.

For my equipment, what we're after is more the low-noise and short-term stability characteristic (90 days) of the Vref, in which the LTZ1000(a) is better at than the 199/399. 

That being said, and LM199, or even 20~40 or so LM399's in parallel for lower noise makes a good Vref too, I will agree! 

At least here in the states, I've not seen a 3458 used in metcal lab as a master voltage - test reference for a long time; that job has gone to the Fluke calibrators anyway.  It might be used as a secondary test ref for sure.  But I know our local metcal just sold off its last 3458, I think they are just all 100% Fluke calibrators now.  But different labs have different setups I know.

But on the production lines, you see lots of 3458's more than any other 8.5 digit DMM running on the test probers.  At least with what I work with.  Almost all the probers I know of are designed to work with the 3458's as the DMM device, but you do see other vendors sometimes.  Not often but there are other DMM's at work.  Generally you will need several DMM's at a probing station if you want any throughput, and in general you want all the same model DMM at the prober.  Sometimes you don't need that though, it just depends on the product going through the prober.

HP ripping off people with the "better" Vref and charging people $$$$ for it: I 100% agree with you that's a dirty practice.

I'm still not clear on the Volt-Nuts crowd chasing after a 3ppm / yr drift with LTZ1000A vs maybe 1ppm/yr with LTZ1000, and why that is a net benefit, but I am all for people giving it a try if that's what they want.  Go for it!   I am looking at more the net benefit 10 or 20 years later, and from what I've seen - there just isn't a lot a difference between the two devices over long time spans.  If you have a different experience on devices running 20 years, then that's good to know.  I will stand by my advice: If you have a good, quiet stable LTZ1000a, and you've dialed the heater resistor down to 12.5k~13k over 1k, there may not be a huge benefit to swapping out the LTZ1000a with a LTZ1000.  You might be trading out a "good" LTZ1000a for a "bad" LTZ1000.

In any case years down the road they will be drifting at about the same rate, and I have seen that over and over again.  That's all.  If you see a good benefit over the first 4 or 5 years, then that's a good thing to know.

I keep telling my buddies that want to play with these Vrefs at home, and this will apply to 199/399/LTZ1000(a):  Get an older one that's been in use at least 4~5 years.  That will get you a head start on good stability.

I guess that's all part of the fun!
Title: Re: Ultra Precision Reference LTZ1000
Post by: TiN on July 21, 2014, 04:51:11 am
Quote
Generally you will need several DMM's at a probing station if you want any throughput, and in general you want all the same model DMM at the prober.
That's why K2002 have so called 3458A-compatibility mode for GPIB interfacing, I guess.

I'm still not clear on the Volt-Nuts crowd chasing after a 3ppm / yr drift with LTZ1000A vs maybe 1ppm/yr with LTZ1000, and why that is a net benefit, but I am all for people giving it a try if that's what they want.

I'm asking that question myself every day :)
Probably real answer is that one have some extra cash to spend on toys for fun, same as buying 8.5d DMM for home lab, and leaving it off most of time.
Title: Re: Ultra Precision Reference LTZ1000
Post by: ltz2000 on July 21, 2014, 08:37:50 am
It seems that this has become a "fight" between the metrologists and the industry guy MisterDiodes. I have no experience of his playground, the semiconductor industry, except once visited a factory and spotted a number of 3458As, but I think he has a good point.

I believe too that the 3458A was not designed for the metrology labs. Of course they market and sell it to every possible application they can, but the emphasis must have been where the money comes from. They could have easily made a sister version, but I guess the market was not big enough.

We think the the 3458A is an expensive high end meter used only for the most demanding applications. But for example in military and aviation $10k is nothing. Hourly service rate can be higher as well as the simplest spare part. The 3458A is specified because it is known to be good, it is used everywhere else, the GPIB commands can be copy-pasted and believe or not because it is a full rack width unit (no need to play with flimsy half rack adaptation kits). It was quite common to see a 3458A in a "wrong company" with a function generator and a programmable power supply measuring something as simple as the supply voltages and their ripple.

For the 3458A, it was also designed for use in up to 55C environments [don't ask me why-- I have no clue]

Business? The ratio of instruments sold must be something like metrology labs 1 - automated test systems 100.

Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on July 21, 2014, 07:15:31 pm
Hello Andreas - long time ago you posted in this thread this schematic and I have a few questions:
T2 is an n-ch JFET boost stage that also acts as a pull up to force the start up of U2A in positive direction, well I am wondering why:
- you have added an extra pull up R13 (47k) ?
- you have added a min. load resistor R17 (22k) but you have already other resistive loads available (R4/R5) ?
- If you add 10k series gate-resistor to T2 (to limit output current of U2A in boundary situations), you could remove (short ?) D2, as T2 fulfills same task - even better to replace T2 with a BSS159 (depl. MOSFET) - did you consider this (removing D2) ?
- Why is R9 present ? Even LTC confirms in its ref. design that it should not be used with LTZ1000A-version.
Thanks and have a nice day !
Best regards,
gazelle

Hello gazelle,

Ok R13 is not necessary if the FET is already populated.
But I also wanted to have a fallback solution in case the FET would not work.
So R13 is populated.

R4 / R5 are not visible to the OP-Amp. The FET (or the diode) act only in one direction
this will make oscillations of the LT1013 rather likely.

I adopted the cirquit from a LM399 (together with a BF245A/B).
There I had (small) oscillations without the pull down.

D2 was also intended as fallback solution to the original cirquit.
Then I decided to let it in to avoid gate currents through the FET diode.

I did not consider a depletion MOS-Fet. (The BF245C were already in my drawer).
Question: how do you plan to limit the inrush current on power on
with a low ohmic device like the BSS159?

By the way the BF245C is no longer produced in TO-92.
So you would have to use the SMD part (= BF545C).

With R9 you have found a weak point in documentation.
It is not populated with my LTZ1000A, but I did not mention this on the plan.
(The cirquit could also be used with LTZ1000 as fall back solution).

With best regards

Andreas

Title: Re: Ultra Precision Reference LTZ1000
Post by: Andreas on July 21, 2014, 09:17:46 pm

Isn't there a conductive path from output of U2A through D2 feeding R4 / R5 ?
I assume D2 is stuffed.

Hello,

D2 is populated.
D2 is normally not conducting since the gate of the FET is more negative than the source.
(and more negative than the zener voltage).

I have around 6V at the OP-Amp (U2 Pin 1) with 7.2V at the Zener.
So I gain about 2V of supply voltage headroom against a 1N4148 diode.

with best regards

Andreas


Title: Re: Ultra Precision Reference LTZ1000
Post by: MisterDiodes on July 22, 2014, 05:00:37 pm
OK, I want to make one thing VERY Clear:  I -really- do appreciate all the work and generous sharing of information provided by DR Frank, Andreas, Digelent, and everyone else and this is a GREAT discussion, and a GREAT collection of very smart guys.  It is an honor to post some data once in a while.   My hats off to all of you because I know how much time and patience all of this takes.  Why do I tell you to wait a few years and re-test Vref when its settled down?  Because that's what we are dealing with here.   What I am trying to provide to the discussion is some feedback from the real world, and trust me - I understand the Volt-Nut vibe - its fun and addicting, for sure!  But I have to mix the desire for ulta-precision with pleasing customers (enough to pay their invoice) and to make a profit in a reasonable time frame: Truth be told, my main goal is to not get a phone call at 2am because some machine went down halfway around the world, so I have learned to really make the stuff last.  Other designers scoff because I tend to over-design, but then again most of the stuff we made in 1985 is still running - except for a brain-dead EPROM here and there.

I'm also old enough and lucky enough to have the stars align to have met the likes of Jim Williams and Bob Pease (working on other projects, not Vrefs) and they really were absolutely brilliant, and great to work with.

I didn't know I was in a "fight" with anyone, that is the very last thing I want to do -  I think what I failed to make clear is the difference between a carefully controlled lab at home or a very carefully temperature controlled metcal lab (I think this is Dr Franks expertise) vs. a production environment where you can't trust how abused these things will get, and how long does something last - and that is something I deal with on a daily basis.

As David Jones points out, if you are designing a Vref for a product, the "Ambient temperature" can be "a real trap for young players".  And what you will find missing on almost all high end DMM / Vref datatsheets is the RATE of CHANGE of ambient temperature that is allowed, and this will bite you in the butt every time when you deliver a product if you're not ready for it, and no SPICE simulator will handle this at all. This is one of the places where THEORY ends and REALITY begins.  For instance, the '3458 is rated for 55°C ambient, but you'll see in a moment why that is only part of the story.

Now:  Here are some real world numbers I jotted down from a facility I was at yesterday.  I tried to take pictures for you guys but it is not allowed.

The setup:
Test Prober station #6 has 4ea HP/Agilent 3458a's.  All are on 90-day recal schedule, and they are staggered so that there is only one new unit running at a time.  They range in age from 4yrs old to 14 yrs old.  None have had their Vref replaced or modified, but two of the older units have new VFD front panel displays replaced. There is one box with a pretty dim display that will need a repair soon, but as a computer-controlled DMM it is running fine.

 These are in a 19" rack, stacked near the middle.  Overhead are some test prober relay switcher boards and a couple blade PC's (one is running the prober & DMM's, the other one is dedicated to data logging and sending data to the inventory-control system, because at this station every working device is serial numbered and sorted into quality bin lots).  Under the DMM's are some power supplies, more relay boards and more power supplies.  Because the prober is known to create a small amount of dust, it is surrounded by a plexiglas glazing safety shield doors and the enclosure is under slight negative pressure to keep the dust inside. The doors are only opened up when a stack of wafers are installed or removed.   The whole thing is in a cabinet against a wall, on the other side of the wall is an air compressor and chiller and a bank of vacuum pumps.  There is also a high-pressure Osmosis - De-ionized water processing station. In other words the wall is fairly warm to touch.  To the right of the cabinet is another machine's motion control cabinet and power supply.  It is very warm also.

The room itself is running about 27~29°C on a very warm day outside.  You have to wear a Tyvek bunny suit, hood, mask, booties and gloves, and you wish they would have the temperature lower, but that's what we have on the menu today.

Inside the '3458a boxes, with the machine idle, bottom to top, the interior temps are 38, 41, 43 and 40°C.

Now here's the fun part:  When the prober starts, and then when the machine on the right starts, and then when you notice more vacuum pumps are online on the other side of the wall... The interior DMM temps are now reading 43, 46, 50 and 44°C when I check again, about 15 minutes after everything starts up.  Typically a 5 degree jump in about 15 minutes, or a rate of change of 20°C / hour.

NOW:  What makes this work in this situation is the data being collected is 6 or 7 digits, and the software compensation / ACAL routines running inside the '3458 are able to compensate well enough so that the testing operation is still working right.  AND the Vref Die temp is running at the higher temperature for stability during temp changes. The manager mentioned that they do get special support from Agilent in the form of customized firmware, and they do calibrations at more like 34 or 35°C.

So this is a different situation that you would encounter in a Volt-Nit or metrology lab situation.  BUT as a designer, I have to plan ahead for major abuse.

Bottom Line:

YES, for a cooler, temperature controlled lab, running the LTZ1000(a) at a lower temp makes perfect sense.  If your VRef is going into a very abusive environment where the temperature swings are sudden and relatively rapid - well then you really need to raise the Vref die temp to keep your Vref relatively stable, no matter what's going on around it.  That's one of the reasons '3458 have the die temp set as they are, and I've never really seen one that has "degraded performance" if the DMM has to work hard for a living, at least not over the 90-day calibration period (recommend by Agilent when these units are used like this - nobody ever expects these to run over a year without recal in this setup).  Even then you will need some compensation software to help smooth out the dips and glitches, and the '3458's do a pretty good job at that.

They have tried other DMM's on this rack, but the '3458's run the best while being abused.  At least in this setup.  I know there are other brands and I will not argue with you that they work well also in other situations.

Oh and by the way:  This is something else Vishay leaves off the data sheets of their very expensive magical resistors:  See how the tempco acts during high-rate-changes of temperature, and compare that to a wirewound - and then remind me why the Magical Vishays cost 10 times as much.  Lets just say I have never seen a 10 times performance increase, but that's just me.  I have never seen any sort of cost benefit ratio at all, at least for the stuff I design (like 20 / 24 bits DACs/ ADCS and high-precision current sources).  The main point I keep streesing:  After 10 or 20 years, these Vrefs tend to settle down and become very good performers even without any voodoo, magical datasheet gimmicks at all.

Your milage will vary, and to all Volt Nuts: I love reading about what you do and how you get there.   Honest!    The generous sharing of information here is fantastic!
Title: Re: Ultra Precision Reference LTZ1000
Post by: Dr. Frank on July 22, 2014, 05:41:50 pm
Mister Diodes,

me also, I do not see any "fight" here, but a objective - excited discussion.
And I really appreciate your very interesting descriptions from those rough industrial conditions.
By the way, the high MTBF of the 3458A is another argument for these applications, I think.

Anyhow, what we have found out here in this thread, is that it would have been quite easy and cheap for HP to offer a metrological option also, let's say OPT 03 with 1ppm/a  @ Ta < 35°C, as it was done by the FLUKE 3458A/HFL modification, with 3ppm/a DCV stability, a more stable reference resistor,  and specified Ohm Transfer stability.

Reading again the HP Journal 4/1989 about the LTZ reference, the HP engineers obviously did not think about that at that time, 8ppm/a was top notch for them.


I was one of the early users of this instrument, at university in 1990 or so.
The laboratory was not air conditioned (+10..+15°C), and the instrument was placed in a rack with other instruments (+10°C), so it had up to 50°C internally.

Therefore it was good, that the LTZ was running on high temperatures.

On the other hand, a better annual stability would have been better for the calibration of the current sources and the thermometers over the years.

But it was ok, 100ppm was required.

And the 100k/16Bit sampling capability @low SNR was the key feature of my experiments.

Well, that's yet another use case besides industrial applications.

Frank
Title: Re: Ultra Precision Reference LTZ1000
Post by: ltz2000 on July 23, 2014, 11:52:20 am
Andreas,

Slightly off topic, but could you (or other members) recommend an easy and economical way for recording humidity. You seem to have some sort of electronic sensor with voltage output (?) in your automated data collection system.

And totally off topic, what is the name and version of your schematic editor. The "style" looks very familiar but I can't remember the name of the software.

https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=102804;image (https://www.eevblog.com/forum/projects/ultra-precision-reference-ltz1000/?action=dlattach;attach=102804;image)
Title: Re: Ultra Precision Reference LTZ1000
Post by: babysitter on July 23, 2014, 02:31:37 pm