EEVblog Electronics Community Forum
Electronics => Metrology => Topic started by: janaf on February 13, 2015, 04:59:57 pm
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I bought a couple used LTFLU from ebay thinking that if they are good enough for the Fluke, they are good enough for me :) They should arrive within a week.
Now I have been searching to see what I can find out about the LTFLU reference zener. As some know I'ts a custom Fluke component from LT and used in some of the very best voltage standards like the Fluke 732B. The part was never sold to end users.
While digging in to the documentation, mostly the for the 732A, where there is a schematic and BOM, I realize a few things.
- While the LTZ1000 docs are not the best, for the LTFLU they are almost non-existant.
- Some DIYers in china have implemented boxes with the LTFLU
The main differences between the actual LTFLU IC and the LTZ1000 are:
- The LTFLU has no heater built in while the LTZ1000 has
- The LTFLU has the temperature compensating transistor in series with the diode, not parallel as in the LTZ1000.
- Therefore the LTFLU circuit needs a lot more temperature and current compensation than LTZ1000 circuits do
- i.e the LTFLU is more complicated to implement than the LTZ1000
- The LTFLU is in a 4-pin can, the LTZ1000 in an 8-pin. They are far from drop-in compatible.
A schematic for the LTFLU circuit is available in the 732A instruction manual, page 8-11 (93), figure 85, part U2. Available for download on the net. I don't know if it's identical to the 732B, I have not found a schematic for the reference board of the 732B.
Replicating / cloning the 732A or 732B is definitely out of the question. Those boxes contain several advanced custom components and, as far as seen in a tear-down thread on this forum, also advanced thermal engineering &hardware. https://www.eevblog.com/forum/testgear/fluke-732b-dc-standard-teardown/ (https://www.eevblog.com/forum/testgear/fluke-732b-dc-standard-teardown/)
As far as I know the 732B was improved over the A, by placing more of the critical components in an oven and the B model also has better current compensation for the zener.
IMO, there are two main problems with making a voltage standard circuit.
- To make a stable enough reference voltage
- To amplify and trim the reference to 10.000000V and keeping it stable.
The second is much more difficult than the first.
- With the LTFLU and 732, the zener an 10V circuits are as far as I can see, integrated into one loop
- With the LTZ1000 you typically have one 7.1V zener circuit and another amplifies to 10V (even if it's possible to combine them!)
I'ts now obvious to me that it will be more complicated to get good results with the LTFLU than with the LTZ1000, so why bother? I'll quote what I heard in an interview with a guy who's interest in life was old English sports cars, on why he liked tinkering with them: "They give you such interesting problems to solve!". I guess it's the same with me.
I guess there could also be interest from 732 owners on understanding the inners of this circuit.
As soon as the mail arrives I will throw the parts on a good SMU and get some basic characteristics. Does anybody have a pin-pout?
Yes, I should have searched the info before buying the components. :palm:
By the way, I don't know if the LTFLU and SZA263 are identical or not.
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I attach a copy of the LTFLU section from the 732A datasheet.
I have started cad-ing the schematics of the reference board.
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Bookmark
Can you share were you bought them. I just searched ebay and could not find any (incl. completed listings)
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There is an auction that shows a LTFLU-1 installed in a Fluke 8842A. I noticed this a while ago but did not think it was the same part. It would be interesting if a member here had one that could verify that this is authentic. I haven't seen anyone mention this previously. The date code appears to be 1989 and the other surrounding parts are from 1991.
It could be that these meters are the source of some of these LTFLU references.
http://www.ebay.com/itm/Fluke-8842A-5-1-2-5-5-digit-Digital-Multimeter-Calibrated-with-NIST-Cert-LTFLU-/321575961800?pt=LH_DefaultDomain_0&hash=item4adf6bc4c8 (http://www.ebay.com/itm/Fluke-8842A-5-1-2-5-5-digit-Digital-Multimeter-Calibrated-with-NIST-Cert-LTFLU-/321575961800?pt=LH_DefaultDomain_0&hash=item4adf6bc4c8)
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Sorry I bought the last ones 8)
I think he also had some SZA263 but it seems the shop is closed now, vacations in China.
It's the guy who also sells salvaged foil resistors; hifi-szjxic
(I stay away from the resistors, if they have been subject to high temperatures, they may be at end-of-life where they deteriorate faster and faster.)
Bookmark
Can you share were you bought them. I just searched ebay and could not find any (incl. completed listings)
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First part of the schematics around the LTFLU on the 732A. Not completed! Working on it now and then.
In the Fluke schematics, E23, E24, E24 are all marked 10V Hi while E26, E27, E28 are marked Lo. As far as I understand:
E23, Supply Hi
E24, 10V guard
E25, 10V hi
E26, low for front panel stuff
E27, 10V low
E28, ground
E24-E25 are interconnected via 51R R26 and E27-E28 via 51R R64.
Several other components are somewhat exotic / obsolete.
Almost all resistors on this schematic are selected / matched / marked "Return board to Fluke for repairs", no specs and sometimes no value given.
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Did you buy a complete "RefAmp-Set" including the trimmed resistors or only the zener.
I think without appropriate resistors you will have much fun to find out how to trim the tempco.
With best regards
Andreas
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Hi group,
I can confirm that the LTFLU was used in the 8842A. Here is a picture of the board showing the LTFLU:
(https://www.eevblog.com/forum/projects/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=136055;image)
And here is the reference schematic form the 8842A manual. The adjustment is done by supply the reference and the resistor network as a matched set.
(https://www.eevblog.com/forum/projects/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=136057;image)
(https://www.eevblog.com/forum/projects/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=136059;image)
Regards,
Jay_Diddy_B
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Nope, no complete set, just the LTFLU ICs.
Even with the Amp set, which contains two resistors R5 and R9, there seems to be a ton of trimming to be done.
I'm sure these will keep me busy for a while.
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Jay_Diddy_B, thanks a million! This seems a lot more straight forward than the 10V implementation of the 732.
If you happen to have access, it would be great to have some values for the unmarked resistors R701 and two Z701. Would be a lot of help!
I wonder what they mean by -0.05V near Z702 / 3K2 on the base of U701?
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I just downloaded the 8842A manual. It has the BOM and schematic, but of course no values indicated for the ref set resistors. Are any of those exotic divider resistor chip with two resistors physically "interwoven" to ensure ratio tracking?
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Hi,
I measured the values of the unknown resistors, But I think that there is more to these resistors than shown on the schematic. I suspect that the top of Z701 may be a divider.
I measured 5.081K from pins 1 to 2 and 0.7983K from pins 2 to 3. But I do not get the correct operation in my LTspice model. If I use 0.07983K then I measure the same voltage in LTspice as I do in the unit.
(https://www.eevblog.com/forum/projects/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=136067;image)
The results:
(https://www.eevblog.com/forum/projects/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=136069;image)
(https://www.eevblog.com/forum/projects/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=136071;image)
I have attached the LTspice model if you want to play along.
Regards,
Jay_Diddy_B
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Hello Jay_Diddy_B,
Im missing the 4.445 K resistor in your model.
And the LF411/412 are J-FET amplifiers with low input bias (e.g. like LT1022 but not LT1013).
(dont know if this matters).
With best regards
Andreas
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With the 4.445K resistor and changing R7 to 20K I get near +/-7V at room temperature. I've also used the 6.2V zerer which may of course be wrong.
I'm wondering what the 20K divider is. The ratio would have to be "ppm perfect" and stable as anything.
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With the 4.445K resistor and changing R7 to 20K I get near +/-7V at room temperature. I've also used the 6.2V zerer which may of course be wrong.
I'm wondering what the 20K divider is. The ratio would have to be "ppm perfect" and stable as anything.
Jan and the group,
If you look at this picture:
(https://www.eevblog.com/forum/projects/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=136055;image)
The 20K resistors are part of the larger resistor module(Z702). The smaller resistor module is the Z701. Fluke engineers placed these so they are in thermal contact.
I may have a 'parts unit' 8842a that I am unable to repair. I will look for it later.
Regards,
Jay_Diddy_B
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Hi,
A little more research.
I have attached a pdf with the Fluke 431B schematic. It uses a reference similar or the same as the LTFLU.
Here is the reference section from the 335D voltage standard:
(https://www.eevblog.com/forum/projects/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=136151;image)
The manual says that R13 is selected to set the tempco. R9 trims the output voltage. This gives a clue as to how the LTFLU chips should be trimmed.
This is a heated reference.
The heater is on the die, similar to a LM399. The heater is fed with constant voltage and the heater is a non-linear PTC that controls the temperature. A1 is a 723 voltage regulator.
Jay_Diddy_B
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As we know that, with your numbering, point nr 3 is GND, nr 1 is -7V and nr 2 approx -0.1V, the measured resistors values don't add up, unless the base is reverse biased, current flowing out of the base of the amp transistor :-\
I measured 5.081K from pins 1 to 2 and 0.7983K from pins 2 to 3. But I do not get the correct operation in my LTspice model. If I use 0.07983K then I measure the same voltage in LTspice as I do in the unit.
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My fluke 8840 also has a SZA263.
(https://www.eevblog.com/forum/projects/the-ltflu-%28aka-sza263%29-reference-zener-diode-circuit/?action=dlattach;attach=136157;image)
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My fluke 8840 also has a SZA263.
(https://www.eevblog.com/forum/projects/the-ltflu-%28aka-sza263%29-reference-zener-diode-circuit/?action=dlattach;attach=136157;image)
The difference between the 8840A and the higher specification 8842A is the quality of the resistor used in the reference circuit. The catalogue talks about higher accuracy and longer calibration cycles.
The blue resistors in your 8840a versus the hermetically-sealed, white resistors in the 8842A.
Regards,
Jay_Diddy_B
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Hi,
Here a verry happy owner of two Fluke 8840A DMM's.
My first fluke 8804A multimeter I bought in 1986.
I had to do al lot of overtime, to buy him :D
The 8840A is incredibly stable...
This is the 8840A from 1987 He never been calibrated after 1987.
Its standing on the shoulders of my 3458A, he is in good company...
(http://www.bramcam.nl/Diversen/Fluke-8840A-01.png)
Jumpt one digit, i switched of my voltage reference and it was not completely finished after turning again.
(http://www.bramcam.nl/Diversen/Fluke-8840A-02.png)
Kind regarts,
Blackdog
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This is the 8840A from 1987 He never been calibrated after 1987.
So it will definitively go wrong: 1990 the definition of Volt has changed by 9.264 ppm.
http://www.nist.gov/calibrations/upload/tn1263.pdf (http://www.nist.gov/calibrations/upload/tn1263.pdf)
With best regards
Andreas
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Obviously Fluke made very different implementations with the LTFLU with the 8840A DMM's, 731B, 732, 8508A etc. No doubt it takes a LOT to achieve top performance like for the 732B. Not aiming there as mentioned in my first post...
One thing; the parts I have bought are four-pin, so there can't be any heater in the can like shown in the 335D schamatics?
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Hi Andreas,
Do you not think it's great that this instrument is still as accurate after nearly 30 years?
I know about the change, but thanks for the document!
10PPM is one digit in this instument, far below its specs.
Kind regarts,
Blackdog
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Hello,
don´t take it that serious: I only forgot the "irony punctuation".
Of course its a very good long term stability for any device. :-+
I only mentioned it because the 8840A was the only instrument at work
that had the precision to get the KJ90-sticker in 1990 after calibration. (re-adjustment).
With best regards
Andreas
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Fluke ended up partnering with Linear Technology, and they designed the LTFLU-1AH [there was never a non-'A' version].
This is a mistake. There are both 'A' and a non-'A' versions of the LTFLU-1.
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Hi,
A little more research.
I have attached a pdf with the Fluke 431B schematic. It uses a reference similar or the same as the LTFLU.
Here is the reference section from the 335D voltage standard:
...
The manual says that R13 is selected to set the tempco. R9 trims the output voltage. This gives a clue as to how the LTFLU chips should be trimmed.
This is a heated reference.
The heater is on the die, similar to a LM399. The heater is fed with constant voltage and the heater is a non-linear PTC that controls the temperature. A1 is a 723 voltage regulator.
Jay_Diddy_B
That is not correct.
The 332/335 reference amplifier from T.I. had no internal heater.
The 4 pole package was externally temperature stabilized by a KLIXON oven.
Assembly can be found in this thread:
https://www.eevblog.com/forum/testgear/fluke-332baf-in-the-slaughterhouse/msg393627/#msg393627 (https://www.eevblog.com/forum/testgear/fluke-332baf-in-the-slaughterhouse/msg393627/#msg393627)
There you can also find a T.C. estimation of the reference amplifier w/o oven stabilization, i.e. about 2ppm/K.
Frank
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This is the 8840A from 1987 He never been calibrated after 1987.
So it will definitively go wrong: 1990 the definition of Volt has changed by 9.264 ppm.
http://www.nist.gov/calibrations/upload/tn1263.pdf (http://www.nist.gov/calibrations/upload/tn1263.pdf)
With best regards
Andreas
It depends, where this instrument was bought or manufactured.
Afaik, the change of the S.I. Volt in 1990 in Germany / Europe was much smaller, than in U.S.
Frank
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Fluke 732B
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Fluke 732B
You got a box full?!? :-+
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All of the photos above was taken from bbs.38hot.net
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Hi group,
What happen in 1970 that caused a shift in the Volt?
(https://www.eevblog.com/forum/projects/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=136304;image)
This graph shows the uncertainty between various government labs. It would be kind of interesting to plot the typical uncertainty found in home labs over the same period.
In my own case it would be some thing like this:
Early 1970 around 2% analog meter. 20,000 ppm
1979 bought a Fluke 8050A, from Fluke, 0.03% 300 ppm
1990 HP3455A / 3456A 25ppm
2004 Fluke 732A 6ppm
2008 Datron 1281
Regards,
Jay_Diddy_B
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Hi group,
I found my 8842A parts unit. This unit had the Motorola SZA263 reference. Here is a photo of Z701 resistor network:
(https://www.eevblog.com/forum/projects/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=136434;image)
I removed the resistor network. Between pins 1 & 2 I measured 13.4569 k Ohms and between pins 2 & 3 I measured 0.99855 k Ohms.
Regards,
Jay_Diddy_B
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Hi group,
What happen in 1970 that caused a shift in the Volt?
(https://www.eevblog.com/forum/projects/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=136304;image)
Regards,
Jay_Diddy_B
That's no volt shift, that's a redefinition of the S.I. volt, and a different mise en pratique.
Until 1972, the national volts were defined and realized by Weston cells, each having a nominal value of about 1.018638 Volt.
Afaik, there also was no precise definition of the Ohm at that time, and the Ampère definition also was not so good to be realized, so the S.I. volt was very unprecise in terms of the base units.
As a consequence, each of the different metrology institutes had their own volt, (because the chemistry of every Weston cell always is a bit different, but the assigned value was always assumed to be the nominal one), and all differed and varied , as it is obvious in the curve.
In 1972, the single junction Josephson volt delivered an independent definition and realization, only based on the definition of h, e and the second.
Latter constants of nature were already relatively precisely known, and so was the definition, then. Anyhow, in 1972 the responsible BIPM conference probably had chosen the wrong value for the Josephson constant, off by about -7ppm, or so.
But as a single junction delivered a few mV only, the realization (mise en pratique) was also not very precise, on the order of about 10nV / 2mV ~ 5ppm, I would bet.
You can easily see, that consecutive measurements in one metrology institute already varied by several ppm.
Until 1990, the JJ array (10V) had been developed, which greatly increased the uncertainty of the realization of the volt (factor of about 5000 better), and also e and h meanwhile had been determined more precisely.
So in 1990, the redefinition gave a much more precise value for the Josephson constant, a much more precise mise en pratique, and just recently, then, the von Klitzing Hall effect delivered the quantum Ohm. So the realization of Volt and Ohm was uncertain to better than 1e-9, but within the S.I. both were still unprecise to around 0.2 ..0.3 ppm, and that's still the situation of today.
The next step will probably happen in 2018. Then, Volt and Ohm may get exact values in the S.I., i.e. zero uncertainty for the Josephson and Klitzing constants, as then e and h will be 'exact', as is already the second.
Frank
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Dr. Frank,
Thank you for your explanation. It is very clear. How much of shift do you think we will see in 2018?
Regards,
Jay_Diddy_B
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Dr. Frank,
Thank you for your explanation. It is very clear. How much of shift do you think we will see in 2018?
Regards,
Jay_Diddy_B
Well, less than these ominous 0.2 or 0.3 ppm.
According to a recent publication by our German PTB, which obviously includes the latest intermediate measurement results from the Watt balance and the Si sphere, they expect a jump of about 0.02 ppm for the Ohm, and about 0.1 ppm for the volt.
Frank
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Hi,
Dr. Frank,
Thank you for your explanation. It is very clear. How much of shift do you think we will see in 2018?
Regards,
Jay_Diddy_B
Well, less than these ominous 0.2 or 0.3 ppm.
According to a recent publication by our German PTB, which obviously includes the latest intermediate measurement results from the Watt balance and the Si sphere, they expect a jump of about 0.02 ppm for the Ohm, and about 0.1 ppm for the volt.
Frank
I think I will put my reference building project on hold until we know what the right value is. >:D
Jay
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I think I will put my reference building project on hold until we know what the right value is. >:D
Jay
No, the correct consequence would be, to improve all references of the different projects here, in the next 4 years to a level of 1e-9 stability.
Currently, 0.3ppm is good enough, but then, that will be the new uncertainty limit.
Frank
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While I like the attitude I can not see how the current reference projects could be improved by orders of magnitude.
It think it would take radical new approaches, new components, new paths. And we still have the low frequency noise floor to cope with.
No, the correct consequence would be, to improve all references of the different projects here, in the next 4 years to a level of 1e-9 stability.
Currently, 0.3ppm is good enough, but then, that will be the new uncertainty limit.
Frank
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I received some used LTFLU-1TH in the mail today.
So now, I'll find out if I burned money or not.
- Six of them, all 4-pin.
- Date codes? 89 and 90
- One looks externally bad,
- Four others are used but appear decent,
- One looks new, never soldered, leads un-cut.
Does anyone have the pin-out?
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I received some used LTFLU-1TH in the mail today.
So now, I'll find out if I burned money or not.
- Six of them, all 4-pin.
- Date codes? 89 and 90
- One looks externally bad,
- Four others are used but appear decent,
- One looks new, never soldered, leads un-cut.
Does anyone have the pin-out?
As in the 732B both the SZA263 and this LTFLU have been assembled in the same place, it must be the same pinout as the SZA263.
Therefore, grab a 732A, or 731A, or a 5200A service manual and search there for the pinout.
Frank
Here's a snapshot from the Fluke 5720A calibrator, which also uses two LTFLU as reference, U6, U7.
The pinout should look like this, as identified from a photo of the 5442A.
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I looked in all manuals I can find but none give the pin numbers, only generic symbols.
I think I'll measure, using 1uA current limiting.....
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Dr Frank; Ah, yes, the Fluke 5720A schematic snapshot you posted has the pin numbers! Thanks
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Mickle T. has one photo, showing the pinning.
My CAD drawing is correct, therefore. :-+
Frank
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I think what Frank meant, correct me if I'm wrong, was that there is a need to develop non-JJ, every-day-usable standards, transports, calibrators, that are significantly better than now.
What I meant was that break-trough performance leaps will take much more than tweaking, even the most brilliantly built reference from single users will not be enough. There has to be some break-through scientific development.
In the mean time, I bought some used LTFLU from ebay :-/O
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Back on subject.
I did some measurements on the LTFLU ICs. I've so far measured the zener only, at room temperature only.
There is one big difference compared to the LTZ1000 zener: In the normal current range of a few mA for the LTZ1000, I measure a resistance of approximately 33ohm. For the LTFLU it's around 7ohm at current of 3-10mA!
The zener voltage of the LTFLU is a bit lower than the LTZ1000's, around 6.3V vs 6.5V for the zener only.
I attach a file. Comments welcome!
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I think what Frank meant, correct me if I'm wrong, was that there is a need to develop non-JJ, every-day-usable standards, transports, calibrators, that are significantly better than now.
What I meant was that break-trough performance leaps will take much more than tweaking, even the most brilliantly built reference from single users will not be enough. There has to be some break-through scientific development.
In the mean time, I bought some used LTFLU from ebay :-/O
Yes, that's exactly, what I wanted to focus on..
The uncertainty difference between the upcoming exact definition by JJ array and the usable standards is much too big..
I doubt that zener references might be improved by one or two orders of magnitude..
I have read about MEMS voltage references, which have a totally different working principle..
and might be an alternative..See here, for example:
http://lib.tkk.fi/Diss/2006/isbn9513868605/article4.pdf (http://lib.tkk.fi/Diss/2006/isbn9513868605/article4.pdf)
Well, and JJ arrays on liq. Nitrogen temperature also have been demonstrated.. about 1e-8 uncertainty..See here:
http://juwel.fz-juelich.de:8080/dspace/bitstream/2128/2069/1/19406.pdf (http://juwel.fz-juelich.de:8080/dspace/bitstream/2128/2069/1/19406.pdf)
I think, a compact JJ array, at much lower price would be possible for everyone..
Like in the Rb atomic clocks, the microwave part of the circuitry might be cheaply be realized, also a simple current controller, and so on..
Only the superconducting array would be the really expensive part.. either on Nb base (with He4 cooling) or on 77K with High Tc superconductors (which would be not as stable as a Nb system).
Frank
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Thanks for the feedback and the info is really interesting!
What your little bird tells about the zener size matches my resistance measurements, about 1/4 of the LTZ1000. I guess the the lower zener voltage of the LTFLU also indicates that. It of course means the LTFLU is less sensitive to the current in that respect, or proportionally more stable given the same current stability. The less fun part with the LTFLU is that there are three different currents added to the zener....
Measurements; they are done in pulse bursts at 1kHz, ie one sweep of 100 points takes 0.1s. I could reverse the sequence (highest current first) or use a much longer delay to see if it makes a difference. I'll dig into the docs to see what the actual pulse length is.
If time allows I'll do measurements on the transistors in the weekend. Then I'll need to make arrangements to measure at different temperatures.
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Back on subject.
I did some measurements on the LTFLU ICs. I've so far measured the zener only, at room temperature only.
There is one big difference compared to the LTZ1000 zener: In the normal current range of a few mA for the LTZ1000, I measure a resistance of approximately 33ohm. For the LTFLU it's around 7ohm at current of 3-10mA!
The zener voltage of the LTFLU is a bit lower than the LTZ1000's, around 6.3V vs 6.5V for the zener only.
I attach a file. Comments welcome!
It looks like you are using continuous current to measure the LTZ-A. Because it [intentionally] has low junction to case thermal conductivity, you need to measure it with short pulses, with a rest time in between pulses to let the device cool. The LTFLU will not be affected as much, because it is direct-mounted to it's package. If you do this the upward slope that the LTZ curve shows will probably vanish.
A "little bird" told me that the LTFLU has 4 times the area for the buried Zener, and so you can run it at 4 times the current [resulting in half of the DC-10Hz noise that the LTZ has]. The interconnects on the LTFLU were done better, and it has much lower interconnect resistance. The transistor in the LTFLU will actually carry the same current as the Zener, unlike the LTZ where the transistor would be destroyed if you tried to do this-- it is a small signal amp only.
There really needs to be a new super-Zener reference chip designed, but who will pay for the development? I'm guessing a minimum of US$50K, but more money is likely needed. We know a lot more these days than we did back when the LTFLU and LTZ were designed, and there are some new and possibly better processes that can be used; for example, copper [and/or low-TCR] interconnects, MEMS processing to eliminate die separation stress, enlarging the Zener area, etc. These might lead to much better performance over time.
Ok, I can't decide from that info which reference is more stable, potentially..
But didn't your little bird, i.e. Mr. Dobkin, tell you recently, that the LTFLU and the LTZ1000 were very similar, especially concerning the buried zener?
I assumed, that LT used the same bipolar design cell, and only arranged the zener and the transistor in a different way? And both components drift the same direction, statistically, i.e. about -1ppm / year?
In the end, both chips really look totally different, anyhow.
Frank
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Ok, I can't decide from that info which reference is more stable, potentially..
Frank; was that referring to what I wrote "proportionally more stable given the same current stability"? I was referring to the lower local resistance only. My mind was into "how important are each of those those expensive high stability resistors?" I have recently been measuring voltage from the LTZ1000 circuit, making small incremental changes to each resistor value. A lower zener resistance should make the external current setting resistor (R1 / 120R on the LTZ1000 datasheet shematic) less critical. But the LTFLU implementation has to be different anyway and I wasn't really going to get into that here. For the LTZ1000 circuit, my measurements pretty much show the same thing as the LTZ1000 datasheet except that R1 is less critical than given in the datasheet while R2 is more critical than the datasheet indicates. I will get back with the LTZ1000 measurement results when I have double-checked them.
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You are right. I re-did the measurements, ramping down instead of up, doing fewer points and then the results show lower resistance for the LTZ1000AHC but for the LTFLU, the difference is marginal. Better insulation of the LTZ1000AHC is one reason, the other, the apparent difference in die size.
I attach a new file with some updated plots.
It looks like you are using continuous current to measure the LTZ-A. Because it [intentionally] has low junction to case thermal conductivity, you need to measure it with short pulses, with a rest time in between pulses to let the device cool. The LTFLU will not be affected as much, because it is direct-mounted to it's package. If you do this the upward slope that the LTZ curve shows will probably vanish.
-
Now I need some good advise :-//
What / how to map the LTFLU transistor to get a basis for making a circuit around it. I Never did any transistor characterizing before.
I have two channels SMU that each can measure in constant current or constant voltage mode, can sweep either. (Voltage and current limits available independently as well). The two SMUs can be synced. Both voltage and current are measured independently for both channels.
For now, I think I will measure
- room temperature
- static data, no dynamics / frequency dependency
Later
- combine with the diode
- measurements at other temperatures
- several devises
The number of measurements must be held at a manageable number.
First measurements could be:
- Constant CE voltage (like 1V, 2V, 5V), sweeping BE voltage (up to 0.1mA?)
- Constant BE current (like 10uA, 100uA, 1mA, 10mA?), sweeping CE voltage
- Constant BE voltage (like 0.4, 0.45, 0.5, 0.55, 0.6V), sweeping CE current
Some results will be redundant but that's OK for double-checking.
Once a clear measurement matrix is established, I could automate the whole sequence.....
Feedback appreciated!
-
Here are some more measurents made on the LTFLU. This time on the transistor.
They are still just at room temperature as I'm waiting for some parts to build a small temperature chamber.
-
I just updated the file with the previous post because one of the figure legends was screwed up.
Anyway, I'd appreciate any feedback of what kind of measurements / plots would be useful for making a circuit around the LTFLU.
-
In your LTFLU_Measurements_201.pdf file, page 3, you show what looks like
a curve trace of the NPN transistor in the reference, except that the base-emitter
steps are in volts. Shouldn't that be in amps (probably microamps)?
I'm wondering since I'm repairing a Fluke 5440A which uses 2 SZA263s in it's
ovenized voltage reference and one of them is acting up. Using a Tek 576,
I can see the zener just fine but the transistor seems a bit odd. Without
the spec sheet, it's hard to know what to look for and the datasheet for the LTZ1000
has some scary warnings that imply that its on-die transistor isn't very robust.
......jim (narem at narem dot com)
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Fluke Motorola ref-amp were also used in the 8800A and 8810A meters, predecessors to the 8840/42 -- these are currently very cheap on ebay if anyone wants to get the Motorola version of the ref-amp with the selected WW zero-TC current-setting resistor. Oh and they are the reference in the Fluke 510A AC standards, too. Fluke really used a lot of these in all kinds of gear, including the 8502/5/6 DMMs, which I think maybe all had the LT versions...
-
Anyone mind if I necro this thread?
Any thoughts on why Keithley decided on the LTFLU over the LTZ on the new DMM7510?
-
Any thoughts on why Keithley decided on the LTFLU over the LTZ on the new DMM7510?
https://en.wikipedia.org/wiki/Danaher_Corporation
-
I'm assuming you mean it's simply cheaper, I'm guessing more too it than that.
Perhaps lower noise?
-
I'm assuming you mean it's simply cheaper, I'm guessing more too it than that.
Perhaps lower noise?
I think that main reason is implementation cost. Fluke/Danaher they have lot of experience with LTFLU and thats why they used it also in Keithley instead of LTZ1000.
-
Well perhaps.
Isn't everything that used the LTFLU, including the 8508A discontinued, why was LT still making it vs obsolescence? Or did Keithley find a pallet of these in Fluke's basement when the thought of the 7510 came up? :-DD
I presume LT was either still making them, or easy for them to do so, otherwise the LTZ would have been cheaper. Though I can't find a new price for the LTFLU anywhere.
Too bad for the secrecy, I guess we're not likely to find out more.
-
Well perhaps.
Isn't everything that used the LTFLU, including the 8508A discontinued, why was LT still making it vs obsolescence? Or did Keithley find a pallet of these in Fluke's basement when the thought of the 7510 came up? :-DD
I presume LT was either still making them, or easy for them to do so, otherwise the LTZ would have been cheaper. Though I can't find a new price for the LTFLU anywhere.
Too bad for the secrecy, I guess we're not likely to find out more.
Why do you think the 8508A is discontinued? It is still available on Flukes site, no hint of obsolescence.
The LTFLU is also used in all actual Fluke calibrators and references, like 5730, 732B, maybe very probably also in the 5790B, which are also not discontinued.
The LTFLU is a proprietary part, manufactured by Linear Technology exclusively for Fluke.
So no wonder you won't find a price, and can't buy it from them.
There's also no reason to believe that LT stops producing the LTFLU, like the LTZ1000, which is also still available, despite the limited sales volume.
Fluke for sure follows the policy to focus on the reference amplifier topology based on the SZA263/LTFLU type.
They also acquired Datron, including the 7001 reference (based on LTZ1000), and a few years later terminated this instrument.
That at first eliminated a competitor, but also saved cost, as their 732B was as good as the 7001, and they wouldn't have to maintain two different systems any more.
That includes all the validation / verification / monitoring for the references, where now they have to maintain one process and production line only.
Frank
-
Why do you think the 8508A is discontinued? It is still available on Flukes site, no hint of obsolescence.
A reference here:
https://www.eevblog.com/forum/testgear/its-no-fluke/msg14285/#msg14285 (https://www.eevblog.com/forum/testgear/its-no-fluke/msg14285/#msg14285)
And a search of a couple T&M sites also show it discontinued. Perhaps you can still get it from Fluke directly.
No worries though, maybe rolling off the line as we speak! :-+
-
Rumors only, obviously..and 5 years old..
If you search for really terminated products on Flukes site, you will get an explicit note, that they are terminated.
Very funny, as I'm reading your response, this ad just appeared on the bottom of the forum: :-DD
So, let's get a quote for it..
-
How a query for the 7001 looks like
-
Very interesting, thanks DM. :-+
-
But, that would be really expensive, and with the market as slow as it is for these kinds of parts, who is going to pay for the design work?
Well, let's suggest LT that they advertise a PhD or master thesis for such a redesign with all ideas given by the forum verified by the student :)
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But, that would be really expensive, and with the market as slow as it is for these kinds of parts, who is going to pay for the design work?
Well, let's suggest LT that they advertise a PhD or master thesis for such a redesign with all ideas given by the forum verified by the student :)
That's a good idea. Because their China counterparts can threaten their cushy market with the guys at bbs.38hot.net ;)
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The Zener in the LTFLU has about 4X the area that the LTZ's Zener has, so they can get away with the higher currents without reducing reliability or increasing time-drift [due to the interconnects changing properties].
Hi Ken,
Thanks for sharing!! You mentioned in the LTZ1000 thread reply from Bob Dobkin and he said that "You can safely bias the LTZ1000(A) with 20mA." So It looks like we can increase current also with LTZ1000.But I do not know if this can ruin long term drift and etc.
Let me know if you know cheap source of LTFU (including not working). Die image can follow ;)
-
By the way, I recently repaired Fluke 341A calibrator, which uses SZA263 reference. Here is the topic: https://www.eevblog.com/forum/metrology/fluke-341a-dc-voltage-calibrator/ (https://www.eevblog.com/forum/metrology/fluke-341a-dc-voltage-calibrator/)
One can see the reference in the video at 16:07
Here is the schematic from Fluke web site: http://assets.fluke.com/manuals/341A_343imeng0000.pdf (http://assets.fluke.com/manuals/341A_343imeng0000.pdf)
-
.....
Let me know if you know cheap source of LTFU (including not working). Die image can follow ;)
-
.....Let me know if you know cheap source of LTFU (including not working). Die image can follow ;)
[/quote]
Shenzhen...? :-//
http://www.alibaba.com/product-detail/-electronic-parts-and-components-Transistor_60305153923.html (http://www.alibaba.com/product-detail/-electronic-parts-and-components-Transistor_60305153923.html)
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Hi plesa,
Let me know if you know cheap source of LTFU (including not working). Die image can follow ;)
I've bought here today 5 pieces of LTFLU-1ACH. Price per piece: U$ 25.-- plus shipping plus Paypal fee.
http://goo.gl/aD205s (http://goo.gl/aD205s) (Link points to de.aliexpress.com)
Cheers,
BU508A
This is a photo I got from Barry (seller at this webshop)
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This is a photo I got from Barry (seller at this webshop)
Hello,
for me they look like fakes.
The printing orientation does not comply with the original ones.
(the pin 1 marker of the package in direction of the LT logo).
With best regards
Andreas
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This is a photo I got from Barry (seller at this webshop)
Hello,
for me they look like fakes.
The printing orientation does not comply with the original ones.
(the pin 1 marker of the package in direction of the LT logo).
With best regards
Andreas
I also suspect that these are fake ones.
-
For sure they are fake. All the same manufactured date? These could only be obtained as salvage surely?
Also, aren't the LTFLU normally matched with individually laser cut precision resistors as a set?
-
Hi Andreas,
for me they look like fakes.
The printing orientation does not comply with the original ones.
(the pin 1 marker of the package in direction of the LT logo).
The more I'm thinking about it, the more I think you and plesa are right.
I did some checkings before (they weren't obviously not good enough),
but this little detail did not come to my mind.
:palm: Pity me ... :-[ ;D
Ok, in Germany we would say "Lehrgeld bezahlt."
Maybe we can turn this little misfortune of mine into something useful.
What my plan was (and, well, still it is):
I have two Fluke 3330B, one operational, one for the parts.
Both are having a working 10V reference board, based on a SZA263.
Schematics: http://bama.edebris.com/download/fluke/3330b/3330b_partA.pdf (http://bama.edebris.com/download/fluke/3330b/3330b_partA.pdf) look at page 11
Now I want to build some PCBs and compare the new LTFLU-1 with
these old but rock solid boards from Fluke and find out, what can be improved
in respect of noise, stability etc..
Here is my suggestion:
- do some measurements and publish the data of the 10V Fluke 3330B reference boards
- build some PCBs based on the design of the boards above (I'm aware that, for example,
it is hard to nearly impossible to get the matching resistors from Fluke) but maybe some
low tempco high precision resistors will do a good job as well
- do the measurements on the new (fake) LTFLU-1 based references (probably I'll get some
day genuine ones ;) ) and publish the data
- identify some parameters which can be easily measured to determine, if it is fake or not
- open here a thread with information regarding fake units, how they can be identified and their sources
to warn other ones
Any ideas, suggestions etc. from your side are highly welcome. :-+
Regarding this fake pieces I'll keep you updated (if you are interested in).
Regards,
BU508A
actual measurement gear:
- HP 34401A (calibrated)
- Keithley DMM7510 (calibrated)
- looking for a 3458A
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For sure they are fake. All the same manufactured date? These could only be obtained as salvage surely?
Also, aren't the LTFLU normally matched with individually laser cut precision resistors as a set?
It is not manufactured in pairs, matching is later process step I suppose at Fluke. Probably after ageing procedure.
This set they are not selling,so it is more their internal set of matched components.
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@ BU508 your idea is very good but I have one problem. Some times a fake component mean "work" but far out of spec like fake sanken power transistor for audio or its not work at all like fake atmega uC now my question let say your parts is fake. Is it "work"
Sent from my GT-I9500 using Tapatalk
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'Any ideas, suggestions etc. from your side are highly welcome.'
BU508A, I think it would be interesting if you could get a REFAMP thread going. This one hardly went further than identifying the 4 pins ...
I have 2 old HP 'AMP. REF' original parts (HP used the SZA263 years before Fluke https://www.eevblog.com/forum/projects/pearls-of-the-past-%28only-for-the-voltaically-inclined%29/msg334139/#msg334139 (https://www.eevblog.com/forum/projects/pearls-of-the-past-%28only-for-the-voltaically-inclined%29/msg334139/#msg334139)) - one running, and also a LTFLU-1 from eBay (same seller as the OP of this thread bought from). But I haven't done much trying to adjust current for near zero TC yet.
I have sold my 2 HP 3458A LTZ1000 boards because I found it very frustrating to try to 'characterize' them with LM399 based instruments (34401 and 2x 34970). Having a reference that is more stable than your measurement instruments is nice - but then : What do I do now? If you can buy 3 or more 3458A meters you can at least start to study relative drift and group stability. But I don't have the cash for that :-\ .
So I will use more time on studying electronic circuits. A first goal for a new thread could be to present the (5-10 different) REFAMP based Fluke schematics and try to understand how they work and how to stabilize them.
Just my 2 satang.
-
I have sold my 2 HP 3458A LTZ1000 boards because I found it very frustrating to try to 'characterize' them with LM399 based instruments (34401 and 2x 34970). Having a reference that is more stable than your measurement instruments is nice - but then : What do I do now? If you can buy 3 or more 3458A meters you can at least start to study relative drift and group stability. But I don't have the cash for that :-\ .
So I will use more time on studying electronic circuits. A first goal for a new thread could be to present the (5-10 different) REFAMP based Fluke schematics and try to understand how they work and how to stabilize them.
Just my 2 satang.
The sale of these 2 references was a big error..
Instead, building some additional LTZ or SZA based references and comparing all of them would have been the correct action.
It is NOT necessary to have a better reference, several ones being 'on par' also do the job (man-with-3-clocks-problem)
You also don't need a 3458A as a reference (especially as it is less stable than a well / better designed external reference), you only need a device which is able to do comparisons between different references.
That can be done by LM399 based devices also, when you use them as a difference voltmeter.
If you are doing experiments with the FLUKE RefAmp, your findings will be appreciated.
Frank
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Well ... I guess the judgement of error or not is in the eyes of the beholder. Most things in life - including precision voltage stuff - is in my opinion a matter of priorities and choices.
Both boards are sold to members here, so my guess is that the refs will get a new life in this forum.
The first one went to Australia. The new owner has a 34461 and a 3458. By good luck his measurements are as close to mine as possible to measure (10uV=1ppm on 10v range). I have been following the board more than a year with three Agilent LM399 based 6.5 digits meters. There has been no changes except from temperature variation, and I have data from 20degC to 30degC showing an average TC for all three instruments much less than 0.5ppm/degree (from 7.5 digits RS232 readings).
Even better - here in the tropical climate of the jungle of NE Thailand I have never seen the board go lower than 7.20880 or higher than 7.20884 (maximum change of around 40-50uv or ca. 6-7ppm for 7.2v) on any of the three Agilent meters for temperatures between 11degC and 39degC, and relative humidity from around 40-85%.
Given the fact that the HP LTZ1000 ref board in question is probably 20 years old and the three Agilent meters (34401 + 2x 34970) are 12-16 years old, and the only changes I have observed over more than a year are repeated cycles over temperature change, I do not expect to be able to detect any long time change at all (if there is one, it must be sub ppm).
So I decided that sitting year after year watching a heated avalanche diode (LTZ1000) is not my preferred use of time. I will do other electronic circuit experimenting stuff.
The new owner is happy. He is now more confident that the 34461 is not only within spec, but closer to real value. And I 'know' (very confident) that one of my meters is quite spot on for the 10v range (because it shows the same value as the new in calibration 34461).
The second 3458 board with the AD5791 evaluation board is on it's way to Paris, France. The new owner, also a member here, might do more with the 20bit DAC. That would probably be of interest for many here.
So all in all, I am happy that the LTZ1000's have got new homes. I am very pleased by deciding that my three Agilent LM399 based instruments (that I 'know' better than my wife and children) are the absolute and final limit for all my future precision experiments. Jedem gefällt das Seine.
I might take your advise and try to post some of my opinions and experiments with temperature compensated reference amplifiers.
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I thought it could be useful to present some diagrams that in my opinion are essential for understanding REFAMP devices.
The first picture is a generic diagram for a series regulator with negative feedback and error amplifier and an undefined reference. The second picture is a schematic for a discrete transistor implementation of such a regulator with zener diode reference (unregulated zener bias).
The third picture is the HP 3450 implementation of such a regulator where is added a differential amplifier (for higher error amplification and hence better regulation) between the error amplifier output and the series regulating element. The fourth picture is the Fluke 341 implementation of the same thing. Although these devices are from around 1970 or so, the principle of the modern Fluke 732B is the same as these old discrete transistor implementations.
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Well ... I guess the judgement of error or not is in the eyes of the beholder. Most things in life - including precision voltage stuff - is in my opinion a matter of priorities and choices.
Hi, your original justification was quite different:
I have sold my 2 HP 3458A LTZ1000 boards because I found it very frustrating to try to 'characterize' them with LM399 based instruments (34401 and 2x 34970).
That reason is objectively wrong, as you can still compare 2 or more of these more stable references in differential mode.
But I can understand your latest reason, that you wanted to prioritize your researches differently.
A pity though, because there will be no comparison between these references possible, anymore.
Frank
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I thought it could be useful to plot the simplest circuit of 10V refamp both for reasons of understanding and for implementation, so here is my circuit:
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=206113;image)
The elimination of second divider makes Vc=Vb of Q1, which is actually a good thing.
The elimination of the output drive transistor makes the opamp a bit hotter but no big deal. The output is still short protected.
R1 and R2 should be matched pair of PWW or alike just like before.
R3 is for the temp-co adjustment, the value is varies according to the refamp. Weight is about 1/250, any reasonably good resistor will do.
(weight refers to the fraction of relative change of the output if R3 changes)
R4 is the current bias resistor with weight even less than that of R3.
C1 provide enough phase margin not only stabilize the whole circuit but also makes the output capacitor loadable.
U1 is OP97F just like that in 732B, where 11.5V supply is used. If a rail-rail output is used such as OPA188 or LTC2057, supply down to 10.5V is possible making the circuit 3-rechargeable-lithium-battery operational.
The principle can be now explained with ease: if for any reason the 10V is dropped, the divided voltage will also be dropped feeding to Q1 and the voltage of the collector will rise, U1 gives more thrust to compensate the drop thus remain the 10V stable.
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Having the amplification part directly at the reference amplifier was a big advantage in times when the OPs were not that good. As the OP in that circuit is not really critical - like R3 it only enters to a small part at the output. So you could use that circuit even with something like a CA3140 with hardly more drift. An extra OP for amplification of a 7 V reference like LTZ1000 should be good quality, as the drift fully appears at the output - though this is not a big deal anymore with modern OPs like OP177 or LTC2057.
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@DiligentMinds.com :
Thanks very much!
That's also true for me. I do have quite some LTZ1000, but few LTFLU/263 for experiment only.
Indeed, resistor based boost circuits are troublesome, one of my friends here have successfully developed a PWM circuit that perform very nice and I hope I can get some of his board soon.
On the other hand, that refamp circuit may be regarded as a 7V source, just fetch it from the joint of R1 and R2. I have made all the alterations of my 732A and 732Bs leading-out their internal 7V outside for measurement.
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I'm looking forward to seeing how things went with your friend's PWM boost experience. Are you going to post this on this forum?
Sure I'll post the test result when I get it. What I know is that he made a lot of versions and took long time to test for the noise, tempco, especially aging.
In the mean time, I have an idea of modifying the resistor-divider-based boosting circuit by employing a LTC1043 to reduce the sensitivity of R1/R2 to <5%, that is, more than 20 times better.
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Just a quick update:
I received today the 5 "LTFLU-1ACH", here are some pictures of them.
So, how to identify them, if they are fake?
I have here a reference board from a Fluke 3330B which runs with a SZA263 and my first idea was:
- solder out the original SZA263
- solder some wires to the solder points
- solder the new LTFLU-1ACH to the wires
- do some measurements and post the results here
But I do not really dare to solder out the SZA (which maybe would be a mistake) but I think using this board as a reference for comparison would be helpful.
Any suggestions / ideas?
Cheers,
BU508A
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I suggest that you make a board of you own with sockets for LTFLU-1. The simplest one will do, or use boardless solder technique, to test the tempco, stability and of course to test whether they are fake or not.
-
Just a quick update:
I received today the 5 "LTFLU-1ACH", here are some pictures of them.
So, how to identify them, if they are fake?
I have here a reference board from a Fluke 3330B which runs with a SZA263 and my first idea was:
- solder out the original SZA263
- solder some wires to the solder points
- solder the new LTFLU-1ACH to the wires
- do some measurements and post the results here
But I do not really dare to solder out the SZA (which maybe would be a mistake) but I think using this board as a reference for comparison would be helpful.
Any suggestions / ideas?
Cheers,
BU508A
Where did you purchase them?
I you want to check if they are genuine I can make teardown like we did with LTZ1000A from TiN or fake LTZ1000.
There will be signature on die for sure.
But this will damage the package, but we can have hire picture of LTFLU.
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So, how to identify them, if they are fake?
Any suggestions / ideas?
Hello,
on LT parts the pin 1 marker (nose) of the case is always on top side of printing.
(see also picture of 8842 on first side of the thread)
On your parts the pin 1 marker is at the right side.
So for me they look like fakes or at least re-stamped parts.
You cannot simply drop in a RefAmp into another cirquit.
You allways need to exchange the adjusted resistors of the complete RefAmp-Set together with the RefAmp.
with best regards
Andreas
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Hello Andreas,
thank you for your reply.
You cannot simply drop in a RefAmp into another cirquit.
You allways need to exchange the adjusted resistors of the complete RefAmp-Set together with the RefAmp.
Yes, I have suspected that, because Fluke states in the service manual of the 3330B that
the SZA263 (U2) and the resistors R2, R11 and R15 are selected parts and for a replacement they have
to be ordered together (they have together one part number).
I am just wondering, why they left out R12, which has to be ordered together with R13 as one bunch.
Do you think it could be of any use if I take an experimental PCB and assemble a similiar circuit based on the 3330B design but with standard components?
Best regards,
BU508A (Andreas, too ;) )
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Having a simple board with standard components would be one way to check if they at least are similar to the original.
You likely don't even need a printed board.
If they are fakes, chances are they are not even close in function - more like getting To99 versions of µA741 or µA723.
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I had the ridiculous pair of fake LTZ1000A - for some reason I was ridiculed by a troll for trying to test it before actually using it in a very expensive board with all the support circuitry etc. Simply measuring the heater was enough to prove they were fakes. Also, lopping off the can and looking at the die - Just like with TiNs real LTZ, plesa made a scan of my utter fake.
However this fake is impressive in that the leads are looking gold plated, etc. The giveaway are the exact same date codes and the tab orientation being wrong.
If you haven't okayed this AliExpress item I suggest lopping a sacrificial one and comparing the die with the published ones on here. In my LTZ case it was obviously fake.
Despite my protests and evidence to AliExpress the same seller carries on selling fake LTZ1000's. I did of course get a full refund. Strangely there are customers who are happy with them and upvote the seller! :palm:
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Do you think it could be of any use if I take an experimental PCB and assemble a similiar circuit based on the 3330B design but with standard components?
Hello Andreas,
should be possible.
There are enough infos in the tread how to adjust the T.C.
With best regards
Andreas
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However this fake is impressive in that the leads are looking gold plated, etc. The giveaway are the exact same date codes and the tab orientation being wrong.
Are gold leads an indicator of fakes !, I have a number of LTZ1000's which do not have gold leads and TIN now has 4 of them for further testing, (note the heater test showed approx 270 ohms, and they work in circuit!).
But as for these REF amps a mock up test circuit is the first step then if still in doubt hopefuly a sample will be sent to Plesa for some 'close ups'.
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Hi,
I found this on Alibaba:
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=211354;image)
What's the chance of these been fake, given the price and the ability to supply 1E27 parts per day!!!
(Given the world population is about 7.4E9)
Giving the price to 6 decimal places, is obviously an attempt to appeal to 'Volt nuts'.
Given the nature of this part, it was only used by Fluke, the chance of finding large quantities of New Old Stock, NOS, is very slim.
Regards,
Jay_Diddy_B
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However this fake is impressive in that the leads are looking gold plated, etc. The giveaway are the exact same date codes and the tab orientation being wrong.
Are gold leads an indicator of fakes !, I have a number of LTZ1000's which do not have gold leads and TIN now has 4 of them for further testing, (note the heater test showed approx 270 ohms, and they work in circuit!).
They should have the gold plated leads, but if they have been salvaged (by literally setting fire to the donor boards) then the chinese salvagers will spot weld new plain leads onto the stumps as part of the refurbishment. I can only imagine this causes havoc with the internally aged characteristics, etc, not to mention the now introduced thermal noise with the differing metal junctions.
-
They should have the gold plated leads,
The LTZ1000(A) can be ordered with gold plated leads (LEAD FREE) and with tinned leads according to datasheet.
So this is no indicator for the LTZ1000.
With best regards
Andreas
-
...
You cannot simply drop in a RefAmp into another cirquit.
You allways need to exchange the adjusted resistors of the complete RefAmp-Set together with the RefAmp.
...
This statement is true only as far as Fluke boxes are concerned AND if you expect a similarly low TC of the box after changing the refamp.
The refamp is only a transistor and a zener (avalanche mode) configured so as to trying to have the (current dependent) positive TC of the zener being cancelled by the negative TC of the transistor base-emitter junction (sometimes called the '+/-2mV effect').
(http://s002.radikal.ru/i197/1202/7e/ec99d09a44c1.gif)
This effect has been used in power supplies long before Fluke made their first voltage standard.
(https://www.eevblog.com/forum/metrology/the-ltflu-%28aka-sza263%29-reference-zener-diode-circuit/?action=dlattach;attach=205547;image)
ANY discrete NPN transistor and avalanche mode operating diode that is thermally connected will to some degree benefit from this cancelling effect. What Fluke has done is to characterize every refamp and supply it with resistors for it's optimal working current.
In HP3450A Hewlett Packard chose to use a SZA263 'drop-in' circuit with a cooling assembly. No resistors or other component are selected to match the refamp. May be they found it more convenient to reduce the TC of the box further by using a constant temperature chamber for the refamp in stead of 'nulling' the TC by supplying matched resistors for the refamp.
(https://www.eevblog.com/forum/projects/pearls-of-the-past-(only-for-the-voltaically-inclined)/?action=dlattach;attach=66329;image)
In the Fluke 732 series and other boxes one will find both temperature chamber (in that case an oven) and optimized resistors for minimum TC. This way the temperature dependence of the box is minimized by both low TC for the reference AND a constant temperature for the reference part of the box.
--- ooo ---
'+/-2mV effect': pp.38 in http://www.onsemi.com/pub_link/Collateral/HBD854-D.PDF (http://www.onsemi.com/pub_link/Collateral/HBD854-D.PDF)
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zlymex, is it possible to take higher res die photo?
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zlymex, is it possible to take higher res die photo?
I found that photo on the web :)
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zlymex, is it possible to take higher res die photo?
I found that photo on the web :)
BU508A offered one LTFLU which he purchased for teardown, It it will be genuine we can have hires photo soon.
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zlymex, is it possible to take higher res die photo?
I found that photo on the web :)
BU508A offered one LTFLU which he purchased for teardown, It it will be genuine we can have hires photo soon.
Good news. I have several pieces of SZA263/LTFLU-1 but very reluctant to tear them apart. :o
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zlymex, is it possible to take higher res die photo?
I found that photo on the web :)
BU508A offered one LTFLU which he purchased for teardown, It it will be genuine we can have hires photo soon.
Good news. I have several pieces of SZA263/LTFLU-1 but very reluctant to tear them apart. :o
We will see. There are some Fluke DMM with LTFLU, but I did not find it cheap enough on ebay for this experiment.
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If we don't end up with a high res photo from BU508A I've seen the Fluke DMMs sell for ~130 on ebay.
I'm willing to de-lid for the cause.
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I think there are die photos of the real LTFLU around somewhere.
The question would be if the cheap Chinese ones are real - if they are not and not working very well, than not much is lost to tear one apart. This might even if give a hint on what chip is inside and maybe even make use of them at all, even not as the original.
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I received today the 5 "LTFLU-1ACH", here are some pictures of them.
Any suggestions / ideas?
1. Fake. Another chip which has the same 4 pin package. Easy to measure if the behaviour is totally different from the LTFLU. Any candidate for a cheap 4 pin metal can IC?
2. Fake. Built from a zener and a transistor. Requires tools for welding the metal can. Correct function but poor stability.
3. Fake. Another chip with the same function. The original Motorola version was freely available and manufactured a long time and in large quantities. But the housing was a little different? Maybe some Chinese or East European copy exists?
4. Real. Fluke and Keithley multimeters are manufactured in China nowadays. Even if real, certainly not the highest grade and possibly even rejected.
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I once bought five LTFLU-1ACH with date code 9515, they looks the same as BU508A's. Functionally are all good by test and two of them were being used in DIY of voltage references with very nice performance. By adjusting the resistors at the collector, a near zero tempco can be achieved. There is this DH80417B in one of my 731A transfer standard, was that made by Motorola?
(https://www.eevblog.com/forum/metrology/teardown-voltage-standards/?action=dlattach;attach=211489;image)
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I assume, that it's from T.I.:
https://www.eevblog.com/forum/testgear/fluke-332baf-in-the-slaughterhouse/msg393627/#msg393627 (https://www.eevblog.com/forum/testgear/fluke-332baf-in-the-slaughterhouse/msg393627/#msg393627)
Frank
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Thanks Frank for the photo. Indeed the same reference IC inside TI's plastic container. I have two Fluke 335Ds but never dare to open those TI modules to see what inside. However, I find photos of other TI modules with Motorola SZA263 inside.
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Hello everyone,
My first post here and work is not terribly permitting of time so please be patient If I don't get back to questions in a timely manner...
Perhaps I can contribute some additional pictures of the die. The first is the Motorola SZA236 removed from a Fluke 732A. The second is an example of the LTFLU removed from a calibrator board. You'll have to forgive the picture quality. I don't have a great tool to do this.
Enjoy,
Dave
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Hello everyone,
My first post here and work is not terribly permitting of time so please be patient If I don't get back to questions in a timely manner...
Perhaps I can contribute some additional pictures of the die. The first is the Motorola SZA236 removed from a Fluke 732A. The second is an example of the LTFLU removed from a calibrator board. You'll have to forgive the picture quality. I don't have a great tool to do this.
Enjoy,
Dave
Great photo, Welcome here!
Interesting to see the first photo, the zener and the transistor were separated.
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(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=214917)
Could someone please explain where the zener can be found on LTFLU and what this multiple taps are for? What is the transistor here?
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(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=214917)
Could someone please explain where the zener can be found on LTFLU and what this multiple taps are for? What is the transistor here?
My guess, red area is two zeners in parallel, green area transistors, blue protection diodes.
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The blue and green areas are pad for bonding wire and probing ( measurement). Similar are on almost all IC including LTZ1000.
I agree that in center area is the buried zener itself and it looks like two mirrored structures.
Transistors are between the blue/green areas and center red area.
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Since the photo of LTFLU die is not very clear, we can only speculate about its layout.
I attached a picture with my guess (everything is described in the picture, green rectangles are possible locations of transistor(s)). It seems to me the symbol used for LTFLU-1 is rather simplified schematic while its internal structure is more complex.
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A better picture. Not much more I can do with a 30x microscope and a phone camera...
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Great photo, Welcome here!
Interesting to see the first photo, the zener and the transistor were separated.
Thanks Zlymex.
I've enjoyed reading through the zener related threads.
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It looks like LTFLU is not glued directly to package. I can imagine some thermal insulation material or something to relax stress to silicon.
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@fitzfish:
That's much better. Very interesting, thanks!
I think stripes at the top and on the bottom of your photo are presumably diffused resistors. They seem to be connected to pads only, so they may be used as a heater during trimming process?
I can also see two structures under the top metal in the center of the die.
It is definitely far more complex layout than the one in LTZ1000.
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Definitely right about the trimming fuses, some are blown and others not. There are a couple more in the top left area - in fact there's one on the thick track just above left of the first zener, a 'fuse after trim'?
P.S. Any chance that the long dark structure running the full length of the top of the die is a heater?
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I was told that the LTFLU has a heater, but Fluke decided not to use it.
Actually, there is another identical 'possibly heater' structure running along the bottom of the die (less clear because of the metalization above). The two appear to be wired in series and are terminated on the second and third pads from top right (1st one is bonded out).
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Thanks @fitzfish for the clear photo, not bad at all for a phone camera.
Some of the pads are for binary fuses. :scared:
Only "1" left un-blown, that seems to me not a trim at all, rather, it's a straight forward go(other devices could be the same).
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=215284;image)
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One more... This one uses highly oblique illumination to try to bring out the features obscured by the washout in the previous pictures that used diffuse direct illumination.
The fusible links are different on this sample indicating that they are trimmed for some purpose. Could the two arrays either side of the center band be a large transistor array that are trimmed to match it's TC to more closely cancel the TC of the paralleled zeners?
The fusible links that have been blown shine brightly with this illumination method.
Enjoy!
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Looking more closely, they don't look like transistors. They are probably just degenerating the compensating transistor with a trim-at-test resistor as you suggest.
The two now more visible devices in the center look like they could be a candidate for the compensation transistor...
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I wonder why the fuse between the Base lead and the upper resistor block (assuming that's what it is) is blown on both dies?
Edit: Or is that one the '16' fuse instead?
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Just a quick update:
I received today the 5 "LTFLU-1ACH", here are some pictures of them.
So, how to identify them, if they are fake?
I have here a reference board from a Fluke 3330B which runs with a SZA263 and my first idea was:
- solder out the original SZA263
- solder some wires to the solder points
- solder the new LTFLU-1ACH to the wires
- do some measurements and post the results here
But I do not really dare to solder out the SZA (which maybe would be a mistake) but I think using this board as a reference for comparison would be helpful.
Any suggestions / ideas?
Cheers,
BU508A
I today had the chance to open the part you have sent to me. While the pin 1 marker doesn't fit with the printing the package seems to contain a real LTFLU. If any pictures more are required please let me know. So no fake at all. Here's a first shot.
Sad it's now broken :(
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I today had the chance to open the part you have sent to me. While the pin 1 marker doesn't fit with the printing the package seems to contain a real LTFLU. If any pictures more are required please let me know. So no fake at all. Here's a first shot.
Sad it's now broken :(
Thank you very much branadic for your efforts. I've sent a second one to plesa, so let's see what he will find.
I'll do a test circuit for my other three LTFLU ones and will keep you updated here.
Thank you very much for looking into it, much appreciated. :-+
Cheers,
Andreas
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Great photo branadic!
I today had the chance to open the part you have sent to me. While the pin 1 marker doesn't fit with the printing the package seems to contain a real LTFLU. If any pictures more are required please let me know. So no fake at all. Here's a first shot.
Sad it's now broken :(
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Updated the picture with a more sharp one, after our microscope was newly adjusted today.
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Hello,
I am absolutely flabbergasted... I would have thought that for certain it was a fake. Wow! So, you might want to buy quite a few of these if you are interested as long as they are available from that supplier. Personally, I like the LTZ1000 better because I don't have to rely on precision resistor pairs to maintain the 7V-10V boost function, but if that is your plan then the LTFLU is probably the way to go [but it must be ovenized to eliminate hysteresis and make the drift rate constant].
I just placed another order of 5 pieces.
My plan is to build some circuits and do some measuring to bring some light into the specs of this circuit,
since there is no datasheet around (wasn't able to find one).
Perhaps it is possible then to compare the LTZ1000 and the LTFLU-1 which is imho interesting, because Fluke is using
the LTFLU-1 in their high precision gear (e.g. 732B or (probably) 8508A).
Btw, nice finding: http://www.edn.com/electronics-news/4389635/Go-inside-Fluke-s-electrical-metrology-lab (http://www.edn.com/electronics-news/4389635/Go-inside-Fluke-s-electrical-metrology-lab) :D
Cheers,
Andreas
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because Fluke is using the LTFLU-1 in their high precision gear (e.g. 732B or (probably) 8508A).
732B, 5700/20/30A, 5500/20/22, and yes, in the latest revision of the 8508A. The original 8508 used an LTZ1000.
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I just placed another order of 5 pieces.
Wow! For me Aliexpress shows "?6,221.33 / piece".
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Wow! For me Aliexpress shows "?6,221.33 / piece".
*lol* You should not believe that. :-) I asked for a quotation in my first order and he is selling it for 25 US Dollars per piece plus shipping plus PayPal fee. So, no worries, I didn't won a lottery nor I am Bill Gates. ;-)
Andreas
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Hello,
branadic sent me some pictures with a higher resolution. Every picture has around 10 MByte,
probably here are some people which can do an analysis of these pictures.
Here is the link:
http://www.mounty.de/LTFLU/index.html (http://www.mounty.de/LTFLU/index.html)
And thanks again to branadic and his efforts.
Regards,
Andreas
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While the puppy is already broken I could do some simple resistor measurements with some probe needles in 4w measurement technics and our K2002. This way we could find out, if the upper and lower bar is a heater and which resistance it has.
We do have a Keithley semiconductor analyzer but it's not worth the effort to dig into this gear.
Any measurement suggestion is welcome. Please briefly describe what, how and where to measure at the die and I will see what I can do.
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I'd like to know the resistance of those binary resistors(If they are resistors). One measurement of a large one(16 or 8) will do as the rest of them can be deducted.
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Revive! (http://www.greensmilies.com/smile/smiley_emoticons_zombie01.gif)
I have module with heated dual LTFLU coming my way :D. (No, not the 732)
Time to give those LTZs a worthy rivalry, what you folks say?
If anyone has ideas or theories to check on real circuit, let me know. I'd be happy to do that for upcoming article. :-DMM
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Is it the precursor to the 732, the 735...
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Is it the Fluke 5440B >:D
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Is it the precursor to the 732, the 735...
You mean one of these ? I only know about two in the wild. A member on volt-nuts group bought the other.
Sorry, this was taken late at night.
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Looking into both the 731(I would assume te 735C is closer to this one), and the 732. I would say that zlymex is correct. I don't spend much time learning about those newer calibrators.
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Is it the precursor to the 732, the 735...
You mean one of these ? I only know about two in the wild. A member on volt-nuts group bought the other.
Sorry, this was taken late at night.
Then you must bought that unit on ebay on March 2014.
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Looking into both the 731(I would assume te 735C is closer to this one), and the 732. I would say that zlymex is correct. I don't spend much time learning about those newer calibrators.
Newer calibrators such as 5720A also use heated dual LTFLU.
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That is certainly interesting, like a single oven variant of the 732.
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Is it the precursor to the 732, the 735...
You mean one of these ? I only know about two in the wild. A member on volt-nuts group bought the other.
Sorry, this was taken late at night.
Then you must bought that unit on ebay on March 2014.
That was me. Do you have any information regarding the 735C? I have no idea what the specs are but I am guessing they are close to the 732A.
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That was me. Do you have any information regarding the 735C? I have no idea what the specs are but I am guessing they are close to the 732A.
I have almost nothing about 735C except some photos from eBay and one cut from probably an article and I forgot the source.
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I have that same article. I guess a teardown will have to be planned on this.
Another big difference is that they used ni-cd batteries that are expensive to replace.
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teardown will have to be planned on this.
My screwdriver is ready! :-/O
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Hello,
branadic sent me some pictures with a higher resolution. Every picture has around 10 MByte,
probably here are some people which can do an analysis of these pictures.
Here is the link:
http://www.mounty.de/LTFLU/index.html (http://www.mounty.de/LTFLU/index.html)
And thanks again to branadic and his efforts.
Regards,
Andreas
I wonder if you already started setting up your LTFLUs?
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Hm-hm..
(https://xdevs.com/doc/Fluke/5700a/img/a11_tin/toys_1.jpg) (https://xdevs.com/doc/Fluke/5700a/img/a11_tin/toys.jpg)
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Who is this Cal person and why couldn't he?
Looks like your dual LTFLU has arrived and your screwdriver is on the job.
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I don't know about screwdriver (only one screw in whole thing), but camera is definately eating battery like crazy...
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Hm-hm..
(https://xdevs.com/doc/Fluke/5700a/img/a11_tin/toys_1.jpg) (https://xdevs.com/doc/Fluke/5700a/img/a11_tin/toys.jpg)
That is the A11 module from a Fluke 5700A/5720A :-* >:D :-+
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I just took Flukes oldest RefAmp, see here: https://www.eevblog.com/forum/testgear/fluke-332baf-in-the-slaughterhouse/msg393627/#msg393627 (https://www.eevblog.com/forum/testgear/fluke-332baf-in-the-slaughterhouse/msg393627/#msg393627) and modified it for 10V reference voltage.
(https://www.eevblog.com/forum/testgear/fluke-332baf-in-the-slaughterhouse/?action=dlattach;attach=82599;image)
That is, to replace the PWW 9k / 7k24 divider by 5k/10k for 10V,
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=268723;image)
and changing the other TF resistors which determine collector voltage and currents, so that these parameters stay unchanged.
This gives a similar circuit like in the 731B and 732A/B, and should behave identical, even if an SZA263 or LTFLU is used there.
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=268725;image)
Afterwards, I made slight disturbances on these 5 resistors by consecutively paralleling a 1M resistor, and then measuring the corresponding influence on the RefAmp, or the reference output voltage.
The relative decreases for each resistor was on the order of 0.15 .. 1%, and the relative changes of both voltages were attenuated, between several ppm and 3200ppm.
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=268727;image)
Dividing the relative resistor change by its corresponding relative voltage change, gives the attenuation or suppression factor for each resistor.
That means, any drift (i.e. time, temperature) of this specific resistor is attenuated by this factor.
As an example, a factor of 500 would reduce the effective T.C. of 50ppm/K for a TF resistor to 0.1ppm/K of voltage output.
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=268729;image)
This calculation was already done for the LTZ1000 circuit, which also gave attenuation factors between 75 and 700 for the different resistors.
Therefore, the 6.8V => 10V amplification resistors have to be ultra stable, as their drifts are attenuated by a factor of 3 only, whereas the other ones should also be PWW types at least, as their factors are all on the order of 170..500.
So the FLUKE type RefAmp circuit behaves - no wonder - very similar to the LTZ1000, in this aspect.
Its advantage is the direct 10V output, whereas the LTZ1000 has the advantage to deliver the raw 7.2V reference voltage as a low impedance output.
Frank
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Nicely done Dr. Frank. The study of suppression factors is something very useful, might save someone a lot of money (by not paying $50 for every resistor in the circuit :) )
If you now parallel that with a 10V LTZ1000 based reference you should get something very stable. I remember in one of Fluke papers that SZA263 drifts down and LTZ1000 drifts up (or the other way around). Averaging the two could provide ultra stable 10V reference, limited only by the output scaling resistors.
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The output scaling resistors are like the more difficult part than the reference chip itself. For a more stable 10 V reference it might be worth of using a more stable 7 to 10 V scaling.
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@ zlymex: You dont plan on opening the A11-module and showing us its glorious guts? Pretty please. ;D After all, its the best realised PWM-DAC as far as i know. Meanwhile my little brain still tries to comprehend the Datron 4911 -PWM-circuit...
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@ zlymex: You dont plan on opening the A11-module and showing us its glorious guts? Pretty please. ;D After all, its the best realised PWM-DAC as far as i know. Meanwhile my little brain still tries to comprehend the Datron 4911 -PWM-circuit...
I strongly suppose, that TiN on xDevs.com will do that (or done??) ;)
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For many years I've owned several Fluke 8800A 5.5 digit multimeters. I use them daily and I really like them a lot. Over time I picked up several more 8800A parts units too. The reference devices in several of my 8800A multimeters are labeled "SZA263" with the Motorola icon. But a couple of them look different (If anyone would like photos of the oddball reference devices I will post photos).
Recently I started a project to convert one of my "parts" 8800A multimeters into a reference voltage source, calibrated voltage divider, and buffer amplifier.
The 2 sections of the 8800A that I will be re-purposing are these:
The first part I will use is the internal +/- 1V references. This is the SZA263 section. The +1V and -1V each have individual trimpots for calibration. The +/- 1V outputs come from voltage dividers with 1K5 source resistance. So they can't be loaded. On the other hand, the +/- 7V outputs don't have individual trimpots, but they do have a low output impedance. I believe the trimpots can be used to bring either the +/-7V or the +/-1V to the exact target output voltage.
I will also use the 8800A input buffer amplifier. It has a very high input impedance, near 0 bias current, and when operating at a gain of 0.1 it can handle inputs of +/- 20V. On the other hand, the maximum output voltage available from the buffer amplifier is +/- 2V. But that's high enough for the things I plan to do with it. The buffer amplifier can be set to have a gain of 0.1, 1 (unity gain), or 10 by logic signals. There is also a separate 100:1 input voltage divider that can be switched in front of the buffer amplifier. It has a 10 meg input resistance, however.
I will be putting control switches onto the front of the 8800A panel in place of the original LED display and control levers:
One switch will have 3 positions, to select input from the front binding posts, or from the internal +1V or -1V reference voltage.
Another switch will configure the buffer amplifier gain to 1/10, 1, or 10.
And finally I will have a switch to enable the 100:1 input divider
The result will provide SZA263-based references voltages of +/- 1V and +/- 100mV
If I use the 100:1 input divider then I can also get +/-10mV and +/-1mV outputs
And finally it can operate with an external voltage reference or even a standard cell because the buffer amp has such a high input resistance
When I get things working I'll start a new thread for this project.
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Sounds great project. Keep us posted! :)
To my shame, I didn't do much yet with my Fluke boards..
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Just for curiosity, I had a look at the schematics of the Fluke8800. The reference circuit be itself is a little odd. While the SZA263 can be a very good reference, the circuit in the 8800 is strange, in that there are quite a few resistors involved to set the 1 V and also the 7 V voltage. The 7 V is not directly the internal 7 V of the reference itself. Also the OP (LM301) used is not very good by today's standard. The second strange point is, the the reference current is set by the 18 V supply and not as usual bootstrapped from the reference itself - so this is not a super stable reference circuit. As the SZA263 relies on the suitable individual resistors to get the best performance, it could be hard to change that.
The input amplifier is good for having the large range and stable divider. However the time 0.1 output is more like the divider just behind the amplifier. So it is high impedance, just like the 1 V output of the reference. So t would need an extra buffer. The x1 and times x 10 modes should be able to give more than 2 V too and could be low impedance.
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Just for curiosity, I had a look at the schematics of the Fluke8800. The reference circuit be itself is a little odd. While the SZA263 can be a very good reference, the circuit in the 8800 is strange, in that there are quite a few resistors involved to set the 1 V and also the 7 V voltage. The 7 V is not directly the internal 7 V of the reference itself. Also the OP (LM301) used is not very good by today's standard. The second strange point is, the reference current is set by the 18 V supply and not as usual bootstrapped from the reference itself - so this is not a super stable reference circuit. As the SZA263 relies on the suitable individual resistors to get the best performance, it could be hard to change that.
Most of the available schematics are for the oldest version, circa 1975. By 1978 Fluke changed U7 to LM308H (in T05 can). I was surprised to see Fluke using the LM301A myself!
Actually the 18V supply regulator itself is referenced to the 7V. Therefore the 18V should be rather stable. It is a somewhat byzantine circuit design overall!
The input amplifier is good for having the large range and stable divider. However the time 0.1 output is more like the divider just behind the amplifier. So it is high impedance, just like the 1 V output of the reference. So t would need an extra buffer. The x1 and times x 10 modes should be able to give more than 2 V too and could be low impedance.
Yes, I was thinking about adding a unity gain buffer for the output. Actually there is already an available opamp in the 8800A that could be re-purposed for this. It is U3, and it is already configured as a unity gain buffer. In the original design it is part of the dual-slope integrator A-D converter. But it could be rewired to perform as a unit gain output buffer with high input resistance.
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Comparison of voltage reference section of 2 different vintages of Fluke 8800A 5.5 digit multimeter:
Older unit (circa 1975) uses LM301A in reference circuit
Newer unit (circa 1979) uses higher performance LM308H in reference circuit
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Comparison of voltage reference section of 2 different vintages of Fluke 8800A 5.5 digit multimeter:
Older unit (circa 1975) uses LM301A in reference circuit
The reference amplifier, U9, in the older unit probably is the old T.I. reference, which was initially used in the 332/335 instruments also.
https://www.eevblog.com/forum/testgear/fluke-332baf-in-the-slaughterhouse/msg393627/#msg393627 (https://www.eevblog.com/forum/testgear/fluke-332baf-in-the-slaughterhouse/msg393627/#msg393627)
Frank
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Comparison of voltage reference section of 2 different vintages of Fluke 8800A 5.5 digit multimeter:
Older unit (circa 1975) uses LM301A in reference circuit
The reference amplifier, U9, in the older unit probably is the old T.I. reference, which was initially used in the 332/335 instruments also.
https://www.eevblog.com/forum/testgear/fluke-332baf-in-the-slaughterhouse/msg393627/#msg393627 (https://www.eevblog.com/forum/testgear/fluke-332baf-in-the-slaughterhouse/msg393627/#msg393627)
Frank
Yes that is the reference device in my oldest Fluke 8800A. Same markings: DH80417.
I have 4 other Fluke 8800A multimeters: They all have a Motorola branded SZA263. According to date codes my most recent 8800A was manufactured ini 1986.
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One thing that continues to surprise me about the SZ263 7V reference as implemented in the Fluke 8800A multimeter series is this:
These multimeters don't use an oven. The SZ263 is simply sitting right there on the main PC board alongside all the other components. From photos posted in this thread it appears that the newer 8840 and 8842 are similar: No oven.
Yet the TC seems to be excellent. I've heard that Fluke factory-selected the appropriate resistors to control the TC. They must have had a great system for doing that at the factory.
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One thing that continues to surprise me about the SZ263 7V reference as implemented in the Fluke 8800A multimeter series is this:
These multimeters don't use an oven. The SZ263 is simply sitting right there on the main PC board alongside all the other components. From photos posted in this thread it appears that the newer 8840 and 8842 are similar: No oven.
Yet the TC seems to be excellent. I've heard that Fluke factory-selected the appropriate resistors to control the TC. They must have had a great system for doing that at the factory.
The SZA263 can very well be trimmed to near zero T.C. by the zener and transistor currents (in contrast to the LTZ1000).
That's the basic principle of the 731A.
Fluke was great at T.C. compensating, either resistor pairs and reference amplifiers, also.
Frank
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Trimming to zero TC was quite common in the early days.
If you really want, one could also adjust the LTZ1000 to near zero TC without the heater. It just would need a rather low current and thus might not be very practical.
Edit: I was wron on the LTZ, it it would need a very high current and thus might not work to get zero TC with just a different current.
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My experimental modified Fluke 8800A is gradually coming together.
I am getting excellent results using the internal buffer amplifier in gain = x1 mode to buffer the +1.000000V and -1.000000V from the SZA263 reference circuit. I've got a very stable output and since it has a low output resistance I can use the +1V and -1V with devices that have 10 meg input resistance.
Problems:
The old-fashioned 2-layer PC board layout may have issues with DC offset voltages up to +/- 10 uV at different VSS (ground) points around the PC board. I can't zero out the DC offset from the buffer amplifier for all 3 levels of gain (10x, 1x, and 0.1x). I can get any one of them < +/- 1 uV but I can't get all 3 gains to zero without individual tweaking of each gain level. Worst case DC offset is about 100 uV at gain 10x.
I think there could be another issue too: The non-inverting input of the buffer amplifier has bias current compensation, but the inverting input does not. This might be an issue when using 10x gain. The buffer amplifier input stage uses bipolar transistors. I suppose Fluke went with the bipolar input stage because an FET input would have too much DC offset voltage drift?
Oh well I'm having a lot of fun chasing microvolts.
I'm also thinking about getting some modern opamps! That could be a really simple way to upgrade ancient gear!
This has gone far enough that I will start a new thread for my next post on the project. Not sure if I should put it in Metrology or in Projects category. Any recommendations from experienced forum members?
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A dual layer board is not a problem per se - it is a question on how good the layout is in using star ground and avoiding thermal EMF at resistors.
Not having bias compensation on the inverting input could be the reason to have different offsets.
A good OP like LT1012 could likely outperform the discrete input stage.
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Interesting to note, there is non-A LTFLU-1CH inside of today's 8842A teardown vid (https://youtu.be/X4_iRB2DIW8?t=11m39s) from Dave. I thought there are only LTFLU-1ACH version in existence. Perhaps same difference, as LTZ, different die attach material?
Now I need to buy 8842A too! :(
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As the LTFLU does not have an internal heater, the higher thermal resistance mounting does not make sense. I would more expect a lower grade (e.g. not so well tested) type of reference. This would make kind of sense as the 8842A is supposed to be only 5.5 digits - so it might not need the very best reference, still a lower grade LTFLU might still outperform an LM399.
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I have been living with 2 x 731B and a DMM7510 for over a year. Both of the 731Bs have a nonovenized SZA263 and the DMM7510 has an ovenized LTFLU.
The DMM7510 measuring the best 731B over large temperature swings in my house/office/lab never wanders more than 1ppm p-p. The std dev is usually less than 1/4 ppm.
Each of the 731B cost me about USD $250 and the DMM7510 is USD $4,000.
I am extremely happy with this 10V setup. No desire to spend any more $$$ trying to get a better 10V. I would go so far as to say I have the best 10V in my neighborhood.
-
TiN.
Good luck buying an 8842A. It looks like 9 were sold in the last 2 days. Strange how that happened. :-DD
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For time being I can only post dual LTFLU-1ACH stability figures (and K2002) over 6 day period (red plot):
(https://xdevs.com/doc/Fluke/5790A/test/ltflu_1.png) (https://xdevs.com/doc/Fluke/5790A/test/ltflu.png)
TC of 2002 is compensated by math. Temperature swing from 24.4C to 30+C ambient didn't cause LTFLU output to go outside of 1ppm window. :-DMM
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I've got 3 731s and re-selected resistors on one of them for near zero TC around room temperature. Fluke got them pretty good, but they can be tweaked a bit better without too much effort- cardboard box and a small light bulb.
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Today I've received some nice LTFLU-1CH pieces.
I've bought them here for USD 20.- per piece plus shipping, plus Paypal fee plus import taxes etc. (64 Euro):
https://www.alibaba.com/product-detail/new-and-original-electronic-component-ic_60518077974.html (https://www.alibaba.com/product-detail/new-and-original-electronic-component-ic_60518077974.html)
This is a picture from the Alibaba website:
(https://sc01.alicdn.com/kf/HTB1X7l5NVXXXXalXFXXq6xXFXXXp/new-and-original-electronic-component-ic-chips.jpg)
And here are some pictures of the LTFLU-1CH I've received:
(edit: added picture of complete collection 20170817)
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Getting the LTFLU chips is one part of a reference. These chips are usually use individually adjusted resistors to make them low TC. So one would need either a set of good individually selected resistors or a good temperature stabilization to make it a useful reference.
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Getting the LTFLU chips is one part of a reference. These chips are usually use individually adjusted resistors to make them low TC. So one would need either a set of good individually selected resistors or a good temperature stabilization to make it a useful reference.
This is very true. I'm working on it and will post my results here. One of my goals is to collect as much data as I can about the LTFLU-1 and post it here, so that there will be probably one day an alternative reference, which can compete with the LTZ1000-CH
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Pretty sure these are fakes, sorry.
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Pretty sure these are fakes, sorry.
Why?
The last time I've ordered LTFLU-1 on alibaba (different seller, though) a lot of people were convinced, that they are fake (including me). I sent one to branadic and he opened it and took some nice pictures from the chip. It looked very genuine. If somebody is interested, I can send him one piece to open and take some pictures from it from under the microscope.
Here are some links:
https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg957684/#msg957684 (https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg957684/#msg957684)
https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg959228/#msg959228 (https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg959228/#msg959228)
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Font is not LTCish enough to me. :) Units apperance from your first link (chips you got before) look way more credible than these.
I can check it for you with my microscope/DSLR setup if you like, as I wanted to build LTFLU version reference module as well (wanted to get 8842A for donoring parts to that purpose, but now after Dave's video chances are small).
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Font is not LTCish enough to me. :) Units apperance from your first link (chips you got before) look way more credible than these.
I can check it for you with my microscope/DSLR setup if you like, as I wanted to build LTFLU version reference module as well (wanted to get 8842A for donoring parts to that purpose, but now after Dave's video chances are small).
Why not getting a 8800A? They have the SZA263 in it and are much cheaper than the 8842A.
Thanks for the offer but I hope that somebody from .de is willing to do the photos. If nobody comes around the corner, I'll come back to your offer :)
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A first check to see if the chips are real would be to check the electrical performance. If they are acting like zener and transistor, chances are they are real or at least a good copy. There are not that many alternative chips that could give a similar function, though just a zener and transistor as separate parts would fit inside such a case.
Good fakes would be revealed looking at the noise and maybe zener resistance. If it shows low noise and low zener resistance there is essentially no alternative to a buried zener to get that performance - thus very likely a real part, maybe wrong date code.
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@ BU508A
You can directly compare the last batch with the current one on an electrical performance level. However, if you want I can again take some pictures from an uncapped part, but if they are again true parts we have killed another one :(
-branadic-
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Hello branadic,
You can directly compare the last batch with the current one on an electrical performance level. However, if you want I can again take some pictures from an uncapped part, but if they are again true parts we have killed another one :(
Thank you for your kind offer. I'll send you one piece of the LTFLU-1CH date code 9052 to you.
Yes, it is a pity to sacrifice them, but I cannot see any other way to verify until we have more electrical data.
I have attached a first draft for a testing circuit for these units. It is based on the 10V reference from a (my) Fluke 3330B.
@ everybody: Any comments to this draft are welcome, especially the critical ones. :-)
Thank you again for your help, branadic. Much appreciated. :-+
Edit: grammar typo
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Building a reference circuit and checking the performance might be good enough for a check. You may not be able to tell apart and LTFLU and the older SZA version, but one could tell the difference between a high class reference and a fake. There is no easy way to make a fake that is close the performance of the LTFLU. With a chip from somewhat questionable source one would need a throughout performance test anyway - even a new one directly from LT would need some burn in tests.
Even opening the can, will only show you which die it is. It could still be a damaged one or a low quality one (e.g. higher than normal noise). So looking at the chip will not show much about the actual quality - getting original chips (recovered from old instruments) with not so good performance is a real possibility.
The circuit combines the 7 to 10 V scaling with the reference. This kind of looks like the way the reference is supposed to be used. However it might be a good idea to look at the reference voltage only too.
I am not so sure about the two diodes at the reference - they would influence the TC quite a lot and might show aging as well.
The extra divider at the OP also might add quite some errors - it might be better to use something like a divider where the diodes and the 866 ohms are.
I would avoid a jumper directly in series with the zener or critical resistors - this only adds possible drift. I would consider using temperature stabilization, even if only crude: to get a low TC from well adjusted resistors would need some kind of temperature modulation to find the right values - so temperature control is needed in one way or the other. The constant temperature tends to be easier and stabilization could also help with the resistors.
The 4 µF filter capacitor looks rather large - it would determine the crossover where noise of the reference is traded in for the OPs noise - a low filter frequency only make sense with a good OP. I would guess there are more modern OPs than the LM308 with better performance. The gain of the transistor stage is a little lower (like half) in this circuit than in the LTZ1000 circuit - so the OPs performance is more important.
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@ everybody: Any comments to this draft are welcome, especially the critical ones. :-)
Hello,
I am not familiar with the 3330B cirquit but is it really intended to supply the LM308 from the stabilized 10V.
And is there enough headroom with the zener?
What is J9013? a current source? or is there a pull up resistor missing for the base of the power transistor.
If it is a 10 V source intended as voltage standard I am missing a short cirquit protection.
Further I would place some (EMI-) capacitors directly at the input and the output.
with best regards
Andreas
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BU508A,
you'd better copy the publicly known 732A circuit. It uses an SZA263, which should be compatible to the LTFLU:
The 732B also first used the SZA, therefore should have a very similar circuit around the RefAmp, like the 732A.
Later 732B models have the LTFLU instead, therefore, I assume that the circuit is dimensioned the same.
At first, get rid of these 1N4148 diodes, they may disturb the constant current to the zener.
Zener cc should be 3mA, so use 1k27.
The collector voltage should be lower, around 7V, as can be derived from the 732A schematic. So use 4k22 over 10k for the divider.
Then, the collector current, i.e. this 39k2 resistor, has to be trimmed to zero T.C. for the RefAmp voltage, i.e. UB or TP 5 in your circuit.
This has to be changed anyhow, due to the lower collector voltage.
The 10V divider should have zero T.C. , maybe by means of oven, and drift free resistors should be used.
I assume, you put the whole assembly inside an oven, at 45 .. 55°C?
Analogue circuit around SZA263 and LTFLU can be found in the manuals for the 5440A and the5720A , page 568.
They each contain two stacked references, so the calculation of the zener currents / resistors is a bit tricky.
Last hint: ZLYMEX published pictures of 732B interior somewhere else, maybe a hand made schematic also.
I don't remember, if this crucial collector-current resistor is visible.. But it would be useful, to know the value, just to have an idea of the ballpark of of the collector current.
In the end, if these are genuine LTFLUs, that easily can be proven by simply building this circuit , and measuring @ room temperature the RefAmp voltage, should be about 6.7V +/- about 10..20%, and determining the T.C. Fakes will quickly be identified by failing on these parameters.
Please don't crack them open!!
Frank
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The circuit with 2 stacked refs will not help very much. It uses a different scheme to provide the collector current (uses the upper reference to supply the lower one and seems to also use parts of the current canceling circuit (and thus a higher voltage but no 10 V of cause) for the upper one.
The 731 and 732 A/B circuits seem to be available: they don't use the diodes but otherwise a similar circuit, but slightly different values (especially smaller capacitors). Large caps might cause extra drift due to leakage and maybe even DA. So especially for the first test I would stay at the smaller values. When changing the caps one might have to check that the circuit will not oscillate - due to the extra gain from the transistor the caps are needed for stability. There is some room for tolerance, but they are not arbitrary. If in doubt a simulation with something like LTspice might be a good idea.
Simple current limiting, like in the later 731 refs might be a good idea, as it reduces the danger of damaging the ref. in case the circuit misbehaves.
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Hello Kleinstein,
thank you for your answer.
Building a reference circuit and checking the performance might be good enough for a check. You may not be able to tell apart and LTFLU and the older SZA version, but one could tell the difference between a high class reference and a fake. There is no easy way to make a fake that is close the performance of the LTFLU. With a chip from somewhat questionable source one would need a throughout performance test anyway - even a new one directly from LT would need some burn in tests.
That is my intention with this circuit: not building a reference yet, because there are better implementations around (e.g. Fluke 732A or 732B)
but to determine some of the parameters. And for this the 10V reference module from the 3330B seems to be a good starting point.
And another adavantage is: I have two of these 3330B modules here, so I can do some comparison. There is only one thing: these modules are having the SZA263 assembled instead of a LTFLU-1.
These are my modules:
(http://www.mounty.de/images/Fluke/3330B/3330B_2560000_1.JPG)
(http://www.mounty.de/images/Fluke/3330B/3330B_3995005_1.JPG)
Here you can find the manual including the schematics:
http://bama.edebris.com/manuals/fluke/3330b/ (http://bama.edebris.com/manuals/fluke/3330b/)
The schematic can be found in PartA, page 11
Here is a picture from it:
(http://www.mounty.de/images/Fluke/3330B/Fluke_3330B_10V_reference_module_revised_version.png)
Even opening the can, will only show you which die it is. It could still be a damaged one or a low quality one (e.g. higher than normal noise). So looking at the chip will not show much about the actual quality - getting original chips (recovered from old instruments) with not so good performance is a real possibility.
I am aware of this and due to the sources I got these pieces from I do not expect high performance units.
But perhaps with cracking open (sorry Dr. Frank) one piece of the LTFLU-1CH we can compare it with the other unit opened which was a LTFLU-1ACH and we will probably spot some differences. This could be also a valuable information.
And yes, I am looking for some damaged units (like 8800A. 8840A etc.) to get some genuine units.
I do not want to dissasemble my 10V ref. modules, they are too valuable for me.
The circuit combines the 7 to 10 V scaling with the reference. This kind of looks like the way the reference is supposed to be used. However it might be a good idea to look at the reference voltage only too.
Yes, I saw this style in several Fluke designes and it was my impression too, that this reference circuit should be used this way.
That is exactly what TP3 is intended for.
I am not so sure about the two diodes at the reference - they would influence the TC quite a lot and might show aging as well.
I am not sure as well. All I could find was a remark about the forwarding voltage of these diodes (2x 0.7V) and, mabye, some temp. coefficent compensation. I have choosen 1N4148 diodes, because I was not able to find out, what Fluke has used here. I know, that they are not a good choice, so they will just act as reminder and placeholder until I'll have a better idea or more information.
The extra divider at the OP also might add quite some errors - it might be better to use something like a divider where the diodes and the 866 ohms are.
My impression was, that this divider is part of the sensing. If the voltage is changing then this will be reflected in the neg. input of the OpAmp and since the pos. input is supposed to be stable, the output of the OpAmp will change to the opposite. But I may be wrong here.
I would avoid a jumper directly in series with the zener or critical resistors - this only adds possible drift. I would consider using temperature stabilization, even if only crude: to get a low TC from well adjusted resistors would need some kind of temperature modulation to find the right values - so temperature control is needed in one way or the other. The constant temperature tends to be easier and stabilization could also help with the resistors.
These jumpers are there for two reasons:
- while open one can do some current measurements
- replace the 50 Ohms trim poti with an external resistor decade for doing some experiments.
Yes, I know, this will add some additional disturbance. But this is just the first attempt and a starting point to learn and improve.
I have also planned to build a temperature controlled enviroment for all measuring. I have also planned to measure the atmospheric pressure as well.
The 4 µF filter capacitor looks rather large - it would determine the crossover where noise of the reference is traded in for the OPs noise - a low filter frequency only make sense with a good OP. I would guess there are more modern OPs than the LM308 with better performance. The gain of the transistor stage is a little lower (like half) in this circuit than in the LTZ1000 circuit - so the OPs performance is more important.
Yes, that is why I have put this remark "will be replaced" into the schematic. You have suggested earlier in this thread to use a LT1012.
I have on my list the LT1007 and LT1012. I am not sure, which one is the better choice, so I will try both. I have the LT1012 lounging around, so I can start with it. The LT1007 I'll order soon (with some other stuff). If you have other suggestions, I'll be glad to hear them.
Thanks for your input.
Andreas
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Hello Andreas,
thank you for your answer.
I am not familiar with the 3330B cirquit but is it really intended to supply the LM308 from the stabilized 10V.
And is there enough headroom with the zener?
Yes, I think so. Fluke did it this way, I just copied it more or less from the original schematic. Please see my answer to Kleinstein.
What is J9013? a current source? or is there a pull up resistor missing for the base of the power transistor.
The J9013 is a so called backward diode, also known as back diode, which is a kind of a tunnel diode.
https://en.wikipedia.org/wiki/Backward_diode (https://en.wikipedia.org/wiki/Backward_diode)
But, to be honest: I have no idea what this diode is doing here and why it is used in this direction.
This is one of the mysteries which I wasn't able to solve. :-//
Sometimes you can find a combination of a Schottky diode with a transistor like this:
https://www.quora.com/What-are-the-different-types-of-diodes# (https://www.quora.com/What-are-the-different-types-of-diodes#)!n=12
Scroll down to "7. Schottky diode -"
If it is a 10 V source intended as voltage standard I am missing a short cirquit protection.
Further I would place some (EMI-) capacitors directly at the input and the output.
No, this circuit is not to be meant as 10V source standard. It is intended to get some data and doing measurements around the LTFLU-1. Later (likely much much later) there will be something like a 10V source standard, based on a LTFLU-1. Well, this is one of my goals. But if can achieve it, I don't know. But I will try (and hoping of a little help from this kind forum. :D :-+ )
Anyway, I will create a website around this project and keep you folks informed about the wins and fails (especially the fails ;D )
Andreas
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In most of the places one can indirectly measure the current, as the voltage drop at the resistors. In addition this is with very little disturbance of the circuit.
For trimming one could add a resistor in parallel. This also the more usual way to trim stable resistors. So for small adjustments to a 10 K resistor one might add in parallel 100 K in series with a variable 1 K for adjustment. It is the other direction, but would cause less disturbance if unused.
After a closer look, the extra divider seems to be not that critical: it is behind the gain of the transistor and thus only enters with a sensitivity of about 1/100. The other place for the divider would not be much different / better. Other circuits seem to use it as well.
The about 7 V value suggested by Dr. Frank is more sensible, as is leaves more voltage for the resistor and thus increases the gain of the transistor stage.
The 731 / 732 work without the diodes and a different resistor value instead. Even if it might work with the right diodes, using just a resistors (e.g. 1.2x K range) is much easier and less uncertainty in finding the right diodes. The diodes are expected to shift the TC more towards a positive value. This would would correspond to a lower current for the transistor. So one could consider leaving the option to have a diode or two, but usually start with a wire bridge. As the diode is more for adding TC - my best guess would be more like 1 higher current diode (like a glass passivated 1 A diode).
There is not much to gain from opening another chip. The other can still be bad. It is only if the chip turn out to be bad, that one might want to open it. Even if only the zener part is working well, I would not damage it.
For the OP, the LT1007 / OP27 are little on the high side with current noise - so not a good choice. A more suitable one for low noise would be more like an LT1001 or OPA177. The LT1012 is lower bias / current noise than needed. An OP07 should be OK too. With the large caps the OP might want a series resistor for protection in case of a short at the output. The best choice of OP somewhat depends on the transistor current - this could be different without the diodes and also different between samples as the current is used to trim the TC.
I don't think there is a big advantage in using such large caps - the 732 / 731 use something more like 1/10 the size, which is much more practical, especially for a first test, where you want to see the reference noise.
The gain setting divider could be just 2 resistors - with optional later trimming with a parallel part.
From the circuit function the backward diode should be kind of a constant current source - so could be a small JFET. Just a resistor should also work, if the input voltage is reasonable stable.
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Hello Dr. Frank,
thank you for your answer.
you'd better copy the publicly known 732A circuit. It uses an SZA263, which should be compatible to the LTFLU:
The 732B also first used the SZA, therefore should have a very similar circuit around the RefAmp, like the 732A.
Later 732B models have the LTFLU instead, therefore, I assume that the circuit is dimensioned the same.
I will do that. Later. But for my first steps here I decided to use the circuit from the 3330B. Reasons:
- I have two 10V modules of this kind (no plans to dissamble them but using as a comparing reference)
- less complex
- probably I can start with some cheap low tempco resistors, e.g. the ones from TT Elecrtronics:
http://www.mouser.de/TT-Electronics/Passive-Components/Resistors/Film-Resistors/Metal-Film-Resistors-Through-Hole/RC55LF-Series/_/N-7gz41?P=1yvjsv8Z1yzv4td (http://www.mouser.de/TT-Electronics/Passive-Components/Resistors/Film-Resistors/Metal-Film-Resistors-Through-Hole/RC55LF-Series/_/N-7gz41?P=1yvjsv8Z1yzv4td)
Joe Geller used them in his SVR.
At first, get rid of these 1N4148 diodes, they may disturb the constant current to the zener.
Zener cc should be 3mA, so use 1k27.
The 1N4148 are placeholders, I will not use them. I have not identified yet, what Fluke has used here, I hope I will and then I will use the proper diodes there.
Btw, the sticker on my modules mention a Zener current of about 31µA, resp. 40µA, so 3mA looks a bit high to me.
The collector voltage should be lower, around 7V, as can be derived from the 732A schematic. So use 4k22 over 10k for the divider.
I will try that.
Then, the collector current, i.e. this 39k2 resistor, has to be trimmed to zero T.C. for the RefAmp voltage, i.e. UB or TP 5 in your circuit.
This has to be changed anyhow, due to the lower collector voltage.
Yes. the 39.2kOhm, the LTFLU-1, the 100 and the 60 Ohms resistors are all part of one set, which can be ordered at Fluke. They have to match. Same is true for these two wire wound resistors 5.9kOhms and 10.932kOhms, this is a matched pair.
The 10V divider should have zero T.C. , maybe by means of oven, and drift free resistors should be used.
I assume, you put the whole assembly inside an oven, at 45 .. 55°C?
Yes. I plan to build a temp. controlled oven with some peltier elements. I will place this in an Aluminium case and will isolate this with foam in a bigger case. All kind of electronics which has nothing to do with the LTFLU-1 circuit will stay outside of this Aluminium case (except for the
Pt-100 sensors, of course).
Analogue circuit around SZA263 and LTFLU can be found in the manuals for the 5440A and the5720A , page 568.
They each contain two stacked references, so the calculation of the zener currents / resistors is a bit tricky.
Yes, I had a look into these manuals. And to be honest: I have some diffieculties to understand how they are working.
Over the weekend I have ordered this book:
http://www.ebay.com/itm/182712686876 (http://www.ebay.com/itm/182712686876)
I hope it will help me understanding, how those things are working.
Last hint: ZLYMEX published pictures of 732B interior somewhere else, maybe a hand made schematic also.
I don't remember, if this crucial collector-current resistor is visible.. But it would be useful, to know the value, just to have an idea of the ballpark of of the collector current.
Thanks for the hint, I will look for it.
In the end, if these are genuine LTFLUs, that easily can be proven by simply building this circuit , and measuring @ room temperature the RefAmp voltage, should be about 6.7V +/- about 10..20%, and determining the T.C. Fakes will quickly be identified by failing on these parameters.
I had the same thought, too, but I was a bit unsure about the absolute values. Except for those two mentioned on the sticker.
Interestingly, they are measured at the base of the transistor, not at the cathode of the Zener / emitter of the transistor.
I did it and found a Uz=5.99034V (measured with a calibrated DMM 7510) for one of the modules.
Please don't crack them open!!
Too late, one piece is on it's way to branadic. Perhaps we can find out this way, if there is a difference between a LTFLU-1CH and a LTFLU-1ACH, if there is one. Or do you know by chance what the difference is?
Andreas
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Hello Andreas,
thank you for your answer.
I am not familiar with the 3330B cirquit but is it really intended to supply the LM308 from the stabilized 10V.
And is there enough headroom with the zener?
Yes, I think so. Fluke did it this way, I just copied it more or less from the original schematic. Please see my answer to Kleinstein.
What is J9013? a current source? or is there a pull up resistor missing for the base of the power transistor.
The J9013 is a so called backward diode, also known as back diode, which is a kind of a tunnel diode.
https://en.wikipedia.org/wiki/Backward_diode (https://en.wikipedia.org/wiki/Backward_diode)
But, to be honest: I have no idea what this diode is doing here and why it is used in this direction.
This is one of the mysteries which I wasn't able to solve. :-//
Sometimes you can find a combination of a Schottky diode with a transistor like this:
https://www.quora.com/What-are-the-different-types-of-diodes# (https://www.quora.com/What-are-the-different-types-of-diodes#)!n=12
Scroll down to "7. Schottky diode -"
If it is a 10 V source intended as voltage standard I am missing a short cirquit protection.
Further I would place some (EMI-) capacitors directly at the input and the output.
No, this circuit is not to be meant as 10V source standard. It is intended to get some data and doing measurements around the LTFLU-1. Later (likely much much later) there will be something like a 10V source standard, based on a LTFLU-1. Well, this is one of my goals. But if can achieve it, I don't know. But I will try (and hoping of a little help from this kind forum. :D :-+ )
Anyway, I will create a website around this project and keep you folks informed about the wins and fails (especially the fails ;D )
Andreas
The part marked J9013 is a constant current diode which is essentially a JFET with the gate shorted to the source. They use a similar circuit in the 731B where the diode is listed as "Diode, FET, current regulator" with part number E505. Unfortunately, Fluke used the tunnel diode symbol for these current regulating 'diodes'.
In this circuit, it provides 1mA to the base of the pass transistor, Q1. Regulation is achieved by the op-amp shunting this current to ground via the 5.6V zener. The zener moves the required output from the op-amp to 5V for a 10V regulated output so it's not near either rail and allows powering the op-amp from the regulated output.
Orin.
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But for my first steps here I decided to use the circuit from the 3330B. Reasons:
- I have two 10V modules of this kind (no plans to dissamble them but using as a comparing reference)
- less complex
Well this PCB can be used to test your LTFLUs, as this is THE intended circuit for this kind of Reference Amplifier.
That means, it always serves two purposes:
1) Provide constant current for the zener and correct collector current for the serial transistor, for zero T.C.
2) create temperature and timely stable reference voltage, which is always between ground and base of transistor (The LTZ1000 is identical in this sense, but has different topology)
3) Always provide a stable amplified reference voltage to 10V or 15V (inside Fluke 335D). Advantage: only one OpAmp is used for both purposes!
The 10V stability depends mainly on the amplification resistors used. The RefAmp voltage in comparison, is much more stable.
The 1N4148 are placeholders, I will not use them. I have not identified yet, what Fluke has used here, I hope I will and then I will use the proper diodes there.
Btw, the sticker on my modules mention a Zener current of about 31µA, resp. 40µA, so 3mA looks a bit high to me.
No, that's not the zener current. If you calculate all voltages in the 3330 circuit, you will see, that the zener current is also 3mA. Would be much too noisy, with 31uA.
Maybe, the LTFLU circuit in the 732B has even higher zener current.
Nope, these 31uA is the collector current for zero T.C.
I have a similar sticker on my 332B/AF reference, 'I =92', that's the collector current, 92uA, which fits to the collector resistor.
Yes, I had a look into these manuals. And to be honest: I have some diffieculties to understand how they are working.
Yeah, I also did not yet calculate the whole circuit, it's really tricky.
But if you do, I bet that the zener current will also be a smooth value of 3, 4 or 5mA. That would be the main Erkenntnisgewinn
from these circuits, how to optimally use the LTFLU.
The collector current again will bring T.C. to zero, and has to be tested for each individual RefAmp (on the order of several ten uA) and that's all, what's critical about the RefAmp circuit.
The amplification to 10V or another random voltage, is present also in these circuits, as otherwise they won't work.
This amplified voltage is not used in these calibrators, therefore the calibrator outputs in their 10V range are principally much more stable , than the 732A/B, as the scaling is done by a much more stable PWM.
If you don't need the 10.000V, simply buffer the raw ~6.7 V RefAmp reference, like we've done for the LTZ1000 circuit, and you're done.
I had the same thought, too, but I was a bit unsure about the absolute values. Except for those two mentioned on the sticker.
Interestingly, they are measured at the base of the transistor, not at the cathode of the Zener / emitter of the transistor.
I did it and found a Uz=5.99034V (measured with a calibrated DMM 7510) for one of the modules.
Well, you really have to understand the RefAmp circuit. The 'raw' reference voltage is ALWAYS the sum of the zener and the base-emitter voltage. That's identical to the LTZ1000 circuit.
Time stability is achieved mainly by the buried zener. Temperature stability is achieved by this zener / Ube combination, as the zener has a positive, Ube a negative characteristic.
The LTFLU / SZA obviously is better in this aspect, as the LTZ1000, due to a smaller zener voltage, which matches better with the T.C. of a pn structure, which in the end allows for a really precise zero T.C. trimming.
The LTZ1000 sucks in this aspect, as it always has about +50ppm/°C w/o oven, and is not really trimmable to zero T.C., any further.
Too late, one piece is on it's way to branadic.
branadic should not open it.. I know him personally, and I think he's a cultivated person..
Frank
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I know him personally, and I think he's a cultivated person
Thanks for the flowers, but if Andreas wants me to make a picture of the chip I will go ahead. Otherwise I can send him the part back. By the way, I still have the other opened sucker on my desk.
-branadic-
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I have analyzed the RefAmp twin applications 732A versus 732B and 5440A versus 5720A.
ZLYMEXs teardowns and a brief analysis for the 10V references are here: https://www.eevblog.com/forum/metrology/teardown-voltage-standards/msg902855/#msg902855. (https://www.eevblog.com/forum/metrology/teardown-voltage-standards/msg902855/#msg902855.)
There are pictures of a 732B with an LTFLU-1 CH, with 6.6542V reference voltage.
In the principle schematic from the 732B manual, it's obvious, that the topology of the 732B is an exact copy from the 732A.
The collector voltage over the RefAmp is 7.024V, nearly the same as the 732A, 7.032V.
So it's very probable, that the zener diode of the LTFLU is also driven with 3.0mA.
The 732B additionally has three current cancellation circuits.
The reference inside the 5720A is a nearly exact copy of the 5440A, but features also additional current cancellation circuits.
In this stacked circuit, the collector-base voltage (Ucb) of each of the SZA263 / LTFLU RefAmp transistors is kept at zero difference.
By the way, that's the same mode as in the LTZ1000 reference.
This is the only difference to the 732A/B modus of operation of the RefAmps.
Also, both stacked RefAmp were operated independently from each other, regarding potentials and currents.
The amplified voltage of 19.8V is analogous to the 10V in the 732A/B, and commonly used for both stacked LTFLUs to create constant currents for the zeners and the collectors.
The 5440A schematic is easier to understand, as all signal paths are contained on one sheet.
With this knowledge, the complete analysis of the circuit is easily done, see diagram:
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=344192;image)
The essential insight yields, that the zener diode current is also exactly 3.0mA, and that the T.C. is trimmed by the collector current, on the order of several ten uA.
So I again recommend to test the LTFLUs with the 732A dimensioning.
Frank
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The circuit inside the 731 and 3330 is very similar - mainly without the diodes and smaller capacitors in the 731.
Die Influence of the diodes on the TC and thus the resulting transistor current to get zero TC is rather small. My rough calculation would give some 500 ppm/K for the zener current and maybe 3-5 ppm/ for the TC at constant transistor current. To compensate it would take current that is some 5 % lower. So nothing really significant and thus not really worth having extra diodes. One diode might make current compensation slightly simpler - but no real need with a single reference.
For temperature stabilization I would not consider an peltier element: It is best to have a temperature slightly above room temperature to reduce the humidity too. Cooling is problematic as is can cause humidity problems. The peltier element itself would provide a significant heat path an thus a rather small (and hard to get) peltier element would be needed. The heater only system could have better thermal insulation and thus a more thermal low pass filtering of external variations. It is also easier to distribute the heaters - compared to using several super small peltier elements.
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Hello branadic,
Thanks for the flowers, but if Andreas wants me to make a picture of the chip I will go ahead. Otherwise I can send him the part back. By the way, I still have the other opened sucker on my desk.
I know, it is not a nice move to crack such things open. But, I really want to know, how the chip inside is looking.
And, beside that. the piece I sent you is a LTFLU-1CH, the one you have on your desk is a LTFLU-1ACH. Curious, if there is a difference.
Maybe in a few days we'll know more.
If you want to send them back to me, please let me know. I'll pay for the returning.
Thanks,
Andreas
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Thanks for the flowers...
You guys are a hoot ;D
Thanks for a great thread, all!
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Opened the part today that I received from Andreas and yes it is again looking like a real LTFLU. Couldn't find a significant difference between A and non-A version, maybe I have to take a closer look. I took a few pictures, I'm sure Andreas will share them here.
-branadic-
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Hello branadic,
thank you very much for taking the pictures. Much appreciated it. :-+
It seems to me (I am not an expert in semiconductor manufacturing) that there is no obvious electrical difference between the LTFLU-1CH and the LTFLU-1ACH. Any ideas, why they put in this "A" into the name?
The LTFLU1-ACH is the left one in these two difference pictures below.
And, as promised, here are the pictures:
(http://www.mounty.de/LTFLU-1CH/LTFLU-1x.jpg)
(http://www.mounty.de/LTFLU-1CH/LTFLU-2x.jpg)
(http://www.mounty.de/LTFLU-1CH/LTFLU-2x1.jpg)
(http://www.mounty.de/LTFLU-1CH/LTFLU-3x.jpg)
(http://www.mounty.de/LTFLU-1CH/LTFLU-4x.jpg)
(http://www.mounty.de/LTFLU-1CH/LTFLU-6x.jpg)
Difference pictures:
(http://www.mounty.de/LTFLU-1CH/LTFLU-diff.jpg)
(http://www.mounty.de/LTFLU-1CH/LTFLU-diff2.jpg)
Link to the website:
http://www.mounty.de/LTFLU-1CH/ (http://www.mounty.de/LTFLU-1CH/)
And, for convenience and compariosn reasons, the link to the LTFLU-1ACH pictures:
http://www.mounty.de/LTFLU-1ACH/ (http://www.mounty.de/LTFLU-1ACH/)
@TiN: If you want, you can take the pictures and store them on xdevs.com as well.
Regards,
Andreas
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I'm pretty sure the difference is the die attach. As you can see in the former opened LTFLU that the die attach is deviated while the latest pictures shows a well defined area for the die attach. They will not have changed the die itself.
-branadic-
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Great shots. What setup was used for such magnification?
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My SZA (older than me) just arrived.
I spent 25 USD and an unlimited amount of time searching on eBay (plus almost double $ for shipping). I was lucky TiN arrived late on this :-).
Meter is almost spot on at 10V so it should worth to salvage it and try something different.
Any good advice to where to start? I have an schematic saved somewhere form some hot Chinese BBS ... I need to start looking at it.(https://uploads.tapatalk-cdn.com/20171010/7983a3514d60742a92de08604f9ddde1.jpg)
Inviato dal mio Nexus 6P utilizzando Tapatalk
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What are your plans? There was no luck, TiN doesn't hunt for 8840A. :)
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My plans are to amuse me in some other voltage reference rabbit hole ... kinda bored of LTZ1000 ones.
Some mini hovenized SZA reference should keep me busy for some time.
Inviato dal mio Nexus 6P utilizzando Tapatalk
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An ovenized reference circuit can keep you busy for a while. However leaving out the oven does not necessary make is much faster: the oven helps to adjust the TC and without the oven the TC needs to be adjusted even more accurate. The oven could allow for a less accurate TC trimming and a good TC trimming would alow a less critical oven - so quite some choices. A crude oven might be easier than no oven.
With the SZA type reference there can be a few more options in how to build the circuit, especially if combined with a 7 to 10 V scaling.
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An ovenized reference circuit can keep you busy for a while. However leaving out the oven does not necessary make is much faster: the oven helps to adjust the TC and without the oven the TC needs to be adjusted even more accurate. The oven could allow for a less accurate TC trimming and a good TC trimming would alow a less critical oven - so quite some choices. A crude oven might be easier than no oven.
With the SZA type reference there can be a few more options in how to build the circuit, especially if combined with a 7 to 10 V scaling.
Wondering if I can use the provided resistors as starting point to find optimal current for TC ... this can be a good way to begin before to think at the hoven.
Inviato dal mio Nexus 6P utilizzando Tapatalk
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The originally used resistors and circuit would be very good starting point. As that reference looks like being used without a heater should have a well chosen set of resistors. AFAIK these references come as a set with resistors. So if they were to sell these refs, this would be more like a set of chip and 2 or 3 resistors.
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As far as I understand the original resistors cannot be used, they are tuned for 6.7V in this instrument and not for the 10V version.
If I run the SZA at 20ma ... can I use it as self heating and use the same method of the "pulsed heater" in LTZ1000 to find the zero TC resistor?
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Surely you can use the original resistors, if you intend to replicate a 10V reference, and if the circuit also delivers 10V as an internal reference.
At least, you should use the same currents, as in this DMM , i.e. about 3mA for the zener, and some tens of uA for the collector current.
If you want to damage the device, go ahead and use 20mA ,but heating the reference that way, I think is quite useless, and a brute force method.
Better build an oven assembly, for 45..55°C, like in the 732A/B and use the raw 6.7V output as an ultrastable reference..the 10V output, anyhow might not be as stable,or you also include these amplification resistors in the oven assembly.
Frank
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Frank,
are you sure 20mA is too much? I understand (from this thread) that SZA236 ha 4 time the area of LTZ1000 and can be run at higher current than LTZ1000.
Than there was this lines few posts ago of Bob Dobkin and he said that "You can safely bias the LTZ1000(A) with 20mA." so this should be good also for the SZA236 that has bigger area.
If than I'm not wrong Fluke 732a uses almost 8mA ... that is half of 20mA.
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For a different (higher) current one would also need a different set of resistors to keep a small TC. If one knows how one might be able to calculate the new values from the old one. Even with a higher current the voltage will not go up by much. Going from 6.7-6.8 V up to 10 V is a second step.
The main advantage of a higher current would be slightly lower noise. The downside is more self heating and thus a higher temperature and in case a oven is used more power needed to keep the temperature. As a crude rule of thumb the oven needs at least the same power as the devices inside, possibly more. The stability will likely not be better at a higher current.
It is more like that at 20 mA there will be a lot of heat and thus the reference would be more sensitive to orientation / air pressure.
The larger die / structure does to directly result in more current capability. The limit due to bond wires can be the same (or may be even lower).
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are you sure 20mA is too much?
The SZA263 should be used with a Zener current of 3mA and a Collector current of about 50 uA, otherwise will be quite impossible to find a zero Tc bias.
The collector current is used to trim the Tc: increasing current will make the Tc more positive, and vice-versa.
In the Fluke 8800 the resistors are calculated for the internal bootstrap supply of 18V, so are useless if you need 7 or 10 V.
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Also more heat on die means larger thermal gradients, and larger thermal EMF's around other parts.
Heat and oven itself does not make the reference better or more stable, actually the opposite, accelerate aging and increase noise (typical for most of semiconductors, not just the reference/zener junction). But with ultimate precision requirements, having ambient temperature variations is larger evil than these introduced instability factors from the small oven, so hence designs like LM399, LTZ1000 exist and sell for their niche field.
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>> The SZA263 should be used with a Zener current of 3mA and a Collector current of about 50 uA, otherwise will be quite impossible to find a zero Tc bias.
Mostly looking at zlymex designs I missed this part at 8810a ... so they are using 18V rail.
Scaling this resistors to have the same transistor and zener currents at 10V should be a good place to start ... but where to find this resistors?
So let me guess in 732A they can go away with 8mA because they are running in a oven?
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So let me guess in 732A they can go away with 8mA because they are running in a oven?
In the 732A the Zener Current is 3 mA. It is supplied via R4 connected from the 10 V Output to the Zener cathode (~6.3 V).
So it see 3.7 V across, and with R4=1.27k the Zener current is ~2,913 mA.
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Mimmus, as I have demonstrated in this blog,ALL reference amplifiers of Fluke have exactly 3mA. I don't get it, where from you take 8mA.
Also, in particular, what has the Motorola SZA263 to do with the LTZ1000 from Linear Technology????
So, if you intend to violate this great chip...go ahead.
You're sick and tired of the LTZ?
CORRECT...
The SZA263 probably offers a positive drift /time in contrast, so it might deliver a very nice complementary reference for your LT references,which all feature a negative timely drift, typically.
Frank
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My mistake about actual current of 732a, off course it is 3mA.
Frank (and the others) I hope that with your help no SZA263 will be harmed during this experiment :-)
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If you need a 10 V Ref. you can duplicate the circuit of the Fluke 731B. With a better power supply, a precision opamp like OPA277 (in place of the original LM308AH, I don't like AZ opamp ::)), and only four high precision resistor, you can realize a very high grade reference without any oven. But since only few components are needed, ovenize them will be not a problem. :-DMM
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Anybody checked what proposed by (z)Lymex few posts ago:
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=361134;image)
that's to eliminate R1/R2?
I'm interested in using only this 4 resistors as I already have some Vishay metal foil resistors (R401/R402) in the right value to get something like 9.999XXXV but I do not have the second divider (R1/R2).
My idea is to check this SZA with this schematic and than if it behaves than order some vishay VHP goodness for (R401/R402) (and eventually some others for R1/R2).
How they decided the ratio of the second divider (R1/R2)? I suppose must be in something slightly more (or equal) to achieve Vzener+Vbe?
How this missing two resistors affect the temperature compensation current when you trim output with R401/R402 ratio variation?
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The two resistors R1/R2 set the voltage over R3. So depending on R1/R2 a slightly different, especially larger value might be needed. A voltage slightly higher than the base voltage might have a slight advantage. Transistor gain slightly depends on the emitter - collector voltage, but this should not be a very large effect - so I doubt it is used to also compensate higher order TC.
Similar to the LTZ1000 circuit, the OP is not that critical. The transistor will have a gain of something around 100. So 10 µV of drift of the OP would result in 0.1 µV of drift for the 7 V reference, or 0.15 µV at the output. The rather high impedance at the R3 side can be an issue. So don't forget the bias current.
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Anybody checked what proposed by (z)Lymex few posts ago:
that's to eliminate R1/R2?
I'm interested in using only this 4 resistors as I already have some Vishay metal foil resistors (R401/R402) in the right value to get something like 9.999XXXV but I do not have the second divider (R1/R2).
My idea is to check this SZA with this schematic and than if it behaves than order some vishay VHP goodness for (R401/R402) (and eventually some others for R1/R2).
How they decided the ratio of the second divider (R1/R2)? I suppose must be in something slightly more (or equal) to achieve Vzener+Vbe?
How this missing two resistors affect the temperature compensation current when you trim output with R401/R402 ratio variation?
Should work, didn't test that. That's the same mode as the LTZ1000 circuit, as the base-collector voltage U(BC) =0.
That modus operandi is also used in the 5440 and 5720 reference circuit, see schematic on previous page.
As the collector voltage is a bit lower, you need a bigger collector resistor, for zero T.C. compensation.
Frank
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Although the "(z)Lymex" circuit might work, it is imo not in the spirit of the original use of the "refamp".
In order to understand the refamp one should imo think of it as a transistor amplifier with a TC-matched reference emitter offset.
This simple linear regulator block diagram describes the common use of the refamp:
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=205545;image)
In the context of the SZA263 / LTFLU-1 refamps *both* the roles of Voltage Reference and (first) Error Amplifier are played by the refamp. The "(z)Lymex" circuit (with the transistor in "soft saturation") is more in the spirit of LTZ1000 or maybe an actively controlled 1N829A circuit.
HP 3450A (revised manual dated 1969 - the instrument itself or a refamp-based predecessor is referenced in HP Journal year 1963) shows this very clearly:
(https://www.eevblog.com/forum/projects/the-refamp-general-stuff-on-temperature-compensated-avalanche-zener-diodes/?action=dlattach;attach=87070;image)
Note the "low" resistor values (high current) in the output voltage divider, the base series resistor for the refamp, the high value refamp collector resistor and the superimposed collector bias supply 16.2v. All this for high gain combined with low loading of the 10v reference sampling resistor divider.
The following differential stage (with the the operating-point setting "second" voltage divider discussed in posts over) is just a 2nd error amp gain stage and nothing but that. It is this second stage role that is played by op-amps in more modern versions of the circuit. [The driver and/or series element (Control Element in the first figure) sometimes is and sometimes is not added to the op-amp.]
A similar circuit from Fluke 341A (manual dated year 1969):
(https://www.eevblog.com/forum/projects/the-refamp-general-stuff-on-temperature-compensated-avalanche-zener-diodes/?action=dlattach;attach=176386;image)
This circuit might help answering a question raised in a post above concerning series diode for zener bias current.
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PS Simpifying the two circuits "as much as possible" will be recognized by many as one of the ways to make simple but good linear regulators often used a few decades ago (and by some people even today):
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=205547;image)
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The "(z)Lymex" circuit with one less divider is not that much different from the others. The main difference is working at a emitter to collector voltage of around 0.6 V instead of something in the 1 V range. At around 3 V the voltage for the collector resistor is not that high and transistor gain is limited, but it is still not that bad. Using a low voltage at the transistor is attractive as this leaves more to the resistor and thus a higher gain. This a little different with a 15 V reference circuit or when an extra higher auxiliary voltage is used. So with just the 10 V circuit the version with the extra divider is not that attractive: less voltage gain for the transistor (and thus a slightly more sensitive resistor and more sensitivity to OPs drift) and two extra parts. The only advantage I see would be not having the OPs bias at the critical divider. However with modern OPs with bias levels below nA, this should not be a bigger issue. The more critical point to OPs bias would be the collector anyway.
One thing one might consider would be adding another capacitor and a non critical resistor to improve tolerance to capacitive loading.
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it would be fun to try a 10V ref based on this, but when one potential supplier according to google search says they can provide 10,000 per month of the chip you just know it will be difficult to source...
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According to my ltspice simulation, I noticed that the base of the SZA reference (7V ish) is way more stable than the 10V node.
Apart for (z)lymex did anybody wired this out the 732a or verified it?
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The base is expected to be more stable - this is the raw 7 V reference point. The 7 to 10 V amplification (or should I say 10 to 7 V division) only adds errors to this.
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The 7 to 10 V amplification (or should I say 10 to 7 V division) only adds errors to this.
I think by calibrating the 7V to 10V divider ratio in two different times you can compute what was 7V and 10V drift eliminating most uncertainty from this weak point.
I made some math on a xsl with simulated values in LTSpisce and it seems to works, don't know if this works also in reality.
Maybe the better is to use a pot to set ratio back to original divider ratio after a drift.
Only thing is that you need a 3458a to do this calibration and the measurement can be quite tricky to do.
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One usually does not need a reference of exactly 10 V. The important part is usually to have a voltage with approximately (like 1 or 10%) the right size and know precisely the value.
The high end zeners (LTFLU and LTZ1000) tend to have very low drift. You have to pay quite a lot to get resistors more stable. The simulation can show you how much a change in resistance will effect the final voltage. This sensitivity is than multiplied with the estimated drift rate of the parts. Chances are that drift of the 2 critical resistors for 10 to 7 V division will be a major contribution, followed by the LTFLU and than the other resistors and the OP.
The stability does not always follow a simple linear relationship. It is quite often a combination of a linear part, a few exponential contributions and also effects from the environment (temperature, humidity, pressure, magnetic field, radiation, gravity).
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I think by calibrating the 7V to 10V divider ratio in two different times you can compute what was 7V and 10V drift eliminating most uncertainty from this weak point.
I made some math on a xsl with simulated values in LTSpisce and it seems to works, don't know if this works also in reality.
Maybe the better is to use a pot to set ratio back to original divider ratio after a drift.
Only thing is that you need a 3458a to do this calibration and the measurement can be quite tricky to do.
If I understand you right, you got an error in reasoning.. your argument obviously contains a circular reasoning..
You cannot extract the drift of the raw RefAmp (7V) voltage, simply from measuring this 7V / 10V several times.
The 7V drift independently from the 10V, so the 10V drift consist of the 7V drift PLUS the divider resistors drifts, which is then logically higher than the 7V drift.
This 10V/7V ratio divider is the really weak point.
The ratio calibration itself is very easy to do, and it's not tricky at all.. you can use a 3458A, or a Fluke 720A, for 0.1ppm accuracy, and to remove this ratio uncertainty instantly.
You can even use any good 6 1/2 digit DMM with ratio function to measure the 10V drift with about 1ppm accuracy.
If the 732A/B would provide an additional buffered RefAmp output, this would give much better uncertainty than these 10V.
That's a hint for the development of the 732C, probably. ;)
Frank
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Hi Frank, thank for your response, I try to better explain my idea.
I'm not talking to null out the reference drift (we all know this is not possible) but only the drift that is directly caused by the 7V to 10V ratio change, that we know is the major contributor to the overall drift.
What I plan is to follow this protocol to do this:
at beginning of the observation period:
- I can determine what is the suppression ratio of the divider on the 10V and on the 7V nodes.
- I can also determine with sufficient accuracy what is the divider ratio.
Than let say that after 1 year:
- Check that suppression ratio is not changed
- Check how much divider ratio is changed
At this point:
- If divider ratio and suppression ratio is not changed (significantly) you can assume that the divider has not changed (significantly) and so have more confidence that the hole reference is quite stable.
- If you find that suppression ratio is changed a lot, you know something strange happens and you cannot assume anything.
- If suppression ratio is constant but divider ratio is changed a few ppm, you should be able to calculate what correction factor you can apply to the two outputs to calibrate out those changes. This should be totally doable because you know that divider ratio has not change that much and you know also suppression ratio remained constant during this observation period.
After this you still remain with:
- drift caused by the current setting resistor (very high suppression ratio)
- drift caused by the nulling TC resistor drift and generally with eventual changes in TC (I suppose also this are almost negligible if you used good resistor)
- drift of the Zener or other components (OP Amp offset, etc)
- drift of other external causes (that can be mitigated some way)
but because those drift have less impact on the REFENCE you end up with a better predictable reference.
So you moved from a concept of totally stable reference to a more predictable one.
Than I think having a stable 10.0000X V is still a great advantage for comparing multiple references or also for averaging purposed without having to be worried too much for the drift of the averaging resistors.
PS: I only make "serious" electronic from a couple of years ... hope I pass DR. Frank "peer" review and if not I hope to have learn something more.
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Mimmus78,
now I understand your idea.
You're mainly interested in the influence of the different resistors of the circuit, or how their individual drifts influence the overall drift of the RefAmp reference voltage, and the 10V.
The suppression ratio is then the relation between the individual drifts and the drift of the outputs.
These parameters have been determined experimentally also on the LTZ1000 circuit by several people, but far easier.
If you simply vary each resistor by a small amount, say a few percent, by adding another resistor in parallel, you can directly determine these suppression ratios.
They will be in the same ballpark, like between 100 and 2000, as both circuits are similar.
The 7V / 10V stepup resistors have a suppression ratio of about 1, which makes them very critical.
So you can calculate from the known T.C.s and the estimated timely drifts the overall changes over temperature and time directly.
Btw.: I think, as these suppression ratios are partly quite high, that you can't really measure them, if you make comparing measurements 1 year apart.. the changes are probably too small, compared to the stability of your measurement equipment.
The only thing you have to measure over 1 year, or so, is the absolute drift of the RefAmp, and the 10V output.
Frank.
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One could also determine suppression factor from a simulation of the circuit, if a suitable model for the zener is found (correct differential resistance). The factor should be rather higher for the 2 resistors to set the currents and rather low (more like 3, as about 1/3 of the output is set by the resistor ratio) for the 10 to 7 divider.
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Well I think in reality my method is applicable only to the divider where you can easily check with very good accuracy divider ratio by two relative measurements.
Fortunately this divider is also considered the main cause of the drift on the 10V node, so it still makes sense to focus just on it.
Measuring the others resistor is unpractical and than they have a so high suppression factor that their effects cannot get out of the hole system drift.
I think is better to determine suppression factor of the divider sample by sample and not by simulation.
This suppression factor need to stay fair constant in the period of observation as only in this case your calculated 10V drift can be considered valid.
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There is no real need to get a very accurate suppression ratio. This number is mainly important in when you decide on which resistors to spend money and which can be slightly cheaper ones. So simulated numbers are OK. Especially for the 10 to 7 divider the number should be very close to the calculated one. It has 2 Contributions: the main part is the divider directly which should be factor of a little under 1/3 depending on the resistors only and a smaller effect (expected to be about 1/100-1/500) of the 10 V value on the 7 V reference. One might need to measure the second part to get a good number, as here parameters like the zener resistance enter. So a simulation could only give a rough number.
It makes some sense to make the raw 7 V reference available externally, so the 7 to 10 V step can be checked independently and if needed corrections can be us. However as the ratio should be rather stable and thus only a small correction, there is likely no real need to also do a correction of the influence of the divider to the raw reference. The current setting resistor is expected to have the same effect and is usually not more stable than the critical divider.
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When did Linear change from the LTFLU-1CH Zener to the newer -1ACH?
I have seen photos of the -1CH Zener with date codes from 8915 to 9052.
I have seen photos of the -1ACH Zener with date codes of 9515 to present day
Does anyone have LTFLU parts with date codes between 9052 and 9515 and which part number is it?
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I found another interesting implementation for this reference. This should avoid other two important resistors so it should be more stable than "usual 10V design". Off course you online get 6.7V (negative)...
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I found another interesting implementation for this reference. This should avoid other two important resistors so it should be more stable than "usual 10V design". Off course you online get 6.7V (negative)...
Inviato dal mio ONEPLUS A5010 utilizzando Tapatalk
it looks similar to this shematic here, posted #94 https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/75/ (https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/75/)
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No, this is just a simplified version of the classical circuit where R1/R2 ratio is still very critical as this scale the reference out to 10V.
In the classical version you can get the 6.7V out at the base of the reference transistor, this node should be very stable but in my 731 this point is more nosier o susceptible to disturbance than the "direct output" so I don't know how much is useful other than a sanity check.
In this alternative version you get the 6.7V just out of the buffer ... if I'm not wrong it should be better and more stable.
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This version should give you a stable -6.7V and a not so stable +10 V. The 10 V value depends on R5/R6 even more than the usual version that starts with +7 V and than amplifies to + 10 V (here only about 1/3 of the output depends on the resistors).
There is no magic way around the scaling from 7 to 10 V. Starting with 6.7 V might allow to use a capacitive divider (e.g. charge pumps with LTC1043) and only do the fine adjust from a nominal 10.1 to 10 V with resistors. However this can add noise.
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This version should give you a stable -6.7V and a not so stable +10 V. The 10 V value depends on R5/R6 even more than the usual version that starts with +7 V and than amplifies to + 10 V (here only about 1/3 of the output depends on the resistors).
There is no magic way around the scaling from 7 to 10 V. Starting with 6.7 V might allow to use a capacitive divider (e.g. charge pumps with LTC1043) and only do the fine adjust from a nominal 10.1 to 10 V with resistors. However this can add noise.
Why they used trimpot there? Just to trim the current compensation a little bit?
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The trimm-pot would adjust the upper voltage (around 10 V) and also the Zener current. This will have a slight effect one the TC of the -6.7 V output. So maybe It a TC adjustment and the possible 10 V output is not really used.
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The trimm-pot would adjust the upper voltage (around 10 V) and also the Zener current. This will have a slight effect one the TC of the -6.7 V output. So maybe It a TC adjustment and the possible 10 V output is not really used.
Well most probably is a crude 10V adjust to get the voltage up to 10V.
This may be required by the pre-characterization of the references as every chip has his own compensation resistors and maybe this coupling was done at 10V.
I think that if you lower this 10V to something like 7.2V this should bring even more rejection factor to this "divider" so that any good resistor couple will be practically ininfluent.
In this case you end up with just 2 resistors with already very good rejection factor.
Only cons is that you need to source other two custom resistors.
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Anyway as the lower side of the reference is at negative -7V ... I guess there should be +3V at the top and not 10V.
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There is neither 3v nor 10v "on the top". There is something between 9.9v or so (6.5X15.4/10.05) and 11.9v or so (6.9x17.4/10.05) on the left leg of CR1 - depending on vref, pot setting and component values. (No sane person would adjust a refamp to 10.000xx.. volt with a 2 kohm potentiometer).
The voltage arises because U1A has its non-inverting input grounded and therefore serves as an inverting amplifier of the output reference voltage. U1A is the (very stable) power supply for the refamp. R1 is only a pull-up resistor in order to guarantee positive start-up.
This circuit is similar to some older Fluke circuits and others using appr. +- 7v ref. I am sure you did not "find" it laying around somewhere, so maybe you could give information or link to where it originates.
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=388910;image)
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I forgot to say: R3 adjusts the bias zener current through R2 and CR2 (in this case some 2.5+ mA - iirc it usually is about 3 mA). R15 and R16 sets the collector current of the refamp so that the tempco of the BE-diode and the zener cancel.
This circuit is NOT designed to deliver anything like 10v as mentioned in some posts over. Also - it does not tap the base voltage of the vref directly. The refamp transistor is "saturated" so that the collector is at virtual ground (because the base is grounded). Hence one gets -6.5 to -6.9 volt "zero tempco" voltage on the reference output (relative to ground).
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I forgot to say: R3 adjusts the bias zener current through R2 and CR2 (in this case some 2.5+ mA - iirc it usually is about 3 mA). R15 and R16 sets the collector current of the refamp so that the tempco of the BE-diode and the zener cancel.
This circuit is NOT designed to deliver anything like 10v as mentioned in some posts over. Also - it does not tap the base voltage of the vref directly. The refamp transistor is "saturated" so that the collector is at virtual ground (because the base is grounded). Hence one gets -6.5 to -6.9 volt "zero tempco" voltage on the reference output (relative to ground).
Well after you make the math it's pretty obvious that total differential voltage is more than the usual 10V ... even by just looking at R3.
What I don't understand is why they used much more driving voltage than needed. Wasn't the module been more stable if those two resistors had a smaller ratio?
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The higher voltage increases the amplification provided by the transistor. So the OP gets less important. This might be a relict from old times when an 741 was considered a good OP. If for some reason you have the +15 V available, there is no reason to not use the voltage available - the heat would otherwise end up in the OP and nothing is gained.
It depends on the details of the zener diode, whether CR2 is actually needed and if does some useful compensation. The first order TC compensation can be done without the diode as well, just through the Zener current and maybe transistor current. With an upper voltage at 10 V the effect of the diode is not very large anyway.
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This circuit is similar to some older Fluke circuits and others using appr. +- 7v ref. I am sure you did not "find" it laying around somewhere, so maybe you could give information or link to where it originates.
hello,
the reference circuit is for Fluke 8520A 5-1/2 digit dmm.
regards.
-zia
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Thanks to mimmus78 and zhtoor for providing and identifying this refamp-circuit from Fluke 8520A (5.5 digit DMM).
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=388910;image)
The topology is the same as that used in Fluke 8505A (6.5 digit DVM w/ claimed 24 bit discrete ADC and "7.5 digit performance and display in the native 10v range averaging mode").
I find it very interesting that the essentially same reference circuit can be improved an order of magnitude or more by selecting refamp (-set) and refining the circuit a bit.
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I had the good fortune of finding 3 SZA chips on ebay (they are from hifi-szjxic, so I am hoping they are not fakes).
Last night I read through Lyzmex's excellent thread over on 38hot: http://bbs.38hot.net/thread-177-1-1.html (http://bbs.38hot.net/thread-177-1-1.html)
However, I noticed that all of the circuits in that thread, and all of the circuits in this thread (with the possible exception of the fluke 8520 circuit?) are true "refamp" circuits, in that they utilize the amplification (beta) of the transistor.
However most of these circuits suffer from having an "attenuation" of only 3 for the most sensitive resistor pair.
One of the big advantages of the classic bootstrapped zener circuit is that the resistors have very good attenuation (if the zener has good dynamic impedance). For example, the LM399 has an impedance of about 1, so a 1k zener resistor will attenuate op amp voltage output errors by 1000.
I'm interested in seeing if we can gain the advantage of the bootstrap circuit, and get good resistor attenuation, by discarding the "amp" functionality from the "refamp".
Starting from the bootstrap circuit, the simplest thing to do is simply insert the Vbe junction into the circuit, ignoring the collector pin (see attached 1).
I think this would as expected if you simply threw a 2N3904 and a 5.6V zener onto a breadboard, but I suspect that this won't work with the SZA, as I suspect the transistor cannot handle the full zener current through its base. Can anyone confirm that? Is there a way I could find out how much current the transistor can handle, without sacrificing an SZA to find out?
Assuming the transistor can't handle the full zener current, we need to have independent resistors for the base and zener currents (another way of stating this is that we need to add an extra resistor to inject additional current into the zener). This is circuit 2 (attached).
However, this isn't ideal, as my understanding is that the collector current is what gives you the ability to tweak the zero-tempco point. So we can also add a third resistor to have independent control of the collector current. (see circuit 3).
This is still the classic bootstrap circuit, and the base pin (the most stable point of the circuit) is what controls the op amp loop, so this should be as stable as we can get.
Does this approach work? I plan on trying it out when my chips arrive.
I think the biggest disadvantages of this approach are that you only get a 7V output, and it is a high impedance output. However, for me, that's ok.
Any feedback / thoughts are very appreciated! :-+
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Starting from the bootstrap circuit, the simplest thing to do is simply insert the Vbe junction into the circuit, ignoring the collector pin (see attached 1).
Hello,
if you short the base + collector you will get a better "ideal" diode (the base emitter resistance cancels out by beta).
This will also help to increase zener current.
with best regards
Andreas
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Using the amplification of the transistor makes absolute sense - it reduces the effect of the OP. It is not that important anymore with modern OPs, but the extra amplification of the error signal essentially comes for free (no extra costs and not much (if any) noise).
The simple circuit to give a roughly 7 V reference should not have a high sensitivity to resistor drift. Those to output a higher voltage naturally have the more critical resistors as these set the gain. AFAIK these resistor attenuation is a little less than with the LTZ1000, but not that much.
Using a 5.6 V zener and a small transistor can be a first approximate circuit for tests.
Even if the transistor can handle a high base current, there usually is extra series resistance, that gets more important when the base current is high. So the stability will likely suffer if the main part of the transistor current is not flowing through the collector as intended.
The LM399 has internal resistors to set the actual zener current. So one can not directly compare the differential resistance to bare zener refs - there are critical resistors inside the LM399. So the drift is not avoided the resistors are just in the case.
It is a good idea to do a simulation (e.g. LTSpice) of the circuit to check both the sensitivity to resistors and also to check the loop stability. Due to the extra gain from the transistor the loop stability is usually not that simple and usually needs at least one extra capacitor.
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Ah, I hadn't considered that the same resistor pair could have different attenuation, depending upon where you "tap" it (the 10V node or the 7V node of the divider).
I'll play around in LTSpice this evening and breadboard up a dummy circuit while waiting for the chips to arrive.
Andreas, in the situation where the base and collector are shorted, does the overall transistor current then allow for tweaking the zero-tempco point?
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With base tied to collector the transistor current can still be used to adjust the TK.
The normal circuit is not that much different: quite often the base and collector will still have the same potential, just with the control loop taken from the collector to use an amplified signal to control the rest of the circuit to make the the difference zero.
The Fluke 8520 circuit is kind of typical for a circuit without extra voltage gain. The hight of the auxiliary voltage (here the positive reference) will set the gain of the transistor.
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I had the good fortune of finding 3 SZA chips on ebay (they are from hifi-szjxic, so I am hoping they are not fakes).
Last night I read through Lyzmex's excellent thread over on 38hot: http://bbs.38hot.net/thread-177-1-1.html (http://bbs.38hot.net/thread-177-1-1.html)
However, I noticed that all of the circuits in that thread, and all of the circuits in this thread (with the possible exception of the fluke 8520 circuit?) are true "refamp" circuits, in that they utilize the amplification (beta) of the transistor.
However most of these circuits suffer from having an "attenuation" of only 3 for the most sensitive resistor pair.
One of the big advantages of the classic bootstrapped zener circuit is that the resistors have very good attenuation (if the zener has good dynamic impedance). For example, the LM399 has an impedance of about 1, so a 1k zener resistor will attenuate op amp voltage output errors by 1000.
I'm interested in seeing if we can gain the advantage of the bootstrap circuit, and get good resistor attenuation, by discarding the "amp" functionality from the "refamp".
Starting from the bootstrap circuit, the simplest thing to do is simply insert the Vbe junction into the circuit, ignoring the collector pin (see attached 1).
I think this would as expected if you simply threw a 2N3904 and a 5.6V zener onto a breadboard, but I suspect that this won't work with the SZA, as I suspect the transistor cannot handle the full zener current through its base. Can anyone confirm that? Is there a way I could find out how much current the transistor can handle, without sacrificing an SZA to find out?
Assuming the transistor can't handle the full zener current, we need to have independent resistors for the base and zener currents (another way of stating this is that we need to add an extra resistor to inject additional current into the zener). This is circuit 2 (attached).
However, this isn't ideal, as my understanding is that the collector current is what gives you the ability to tweak the zero-tempco point. So we can also add a third resistor to have independent control of the collector current. (see circuit 3).
This is still the classic bootstrap circuit, and the base pin (the most stable point of the circuit) is what controls the op amp loop, so this should be as stable as we can get.
Does this approach work? I plan on trying it out when my chips arrive.
I think the biggest disadvantages of this approach are that you only get a 7V output, and it is a high impedance output. However, for me, that's ok.
Any feedback / thoughts are very appreciated! :-+
I got a LTFLU in my Fluke 8842A multimeter :-DD
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Hi plesa,
Let me know if you know cheap source of LTFU (including not working). Die image can follow ;)
I've bought here today 5 pieces of LTFLU-1ACH. Price per piece: U$ 25.-- plus shipping plus Paypal fee.
http://goo.gl/aD205s (http://goo.gl/aD205s) (Link points to de.aliexpress.com)
Cheers,
BU508A
This is a photo I got from Barry (seller at this webshop)
EDIT: I guess I quoted the wrong post. This is the photo from post #87 https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg901493/#msg901493 (https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg901493/#msg901493):
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=210967;image)
Was it ever established if this was the real thing or not? There is now a seller with 2000 (?) pcs with the same date code 0625. 2pcs costs some 55ish USD. Wondering if I should buy 2 pcs for fun.
I emailed him and he sent me this picture:
(http://i.ebayimg.com/images/g/lrgAAOSwPDdbFOqj/s-l1600.jpg)
https://www.ebay.com/itm/2PCS-X-LTFLU-1ACH-CAN4-LT/332495632007?hash=item4d6a48b687:g:QCkAAOSwFyhaPL0K (https://www.ebay.com/itm/2PCS-X-LTFLU-1ACH-CAN4-LT/332495632007?hash=item4d6a48b687:g:QCkAAOSwFyhaPL0K)
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Hi plesa,
Let me know if you know cheap source of LTFU (including not working). Die image can follow ;)
I've bought here today 5 pieces of LTFLU-1ACH. Price per piece: U$ 25.-- plus shipping plus Paypal fee.
http://goo.gl/aD205s (http://goo.gl/aD205s) (Link points to de.aliexpress.com)
Cheers,
BU508A
This is a photo I got from Barry (seller at this webshop)
Was it ever established if this was the real thing or not? There is now a seller with 2000 (?) pcs with the same date code 0625. 2pcs costs some 55ish USD. Wondering if I should buy 2 pcs for fun.
I emailed him and he sent me this picture:
(http://i.ebayimg.com/images/g/lrgAAOSwPDdbFOqj/s-l1600.jpg)
I already order from those little devils :-DD
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You guys do realize these FLUs are genuine fakes, outside of maybe a very few being removed from broken equipment, there will not be any real, brand new genuine Fluke LTFLU-1AHs on the open market. LT knows where everyone of those chips goes and who bought them, just like the LTZ1000/As and there are never any quantity of genuine 'surplus' chips, especially in any large quantities on the open market. It isn't that hard to put a transistor and zener diode on a chip, just about anyone can do that, what they can't do is produce a genuine part. By all means test the little buggers but they aren't going to be the same. Linear Tech is the sole source of these, they're not like the old Motorola SZA263s, they are similar of course but not the same.
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You guys do realize these FLUs are genuine fakes, outside of maybe a very few being removed from broken equipment, there will not be any real, brand new genuine Fluke LTFLU-1AHs on the open market. LT knows where everyone of those chips goes and who bought them, just like the LTZ1000/As and there are never any quantity of genuine 'surplus' chips, especially in any large quantities on the open market. It isn't that hard to put a transistor and zener diode on a chip, just about anyone can do that, what they can't do is produce a genuine part. By all means test the little buggers but they aren't going to be the same. Linear Tech is the sole source of these, they're not like the old Motorola SZA263s, they are similar of course but not the same.
I got that in mind when I order it, I add in my comments of the order "Must be genuine"
I got in mind as well that Fluke probably assemble production in China like many other instruments producers and there could be some leftover ...
I will test them and we will see ...
A least I have a genuine LTFLU in my Fluke multimeter :-DD
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even this is the genuine one, it's probably a remark, refurbished, and pins are probably ’re-connected‘. :palm:
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I got my LTFLU's today, based on the logo it look genuine compared to the logo from the company.
Just the date code is suspicious : they all got 0625, mayby all the same batch ...
I check the presence of a zener between pin 1 & 4.
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HELP! Could anyone give me the pinout diagram of LTFLU-1ACH?
THANK YOU!
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HELP! Could anyone give me the pinout diagram of LTFLU-1ACH?
THANK YOU!
I can't believe that I've made such a STUPID mistake!!! |O |O |O
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Hi!
I've built the circuit from Dr Frank, post #155: https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg1066470/#msg1066470 (https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg1066470/#msg1066470)
I've bought - from two different chinese sellers - the LTFLU-1ACH with the printing "wrong" (LT-logo not at the tab) and the very same datecode "0625"; 10 + 10 pcs for a total of 20 pcs. What I don't understand is if the reference needs to be heated, I am seeing voltage drift by just getting close to the LTFLU, and quite obviously I do not have zero tempco. A 3D-printed "hood" for the LTFLU slows down the short-term voltage-changes, but long-term (minutes, hours) it drifts according to room temperature. I'm measuring the circuit with a DMM7510 (which uses a heated LTFLU i think).
Is it possible to design the circuit for (near) zero tempco, or is it neccesary to keep the reference heated at a constant temperature to keep the output stable? Or - worst case - is the component totaly fake and not worth the effort/time?
If someone wants to cut up one of my LTFLU:s and watch them in a microscope, I'm all in. Probably it's not needed since I seem to have components from the same batch as others, regarding the rotated print and the same datecode.
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If the part performs reasonably well, especially low noise, chances are the part is good. Only way to have a working fake i could imagine would be a selected 2DW232 reference with a transistor - but even this would show up as a rather low voltage.
Even a well working Chinese copy might be worth a circuit to do the real long term test.
So the suitable test would be a simple unheated test-circuit with a roughly adjusted current and than a noise measurement. This could not tell the difference between an LTFLU and SZA263 (old Motorola), but between :-+ and :--.
The LTFLU can be used heated or not heated. The non heated circuit would need carefully chosen resistors for the given unit to get a low TC. AFAIK Fluke has these as a set in there BOM. At least the linear TC can be adjusted, some square law part will likely remain. So this might work well over a limited temperature range. AFAIK the reference in the Fluke 8046 and DMM7510 is not heated - but likely uses extra numerical corrections.
The other option is to use less accurately set current (and thus a larger TC) and than add temperature control, usually for the whole reference circuit including scaling to 10 V or whatever. This is used in the Fluke 732 etc. Depending on the quality of the current adjustment the temperature does not need to be as stable as the LM399 or LTZ1000, as the TC to start with can be smaller (e.g. around 5 ppm/K vs 50 ppm/K for the LTZ). Fine tuning (to get near zero linear TC near the set temperature) could be done with the temperature or current if really needed.
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Fluke 5500A also doesnt use a thermostat for the LTFLU: https://www.eevblog.com/forum/metrology/fluke-5500a/msg1432755/#msg1432755 (https://www.eevblog.com/forum/metrology/fluke-5500a/msg1432755/#msg1432755) Picture A6_2
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If the part performs reasonably well, especially low noise, chances are the part is good. Only way to have a working fake i could imagine would be a selected 2DW232 reference with a transistor - but even this would show up as a rather low voltage.
Even a well working Chinese copy might be worth a circuit to do the real long term test.
So the suitable test would be a simple unheated test-circuit with a roughly adjusted current and than a noise measurement. This could not tell the difference between an LTFLU and SZA263 (old Motorola), but between :-+ and :--.
The LTFLU can be used heated or not heated. The non heated circuit would need carefully chosen resistors for the given unit to get a low TC. AFAIK Fluke has these as a set in there BOM. At least the linear TC can be adjusted, some square law part will likely remain. So this might work well over a limited temperature range. AFAIK the reference in the Fluke 8046 and DMM7510 is not heated - but likely uses extra numerical corrections.
The other option is to use less accurately set current (and thus a larger TC) and than add temperature control, usually for the whole reference circuit including scaling to 10 V or whatever. This is used in the Fluke 732 etc. Depending on the quality of the current adjustment the temperature does not need to be as stable as the LM399 or LTZ1000, as the TC to start with can be smaller (e.g. around 5 ppm/K vs 50 ppm/K for the LTZ). Fine tuning (to get near zero linear TC near the set temperature) could be done with the temperature or current if really needed.
I think the LTFLU in DMM7510 is heated.
(https://www.eevblog.com/forum/blog/eevblog-731-keithley-dmm7510-7-5-digit-multimeter-teardown/?action=dlattach;attach=228793;image)
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So Andreas left some LTFLUs here on Metrology Meeting 2019 for x-ray analysis.
Since I couldn't resist, yes I've asked Andreas before, I dirty hacked together some components on a breadboard/veroboard. The circuit is based on the schematic presented by Frank here:
https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg1066470/#msg1066470 (https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg1066470/#msg1066470)
I used what I had at hand: LT1006, simple 1% through hole resistors and a precision socket for the reference. R15 is made by 9.1k||180k, R7A simply by paralleling 2x 10k, R7B is 10k and instead of R13 I connected my GenRad 1434-G.
The output voltage connected to my R6581D and the circuit powered with 15V from a lab power supply I used my Ersa icon set to 150°C and touched the top of the TO package to slightly heat up the reference and read the change of output voltage in particular its direction when warming up. This way I was able to determine a resistor value of ~23k needed to have almost no change in output voltage, thus zero t.c.
I then started overnight measurement and went to bed. This morning I stopped the measurement, checked the readings and continued measuring. During the day the reference stabilized, obviously the reference had to relax after my heat treatment.
The veroboard is clamped in a bench vise hanging in free air, so results are not to bad considering that everything is hacked together with cheap components, not packaged and not ovensized.
Need to prepare a temperature sensor and mount it close to the reference so that we can get some temperature readings too.
And yes, output voltage is not trimmed to 10.00000V, but that wasn't the purpose of this quick look onto the part that where called to be LTFLU fakes.
-branadic-
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The settling / dirft of the reference circuit could very well be to a large part from the resistors used for the 7 to 10 V part. To really check the reference one could measure the 6.9 V point at the base of the reference transistor (a series resistor for safety would be a good idea here).
Still I would not complain about such a reference.
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Here I have some pictures and the schematics of my crude prototype which I had with me at the Metrology Meeting.
And when I was going to do some measurements together with HighVoltage, it failed. Reason: for any unknown reason
the B-E diode of the 2N2219A became high-impedance. No idea why. I changed the transistor and everything
is nice and dandy since then.
Ok, as promised, here are some pictures.
Let's start with the schematics, made with a high sophisticated CAD system ;)
(https://i.imgur.com/3fkM5fz.jpg)
This is the Bopla plastic case opened. On the top right there is one of two
Makita 18V LiIon batteries, used for the power supply. In the socket,
there is a 15V LDO from TI.
(https://i.imgur.com/Qb3TYaA.jpg)
This is the on/off switching, realized with a latching relay and some reed sensors.
Also some filtering with Wima capacitors on the left.
(https://i.imgur.com/9TI4Jji.jpg)
This is the ground plane, made of 1mm copper.
(https://i.imgur.com/iCX8IoY.jpg)
Inner case opened and overview:
(https://i.imgur.com/JvKTLmL.jpg)
Detail view of the LT1028 and the air wiring (no need of guardening the lines)
(https://i.imgur.com/3LIPkYC.jpg)
Detail view of the LTFLU-1 in it's socket (coming from Fischer electronic, they are also doing cases)
The yellow resistors were made by forum fellow Edwin G. Pettis. Thanks again for the really fast delivery.
(https://i.imgur.com/O3WKyZC.jpg)
Detail view of the outgoing binding posts:
(https://i.imgur.com/HxvsQnh.jpg)
And here I've assembled the reference prototype with my resistor decade (top left),
the GW121 measuring the air temperature and the DMM 7510
(https://i.imgur.com/ySpg2Qi.jpg)
It is continously drifting upwards. I think, the main reasons for this is:
- too short power on time, so it did not found its thermal balance
- I did not set up the tempco point of the LTFLU-1, this is the next thing on my list.
- my bizarre resistance decade box is probably not the best in respect of thermal drift
(https://i.imgur.com/n743BZY.png)
These are the settings used for the measure:
(https://i.imgur.com/r70e0EW.png)
More to come, but don't know, when.
Next step is getting the 0.something ppm tempco point.
Will let you know about my (probably slow) progress.
Best regards,
Andreas
Edit: some typos
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The LT1028 is not the right OP to choose here. The signal from the reference is high impedance (a little less than R5). So a more suitable choice would be the normal OP07, or maybe ADA4077. The bias and maybe also the low frequency current noise of the LT1028 are a problem. There is no need for a super low noise OP, as the SZA263 reference should be the noise setting part, not the OP. The transistor inside the reference provides a gain of some 100 and this way effectively reduces the noise and drift of the OP.
In addition the LT1028 is tricky with unity gain.
The other odd point in the circuit is the large capacitor value for C3, especially compared to C2. With this choice a low noise OP makes some sense, though not the LT1028. C3 together with R5 sets the cross over from where the OP is responsible for the noise.
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So Andreas left some LTFLUs here on Metrology Meeting 2019 for x-ray analysis.
Since I couldn't resist, yes I've asked Andreas before, I dirty hacked together some components on a breadboard/veroboard.
The diagnosis is absolutely clear: you are infected with the LTFLU-flu. There is no remedey nor escape. Get along with it. ;D
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The LT1028 is not the right OP to choose here. The signal from the reference is high impedance (a little less than R5). So a more suitable choice would be the normal OP07, or maybe ADA4077. The bias and maybe also the low frequency current noise of the LT1028 are a problem. There is no need for a super low noise OP, as the SZA263 reference should be the noise setting part, not the OP. The transistor inside the reference provides a gain of some 100 and this way effectively reduces the noise and drift of the OP.
In addition the LT1028 is tricky with unity gain.
The other odd point in the circuit is the large capacitor value for C3, especially compared to C2. With this choice a low noise OP makes some sense, though not the LT1028. C3 together with R5 sets the cross over from where the OP is responsible for the noise.
Yes, I am aware, that the LT1028 is not the right choice here. The reason, why I've used it: I want to to some noise measurements. But, when I had assembled it,
it dawned to me, that using this OP here is not necessary: If I want to do some noise measurements, I should do this directly at the collector of the transistor. :palm:
In the next version I'll use probably the LT1008 or LT1012 which are, as per datasheet from LT, the recommended replacements for the originally used LM308A.
It is a prototype and I'm playing around with it.
C3 comes directly from the original Fluke schematic for the 3330B. I have one of the original reference modules here, where I can do some measurements for comparison.
See attached schematic from the 3330B reference board.
Edit: some typos
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To measure the noise of the reference it is OK to measure the 10 V - no problem there, just some filtering from C3 included. The position for the filtering is in deed not that bad, as the relatively high impedance is there already and some capacitance may be needed for stability anyway. For noise tests one may consider reducing C3 (down to maybe 100 nF), to see the unfiltered noise of the reference.
The LT1028 is not only not needed, it is rather noisy in the form of too much current noise. Drift of the bias current could also effect the stability. A good choice for some 35 KOhms source impedance would be something like ADA4077, OP177, OP07 and similar. As the OP is mainly relevant for the higher frequency noise one could also use some low noise JFET type like OPA141 or similar. The LT1008 / LT1012 are more on the other side with very low current noise, but quite some voltage noise.
Below the filter frequency the transistor supports the OP - so the low frequency noise and drift of the OP are not that important. The transistor gain may be just enough so that even the noisy LT1028 may not be noticeable.
Looking directly at the transistor collector would measure the noise before the amplification of the OP and at a relatively high impedance node. I don't think this would be a good idea. If at all it might make sense to look at the base of the transistor to check for long term drift and TC from the reference only, without the scaling resistors.
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What is function of D3 D4?
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D3 and D4 have some effect on the temperature effect. Other circuits can work without it or with only one diode. This may effect the second order TC.
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D3 and D4 have some effect on the temperature effect. Other circuits can work without it or with only one diode. This may effect the second order TC.
That's right. This is from the Fluke 731B and as you can see, it has only a 1.27kOhm resistor and no diodes.
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The transistor gain may be just enough so that even the noisy LT1028 may not be noticeable.
What I do not understand is: why are you saying, the LT1028 is noisy?
Looking at the datasheet, it's specs are very good in respect to noise. As far as I know, there aren't much other OpAmps around which can beat the LT1028 in that discipline, imho.
*scratching head*
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The transistor gain may be just enough so that even the noisy LT1028 may not be noticeable.
What I do not understand is: why are you saying, the LT1028 is noisy?
Looking at the datasheet, it's specs are very good in respect to noise. As far as I know, there aren't much other OpAmps around which can beat the LT1028 in that discipline, imho.
*scratching head*
You have to remember: datasheets are written by marketeers not engineers.
The zener must be treated as a high impedance source, so current noise specs come into play.
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What I do not understand is: why are you saying, the LT1028 is noisy?
Looking at the datasheet, it's specs are very good in respect to noise. As far as I know, there aren't much other OpAmps around which can beat the LT1028 in that discipline, imho.
*scratching head*
A very good zener reference chip like this will have a noise density on the order of 50-100nV / rt Hz. The R5 collector resistor (39.5k) will add 28nV / rt Hz to the Zener noise. These random noise sources add together via RSS. So using an opamp with a 1nV / rt Hz noise density will not help reduce any noise. All of the opamps with low VOLTAGE noise will have a very high CURRENT noise. In this case it's 10pa / rt Hz. On three LT1028A opamps that I tested the current noise was 15-20pa / rt Hz at 10Hz. When the 10pa / rt Hz interacts with the R5 impedance of 39.5K it will generate an additional 400nV / rt Hz of noise. The current noise of the LT1028A will generate 4-8 times the voltage noise of the zener.
I believe the Fluke 732A voltage standards use a LM308 in this application. The LM308 will add a total of about 50nV / rt Hz to the output noise. The newer LT1008 will add about 20nV / rt Hz to the Zener noise, so it will not be noticeable.
I don't know if the voltage gain of the LTFLU internal transistor will suppress the LT1028A opamp current noise.
Every opamp has an optimum source impedance for lowest added noise. See the attached Design note from Linear Tech for the math.
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Isn't C3 meant to be a lowpass filter taking away the LT1028 input current noise from R5 and the zener noise away from the amplifier input? With 39K and 4u7 it works above 0,8 Hz.
If i had a 1nV/sqrt(Hz) amplifier around i would just do some measurements with different values of C3. Maybe it needs to increase a little.
I would also do some tests with an extra miller capacitor for the LTFLU transistor in order to keep it quiet.
Regards, Dieter
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@soulman and @chuckb
Thank you for your explanations. Now some things makes more sense to me and I hopefully have my lesson learned. ::) :-+
Also a big Thank you to branadic who did some black x-ray magic and scanned the LTFLU-1 devices and one SZA263.
Here are the pictures he sent me:
LTLFU-1 ACH:
(https://i.imgur.com/fZNeelD.jpg)
Zoomfactor 5:
(https://i.imgur.com/yJwtRkV.jpg)
next piece LTFLU-1 ACH
(https://i.imgur.com/HPiKVId.jpg)
and another one
(https://i.imgur.com/d7yBPMV.jpg)
and the last one of the LTFLU-1-ACH
(https://i.imgur.com/jq7IGyc.jpg)
This ones are LTFLU-1 CH (without the "A", which is a newer version)
(https://i.imgur.com/XEExjbA.jpg)
Zoomfactor 5 of the LTFLU-1 CH
(https://i.imgur.com/Pan9IqZ.jpg)
And here, for comparision, some pictures of the SZA263
This one is coming from this message https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg911832/#msg911832 (https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg911832/#msg911832)
(https://i.imgur.com/opKFJ3q.jpg)
X-Ray picture made by branadic:
(https://i.imgur.com/6Lc4gZs.jpg)
I did not expect, that the SZA and the LTFLU are that different.
Thanks again for doing the pictures. :-+
Regards,
Andreas
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D3 and D4 have some effect on the temperature effect. Other circuits can work without it or with only one diode. This may effect the second order TC.
That's right. This is from the Fluke 731B and as you can see, it has only a 1.27kOhm resistor and no diodes.
Here is an additional reference circuit, this time from the Fluke 332D (with an oven):
The A2 is a LM301A
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I did a quick simulation of the circuit in LTspice:
The model for the Zener is essentially noise less, so this important noise source is missing. To get some noise for the OP I added a resistor (r11) in series to the input.
One can still see that the cross over is not at the frequency one would expect from just R5*C3 (0.8 Hz) but at a much higher frequency of some 80 Hz. This is due to the extra gain for the Zener signal.
So it is possible to get some filtering from C3, but it is not as effective as one might expect from the relatively high impedance node.
C3 is also not really effective in filtering the current noise from the OP. This noise has quite some 1/f part that is not effectively filtered.
p.S: the slightly more effective position for filtering would be a miller cap, from collector to base of the transistor. Here the impedance of the divider is added.
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Isn't 80 Hz the cutoff frequency of R2 and C2? Obviously R2C2 does not match R5 C3, and it does not need to since impedance and noise are lower anyway. BTW in the proposed circuit opamp inputs are not at +/- 7 V but at 8.7 V or so.
What happens if you make the zener the dominant noise source? Can you give the zener the 7 Ohms differential resistance (maybe a series resistor) that was determined before and feed noise with a 1K plus capacitor?
Regards, Dieter
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Having the cross over at about where R2C2 is, is more coincidence. The cross over frequency is set be R5*C3 divided by the voltage gain of the transistor stage that is at around 100, maybe a little less if the voltage at the OP inputs is higher and thus less voltage at R5.
I have checked how noise (or a signal) from the zener propagates: it looks like a simple 1st order low pass with 63 Hz cross over. The frequency does not change with C2, but it does change with R2 and thus the DC voltage over R5. A smaller R2 reduces the gain of the transistor and thus cross over frequency.
The impedance of R2,R3,C2 has an odd effect: with a fixed C2 the noise goes down (not relevant in the sum) when R2 and R3 get higher values (at least without the current noise from the OP). C2 seem to be chosen just large enough that in the relevant frequency range R2 and R3 don't contribute much to the noise. With a BJT based OP it would make sense to go here up to the 40 K range so one has a matched impedance on both sides of the OP.
P.S. the zener model has a differential resistance of some 19 Ohms.
I also checked the effect of a current signal injected at the OPs input: It has an effect like seeing a resistance of some 700 Ohms at the lowe frequencies, which would be the 40 K divided by the transistor gain. So some 15 pA/Sqrt(Hz) from the LT1028 would result in some 10 nV/sqrt(Hz). So this is likely still less than the zener noise at 10 Hz.
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Sometimes the dynamic behaviour of those regulators is surprising.
BTW in the meantime i put the recommended ADA4077 into the reference circuit of one of our HP 3456A and made some changes to get the extra bandwidth. I am using it with a snubber on the -12 V reference output that consists of a 4,7uF MKS parallel with 1 Ohm plus 100 uF. Then the OpAmp sees a 1 Ohm resistive load up to a time scale of about 10 or 20 usec and stability is almost guaranteed. Maximum output current of the Opamp is about +/- 10 mA and enough to control output voltage across the 1 Ohm resistor. The snubber buffers fast current spikes from the ADC and residual noise remains well below 100 or 200 uV (looking at it with a scope). Since the ADA4077 shows an offset voltage of 50 uV i will also try a ADA4522.
I am not sure whether a 100 uF electrolytic cap is ideal for C3 in the proposed circuit.
Regards, Dieter
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For the filtering cap it really depends if filtering of the reference is needed 7 wanted.
Instead of C3 one could use a miller cap - this give a cross over frequency at 3 times lower, as there is the additional impedance from the divider at the base. Still filtering would be at relatively high frequencies only.
The main reason I see for filtering would be if some AZ measurements are used (either from an external meter or if the reference us used with an ADC). The filter would kind of bridge the pause caused by the zero reading. So it would need at least some 20 ms (maybe longer, depending on the meter). Currently C3*R5 /70 is at about 3 ms. So the 4.7 µF cap does not really help much. There is probably not much room for changing R5, as this is more like one point to adjust the TC. Even with 5 µF as a miller cap one would only be at some 10 ms (e.g. C * (R5/70 + R1||R7)).
In principle an electrolytic cap could work. Due to the transistor gain the effective resistance is at some 700 Ohms, both for the cross over frequency and the leakage / bias related error. With some 100 µF would would be at some 60 ms and thus just enough to cover 1 to maybe 3 PLC auto zero. Besides leakage, there can be also some DA (hard to tell apart from the experimental side) with an electrolytic cap. So it may take quite some time (e.g. hours) to stabilize after turn on.
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All this talk couldn't let me sleep... :o
UPDATED 6/20 version:
(https://xdevs.com/doc/xDevs.com/XACU/ltflu_xacu_1.png) (https://xdevs.com/doc/xDevs.com/XACU/ltflu_xacu.png)
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A few words to the x-ray images shown above.
With some imagination you can see the rectangular die as a slight shadow on the squared metallic piece of the package. You can also see the die attach, but you can't see any bond wires as aluminium bond wires are transparent to x-ray (other then bond wires made of gold). Comparing this images with the pictures of the already cracked open parts in previous posts indicates, that there is at least the same thing inside. Finally only electrical characterisation can show, if they behave like a LTFLU even though the package marker is at the wrong place.
-branadic-
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@TiN: the circuit shown looks really odd in some points.
Like with the LTZ1000, there is little need for the OP used with the LTFLU/SZA263 voltage loop to be an AZ type.
The extra resistor between the divider and base of the reference does not really make much sense with such a small cap at the transistor.
For the switched capacitor circuite C221 directly in series with the switch does not make so much sense. It looks a little like a capacitive inverter, but not really.
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@TiN: the max LTC1050 voltage is 16V.. (abs max 18V).
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Yup, you guys right. Haste is never good in circuit design.
Pin 3 from DG444 missing AGND connection, it is the inverter there. Power is valid note too. As of opamp choice, have to admit, I'm copycating Fluke ref circuit, where LTC1150 is also chosen. There is also little PCB oven around the ref, aka K7510 design. I have only two salvaged SZA263 chips to try this , not even LTFLU. Ordering bigger board, so this will be just a test vehicle on free area, not a standalone proper ref.
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Reuploaded updated schematics version.
* Fixed inverter missing ground connection
* Fixed U128 power to meet rated spec
* Cleanup
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Since Frank has mentioned it one or two times that he recommends to use the 732A reference as an example, I've found on KO4BB this nice redrawing of the reference circuit:
(https://i.imgur.com/Gy0AAyz.png)
The original can be found here:
http://ftb.ko4bb.com/manuals/93.200.148.254/Fluke_732A_Redrafted_schematic_of_reference_circuit_of_voltage_standard.pdf (http://ftb.ko4bb.com/manuals/93.200.148.254/Fluke_732A_Redrafted_schematic_of_reference_circuit_of_voltage_standard.pdf)
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Indeed a nicer drawing than the convoluted manual schematic :)
I have a few DH80417B-based refs which are quite old and which i want to TC-adjust. Afaik the reference-collector-resistor needs to be adjusted to get around zero TC. Is that correct or do the other resistors have a significant impact on the TC (Fluke WW, <=1ppm stated)?
I know that Conrad TC-adjusted his 731B, but he didnt recall which resistor/-s he adjusted.
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The collector current is at least the obvious point for adjusting the TC as the effect is predictable. Doubling the current gives some extra 18 mV BE voltage and some -60 µV/K of additional change or some -10ppm/K.
The main Zener current would also effect the TC, but often one is limited here: to low a current would increase the noise and too much current would make it run hot. How much the zener current changes depends on the details of the zener. Still the current setting resistors need to be stable. The TC of the current setting resistor would have an effect by attenuated by something like 1 K divided by the 10Ohm of zener differential resistance.
The series diodes to the main current path may be useful if the TC is otherwise far off. Expect them to give some +10-20 µV/K for each diode, depending on the output voltage/ current setting resistor.
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Thanks, was a bit unsure due to the instability of my measurement-setup when substituting the collector-resistor with my resistor-decade. Seems i need it to give more temp-stabilizing time for evenly distributed temp on the pcb. Hope to get it near zero TC without using extra diodes like done in the 3330B-reference
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Minor relation, but here are some test results on dual SZA263 reference out of Fluke 5440B board (bought just a board on ebay, to steal a ref).
(https://xdevs.com/doc/Fluke/5440B/img/f5440b_ref_a_1.jpg) (https://xdevs.com/doc/Fluke/5440B/img/f5440b_ref_a.jpg)
As you can see, board have two Motorola SZA263, year 1982 vintage, some teal epoxy Fluke PWW resistors. There is nothing else to it, no voodoo slots. Ref assembly sits in isothermal oven box on the main DAC/REF PCBA in calibrator. All oven heaters and control circultry is on mainboard, not here.
This module was unexpectedly easy to get working. Just +30 and -15V supply at the input, and +13V DC output, no whackers with external signaling required.
It also maintains output voltage with reduced supply, down to +23V and -1V.
(https://xdevs.com/doc/Fluke/5440B/img/f5440b_ref_r11res_1.jpg) (https://xdevs.com/doc/Fluke/5440B/img/f5440b_ref_r11res.jpg) (https://xdevs.com/doc/Fluke/5440B/img/f5440b_ref_sza_1.jpg) (https://xdevs.com/doc/Fluke/5440B/img/f5440b_ref_sza.jpg)
Little trap for young players - Hi sense and Hi force of the output must be connected externally to close the feedback loop.
My love to carbon resistors was rewarded here as well, in shape of R11. Replaced it with 1W metal film 1 Kohm already. :box:
(https://xdevs.com/doc/Fluke/5440B/img/f5440b_ref_back_1.jpg) (https://xdevs.com/doc/Fluke/5440B/img/f5440b_ref_back.jpg) (https://xdevs.com/doc/Fluke/5440B/img/f5440b_ref_cooked_1.jpg) (https://xdevs.com/doc/Fluke/5440B/img/f5440b_ref_cooked.jpg)
Magic Fluke shapes around SZA263 chips supposed to keep temperatures uniform among all pins. And it's doing that job just fine.
Colorful picture of the running board as well:
(https://xdevs.com/doc/Fluke/5440B/img/f5440_ref.png)
This was taken before replacement of carbon R11 1K resistor.
Results from 8 hour tempco sweep from +18C to +32C:
(https://xdevs.com/doc/xDevs.com/FX/fxtc_run3_1.png) (https://xdevs.com/doc/xDevs.com/FX/fxtc_run3.png)
Bunch of lines on other channels are LTZ1000A references.
Overall I'm quite impressed with performance of this cute unheated +13V reference, delivering respectable <0.45 ppm/K. Heck, that is lower than some of LM399's!
Ref was powered by Keysight E36312A (https://xdevs.com/review/e36312a/) benchtop linear PSU, with +25.00 and -12V supply voltages. It took about 24mA for positive rail and ~3mA for negative.
With ovenized assembly, like it is used in Fluke 5440 calibrator there is little to worry about tempco, even if oven temperature kept within easy +/-0.1C.
RAW-datafile (https://xdevs.com/datashort/test2_k7168d_scan_fx7_trim_tc_jun2019.dsv) for those who want split ppms.
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I rearranged LTFLU circuit (RC55Y resistors + LT1006) on my protoboard. I used a NOMCA16035001 resistor network configured to 5k:11k for R7A/R7B. With the current value of 25k343 for R13 I get the following temperature response, with almost zero t.c. at about 32°C. Need to trimm it to 45°C and put it into a 45°C oven. The shape of the curve looks like a typical temperature compensated zener, so no flat temperature response at all. This also explains why Fluke put the reference into an oven, instead of trimming it to zero t.c. over a wide range. Obviously the device I have shows some sort of popcorn noise.
a= -3.077749294582220e-02 b= 1.738680963044457e+00 c= -2.776202459721796e+01
-branadic-
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What happened between 7:30h and 9h in the run? That's no popcorn noise and doesn't seem temperature related.
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I have no idea, I was sleeping during that time :)
-branadic-
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So I changed R13 last night by estimating the value required for 45°C (linear correlation expected). I guess I found the correct value, R13 is now 22k. Time to replace the decade resistor box by a proper resistance and repeat the measurement.
Next step is to adjust 10,00000V output and add an LDO, as by now the circuit is powered from a linear lab power supply directly (15V). Afterwards I need to build an oven around the reference.
-branadic-
-
..
The shape of the curve looks like a typical temperature compensated zener, so no flat temperature response at all. This also explains why Fluke put the reference into an oven, instead of trimming it to zero t.c. over a wide range.
-branadic-
Hello branadic ,
I think you have demonstrated, that the RefAmp can never be trimmed to zero over a broad temperature range, but only at a single temperature point!
The 731B has a specified T.C. of < 1ppm/K over 10..45°C , and that fits perfectly to your measurement.. so nothing more can be expected, anyhow.
If you put this pre-T.C.-trimmed circuit into an oven, the stability requirements on the oven will be much less demanding compared to that 50ppm/K T.C. of the LTZ1000, I assume.
Could you please provide your calculus, how you estimated the correct R13 value for 45°C from the initial 25k set up?
Maybe it's sufficient to add an oven around the LTFLU, like it's done in the 335D reference assemblies (with the SZA or DH80... chips) ?
THX - Frank
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Hi Frank,
I simply had two temperature runs with two different resistor values and zero t.c. point close to 30°C (25k343) and 35°C (24k), so I assumed a linear correlation to estimate the resistor value for 45°C (22k) and that seemed to work.
I was planing an oven circuit as used in 732B based on TL062, but with a setup as used in Fluke calibrators, thus a single sided ceramic, with a ceramic thickfilm heater (in my case BPR10101) glued to its back. First successful processed ceramic samples are on my desk.
-branadic-
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Are there still suitable small ovens available like the ones made by Klixon, which only heat the Refamp-Chip?
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I simply had two temperature runs with two different resistor values and zero t.c. point close to 30°C (25k343) and 35°C (24k), so I assumed a linear correlation to estimate the resistor value for 45°C (22k) and that seemed to work.
-branadic-
A glass thermistor / precision resistor network that provided 25k343 ohm at 30 deg C and 22k at 45 deg C could flatten the temp co curve of the reference. This would be useful for low power applications.
This is similar to what lymex proposed for flattening the temp co curve of precision resistors. Good quality glass thermistors can have very good stability.
Of course you may want an oven to help stabilize the resistors in the 7V to 10V stage, reduce relative humidity and minimize hysteresis.
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Next step is to adjust 10,00000V output and add an LDO,
interesting would be determing the PSRR before stabilizing the voltage.
with best regards
Andreas
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Lymex mentioned the possibility of substituting the R1/2 (main-divider without stability-attenuation)-divider in the usual RefAmp-circuit with a LTC1043.
So i tried to simulate it with the 731B-circuit as the basis and it works.
LTC1043-blocks are used as /3 and *2 to get ~10V.
The calculation is as follows: Ub (6.807V) /2 *3 = Vout (10.210V).
Outputvoltage ~10.210V, sadly a bit more then the wanted 10V +-100mV to measure references in opposition mode with a DMM in 100mV-range.
Have to read the thread again for info on real measured usual Ub @ 3mA Iz and ~100µA Ic.
Simulation in LTSpice is attached. Also the minimized version which uses only 1 LTC1043 as /3*2-divider from Andreas.
What i still dont get is the exact control loop of the RefAmp-circuit and why it attenuates some resistor-variations. For example i dont know why the resistor-divider (R28/19 in the simulation) doesnt massively influence the outputvoltage. Maybe someone can explain it to me?
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The R28/R19 divider (together with the main divider) sets the emitter-collector voltage for the transistor in the reference. This voltage only has a limited effect on the gain of the transistor stage. Similar R32 set the current for the transistor stage and this way on a logarithmic scale a small contribution to the V_BE part of the reference voltage.
The transistor stage has a gain of around 100, that is about set by the voltage at R32 (slightly reduced by the Early effect) divided by kT/e.
Changes in R32, R28, R19 are about attenuated by this factor, so that changes of these resistors are less critical. The capacitor C2 reduces the gain of the transistor stage towards higher frequencies to ensure stability of the OPs FB loop. So the low frequency range (up to a few 10-100 Hz) is dominated by the transistor in the reference, while the higher frequency part is mainly controlled by the OP and C2 acts as low pass filter for the reference part.
For reducing the output voltage, one could modify the FB divider to also use a small resistive contribution (e.g. have the divider from V_out to the the buffer LT1012 output) to go to the transistor. As the part effected by these transistors is only small (e.g. 200 mV instead of some 3.5 V), stability of these transistors would be less critical than a full divider by about a factor of 20. This is still a little more critical than the other resistors (R32, R28,R19), but not that much.
There may be a better version for the LTC1043 divider that directly gives 2/3 in a single stage, a little like the 1/3 stage.
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LTC1043-blocks are used as /3 and *2 to get ~10V.
Hello,
there is also a possibility to do this with one single LTC1043: (and without extra measures to synchronize the clocks).
https://www.eevblog.com/forum/projects/building-a-7-decade-voltage-calibrator/msg298350/#msg298350 (https://www.eevblog.com/forum/projects/building-a-7-decade-voltage-calibrator/msg298350/#msg298350)
With best regards
Andreas
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I can only find the /2*3 and *3/4-implementation in the thread. Do you have the /3*2, so i can implement it in the simulation?
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https://www.eevblog.com/forum/projects/ppmgeek!-5-5-digit-dvm-volt-ref-cal-(for-arduino-or-any-uc-w-spi)/msg296127/#msg296127 (https://www.eevblog.com/forum/projects/ppmgeek!-5-5-digit-dvm-volt-ref-cal-(for-arduino-or-any-uc-w-spi)/msg296127/#msg296127)
with best regards
Andreas
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Thanks, RefAmp-circuit with minimalized /3*2-circuit added to the post from me above.
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So here is a picture of my current LTFLU protoboard. I can hear them voices "Don't use sockets, sockets are bad!", but I did since it's only for verification and not the final board. More to come soon, hopefully.
-branadic-
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The sockets are fine, the leads with gator clips are however... :popcorn:
I usually just solder twinax wire or shielded UTP cable during test, so i can mangle with the board without worry about value changing because of connection to DUT.
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Modified circuit today, LTFLU is know running from single 12V supply and decade resistor box was replaced by fixed resistors. Though the output is not proper adjusted to 10,00000V yet readings are now stable.
So I'm on track with the idea to power reference and oven from battery (12x NiMh) with LT1763 LDO set to 12V. Propably I use BMON designed by Andreas or parts of the circuit for a stand-alone reference. The oven circuit will be a copy of Fluke 732B, but with AD822 instead of TL062.
-branadic-
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After i made a LTFLU reference circuit i got a first TC determination. The result is way off, needs adjustment first. This made me think a little.
This type of reference (also the LTZ1000) includes a voltage sensing transistor on chip. If i assume about -2 mV/K as the Ube contribution of that transistor, this amounts to -0.002 V / 7 V * 1E-6 = -286 ppm/K. Is that correct? This terrible TC is there even after compensation by a positive TC of the zener of similar size. So to get the reference down to TC < 0.1 ppm/K, one needs a factor 3000. If the TC compensation is adjusted to 1 %, the oven still needs an ambient temperature regulation of 1/30, that is < 30 mK/K(ambient) or so.
I know the LTZ1000 thermostat is very fast and oscillates at about 50 Hz if you have enough gain. So speed may be important. On the other hand i remember reading in this forum someone was using water to attach "thermal mass", with very interesting results. Arroyo TEC controllers appear to reach stability levels of some mK, too. What is a practical solution for the LTFLU?
Regards, Dieter
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The TC adjustment can be quite good, better than 1% if needed. It just takes times and effort to find the right transistor current.
Even with a crude oven this would be needed only for the linear TC at the set temperature, no longer over a larger range including square parts.
A temperature regulation to 30 mK/K is not that demanding. A fast loop makes things a little easier, but is not absolutely needed. Extra thermal mass is a two sided thing and may help if the regulator is slow - I prefer good thermal conduction, so that one has only 1 temperature to worry about and not also gradients.
I would start with the oven and than adjust the TC of the reference to the required level for given oven quality. So a better TC adjustment could compensate for a not so well done oven. So the 30 mK/K would be a staring target.
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If i assume about -2 mV/K as the Ube contribution of that transistor, this amounts to -0.002 V / 7 V * 1E-6 = -286 ppm/K. Is that correct?
This terrible TC is there even after compensation by a positive TC of the zener of similar size.
No, that overall T.C. is indeed reduced to about +50ppm/K for the LTZ1000. In some special applications, it's reduced to < 5ppm/K by adding a resistor in series with the zener.
LTFLU can be easily reduced to ~ 0ppm/K by selection of the collector current @ 3mA zener current.
So to get the reference down to TC < 0.1 ppm/K, one needs a factor 3000. If the TC compensation is adjusted to 1 %, the oven still needs an ambient temperature regulation of 1/30, that is < 30 mK/K(ambient) or so.
That's proven to be no problem, the factor is indeed 500 only, and the regulation is even much better.
I know the LTZ1000 thermostat is very fast and oscillates at about 50 Hz if you have enough gain. So speed may be important. On the other hand i remember reading in this forum someone was using water to attach "thermal mass", with very interesting results. Arroyo TEC controllers appear to reach stability levels of some mK, too.
Really? never heard of this..especially no oscillation problems. That measure seems to be counter productive, as the normal circuit w/o these artifical measures works just fine, giving near zero overall T.C. by trimming.
Anyhow, I always try to add some thermal around the whole assembly, so that all affected components stay on the same temperature.
What is a practical solution for the LTFLU?
Regards, Dieter
?? LTFLU always requires an external oven, and its own T.C. is already near zero, so requirements on the oven are for sure relieved.
Main advantage of external oven is the stabilization of the step-up resistors.
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The 50 ppm/K of the LTZ1000 are the final result of a compensation that involves the sense transistor with it's 300 ppm/K, right? During all my professional life i learned that compensations to better than 1 % or so aren't very reliable. More like pieces of art instead of solid engineering. So in the LTZ1000 the compensation is about 50 / 300 = 16 % and it relies on an almost perfect thermostat instead. Will have to run the LTZ1000 without thermostat to have a look.
Of course nobody wants to have an oscillating thermostat and there were no "special measures". I just observed oscillations when building the circuit first without damping capacitors. I had noticed before that many schematics proposed in this forum and on the web were fairly chaotic concerning those bandwidth limiting parts. So i added/tuned the caps later observing the dynamic behaviour of the circuit.
For the LTFLU i remember now we have some old, non-working FEI rubidium oscillator. Maybe that one can donate its thermostat for the LTFLU reference.
Regards, Dieter
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In the LTZ and LTFLU the compensation in temperature drift is with transistor and zener diode directly on the same die. So there is no problem to get a good compensation and both parts are made to be very stable. So the "art" is already in the chip. The external part is not that critical:
Doubling the current through the transistor increases the voltage by some 20 mV and gives a change in the TC of some 66 µV/K or about 10 ppm/K of the total reference voltage. So to adjust the overall TC to 1 ppm/K the collector current only need to be within about 10% of the ideal value. The adjustment is quite predictable and the oven could be used to change the temperature, so that it does not take many iterations to get a good setting.
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While glueing a PT1000 temperature sensor to the bottom of a LTFLU i had the idea that the transistor inside the LTFLU should make a good temperature sensor, too. In order to measure its Ube one can split the 1K3 resistor that generates the zener current into a voltage divider 6:1, e.g. 1080 + 220 Ohm. After that the scheme should become somewhat similar to a LTZ1000, except the heater is external. Current variations in the transistor will probably not disturb temperature measurement, if the 10 V step up OpAmp is of good quality.
Need to work it out, though. So, next thing is to glue a heater transistor to the bottom of a LTFLU, for constant voltage heating (linear!).
Regards, Dieter
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Here is what I'm currently working on. It's a Fluke style solution, thus a 20mm x 40mm ceramic board with LTFLU circuit and NTC on it. I use LT1006 for +12V power rail only and NOMCA16035001 resistor network for what I call R7 divider.
I'm designing a LTZ1047 like mainboard, lets call it LTFLU1047, with BMON (battery monitor and charging circuitry) and with a modified copy of F732B oven circuit, but use AD822 instead of TL062 for the oven, so again +12V rail only. The ceramic board will be thermally glued to a BPR10101 resistor, with the heater resistor perpendicular mounted to the mainboard. It will have some thermal shielding around it, that's for sure.
Single sided ceramic boards of that size with 12µm thick silver traces (silkscreen printing + burning --> thickfilm technology) can be bought commercially rather cheap. Lowest offer is $150 for tooling and $0,6 for each board.
Couldn't find any 100R heater resistor by now fitting the same size as my ceramic board, but will give it a try.
-branadic-
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The ceramic board will be thermally glued to a BPR10101 resistor, with the heater resistor perpendicular mounted to the mainboard. It will have some thermal shielding around it, that's for sure.
I thought of using aluminium substrate PCB for projects like this.
Did you consider this? For this particular project, what are advantages of ceramic PCB over aluminium one?
Lowest offer is $150 for tooling and $0,6 for each board.
Aluminium substrate PCB 40x20mm, 1,6mm thick, 6/6mil goes for ~50USD per 5 pieces (DHL shipping included) from random first asian vendor. Or ~60USD per 100 pieces.
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Aluminium is not the best material because of it's thermal capacity and worse thermal conductivity compared to ceramic. Furthermore you have additional thermal resistances (aluminium --> glue --> foil) which worsens the oven functionality, since the temperature sensor is mounted at the LTFLU circuit.
-branadic-
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Another problem with an aluminum core PCB is the different thermal expasion. It is much higher than that of SMD resistors.
There is a chance the glues could show some creeping / drift.
Heat capacity should not be that much different to make a difference and also thermal conductivity is good with an aluminum core.
The DMM7510 shows that an oven is even possible with just FR4. After all with a tuned TC the oven is much less critical.
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It is heavily different... please make the experiment, take a plate of aluminium and same size Al2O3 ceramic, heat both to say 350°C, wait half a minute and touch the aluminium with left hand and ceramic with right hand. Let me know which hand hurts. ;)
I'm sure you will agree that DMM7510 is not a metrology grade meter and that it's more due to cost aficionados to make such a piss pour oven, nothing really serious.
-branadic-
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Just a few comments:
Aluminium has 5-10 times better thermal conductivity than Al2O3 - depending on exact composition and manufacturing process of both materials. Aluminium spreads heat from resistor better than ceramic substrate, creating lower thermal gradient on back side of board. Aluminium nitride ceramics is much better than Al2O3, berrylium oxide is even better than aluminium itself, but those are out of question, probably.
Typical thermal conductive dielectric for aluminium PCBs has one order of magnitude worse conductivity than Al2O3, accounting for its low thickness it may mean somehow worse conductivity from back side of board to front side of board. Question is how much it matters in well insulated system.
Thermal regulation via the PCB with its thermal mass and thermal resistance (which is suboptimal for both Al2O3 or Al substrates anyway) may need to be adjusted for somehow slower response - in comparison to Al2O3 board. That may be problematic if you need to power up reference from cold state to full specs fast, but this is not the usual use case of high-class references.
One may consider placing the temperature sensor to aluminium substrate directly; here would aluminium (with its better thermal conductivity compared to alumina) fit better and make thermal feedback loop tighter (that is always desirable for temperature regulators).
Thermal expansion of Al2O3 is usually stated 5-10 E-6/K, while aluminium itself 12-14 E-6/K. FR4 PCB has two orders of magnitude higher thermal expansion, so I expect resistors designed for PCB mounting being OK with both aluminium or AL2O3 substrates.
Bottom line - alumina ceramics is suitable material for your project and fortunately material science and cheap mass production is bringing new suitable material possibilities to the table. Decades since original Fluke references inception we have more options to choose from.
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The new Fluke 732C uses a ceramic PCB, and they claim that this is the reason that the 732C no longer has seasonal variations (due to humidity) as earlier versions of the 732 have.
@branadic:
Can you say who is the vendor you found that has such good prices?
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The expansion of FR4 is a little complicated: it has a "normal" thermal expansion part that is not that bad (some 5-10 ppm/K), humidity effect and a delayed part from the epoxy part creeping relative to the glass-fiber part, especially at higher temperatures like 70-100 C. When coming from higher than some 150 C one can also have some structural relaxations.
Aluminum CTE is normally quite a bit higher, more like 22-23 ppm/K.
So the ceramic carrier parts are likely better of with ceramic or FR4 than an aluminum based board.
For the OPs in a SOIC8 case, the thermal expansion of the plastic and copper carrier is likely better matched to aluminum.
Anyway for a reference oven the temperature is likely relatively constant and power consumption is constant. So thermal conductivity and heat capacity are not that important.
What hurts, when touching a hot piece of aluminum is the high thermal conductivity - lower conductivity can ease things quite a bit.
For the DMM7510, I don't think the oven temperature control is the real problem, the weak point is more the FR4 board and using numerical corrections for temperature effects (which has some acceptance problems). The other point is the Keithley typical extra noise for longer integration (which likely is a software/statistics problem taken over from the old days when they had the brown cases).
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The new Fluke 732C uses a ceramic PCB, and they claim that this is the reason that the 732C no longer has seasonal variations (due to humidity) as earlier versions of the 732 have.
Humidity changes do influence board itself, as well as components on the board.
For example folks at Fluke are using custom made resistor network, rather than resistor array in plastic SOIC package.
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@branadic:
Can you say who is the vendor you found that has such good prices?
www.xdpcba.com (http://www.xdpcba.com) / www.xdxpcb.com (http://www.xdxpcb.com)
You can't compare single material only, but the complete board setup. So having an aluminium core board also includes the insulator between aluminium and copper and additional thermal resistances. Also keep in mind the different thickness, e.g. 1mm? for aluminum core board and 0.5mm for alumina. Humidity influence was also mentioned, almost no problem on ceramic, but a factor on FR4 and the insulator of aluminium core board. Same for CTE, FR4 with 13 - 17ppm/K in x and y (good match to copper with 16ppm/K), but 80ppm/K in z-direction. On aluminium core board you mix up materials with different CTEs.
So you can use whatever you want, I decided for ceramic.
-branadic-
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Recently i got four LTFLUs of the "LTFLU-1ACH 0625" version mentioned before. Basically they were tested OK.
In the standard application circuit, the first one exhibited a TC = - 40 ppm/K at 0,1 mA collector current of the sense transistor and TC = - 25 ppm/K at 0,04 mA. This means the negative TC of the transistor Ube overcompensates. To tune the TC to zero, collector current would have to be further reduced. Apparently this LTFLU is out of specs.
Now i implemented the idea i mentioned above to use the LTFLU transistor as a temperature sensor. A second OpAmp outputs -200 mV/K.
While this was first meant for the implementation of an oven, i found that with a resistor R24/R25 i could tune TC to less than 1ppm/K without modifying the sense transistor collector current. This method works similar to the 400K resistor in the LTZ1000 circuit.
Already got some promising measurements under ambient temperature variations. Apparently nonlinear TC (including HP3456a TC) is so small that a +/- 0,5 °C oven is enough to keep the reference voltage within 0,1 ppm. Need to put the circuit into a aluminum box to use as oven.
Regards, Dieter
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If the TC is too negative (would need to little collector current for the transistor, a little like with the LTZ) one could try reducing the zener current. This should also result in a more positive TC. The zener current is a second point to trim, though less predictable.
Another option would to use a diode in series to the resistor that sets the zener current. This is found in some circuits too.
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As i wrote, TC compensation was a lucky by-product until the oven is ready. I am not even sure i will need that once the oven works.
I think it may be interesting how the LTFLU transistor can be used as an on-chip temperature sensor. Haven't seen that before.
Regards, Dieter
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I don't think it would really take AZ-OPs in the control loop around the LTFLU. They tend to cause more trouble due to EMI issues going out from the OPs. RF signals originating from the OPs could effect the reference reading and the level of interference could be effected in a hard to predict way by thinks like attached cables.
At least for the OP for the voltage loop an normal OP (like OP07, LT1097,ADA4077) should be OK, as only something like 1/100 the OPs offset would appear at the output.
For the temperature reading this is less clear. Still 1 mK corresponds to about 2 µV at the sensor. So some drift may not be that critical.
However the temperature compensation part could be rather sensitive to the OP. R22/R20 gives a gain of some 100, while R7/(R24+R25) give an attenuation of some 700. So about 1/7 the OPs drift would appear at the output, which is not a good ratio, though probably just acceptable.
Another possible problem could be resistor drift from R20,R21,R22,R24+R25, especially if R20/R21 are not adjusted to keep the current through R22 and R24 low. The high values resistors tend to be more drifty than others. Chances are R20 and R21 also would need to be low drift, not as important as R7, but still with quite some effect.
I would try adjusting the zener current, if less transistor current is no longer an option. The Zener current should also effect the TC, though less predictable.
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Yes, the temperature measurement should be as precise as possible. If we want to get down from 40 ppm to 0,1 ppm, that's a stability requirement of 1/400. I think this is realistic with standard metal film resistors, if they are inside the oven. This is why i gave up on splitting R12 and ordered some foil resistors for that one. And the 100 gain is an example, maybe a -20 ppm/K output is as good. Then the MegOhm resistors will be 100K or so. On the OpAmps: I used what i had around, will certainly try others.
By the way, the schematic is missing some startup helper. Can be made with a zener, like in HP 3456A reference.
Regards, Dieter
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The statement i made above that a LTFLU with -40 ppm/K was out of specs was premature. Today i tested another one of those "LTFLU-1ACH 0625" and its TC is the same within 1 ppm/K in the same circuit.
Also i noticed the 8842A reference schematic in the beginning of this thread showing a 164K resistor instead of the 16K resistor in Dr. Franks schematic, which means running the LTFLU sense transistor at 10 uA instead of 85 uA. Maybe LTFLUs could be configured for different operation conditions. I mean we have seen those fuses in the die images presented above. Interesting enough, in many circuits above that resistor value is marked as configurable with no value given.
Regards, Dieter
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The LTFLU is intended to have zero T.C.between 25 .. 45°C, i.e. operation at room temperature, inside 731B or 334A, 8842A, or similar, or inside a 45°C oven.
Both used cases obviously are different, concerning different setpoints, reflected in the 2nd requirement:
At 3mA zener current, this zero T.C. has to be obtained for collector currents between 20...200µA.
Therefore, if your LTFLUs do not achieve a zero T.C. within these limits, that might be the reason, why they showed up on the black market.
Frank
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How did you determine the current limits 20 .. 200 uA? Where did you get that from? Do we have to vary both temperature and current or can we find the zero TC point at any given temperature within the given range 20 .. 45 °C?
If the difference between LTFLU-1CH und LTFLU-1ACH is that one of them is meant for oven use and the other one for ambient temperature use, i would like to know that.
Regards, Dieter
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Dieter,
I'm refering to this circuit:
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=268725;image)
By changing R13 you can set the temperature at which you have zero t.c. For my device I found it to be:
R13 = 25k343 --> ~30°C
R13 = 24k --> ~35°C
R13 = 22k --> 45°C
with zener current set to 3mA (R12=1k3) and the R7 divider set to 5k : ~11k, but without R8, R9, R10.
You should be able to reproduce similar results.
-branadic-
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The resistor to set the transistor current would be my primary choice to adjust the TC, as the effect should be relative predictable. There is a range of current that is feasible: some 10 µA may be the lower limit with maybe noise increasing to go up a little. However this would mainly be higher frequency noise, not so much the often more critical low frequency noise. An upper limit may be at some 200 µA as than the base current and current noise at the divider inputs get increasingly higher.
Another point to adjust the TC would be the zener current, with more zener current usually leading to a more positive TC. Here the effect could be unit dependent, so its not so clear how much effect a 50% higher current would have. There is no need to stay at 3 mA - if needed some 2, 5 mA or 7 mA may be acceptable.
A third point would be a possible diode in series to the resistor (especially R12 to set the zener current). A diode in series should also give a more positive TC, as the zener current would than go up with temperature. This could also effect the 2 nd order TC and thus the curvature, that could be important for a non heated reference.
Only if the 3 point's above don't give a suitable working point, I would consider adding the extra TC compensation.
The collector current is the most predictable way and thus likely the choice for fine adjustment.
There is no absolute need to adjust the TC zero to the exact oven temperature. It is more like adjusting the TC at the oven temperature to a low enough (e.g. < 3 ppm/K) value. So no need for large range temperature scans. Just 2 temperatures (e.g. +-5 K) near the paned temperature should be good enough.
I don't think there are official specs available to others than Fluke - so hard to tell if out of unknown specs.
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https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg2519910/#msg2519910 (https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg2519910/#msg2519910)
branadic has demonstrated, that the zero T.C. point is only an extremum point over temperature, on a parabola, so for ambient temperature applications, it's necessary to trim the collector current.
All known oven application, beginning with the T.I. chip inside the 332/335 calibrators also have a trimmed collector resistor, or even have a label indicating the zero T.C. collector current.
Therefore, even oven applications need to be trimmed to zero T.C. for best performance.
I would also use the most simple examples of circuit, like inside the 5440 or 537x calibrators, I.e. w/o any additional diodes, or so.
Frank
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I agree that a circuit without an external diode has some advantage. Especially it does not depend on the diode and thermal coupling.
I see two possible reasons for the diode: one is having a little larger adjustment range - this may be needed with some ref Chips to reach the zero TC without to extreme currents.
A second reason could be the 2 nd order TC for a non oven use. The diode might reduce the 2nd order TC. In the extreme case one may even be able to also trim the 2 nd order TC.
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Any idea what the 11 fuses on the LTFLU chip are good for?
PS: the chip (the source - branadic)
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The fuses are used to trim the transistor area by effectively connecting small transistors in parallel. This can be clearly seen on the high-res die photos. A bigger transistor has a slightly lower V_BE for a given I_C. A slightly different tempco as well. Trimming is probably done at the wafer level, before dicing and packaging - therefore I expect parts that are outside the trim range to be scrapped rather than packaged by LT.
Edit: they might also just be unused, as are the two on-die heater resistors that could possibly be bonded out in a different package but are not in the SZA-compatible one. Silicon revisions are expensive, better have some flexibility in the design...
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..it is not a chip with a diode and a transistor wired to the pins, it seems :) It looks like there are 4 zeners in parallel (each with some small resistor in series)..
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Yes i can see the four zeners with their resistors.
The transistors appear to make something like an 8 bit binary DAC. The "bits" are present on individual pads with remnants of previous bonding/sampling. I labeled the corresponding pads by transistor size/weight. If the die were in a 18-pin package each individual bit could be brought out. In the LTFLU we know (with four pins) the bits are hard wired by fuses. The pads were used for burning some fuses.
I understand there is nothing like "the LTFLU" and general wisdom will not tell you what kind of LTFLU you have. When you read free_electrons
"Fluke 732B DC Standard Teardown " thread and his description of a "transistor on top of a zener", reality is just a little more complex.
Regards, Dieter
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So when the chip was tested, they selected the best performing "bit pattern" and hard wired it?
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There are small "fuses" on the chip near the "programming pads". They blew the specific fuses (by applying a short current pulse) either while the chip was still on the wafer or after they bonded the chip to the package.
PS: would be interesting to reverse engineer the chip :)
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So basically, they would have been able to tailor the characteristics of the transistor to match (one of) the zener(s) and end up with an overall low temperature coefficient, eliminating or simplifying the need to temperature control the part? Respect! :-)
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From the above information there are several interesting observations:
1. there are 4 buried zeners in parallel
2. there is an 8bit resolution fuse-programmable setting of either a current via transistor(s) or a voltage(?)
3. there are 2 large resistors (at the top and bottom of the picture) not wired, perhaps the heater
4. there are several other components on the chip (like some resistors, diodes).
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After some more thought, this is what I think about these fuses: at the circuit level, they adjust the effective transistor area, nothing more, nothing less. It makes sense perfect for the steps to be binary - wide trim range and fine steps with a small number of fuses. As a sidenote, Vishay foil resistors use a similar approach to trimming.
The low-level transistor equations feature current density rather than current, so essentially doubling a transistor's area is the same as halving its collector current. So in the classic refamp designs you always see the SZA and current-setting resistor listed as a matched set in the parts list - the resistor is hand-selected for a certain zero-TC current. With the newer LTFLU, the part can be trimmed at the factory and a constant current can be used, and more importantly a constant resistor value - a huge plus for manufacturability. The trimming is almost surely done on an ATE-type system at the wafer level, before dicing and packaging, and most probably at room temperature, probably based on voltage measurements at constant current. In support of this theory, please see the LT1236 teardown on this forum. It has fuse trim pads that are used for coarse trim before packaging, as well as bonded-out trim pins labeled Do Not Connect, used for post-packaging fine trim.
This is how I see it at least, only Fluke and LT\AD really know the details...
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User zlymex did a teardown in past too:
https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg913137/#msg913137 (https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg913137/#msg913137)
PS: Btw, assuming the large resistors there are heaters (perhaps used during that calibration) where is a temp sensor then?
During the calibration they may heat the chip up to the desired temperature (as ordered by Fluke) and set the current (by blowing the fuses) to the zero TC..
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Yes, i noticed that zlymex had seen the binary trim before. Concerning the temperature sensor, i posted a schematic above showing how to extract chip temperature from Ube of the transistor. That seems to work very well.
Ah the other three fuses:
One is between two pads connected to the heater resistors. This must have been some option for selling the chip in a package with the heater pads bonded to pins. The remaining two fuses allow disconnection of base and collector of the largest transistor, while the eight trim fuses are between the zener cathode and the transistor emitters. Disconnecting the large transistor completely avoids leakage when running the chip at low collector current.
And by separating the large transistor, one arrives at something very similar to a LTZ1000, i mean a second transistor for temperature control.
Regards, Dieter
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Here's all the LTFLUs you could ask for:
https://www.jotrin.com/product/parts/LTFLU_1ACH (https://www.jotrin.com/product/parts/LTFLU_1ACH)
Have at it.
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Imagine you buy 10pcs of the LTFLU chips (random prod date). How do you know for which temperature they set the zero TC?
I still think the chips are "preset" for a "zero TC" at a specific temperature (not the ambient one). Small fine-tuning is done by the external resistor then.
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The measurements of my four LTFLU-1ACH 0625 were taken in the basic circuit at 3 mA zener current. All four chips work well, with low noise (< 0,2 ppm or so). I have not seen any unexpected drift yet, but that's a little early to say considering the age of those parts. TC measurements are at ambient temperatures and accurate to about 0,1 ppm/K. The characterization indicates that i got two different kinds of LTFLU reference.
Two of the parts have TC = 7 ppm/K. Their larger Ube indicates they are configured for lower collector currents. Their TC nulls at about 40 uA collector current. According to Dr. Frank this is an acceptable value. Then they exhibit a TC curve very similar to the curve presented by branadic, with a flat maximum at around 30 to 40 °C.
The other two references appear somewhat different, with a large negative TC and with lower Ube. In order to null their TC at room temperature, they would need about 1 mA of collector current. I can null their TC at 0,08 mA collector current using the temperature measurement and compensation circuit proposed above. Then the nonlinear TC contribution exhibits a minimum at some 30 to 40 °C.
Regards, Dieter
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My suspicion comes more less from the general information the chips are delivered to certain vendors only (based on their specific order), and the chips are not offered on the free market, and none from those vendors is using the chip at the ambient temperature (but inside a box at say 35-40C or inside an oven somewhere between 50-60C, or..).
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When i look at the temperature curves of my LTFLUs, they are so flat that i will never see the nonlinear contribution (minimum or maximum) unless i first trim TC to about zero. Then the parabola fit of the maximum is like +0 / -1ppm within +/- 3 °C. In the end, i also want to have the reference inside a precise oven. free_electrons 723B thread gives some insight on how one can do that.
Regards, Dieter
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LTFLU RevEng v0.001
A naive attempt so far ;)
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And the v0.002 with TC.
With 8 emitters :)
PS: 1ppm within 7C..
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dT=20degC
EDIT: dV<+/-1ppm
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So we should not only try and tune TC to zero, but we should also learn how to find the largest possible range of small TC. I have no idea how you did that.
The best i could get until now is about +/- 6 °C for a 1 ppm change below maximum resp. +/- 2 °C for a 0.1 ppm change.
Regards, Dieter
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For a larger range of low TC it would take a way to not only adjust / trim the linear TC, but also to trim the 2nd order TC (square contribution). There are a few different possible variations that also change the 2nd order effects: a different zener current, the hight of the effective voltage driving the zener current. A possible diode in series with the resistor to set the zener current.
For a version with oven, there is no real need to trim the 2nd order TC: with regulation square law contributions are very low. It only needs to get the linear TC at the oven temperature below a certain limit.
The extra trim may be needed for a non heated version. As there can be also effects from additional resistors in the circuit it does not make sense to only look at the reference chip. The resistors can also contribute higher order effects.
The simulations have only limited value, since AFAiK there is no reliable model for the buried zener including the thermal effects. The simulation may still help to get the number on how much change is expected from changes in some resistors.
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So we should not only try and tune TC to zero, but we should also learn how to find the largest possible range of small TC. I have no idea how you did that.
The best i could get until now is about +/- 6 °C for a 1 ppm change below maximum resp. +/- 2 °C for a 0.1 ppm change.
Regards, Dieter
Here is a 23.6degC range span below +/-1ppm ("S" shaped).
Mind it is just a naive simulation (not sure it reflects the chip wiring properly). Pretty sensitive to the external resistors values (the external resistors and the opamp with TC=0 here).
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Your simulation is very interesting. By the way the resistors R2, R4, R6 and R8 should be smaller. As far as i remember, differential resistance of the LTFLU zener as a whole was measured by janaf to be about 8 Ohm at 3 mA and 7.4 Ohm at 10 mA. It has always been said the LTFLU zener should get 3 mA, but the 8842A circuit runs at 4,2 mA = (7+0,5 V) / 1887 Ohm. Collector current is 43 uA = 7 V / 162K.
As far as i understand you got a wider temperature range by increasing the zener current and by adding 16 more transistors. Maybe the transistor configuration isn't really about having a preadjusted collector current (in order to use a fixed resistor), but for a wide flat range. This could explain why many LTFLU applications still have those selected resistors of unspecified value in the collector path.
Regards, Dieter
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The zeners - the structure around the zeners looks more complex, imho. The question is how is the wiring done in reality.. ::)
The resistors - you may get the values by counting the number of squares the resistor is made of, and multiply by a resistance per square (I've guessed 100ohm/square).
But again, it would be great an expert in reading the analog silicon would analyze the area around the zeners..
PS: Zeners - here we did a similar exercise while I had used two additional tempco parameters for the zener - https://www.eevblog.com/forum/metrology/compensating-the-temperature-coefficient-of-a-voltage-reference/msg2387187/#msg2387187 (https://www.eevblog.com/forum/metrology/compensating-the-temperature-coefficient-of-a-voltage-reference/msg2387187/#msg2387187)
The two parameters were not used in this simulation, you may try to add them into the zener's model.
PPS: below
a. with zener's TRS1 and TRS1 tempco parameters, RZ=700ohm, and Rcollector from 15k to 16k step 50ohm
b. with Rc=15.7k you get 24degC span within 1ppm
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Inside our two Fluke 8502A multimeters i found SZA263 references without temperature control.
Zener currents are fixed at 2,72 mA by 2753 Ohm 0.1 % resistors, collector currents of the references are adjusted for each reference chip, in our case 58 uA (107K + 13K6) and 19,99 uA (324K + 26K1).
Those Fluke multimeters were specified for a 4 hour warm-up time and operation at 23 +/- 1 °C (24h) and at 23 +/- 5°C (90 days and more).
For a range of 0 to 18 °C or 28 to 50 °C they gave an extra 2.5 ppm tolerance in the 10 V DC range.
That means a pretty low TC again - less than 0,05 ppm/K on average.
Regards, Dieter
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Meanwhile i implemented the two chinese LTFLU-1ACH references 2 and 4 with the circuit i proposed above (basic circuit + temperature measurement), with TC optimized NOMCA dividers for 10V/7V. Both references are adjusted for low TC at room temperature. The first plot is from HP 3456A with NPLC=1, one measurement every 10 seconds for almost 2 hours. This is a direct comparison of the references. I also logged the temperature output of one reference (200 mV/K).
Noise of measurement is about 0,2 ppm (2 uV), a promising result for the voltage difference between two references. Need to clean the boards yet and put them into their boxes. Also i want to see the noise of the HP 3456A with shorted inputs.
Regards, Dieter
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In the difference more the DMM should not be that critical. Still one should use more than 1 PLC if this is only for a slow measurement. One point every 10 seconds would allow at least 100 PLC.
To judge the temperature effect likely a larger temperature swing would help. This looks like only 1 C change.
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The 1°C temperature increase happened this afternoon (sunny weather here). I just wanted to show that test, because i was very happy with the result. I mean there was some doubt about the noise of the Nomca resistor arrays.
Meanwhile i repeated the two hour log with 10 NPLC. Now the Y scale is 1 ppm! Noise seems to be even lower now and there seem to be some waves in the curve. Hope that disappears once the references are in temperature controlled boxes. The boards fit into Hammond 1550 Z boxes, that i want to keep at constant temperature with Peltier elements.
Regards, Dieter
PS: Sorry, the first measurement was with 10 NPLC and this one is with 100 NPLC.
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I measured the series of the outer two resistors (bottom and top) today (connected to second and third pad from the top row on the left). The sum of them is 200ohms, so each one is 100ohms.
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=798891;image)
-branadic-
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After observing two new LTFLU references for some days, i got a question to the experts.
For the direct comparison of the two voltages i made a 100x preamplifier with a ADA4522 ("null detector"). I am using two 470 Ohm resistors for the inputs and a 47 KOhm resistor for negative feedback, a very basic inverting amplifier circuit. I am measuring the amplified difference voltage using a HP 3456A in its lowest range. Then it has a resolution of 100 nV, so with the preamplifier the resolution is 1 nV. The ADA4522 is specified with 117 nV noise peak-to-peak into 0,1 to 10 Hz. Using the preamplifier i am trying to reduce the difficulties of bringing sub uV voltages to the DVM without thermal EMF.
What would be a realistic expection for the noise in 100 PLC measurements that i am doing every 10 seconds? One measurement takes about 2 seconds, so the Nyquist limit would be 0,25 Hz and i would expect a small fraction of 117 nV, like 20 nVpp, about 2 ppb of a 10 V standard. Plus the noise of the two references.
I am asking this because today i got a sequence of about 300 measurements that exhibited less than 3 ppb noise after subtraction a the thermal drift. Is the reference noise negligible in such a setup?
Maybe i should first measure the preamplfier+DVM noise with a short-circuit on the amplfier input.
Regards, Dieter
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The 100 PLC measurement should have a bandwidth of 1/2 Hz, unless there is extra filtering used and the actual time used is more than 2 seconds. I don't think the 3456 uses this. With AZ a measurement should take more like 4 seconds with the 3456.
1/2 Hz BW compared to 10 Hz would be a little less than 1/4 the noise, so maybe some 20 nV (peak to peak) from the OP.
There will be additional noise from the resistors though. 2 time 470 ohms in series give around 4 nV/Sqrt(Hz) or some 17 nV_pp for the 0.5 Hz BW.
For lower noise one could use a normal non inverting amplifier circuit and connect the amplifiers ground to one reference. This could use just one resistor of lower value.
The white noise of the reference can be quite low, possibly less than the described amplifier. The weak point of the reference is likely more in some low level of popcorn noise - so relatively rare jumps up or down.
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Thanks.
I had the references on top of the 3456A for several days and the stdev of the 100 PLC difference measurements was about 60 to 65 ppb. After repairing the 3456A fan (new oil) the setup is a little different now with very little vibration from the fan and the noise went down to about 3 ppb.
Have to put the preamplifier into a box yet, with thermal clamps for the flat cables from the references. Image shows thermal clamp inside a reference's box.
Regards, Dieter
PS: The feed-through capacity between the case and each signal line is 35 .. 40 pF, helps against RF entering the reference.
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I've been using a diff amplifier with an opamp and 1 input resistor (with one of the Vreferences wired to the amplif's gnd), but nulling it is a nightmare. How do you null your references?
PS: .. for example Vref1=6.9978787V and Vref2=7.1723872V
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[..]
The ADA4522 is specified with 117 nV noise peak-to-peak into 0,1 to 10 Hz. Using the preamplifier i am trying to reduce the difficulties of bringing sub uV voltages to the DVM without thermal EMF.
What would be a realistic expection for the noise in 100 PLC measurements that i am doing every 10 seconds? One measurement takes about 2 seconds, so the Nyquist limit would be 0,25 Hz and i would expect a small fraction of 117 nV, like 20 nVpp, about 2 ppb of a 10 V standard. Plus the noise of the two references.
[..]
The Nyquist limit is .25Hz, that doesn't mean (noise) contributions of higher frequencies are lost, they just can't be distinguished from those of lower frequencies ("aliasing"). To get rid of (noise) contributions of higher frequencies (and prevent aliasing), you will need a low pass filter.
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Tuning the references was a pain. I ended up ordering various values of precision resistors over and over. Especially the adjustment of LTFLU collector current turned out to be difficult. First i used MF resistors but then i found that those have enough TC to screw up everything. In the end i set the same collector currrent of 40 uA for both references and the TCs are within +/-1 ppm/K, with a difference of 0,43 ppm/K.
To tune the 10 V output i added S102K resistors to the Nomca network. With the Nomca network alone i reached 10.002 V, so the tuning resistors get their TCs well attenuated. The final difference between the references is about 3 ppm now and both agree within some ppm with the standard i received at the Stuttgart meeting in June. 3 ppm means 30 uV, times gain=100 gives some mV, so no special nulling required for the amplifier. Each reference output has a 100uF + 3,3 Ohm snubber close to the difference amplifier. The HP3456A that measures the difference is connected between one of the references and the amplifier output, with Guard connected to the common minus of the references.
On the interface cable i have an input that tunes the reference voltage by about +/- 20 ppm, but i did not use that until now. I hope to use that together with the temperature signal to compensate residual TC (MSP430+ADS1256+DAC8554).
Regards, Dieter
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Other people have discussed and explained flicker noise so here is a visual to support the understand of flicker noise floor limitations in Voltage Standard measurements.
I used a K2182 Nanovoltmeter to monitor the 10V output voltage of two Fluke 732A zener references. This is similar to what you would do to calibrate a known Voltage reference to an unknown reference. For a real calibration the interconnect leads would be reversed to average the thermal EMFs. During this test the temperature was stable and I did not see any correlation between error voltage and temperature.
The raw data was sampled every 30 seconds and it has a 2.4 uVpp noise level. If that data is post processed with a 10 minute moving average
the noise level only drops to 1.8 uVpp. With an extreme 2 hour moving average the noise is reduce to 1 uVpp. At these time levels the drift of the zeners starts to add to the noise. You can see a slight downward slope to the traces after 100 hours.
With 2 or 4 independent references you can get some noise reduction and you reduce the influence of one bad device.
A few years ago I compared two Fluke 732A 10V standards with a Nanovoltmeter. These older units use the SZA263 Zener. I don't know how the noise of the LTFLU compares but it should be over 1uVpp. Attached is a pdf of results.
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How stable is a nanovoltmeter? - isn't it a case of when a gnat breathes three floors up, it influences the reading?
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The K2182A and the HP34420A Nanovoltmeters have about 5nV rms noise at 1 second. When the inputs are auto-zeroed the noise gets better at longer duration.
I did some further stability testing of an HP3458A. I added the results to a plot with the HP34420A and the EM A10.
One HP3458A had half the noise of another HP3458A on the 100mV and 1V scales. I have a third unit I will check sometime. The data in the plot is the unit with higher noise.
The top line is the stability on the 1V scale with auto zero turned off. The line does not continue up forever. It levels off before 2uV. This will depend on the meter and ambient conditions.
There are measurements for other nanovoltmeters is the linked thread.
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Thanks for the link to the noise characterization thread. Meanwhile i found that the 3 ppb number i wrote above was a pretty stupid analysis error, so i am still left with 60 ppb as standard deviation for the time being. This compares well to the 3458a autozero noise curves presented by chuckb. From the curves i read an Allan Variance of about 60 ppb at 2 sec. The 2x Fluke 732A comparison seemed to be 2,4 uV = 240 ppb at 30 seconds. Maybe noise wise the LTFLU with its four zeners is more like a LTZ1000.
Regards, Dieter
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The K2182A and the HP34420A Nanovoltmeters have about 5nV rms noise at 1 second. When the inputs are auto-zeroed the noise gets better at longer duration.
Looking at those graphs, it is clearly critical to zero the meter frequently when doing longer measurements...
How is autozero actually accomplished in practice? Does the meter CPU disconnect the DUT from the input followed by shorting the inputs using internal switches, then reads the zero level from that internal short?
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A few years ago I compared two Fluke 732A 10V standards with a Nanovoltmeter.
This is a super interesting test. - One thing that strikes me is that one of the 732As could have been perfectly quiet, and the other one generated all the noise (at an extreme!)?
How big a factor would thermals have in this test? - the two boxes might have differing reactions to temperature changes, so the thermals don't actually cancel out "common mode style"?
One thing is for sure, making any absolute measurement better than 2uV is probably ambitious! - the graphs look like a live demonstration of the Heisenberg uncertainty principle...
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Meanwhile i have done the ADA4522 preamplifier test with shorted input. Input offset voltage after thermal relaxation is -480 nV (spec < 5 uV), noise is 7.1 nV (standard deviation after subtraction of thermal drift). Noise is as calculated by Kleinstein, also the peak to peak estimate. The plot shows raw data. Vertical scale is 0.6 uV.
I know this is off topic, would fit better into the null detector thread. As far as i can see an ADA4522 makes a very good null detector if you put it into a box of constant temperature and adjust the offset. The problem will be low thermal connectors again.
Regards, Dieter
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The ADA4522 is a good amplifier for a low Z source. It has quite some current noise and is thus not really suited for a source with more than some 50 K. Another point is the bias current that can be quite high (more than 100 pA). So again not suitable for high impedance source.
If the high voltage range is not needed, one could use the 5 V supply brother ADA4528, that is supposed to have slightly lower noise.
For normal lab use one should have some higher frequency filtering at the input, to reduce the current spike going out and maybe more important to make it insensitive the RF properties of the input. Without filtering, the source impedance in the 10-100 MHz range may have an effect and this can add some uncertainty.
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Your reply is certainly correct and very general (as always), but does not really apply here. When comparing two voltage references, the source impedance is less than 1 Ohm. So i don't quite get why we should consider 50 KOhm. Of course i can reduce the feedback divider from 470R/47K to 10R/1K to reduce the 7 nV noise to 4 nV, as you proposed above. A 30 uV difference between the two references will then generate a current of 3 uA, so a nV measurement requires a source impedance in the milliOhms.
This ADA4522 preamplifier i made is a cheap and rapid method of getting reference comparisons in the low nV. I did not have to do lengthy experiments modifying input terminals on the DVM or getting into expensive used equipment from ebay or low thermal EMF equipment. From the graph you can see that resolution is about +/- 60 nV, which is already unusual, as most of the readers here know from own experience. My post was meant as a reply to SilverSolder.
I will certainly put the preamplifier into an aluminium box to keep RF away. The input filters you propose are already there. Will have to add a MUX and some low thermal EMF ports to enable comparison to other references, though.
Regards, Dieter
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Not sure I am understanding this right: If the noise of the ADA4522 is 117 nV p-p typical from 0.1 Hz to 10 Hz (from the data sheet), and the gain is set to 100, does that mean the resulting amplifier baseline noise becomes 11.7uV in that pass band? (disregarding resistor noise, thermals, etc for the moment)
The x100 amp idea seems like a practical solution that leverages existing equipment for comparing references.
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The noise specs are relative to the input. With AZ OPs the noise is essentially white noise, so that reducing the bandwidth to a more DMM typical 1 Hz would reduce the number from the 0.1 - 10 Hz range by about factor of 3 (Square root of 10).
Adding such an x 100 amplifier is a way to do low level measurements, like the difference of 2 refs. or thermocouple voltages. The ADA4522 is lower noise than most 6 digit meters (except special low level DMMs like 34420, K2182).
In most cases I would prefer a normal non inverting amplifier configuration over the difference amplifier type, as it has a little lower noise and high input impedance (though still not good for high impedance signal sources).
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Yes, the inverting amplifier isn't the best configuration. In my case it resulted from using the same 15 V power supply for both references and for the difference amplifier - a very simple test setup. As soon as the preamplifier gets its own power supply (maybe batteries or taken from the 3456A front end), a different amplifier configuration can be used with very small currents into either reference. Then the difference measurement will be less dependent on the reference source impedance.
Regards, Dieter
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What is a good target for bias current? - are there many metrology situations where one would be looking into much more than a 100K resistance e.g. a divider of some kind? 1pA would give an error of 100nV in that case - it might even be possible to "null out" this error current on the DMM by adding a constant?
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What is a good target for bias current? - are there many metrology situations where one would be looking into much more than a 100K resistance e.g. a divider of some kind? 1pA would give an error of 100nV in that case - it might even be possible to "null out" this error current on the DMM by adding a constant?
This thread is about the LTFLU and SZA263-- Isn't this thread getting a little off-topic?
Measuring things with null meters is definitely an important metrology related topic, but should we move this to an existing or new thread?
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The reason for asking is that there are several LTFLU based references here that could be fun to compare and characterize, but I am not sure how much the measurement bias currents would influence the devices or measurement process overall (when comparing via a KVD or other resistive device to null the voltages)?
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Our AD587 (Geller) reference drops about 10 ppm when connecting a 10 KOhm load. So 1 mA test current is high, but 1 uA should be OK unless one aims for nV.
Meanwhile i got some more LTFLUs and made a little test rig (example). It is about the minimum to put the zener and the transistor into operation. One can check the transistor beta, should be about 150.
One of the new chips came bad, with a short between collector and Gnd. Those two chips with strong negative TCs mentioned before have low beta of about 30. All others exhibit low TCs at about 40 to 45 uA and will probably make good references.
Regards, Dieter
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Well, and JJ arrays on liq. Nitrogen temperature also have been demonstrated.. about 1e-8 uncertainty..See here:
http://juwel.fz-juelich.de:8080/dspace/bitstream/2128/2069/1/19406.pdf (http://juwel.fz-juelich.de:8080/dspace/bitstream/2128/2069/1/19406.pdf)
Unfortunately, this link is not available. Maybe someone saved this file?
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I think the article is at http://juser.fz-juelich.de/record/26808/files/19406.pdf (http://juser.fz-juelich.de/record/26808/files/19406.pdf)
Well, and JJ arrays on liq. Nitrogen temperature also have been demonstrated.. about 1e-8 uncertainty..See here:
http://juwel.fz-juelich.de:8080/dspace/bitstream/2128/2069/1/19406.pdf (http://juwel.fz-juelich.de:8080/dspace/bitstream/2128/2069/1/19406.pdf)
Unfortunately, this link is not available. Maybe someone saved this file?
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Today i tried to run one of the LTFLU references at higher zener currents. I remember some statements that the LTFLU zener is similar to the LTZ1000 zener, except the LTFLU has four of them. The differential resistance measurements of about 7 to 8 Ohm for the LTFLU and 30 Ohm for the LTZ1000 point to the same direction. So i found it plausible to run the LTFLU at 12 mA instead of 3 mA. Of course the reference needed readjustment for output voltage and TC, but it works well. Have to compare noise yet.
Anybody tried this? What are the long term results?
Regards, Dieter
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I've seen a lot of information on the LTFLU on the bbs.38hot.net site, but it is not accessible today..
Or lymex'es new site:
http://bbs.1ppm.cn (http://bbs.1ppm.cn)
:)
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Received some LTFLU boards today, fabricated on aluminium. Need to solder a first board the next days.
-branadic-
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Almost done with assembly of the first board. All parts are 0805 except the NTC, which is a 100k in 0402. We are getting closer to the first working unit :)
-branadic-
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First temperature measurement performed on the aluminium core board, looks okay. Now I need to trimm the output voltage to 10,00000V and repeat the measurement, to find the final oven temperature setpoint.
-branadic-
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What is your experience with handsoldering wires and SMD tuning parts to that board? I guess the LTFLU is mounted as an SMD part.
Here an image of my latest creation: LTFU with higher currents, Nomca replaced by 10x UPF50B100RV, ADA4622-2 => OPA2189, npn transistor as buffer for reference voltage. The divider now has an output impedance of about 200 Ohms and i hope this will help. The transistor base current contributes roughly 1 ppm for every 10 Ohms of divider impedance. The LTFLU was mounted into the board, since i am using its transistor(s) as temperature sensor.
Regards, Dieter
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Hi Dieter,
my experience with SMD tuning parts are fine by now. Still I have to tune the output by changing one SMD resistors to what I think is 10,00000V as I want to avoid any pot. However, I'm still about 8ppm off, which in theory is okay, but I want to see the limits I can achieve. Tuning takes some time, as with every change in resistance I have to perform a new temperature profile to verify the result.
Also the wires are no problem by now as I currently run the profile using my temperature chamber and not with a heater resistor attached to the back, thus not the final oven. But I'm sure I have to use smaller wires in later assembly.
-branadic-
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Still I have to tune the output by changing one SMD resistors to what I think is 10,00000V as I want to avoid any pot.
How the TC of that tuning resistor translates into the total TC?
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Don't know and I currently don't care about it.
-branadic-
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Hello Branadic,
thanks for the paper.
If you consider the LT1006 as output buffer for the LTFLU then another possibility would be also the OPA187.
This is the low power version of the OPA189 with comparable noise to the LT1006 but lower supply current as the OPA189/ADA4522.
So battery life could be increased.
For my AD587LW references I will check wether the OPA187 is suitable. (The AD587 has 2-3 uVpp of noise so the 0.4uVpp will not be mentioned but the 0.1 mA supply current will extend battery life by 20-30% compared to the LTC2057).
with best regards
Andreas
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The NOMCA resistor array may introduce quite some noise. There may be a slightly less noisy resistor configuration to build a 10 to 7 V divider.
The idea is to have a more even voltage across the resistors and thus better averaging of the 1/f excess noise. I would expect about a 25% reduction in excess noise - so maybe not worth a new board. Similar the matching should also be slightly better, as the weight of the resistors is more even.
In the shown diagram R8 is used for trimming to something a little less than 7 V. It's best use / value depends on the exact ratio needed.
I would be somewhat careful with an AZ OP in the circuit. There is no need for super low drift, as there is some extra gain (e.g. 50-100) from the transistor, that attenuates OP drift. However the extra spikes can be a problem. Different from the LTZ1000 circuit, there is no need for a single supply OP. I would more like consider an OPA196 for low current.
For an extra output buffer an AZ OP is OK.
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Andreas,
already to late, the other boards are already assembled with LT1006. Since LT1006 is the single version of LT1013 and I have only one in the circuit current draw is already reduced.
Different from the LTZ1000 circuit, there is no need for a single supply OP. I would more like consider an OPA196 for low current.
I wanted a single supply opamp, since it simplifies battery power supply and charge circuitry.
-branadic-
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I measured the LTFLU reference in my thermal chamber at constant temperature (48.2°C). However, I'm somewhat limited by my meter R6581D and its tempco of about 1ppm/K or at least by stability of ambient temperature, which changes internal temperature of the meter.
Still waiting for some resistors to further trimm the output voltage.
-branadic-
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Thanks to branadic :-+ I was able to take some more pictures of his LTFLU:
https://www.richis-lab.de/REF04.htm (https://www.richis-lab.de/REF04.htm)
(https://www.richis-lab.de/images/REF01/03_01.jpg)
Quite interesting!
(I didn´t read the whole thread. Sorry... ::))
Only very few transistors are connected to the circuit (tempco-mathing i assume)...
There seems to be a optional heater…
Nice big fusable links:
(https://www.richis-lab.de/images/REF01/03_06.jpg)
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It would be great to find a final result based on your and mine (https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg2586024/#msg2586024) attempts to draw a schematics of the chip - especially the pretty difficult block around the 4 zeners.. :phew:
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Interesting… :-+
Q9... Hm... The two transistors in the middle between the four zener?
In my view that transistors connect the "anode" or "z" with the substrate. Probably to give it some negative potential...
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There are 2 resistors at each zener, one from the cathode to the emitters and the second one going somewhere - in my case to the base of the Q9.
The sheet resistance - my values are rather high, it will be much less, but the first step is to understand how the block around the zeners is actually wired :scared:
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Richard, SZA263 and LTFLU are two different stories:
https://i.imgur.com/opKFJ3q.jpg
https://i.imgur.com/6Lc4gZs.jpg
-branadic-
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I interpreted the structures this way:
From each zener there are four "resistor wires".
Every resistor is connected to the cathode.
Two parallel resistors are connected to the bondpad in the upper right corner.
One is connected to the emitter of the transistor batch. I assume it´s some current dividing.
The last one is a possible low impedance path instead of the resistor for dividing the current.
(https://www.richis-lab.de/images/REF01/03_03.jpg)
I see nothing more…
Richard, SZA263 and LTFLU are two different stories:
https://i.imgur.com/opKFJ3q.jpg (https://i.imgur.com/opKFJ3q.jpg)
https://i.imgur.com/6Lc4gZs.jpg (https://i.imgur.com/6Lc4gZs.jpg)
-branadic-
Uuuuh, thanks for the hint... Sorry!
Edit: Corrected the mistake on my site…
-
Btw, one unit was marked as LTFLU-1CH while the other was marked as LTFLU-1ACH. I leave it up to you to find out what's the difference between both.
-branadic-
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Anyone with the capability to sample the zener voltages of an open LTFLU chip? I am wondering whether it makes sense to implement four seperate zeners on one die. This may be more or less useless because the zeners will be extremely similar.
Regards, Dieter
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Hello noopy,
thanks a lot for these very nice pictures and explanations on your website. :-+
Thanks to branadic :-+ I was able to take some more pictures of his LTFLU:
https://www.richis-lab.de/REF04.htm (https://www.richis-lab.de/REF04.htm)
[...]
One remark about this sentence on your website: "In der rechten oberen Ecke befinden sich die Zeichen FLU1."
The two signs below the "FLU1" are coming from the logo from Linear Technology.
(https://logo.lusha.co/a/b48/b481ca9e-4bee-469c-99f0-4de5faa47635.png)
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These RefAmps all are relatively tightly specified concerning their initial reference voltage span, but especially for the span of the transistors collector current (20..200µA) where zero T.C. can be achieved, over defined operating temperature span, with zener current being 3mA.
Therefore it's clear, that they have to heat the device in situ, in turn to trim the transistor and zener to the correct above mentioned parameters.
Frank
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The adjustment range for the transistor size is surprisingly large: 8 fuse to for a kind of 8 bit weighted sum for the transistor area.
The trim may need some heater to check the TC. However they could as well use the zener current
4 separate zeners with extra resistors for current sharing could be better than one large one where current sharing may not always work.
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thanks a lot for these very nice pictures and explanations on your website. :-+
Thanks for the positive feedback! :-+
One remark about this sentence on your website: "In der rechten oberen Ecke befinden sich die Zeichen FLU1."
The two signs below the "FLU1" are coming from the logo from Linear Technology.
You are right. :-+
I thought most people would recognize the LT-Logo... ;)
...
...
An interesting explanation. Heating for adjusting the number of transistors and finding the right tc.
But wouldn´t that cost a lot of time? And a good accuracy of the temperature would be difficult to get? Hm... :-//
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One remark about this sentence on your website: "In der rechten oberen Ecke befinden sich die Zeichen FLU1."
The two signs below the "FLU1" are coming from the logo from Linear Technology.
You are right. :-+
I thought most people would recognize the LT-Logo... ;)
Yes, sure. :)
But sometimes, you know, you'll never know who will read it. :-+
Edit:
And, to be honest, when I saw this logo the first time, the first thing which was coming to my mind was "Klingons? What the heck .. ?" :palm: :-DD
Anyway, thanks again, I've learnt a little bit on how to look at all these integrated circuits and identify some parts. Sort of.
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Hi Frank,
These RefAmps all are relatively tightly specified concerning their initial reference voltage span, but especially for the span of the transistors collector current (20..200µA) where zero T.C. can be achieved, over defined operating temperature span, with zener current being 3mA.
Therefore it's clear, that they have to heat the device in situ, in turn to trim the transistor and zener to the correct above mentioned parameters.
Frank
what exactly is done during the trimming process that you describe?
I have an idea how metal foil transistors are trimmed to achieve a certain resistance value but how do you trim zeners and transistors
to achieve a combined temperature coefficient near zero?
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Hi Frank,
These RefAmps all are relatively tightly specified concerning their initial reference voltage span, but especially for the span of the transistors collector current (20..200µA) where zero T.C. can be achieved, over defined operating temperature span, with zener current being 3mA.
Therefore it's clear, that they have to heat the device in situ, in turn to trim the transistor and zener to the correct above mentioned parameters.
Frank
what exactly is done during the trimming process that you describe?
I have an idea how metal foil transistors are trimmed to achieve a certain resistance value but how do you trim zeners and transistors
to achieve a combined temperature coefficient near zero?
They burn fusable links like these:
(https://richis-lab.de/images/REF01/03_06.jpg)
By burning the fuses they can adjust how much transistors are in series with the zener. With less transistors you have more current per transistor. The Tc varies with current, so when you adjust the current per transistor you adjust the Tc. => Bob´s you uncle! ;D
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The reference is not an exactly cheap part - though one a price is hard to tell for a part that is fluke exclusive.
So a little more time for testing would no hurt that much, as it would with a cheap OP.
Chances are they would measure the ref. voltage during warm up, possibly with additional power to the heater or a pulse of higher zener current. Form that one can estimate the TC and get the right number of transistors. There seem to be just the 1:2:4:... ratio seta and no extra spare. So they likely do the adjustment in one step and not as coarse and fine tune, as in some cases there is no way back. For some cases one could still try it in steps and burn the corse fuses first only. The really fine tuning of the TC is done with the external resistor(s).
I am not sure if the is includes in normal production test, but the by far slowest test would be checking for excessive popcorn noise. This test could be separate already with the case.
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Hello dietert1, hello Noopy, hello Frank!
Anyone with the capability to sample the zener voltages of an open LTFLU chip? I am wondering whether it makes sense to implement four seperate zeners on one die. This may be more or less useless because the zeners will be extremely similar.
Regards, Dieter
Similar in relation to what property or properties exactly?
@Noopy:
I assumed Frank was talking about another treatment than just triggering fuses.
I thought he was refering to manipulating zeners and transistors directly.
Why bother with all these tedious micro adjusting stuff when I can set the collector current externally?
From my layman's view a reference amplifier has three connectors. What is "e" used for?
Best regards
try
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I assumed Frank was talking about another treatment than just triggering fuses.
I thought he was refering to manipulating zeners and transistors directly.
Hm, I don´t know what that should be. But let´s wait for an answer from Frank.
Why bother with all these tedious micro adjusting stuff when I can set the collector current externally?
In the circuits I know you adjust the current through the zener and the transistor but you can´t or don´t want to adjust the two currents on their own. *
With the adjusting of the current per transistor on the die you can be sure that the current through the active transistors is ideal to compensate the Tc of the zener.
* Not absolutely correct. You do adjust the current but only in a very small window.
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@Noopy: what could be the structure between the zeners?
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@Noopy: what could be the structure between the zeners?
1. In my view the two structures are connected only to the metal layer above them (z/GND).
2. It looks like a transistor or something similar.
3. I didn´t find an obvious contact to the substrat on the die.
=> I asssume it´s something connecting ground to the substrate. Probably a pn-structure (shorted transistor) giving the substrat a little lower potential than ground to get a better isolation.
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I know a little bit about large mosfets made by paralleling lots of smaller mosfets on a single chip and about the negative TC that makes this possible (no unstable hotspots). I have also seen that two 7.5 V zeners in series get you lower differential resistance than one 15V zener.
But i have no idea whether and why it makes sense to divide a larger zener into several smaller ones. I know very little about the current sharing resistors or to what extent they are doing something meaningful. I think you want the current sharing resistors as small as possible for obvious reasons. If they are "small" and the zeners very similar, is there still some reason to divide the zener into four regions? Very similar means equal structure resulting in equal electrical and physical parameters (knee shape, TC, differential resistance etc.)
For me these are the open questions with respect to LTFLU. If possible someone should measure the current sharing resistors and the zener voltage differences of a LTFLU.
Regards, Dieter
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The 4 zeners are not the same. They are always different even on the same die. They do it to lower the noise (2x in this case) and statistically TC.
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Well,
The external fine trimming of the overall T.C. by changing the collector current from 20 to 200µA will change the -2mV/°C very lightly , I would estimate maybe from -2.0mV/°C to -2.1mV/°C.
This should mostly account for the variations of T.C. at different operating temperatures, either 23°C nominal for non stabilized use case, and about 45°C for an ovenized use case.
For this RefAmp, the T.C. of the zener and the transistors BE diode then have to match exactly in terms of these -2.0 ..-2.1mV/°C for the transistor and +2.0..+2.1mV/°C for the zener, which has about 6.4V.
When you convert these latter values for the zener to a relative T.C. = dUref/Uref, that gives a quite narrow window of +312 .. +328 ppm/°C, which the zener structure has to be trimmed for, otherwise you will not get an overall zero T.C.
Even if the collector current variation changes the T.C. of the transistors to a much higher degree, you will anyhow get a quite narrow span for the zener, due to its 13 times bigger voltage.
I don't know, how you can change a zeners T.C., but one parameter is its absolute voltage.. the 1N821..829 have nearly zero T.C @ 6.2V, and higer zener voltages will have more positive T.C.s.
The T.C. of these type of zeners vary greatly from 5ppm/°C (1N829) to 100ppm/°C (1N821), which is accomplished by selection only. Here you see, that precision zener diodes usually have a big manufacturing variation.
So LT had to invent something to precisely trim this T.C. of the buried zener to the desired value.
Frank
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Hello Frank,
is it really necessary to trim the T.C. of the zener?
Trimming the T.C. of the transistor to match both should work too?
Greetings!
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Hello Frank,
is it really necessary to trim the T.C. of the zener?
Trimming the T.C. of the transistor to match both should work too?
Greetings!
That will not work at all.
I think it's impossible to shift the T.C. of a Si pn junction by a factor of 13 by any means.
Check the appropriate diode equations.
Frank
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Hello Frank,
is it really necessary to trim the T.C. of the zener?
Trimming the T.C. of the transistor to match both should work too?
Greetings!
That will not work at all.
I think it's impossible to shift the T.C. of a Si pn junction by a factor of 13 by any means.
Check the appropriate diode equations.
Frank
Right, didn´t see that…
But you don´t see any trimming around the zener.
Perhaps they managed to establish a manufacturing process that is extremly good and then there is some sorting?
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FYI - An info on 1N821-9 and other "zero TC" diodes. They combine a zener with one or two or more pn junctions to get near zero TC.
https://www.microsemi.com/document-portal/doc_download/14616-zero-tc-reference-diodes (https://www.microsemi.com/document-portal/doc_download/14616-zero-tc-reference-diodes)
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@Noopy: by these fuses you can divide the external collector current further down binarily, by sending it through 1, 2...., 16 transistors. As the Shockley equation gives a logarithmic behaviour for the T.C. (I think?!!) this 16 fold current variation will still not give a 13 fold T.C. variation (I assume!??)
@imo: well, I know that already, but does that change anything about the conclusion, that zener diodes obviously have a great production variation of their T.C. ?
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@imo: well, I know that already, but does that change anything about the conclusion, that zener diodes obviously have a great production variation of their T.C. ?
I think the LUTFU is not trimmed for Voltage, the voltage comes from the process variation as-is.
They trim for zero TC by selecting the 1..256 emitters at a specific temperature given by the customer.
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Changing the transistor area is vey similar to changing the transistor current. The voltage is set by the current density. The transistor area change may be good for a factor of 100 at most, probably better less, not to make the transistor too small and thus more 1/f noise. There are not just several large transistors but also a few smaller ones. A factor of 100 in the current shifts the voltage by a little over 100 mV or the TC by 100 mV/300 K = 0.33 mV/K. Changing the external current by a factor of 100 would be possible too, but would change the circuit more than just a simple change in resistor value: the FB divider may have to get lower impedance with more base current.
100 ppm/K for the 6.5 V zener voltage is some 0,65 mV/K, still much smaller then the about 2 mV/K of zener or diode individually. Still this would be too much to adjust with the transistor current density. So it needs reasonable tight control over the process too. Here the buried zener may be more predictable than a classical zener in the 1N82x. At least the LM329 usually come on 50 ppm/ K for the lowest grade - so this seems possible. To get zero TC with a useful transistor size and current one may be able to adjust some 0.4 -0.5 mV/K or some 70 ppm. So the production TC should be within some +-30 ppm/K, preferably smaller. This may mean that the yield may not be 100% but could still be good.
The "E" contact is there so one can set the transistor independent from the zener current.
For the zener diodes at higher current density the TC does not change very much anymore. So there is some room for changing the zener TC, but this would also effect the noise level and heat. So the practical range is smaller (e.g. 2-10 mA).
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Yes, I have forgotten to take the transistor areas into account.
Therefore that structure is analogue to a collector current division from /1, /2, /3, ..., /256.
I still wonder, if the T.C. can be changed that much, as required.
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I've done a naive simulation of my model from past, removed the Q9 and the resistors to its base.
I've changed the number of emitters from 4 to 192 and run a Temp sweep 20-70C of the entire voltage reference, not touching anything else.
The TC uV/C is slope of the Vref from 20-70degC.
Below the TC (y axis) of the Vref vs. number of emitters (x axis).
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Interestingly LTFLU is lower in voltage than any other zener reference and tighter in spec for output voltage. Attached a table I once found on the web.
-branadic-
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The tighter specs could be simply because of stricter criteria for acceptance, that may be needed to get the zero TC point under reasonable (e.g. not to low transistor area, to low or high a current). I would not take the span for min/ max values to serious - those specs may not reflect the actual scattering seen. This are more like test limits that may not be updated if the process technology improves.
The more odd entry in the table is the LTZ1000 with a rather high voltage and AFAIK consistent positive TC of around 50 ppm/K without the heater.
The adjustment range for the TC via current density is not that small. They may have to reject some chips (possibly whole wafers) at the extremes, but that is part of the reason why high performance chips have there price.
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I had a FLUKE 8842A mainboard that was only good for parts, so I decided to cut out the RefAmp assembly (SZA263 Datecode 8430) and make a 10V DC reference (in addition to my previous 2x LTZ1000A, 1x LTZ1000 and 1x FLUKE 731B)
The 7->10V booster was in turn cut out from my own designed LTZ1000 PCB ;) Again recycling! (Have just assembled LTZ #4, currently under burn in with elevated heater temperature, parallelled 1k with 4.7k giving ratio 15.77)
Voltage from RefAmp board was 7.00090V, so I ordered custom precision wire wound Ultrohm Plus resistors (10k and 4.284k 0,01% 3ppm/C) from Edwin Pettis, and without trimming the voltage out is 10.00006 on my FLUKE 8846A (which I know show 3ppm high on 10VDC), so only +3ppm without any trimming, impressive! :)
In the box I put a linear +/- 15V supply, LTC2057 chopper amp, 7->10V boost resistors with copper tape for thermal coupling, and a NPN darlington output buffer stage (inspired from KJ7E, thanks!). I really like those hermetic resistor networks on the 8842A RefAmp assembly, the 8846A is also full of them 8)
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Btw, one unit was marked as LTFLU-1CH while the other was marked as LTFLU-1ACH. I leave it up to you to find out what's the difference between both.
-branadic-
I took a better (cleaner) picture of the second LTFLU. But nothing special to see. Found no difference between the LTFLU-1CH and the LTFLU-1ACH. :-[
https://www.richis-lab.de/REF04.htm (https://www.richis-lab.de/REF04.htm)
The surface and the colours are looking different. Perhaps the manufacturing process has changed. Perhaps it´s coincidence… :-//
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Is there any reasonable source of the LTFLU parts to buy? I found some for 3/4 the price of an LTZ1000 and I'd only pay that much if I had to repair something.
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Is there any reasonable source of the LTFLU parts to buy? I found some for 3/4 the price of an LTZ1000 and I'd only pay that much if I had to repair something.
I've bought some LTFLU-1 from Walton electronics in Shenzhen (Alibaba). They look very genuine.
The pictures above from Noopy came from 2 LTFLUs which I'd sent Branadic for taking some HiRes pictures under the microscope.
https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg1282144/#msg1282144 (https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/msg1282144/#msg1282144)
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Wrote a little report on some nice results with the LTFLU references i showed above.
http://www.cadt.de/metrology/2020-05-18 Vref_Ovens.pdf (http://www.cadt.de/metrology/2020-05-18 Vref_Ovens.pdf)
Regards, Dieter
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Wrote a little report on some nice results with the LTFLU references i showed above.
http://www.cadt.de/metrology/2020-05-18 Vref_Ovens.pdf (http://www.cadt.de/metrology/2020-05-18 Vref_Ovens.pdf)
Regards, Dieter
This looks pretty promising Dieter. I wonder if I can build a nice ovenized 10V reference with my LTFLU in similar way to what Micke did but by putting it in an oven? I'm not sure I fully understand how the NPN darlington output buffer stage works though... I'm not an EE?
Regards,
Bill
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This looks pretty promising Dieter. I wonder if I can build a nice ovenized 10V reference with my LTFLU in similar way to what Micke did but by putting it in an oven? I'm not sure I fully understand how the NPN darlington output buffer stage works though... I'm not an EE?
Regards,
Bill
I did not add an oven, just reference circuit "as-is" from the FLUKE 8842A DMM...
The Darlington output buffer is to protect the SZA263 from short circuit on the output, and increase the output capacity, can´t find my notes right now, but loading output with 100mA was no problem, with not that many PPM´s Voltage drop :) (adding heat sink on transistor could be good though)
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You need two high quality references to do that fine-tuning. I mean unless you want to spend thousands on a HP 3458A or the like.
Later i want to repeat the same for the other LTFLU reference, the one i made in January. That one was tuned using our old Geller AD587 as reference.
Regards, Dieter
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Usually an oven around the reference can keep the temperature stable to better than 1 K , often more like 0.1 K.
So there is no real need to get the TC trimming so extremely good.
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Sometimes i am wondering whether you believe your own statements here. When the LTZ1000 has a typical TC of 50 ppm/K at its operating temperature and is used for an 8 digit multimeter (resolution 0.01 ppm), i know that the temperature stability of the LT1000 oven is about 0.2 mK. Your opinion is welcome, yet irrelevant in this case.
Regards, Dieter
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Sometimes i am wondering whether you believe your own statements here. When the LTZ1000 has a typical TC of 50 ppm/K at its operating temperature and is used for an 8 digit multimeter (resolution 0.01 ppm), i know that the temperature stability of the LT1000 oven is about 0.2 mK. Your opinion is welcome, yet irrelevant in this case.
Regards, Dieter
What about all the resistors and other components around the LTZ itself? Ovenizing all that external stuff seems a good idea, on the face of it?
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With the LTZ circuit the effect of the other resistors is attenuated quite a bit (e.g. a factor of 80 or more). For ultimate TC performance there usually is an adjusted R9 to compensate residual TC and this would naturally also include the small effect of the resistors.
The more critical point for the resistors is usually long term drift, not the TC. Here an oven does not help much.
One still looks for low TC resistors as the long term stable resistors usually also have a low TC. If there were good data on long time drift one would look for these.
For the SZA/LTFLU circuit the attenuation effect is similar, possibly even a little better. Here it is more convenient to have more/all of the circuit also in the oven, as there is no internal heater.
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For the SZA/LTFLU circuit the attenuation effect is similar, possibly even a little better. Here it is more convenient to have more/all of the circuit also in the oven, as there is no internal heater.
That's probably why Fluke put them in an oven as well in the 732B.
Here is a teardown of the 732B with some nice pictures:
https://www.eevblog.com/forum/testgear/fluke-732b-dc-standard-teardown/ (https://www.eevblog.com/forum/testgear/fluke-732b-dc-standard-teardown/)
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For the SZA/LTFLU circuit the attenuation effect is similar, possibly even a little better. Here it is more convenient to have more/all of the circuit also in the oven, as there is no internal heater.
That's probably why Fluke put them in an oven as well in the 732B.
Here is a teardown of the 732B with some nice pictures:
https://www.eevblog.com/forum/testgear/fluke-732b-dc-standard-teardown/ (https://www.eevblog.com/forum/testgear/fluke-732b-dc-standard-teardown/)
It doesn't look like Fluke heated the resistors just out of convenience (i.e. because the resistors were already on the board so might as well bake the whole thing), they actually went to the trouble of making an extra cavity in the oven assembly just to enclose the resistors. Even if suppressed by a factor 80, it might buy us a wider environmental operating temperature range, compared to leaving the critical resistors in free air?
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In the 10 V reference there are also resistors for the 7 V to 10 V step. These resistors are the really critical ones. With the SZA263 and similar this gain stage is often part of the reference directly.
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In the 10 V reference there are also resistors for the 7 V to 10 V step. These resistors are the really critical ones. With the SZA263 and similar this gain stage is often part of the reference directly.
Looking at the schematics in the F732B manual, this seems to be the only mode of operation of this IC. There must necessarily be a voltage differential across a resistor(R3) somewhere. As I see this these gain resistors(R2/3) are always present. And their TC is only attenuated by a factor of 1.4, necessitating ovenising or good TC matching. If this matching can be achieved, no oven is needed as the IC with the proper biasing already has no TC(although in a small range).
(https://i.imgur.com/TMXulGW.png)
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I measured the series of the outer two resistors (bottom and top) today (connected to second and third pad from the top row on the left). The sum of them is 200ohms, so each one is 100ohms.
(https://www.eevblog.com/forum/metrology/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=798891;image)
-branadic-
Today i also measured the LTFLU on chip heater, with the result of 180.5 Ohm, so that's a confirmation. Wasn't able yet to probe the 4 separate zener voltages. Need to get some kind of micro manipulator.
That LTFLU is one i pulled because its transistor had a low hfe. Under the microscope it appears like none of the fuses was blown. Maybe they had to reject LTFLUs with low hfe, since they did not know which one of the eight transistors was bad.
Have another result:
I found the Vpp = 6 stdev repeated so often in this forum is a rule of thumb. Of course with constant noise level the maximum excursion will increase the longer you wait. The diagram shows some experimental numbers i got from my LTFLU logs. Error bars indicate error of expectation value, standard deviation is bigger. For example when looking at subsets of 100 samples, the ratio of Vpp over standard deviation has a standard deviation of 0.74 so likely ratios to be observed are between 4.6 and 6.1
Regards, Dieter
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I interpreted the structures this way:
From each zener there are four "resistor wires".
Every resistor is connected to the cathode.
Two parallel resistors are connected to the bondpad in the upper right corner.
One is connected to the emitter of the transistor batch. I assume it´s some current dividing.
The last one is a possible low impedance path instead of the resistor for dividing the current.
....
I can also confirm that interpretation. Measuring resistors and probing voltages from the chip i arrived at a schematic of the internals. I used noopys image to mark some structures. The path lengths were determined as 118 pixels for R7 and R8 and 317 pixels for R14. A nice agreement with the measured resistances. What noopy called "low impedance path" may be test points for each buried zener voltage, that isn't accessible otherwise.
Regards, Dieter
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What noopy called "low impedance path" may be test points for each buried zener voltage, that isn't accessible otherwise.
Hm... I don´t think that small metal squares are used for probing... They are veeeery small. :-/O
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I think those are simply contacts. Because IC masks cost a lot, it was common practice for a design to allow for possible variants, where these variants have the same masks for all layers except the top metallization. You see this with the LTZ as well, where several heater resistors are fabricated, but only one is actually used. They probably tested all these variants and kept the best performing one. They kept those isolated contacts because the fabrication process requires them, you cannot have an isolated via.
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So by changing only the final metallization they could have connected the emitters to what i marked TP1 .. TP4 instead of using R13 .. R16. The alternative/optional averaging resistors would then be about 35 Ohms. Added those to my schematic.
Regards, Dieter
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I meanwhile played a lot with the oven construction for my aluminum LTFLU board. Turned out to be a rabbit hole.
I bought one of those cheap W1209 thermostate modules and tried using it together with a 10k NTC (some 0402 NCP15XH103) on the LTFLU board and with the heater resistor BPR10J101 attached to its back. Turned out to produce large thermal gradients and large switching noise.
I then designed an aluminum case and had it fabricated at work, with the LTFLU board put upside down into it, so that the components are surrounded by the case. Again, this construction failed.
I then tried several positions of the heater resistor attached to aluminum case itself and even tried a seperate 10K NTC, insulating the room between case and components with styrofoam, also insulated the whole assembly, but whatever I tried, nothing worked. So I skipped the idea with the W1209. It is now used for pre-aging some LTZs, but that's a complete different story.
After some weeks of being disappointed I went back to my initial idea, having the heater resistor directly attached to the back of the board and using the onboard NTC. This setup was then put into a small styrofoam box together with some cotton ball, insulting the complete setup.
This time and since it currently wasn't in use I attached heater and NTC to my Arroyo 5305. After setting all parameters including the ones for the NTC (only Beta and temperature curve is given in the datasheet, so you have to reveal the coefficients for the Steinhart-Hart equation e.g. using the SRS Thermistor Calculator (https://www.thinksrs.com/downloads/programs/therm%20calc/ntccalibrator/ntccalculator.html)) I started the autotune function and indeed, PID parameters where found. Nice, that is something to work with.
Having the oven temperature set to a fixed value and running it all for 24h showed the temperature is rock stable. Even the last digit of "48.50°C" didn't change during that time. Wonderful.
Next thing to do, measure stability of LTFLU with DVM/DMM and the oven control running.
Lesson learned: Switched ovens do not work for precision stuff and you want linear regulation instead. I learned it the hard way and hope that helps someone at some point.
-branadic-
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Those Arroyo Instruments TEC controllers provide for a temperature resolution of about 0.02 to 0.05 mK. The instrument works like many DVMs: On power-on it defaults to a numerical resolution of 10 mK and to a resistance resolution of 1 Ohm for the 10K thermistor. With the command "SETEXTENDEDRES 3 \n" you can turn on the reserve digits and the instrument will deliver 1000 times more resolution. It incorporates an ADS1256 24-Bit ADC for temperature measurement. Don't know (yet) what is the typical TC of its reference resistor.
Regards, Dieter
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Regarding the Arroyo temperature controller, there is a nice video from Marco Reps about this unit.
https://www.youtube.com/watch?v=gP3AXil65V8 (https://www.youtube.com/watch?v=gP3AXil65V8)
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With the command "SETEXTENDEDRES 3 \n" you can turn on the reserve digits and the instrument will deliver 1000 times more resolution.
Thanks, that is awesome! Not sure where you got this information from, but in the Arroyo Computer Interfacing Manual for 5305 there is no such command listed? Are there more hidden commands?
-branadic-
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That information came from P. Corr at Arroyo Instruments, but i remember it's also in one of the documents on their website. This was introduced with one of the firmware updates. I have been using it with a TecPak 585, but as far as i understand it should work on the 5305 as well. and also on the 5235. I posted a little report on my TecPak oven experiments here:
https://www.eevblog.com/forum/metrology/experiments-with-vref-ovens/?action=dlattach;attach=1084858 (https://www.eevblog.com/forum/metrology/experiments-with-vref-ovens/?action=dlattach;attach=1084858)
Regards, Dieter
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Thanks dieter1, unfortunately I wasn't able to find any hint on the Arroyo Instruments website or in the documents over there, even the firmware file doesn't include any hint. I already had firmware 1.44 installed on my 5305, but didn't know about that option. However, having 10µK resolution is crazy and amazing. :-+
As shown in my former post, the temperature of my oven setup at 25°C, some Kelvin above room temperature, shows low noise (<<1mK peak-peak) and good stability over hours, measurement still ongoing.
Next step, get one of the meters free to find the ztc point and measure stability of LTFLU at it. Since Autotune was able to find PID parameters, it should be possible to transfer them to an analog circuit for a fixed temperature, right?
-branadic-
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Depending on how the Arroyo regular is programmed it may not be so easy to transfer to an analog circuit. There are 2 possibly difficult points:
1) the time constants may be quite long, not very comfortable with an analog circuit, though likely still possible.
2) As a high end regulator the Arroyo may use linearization for the TEC to get improved regulation - this can be a little tricky in the analog domain. It may not be needed when using the TEC only at relatively low current and thus in the more linear range.
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The PID parameters found are as followed:
P=0.4760217, I=0.0012075, D=4.1800661
Not sure about any linearization, I guess only Arroyo Instruments e.g. P. Corr can answer that question. I have limited output voltage and output current to what I think an analog oven controller would look like, thus 12V and 130mA for the BPR10J101 resistor on a 12V supply, controller set to heat only.
-branadic-
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The PID parameters looks a little strange. From the ratio of the P and I part would get a time constant there of some 400 seconds (or whatever internal time units are used). This is relatively slow, but reasonable for the thermal system.
The ration of D and P part on the other side suggest a time constant of some 9 seconds for the differential term, which looks reasonable. There is still the possibility the instrument uses an internal time unit and not seconds.
The overall gain factor would probably be adjusted anyway for the analog version and the main part to take from the digital regulator would be the time constants.
The relatively large time constants could be pushing the limits for an analog regulator (especially the simple 1 OP circuit) with rather large capacitors / resistors (e.g. 10 µF and 40 M to get the time constant of the integral part).
A resistive heater is inherently nonlinear. So chances are the analog regulator would ideally also have at least an approximate linearization. The minimal solution may be a relatively close set upper limit for the heater. Getting to high in the heater power would increase the regulator gain and thus can cause instability. At lower heat the gain would be reduced, which only makes the regulation slow.
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Sample rate is more like 31.25Hz, time between two samples about 32ms.
-branadic-
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In my little report under "Reference Oven Experiments" (link above) i had the PID numbers as determined by Autotune for my setup:
TEC:PID? => 4.7796493,0.0258901,180.5900726
They are much bigger, about 10 times for the P term, about 20 times for the I term. The D term normally does not matter and i often set it to zero by hand. I would not trust Autotune with a nonlinear heater setup. The best method is using a TEC mounted on a large aluminum heat sink. P. Corr wrote to me that the regulator calculates at 10 Hz. The power stage is a constant current source and the proportional term is in units of A/K, so P=1 will output 1 A at 1 K temperature error between set temperature and measured temperature.
Regards, Dieter
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dieter1
I have a BPR10J101 (10W, 100R) resistor as a heater attached to my LTFLU on an aluminum board and no TEC installed, which explains the different parameters.
-branadic-
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Waiting for components in one project results in progress in another project.
I've updated my report on LTFLU, but it's still not finished.
Meanwhile I've designed and build an analog oven controller for the reference, but than found that noise is not satisfying. Turned out that noise of the network used is worse than expected for such application, an experience also other members made meanwhile. I've made a new board revision with some minor changes, such as using TOMC network instead of NOMCA and spend a second NTC. Thus, almost all of the former steps had to be repeated.
Things improved, but the project goes on.
-branadic-
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A TEC is nice if one wants a temperature rather close to room temperature. For just a stable 40-50 C it is easier to use just a heater.
With TEC element one usually has quite a fast path from the outside via the peltier element and strong couling to the outside - this makes the oven overall relatively fast to react to external disturbance. Ideally one could us the other side of the TEC element for feed forward correction, but this is hard to implement analog.
I am currently also in looking at on reference ofen. The analog circuit looks similar, though using the output transistor as heater. With the transistor as heater the power is linear and not much power is lost. A similar circuit is used in the HP Model 10811A/B crystal oven (with some extra complications of a 2 nd heater and overly complicated current limit).
For a fixed setpoint I see no need to have the extra filtering for the set point voltage, so no real need for the 2nd OP in the regulator circuit. This takes out some of the OPs noise. For the shown circuit I would prefer the LT1013/LT1006 over the AD822. With a shifted DC level (other resistor values) one would not need a single supply capable OP and could use the more modern OPA202 too.
For the final version, the noise looks not so bad. Remember that one gets the combined noise of the reference and the meter, including it's LTZ1000 ref. It would need a comparison to a 2nd ref with known noise to get a definitive value.
With the oven temperature adjusted so close to the zero TC point, I would not expect much noise from temperature fluctualtions. The TOMC resistor are lower noise than NOMCA, but may still contribute a little to the noise.
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Hello Kleinstein,
I fear you misunderstood. I used a heater resistor BPR10J101, but a TEC controller (Arroyo 5305), that allows heating only mode for the temperature sweeps I've performed. With it, I've then started to develope a discrete analog oven controller.
AD822 was used for the analog oven controller as I want to supply the whole assembly (oven and reference) with a single 12V rail only. Some inspiration for the oven controller came from F732B, though TL062 was replaced by AD822.
By now, not all of the required measurements are done, but F7000 is waiting for exactly the measurement you describe. This is just an update on my current progress to help others contributing to voltage references based on LTFLU.
-branadic-
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With the transistor as heater the power is linear and not much power is lost.
Hello,
that is true but the heater area is punctual and very small.
So you need extra effort for a heat spreader compared to a heater foil.
Otherwise you have large temperature gradients of several deg C within some 10s of mm.
If I would use transistors I would use as many transistors as can be placed on the backside of the PCB.
with best regards
Andreas
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Agree Andreas, that is why I've chosen something in between heater foil with thermal gradients and transistor with its local hot spot and came to the solution thickfilm resistor on ceramic substrate.
(https://www.sager.com/_resources/images/product/4237_bpr101.jpg)
-branadic-
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A agree that a transistor is a much more local heat source, but it depends on the power level and thermal design how bad this is.
With nonlienar (e,g, square law) heater, one could consider to linearize the loop at least a little, e.g. with a divider + diode in the feedback path of the regulator. So one could have something like a divide by 2-3 at low power, going over to some divide by 1.2 at higher power. This could improve the regulation at the lower power end, where the resistive heater is less effective.
Without linearization the loop gain would depend on the power level, with less than ideal gain at low power.
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The TOMC resistor are lower noise than NOMCA, but may still contribute a little to the noise.
At least a factor of 10 difference, something that is hidden in the datasheet, as they are both spec'ed with <-30db. Maybe it's the limit of what Vishay can measure with equipment they have?
Maybe some words to the construction technic used. The design approach here is somewhat similar to the voltage reference of F5700 DAC module, though a ceramic board is used there, which has better heat conductivity compared to aluminum. But they use a ceramic spacer between ceramic thickfilm heater and ceramic reference board, most likely for "act as a heat spreader" purpose.
Not sure what's the power consumption for their oven assembly - assembly only covered with some plastic shield and power consumption doesn't play any rule in a calibrator at all - but with my reference board and resistance heater inside the small styrofoam box - I should post a picture of it at some point - and everything thermally stabilized, the oven draws about 0.9W.
This is larger compared to an LTZ reference - no doubts about that - but probably smaller compared to what Fluke needs for either the F732B oven assembly, which is physical large - or the mentioned F5700 reference module including the oven.
-branadic-
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The < -30 dBi noise level seems to be a common level, like the limit for a simple test system. They also give the same number for the ORN resistors (only 4 resistors), with a measured noise level that is much lower.
10 x better than NOMCA may still not be good enough to ensure low noise also at very low frequencies. I did see quite some noise with NOMCA (50 K and 10 K) already at 25 Hz.
For the in plane conductivity the aluminium core board should be higher than most ceramics (I doubt they go the trouble with saphire or BeO). The advantage of ceramics is lower thermal expansion, but this should not matter so much in a stable temperature oven.
Some 0.9 W sound reasonable for a module of that size - more insulation could probably reduce it a little more. This is still quite a bit higher than the power consumption without the heater - so the minimal temperarure would not be too high.
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Some 0.9 W sound reasonable for a module of that size - more insulation could probably reduce it a little more. This is still quite a bit higher than the power consumption without the heater - so the minimal temperarure would not be too high.
Would be interesting to hear your suggestion on how to improve insulation.
Hear is how it's currently build:
I have a 1cm thick-walled styrofoam box, with some cotton wool on the bottom, the board with the heater resistor attached to its back is laying flat on it. Then more cotton wool covers the top and fills the inner volume of the styrofoam box until a 1cm thick-walled styrofoam lid closes the box. The inside of the insulating box is slightly bigger than the aluminum reference board (20x40cm²) with its heater and has only enough room for bending the wires towards the lid, where they feed through.
Indeed some Aerogel like material could be used, but that is nothing commonly available in your next hardware store. Even a box made out of a bottle with pu foam could be used (already have one), but needs some kind of form, to get it in shape. So no easy task either.
Am I'm missing a much more simple but effective solution, with material that is easy to get? Don't forget, all hardware stores are closed due to lockdown.
-branadic-
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1 cm styrofoam looks already quite good isolation - not that much more to improve. There maybe slightly better isolating gardes, but likely not much difference. A little more thickness can help. A layer of aluminium foil at the outside (and maybe parts of the inside) may help. With the not so good grades of foam quite a bit of the heat goes through as IR radiation. The main point to improbe would be too have the board not too large - not so easy to change and likely not worth a change. The wires may also be important for the heat loss.
So maybe the 0.9 W are not that much to improve on. A small mains transformer often has more no load loss so for the overall power it would not make too much difference.
I don't think the PU foam from the sprax can is an especially good isulator, chances are it would be more one of the not so good grades. Better wait for the hardware stores to open and than get some foam made for isolation.
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I put my 1.2W ocxo into an 1cm thick box made of of expanded polystyren (a white 1cm thick plate made of white balls :) ) and it is not enough..
The thermal conductivity of that material is 0.03-0.04 W/(K.m) so you may somehow calculate the effects..
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P = const * a / d * dT. With a 10 x 4 x 4 cm box a = 200 cm², d = 1 cm => a / d = 2 m times dT = 20 K times const = 0.03 W/mK gives 1.2 W.
Double thickness gives a little more than half of the power.
Regards, Dieter
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Out of curiosity I've put the size of my ocxo into dietert1's calculation.
Pretty good guess :)
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As the batteries for the reference were running out of power and I have to charge them, I took some pictures and measured the actual inner size of my box: 5.5cm x 3.6cm x 3.6cm with a thickness of the walls of ~1.5cm.
I also have to correct myself, the power drawn for the oven is in average 0.484W, not 0.9W as stated earlier. I also took the chance and wrapped some self-adhesive aluminum tape around the outside of the styrofoam box, will see if that makes any difference.
-branadic-
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By the way, the same formula applies for copper, except this time const = 384 W / (m K). So if your wires have a total cross section a = 1 mm² of copper, then d = 1 cm gives a/d = 1E-4 m and with dT = 20 you get P= 0.768 W. If we assume half of the power gets lost in the styrofoam and the other half in the wiring, then this means that your wires are colder than expected inside and hotter than expected outside of the oven (d = 5 cm or so). Otherwise power loss would add up to something like 1 W or more.
Regards, Dieter
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Hello
just want to show my LTFLU. Its case in case. Outside a Zeissler Feltron Alu case
whitch was extended bei an Alubar 12x165x50 mm just to get the right high
for the 4x isolation a´26 mm. So the LTFLU is in Rose ABS case apr. 76x120x55 mm.
Data are a bit slurry.
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@dietert1: I've made a table based on a lecture here
LECTURE (https://www.youtube.com/watch?v=v2UnD9E2ZWc)
taking into account walls, edges and corners of the Box. There are the edge and corner "coefs" which depend on a ratio of the box sizes to the wall thickness, however.
Added the heat transfer via copper.
Added the xls file.
Maybe a topic for a separate thread..
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Here i have a log of two LTFLU references that i made last year (schematic and images above). Nominal oven temperatures are 17.4 °C and 27.69 °C. The voltage difference was recorded with a HP 3456A. Each blue dot represents the average of about 17 000 measurements with 100 PLC and Autozero.
There is a one month gap between 5000 and 6000 hours, when i cleaned the air filter of the HP3456A and used it for other measurements. I also removed three days of data after an unnoticed oven shutdown, when the 17.4 °C oven went up to 28 °C for more than 24 hours. Added an exponential decay fit. Since this was a first crude setup i can't tell which process this may be. There are no significant correlations with ambient temperature, nor humidity, nor barometric pressure.
Next time i want to log multiple references, including a 5x LM399 device and a revision of the LTFLU shown above.
Regards, Dieter
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These are 10 V references both including a gain stage inside the same oven. In the 17.4 °C reference the gain stage has 10x UPF25 100R resistors, in the other one the divider is made from three Dale RS5 wirewound resistors of 750R and 150R plus 180R that i found in a drawer. Impressed by the stability of those wirewound resistors i ordered several lots of Vishay Dale RS02 750R and 330R resistors that are similar except smaller. Those resistors need selection if you want TC below 5 ppm/K, but then i found several with very low TC in the interesting temperature region around 25 to 30 °C.
I am running the reference ovens at relatively low temperatures as i want battery backup. In fact those LTFLU references have seen power outages and oven shutdowns without "jumping around". The 17.4 °C oven was the first serious attempt and it works well, but that TEC takes 1 W or more, while the other one at 27 °C runs from 50 to 100 mW due to self heating.
Another alternative to make a low TC gain stage is a PWM circuit as i recently showed in the LM399 thread. Something similar can be nice with a LTZ1000 reference.
Regards, Dieter
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I guess you mentioned yourself, that your PWM approach might be good for an LM399, but as far as I understood isn't for an LTZ or LTFLU, due to:
"My PWM prototype has an offset of 11 ppm (max of log). The PWM ratio remained unchanged after implementation. In my opinion a more perfect PWM divider would be more accurate, i mean without tuning or "calibrating" it. Accuracy depends on the symmetry of the PWM switches: resistance and/or timing difference. At 10 KHz a 1 nsec timing asymmetry makes 10 ppm. IC muxes have delays of about 50 nsec that can easily contribute an asymmetry of 1 nsec..."
On the other hand the approach using a TOMC1603 resistor network including parallel resistors for trimming the gain as I did on my LFTLU reference (even a TDP1603 would do) worked quite good, as it is part of the ovenized aluminum board with the LTFLU. This is though a totally different story, when the gain stage including the parallel resistors for trimming are exposed to ambient temperature changes.
An approach not discussed yet is the gain stage used in W/F7000, that can now be discussed thanks to the reverse engineering by chekhov. Is uses resistor networks for a coarse amplification and adds additional voltage for fine trimming provided by parallel DACs, so no clocking being involved.
-branadic-
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The on resistor matching is mainly a thing with linearity and only to a small part with temperature drift. An offset due to delays and charge injection is likely quite stable. For a gain stage we don't need good linearity or a very low offset. The main point is long term drift. The more problematic point may be EMI. Chances are a lower PWM frequency could help - however there is a slight complication: in the feedback path it is tricky to use more than a 1 st order filter. So residual ripply could become a problem with a much lower frequency, unless one could reduce the ripple in a different way (e.g. the compensation with an opposing signal).
For a stable ration there would also be the option to use a charge pump (e.g. LTC1043) for the coarse part (Faktor 1.5) and than use resistors for the fine part only.
Anyway it gets a bit off topic from the LTFLU ref., though the ref and gain are closer coupled with the LTFLU - at least in the normal circuit.
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My plan outlined in the LM399 thread is to reduce PWM frequency from 10 KHz to 1 KHz (or 1200 Hz = multiple of 50 and 60 Hz). For this i implemented ripple compensation as shown in the schematic. Then you have a 2nd order filter that behaves better than a third order filter as ripple compensation helps on PWM input and output side. This change gives me a factor 8 or 10, so the 11 ppm becomes about 1 ppm.
Then you want another factor ten of precision. This happens by the implementation of a discrete PWM output stage as we see them in Fluke/Wavetek/Datron calibrators. I already posted a study/proposal for a discrete PWM output stage. By combining everything one should arrive at a PWM with about 0.1 ppm absolute accuracy. If PWM stability will be another factor ten better than absolute accuracy as observed with the current circuit then that PWM gain stage will be good enough for scaling a group of LTZ1000s to 10 V.
Currently i am rebuilding my metering setup so i can check long term stability at a 100 nV level or below. I want to integrate four references: The two LTFLUs with analog gain stage inside oven, the 5x LM399 with improved PWM gain stage and a 10 V double JFET reference without gain stage. And of course there should be at least one unused port for testing other references..
Regards, Dieter
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For testing the reference and gain stages it would make sense to not only compare the scaled 10 V level, but also check drift of the raw reference voltage before scaling. The dirft of the 2 parts can be comparable.
For the PWM switches I would not count much on better performance from discrete switches compared to modern CMOS switch chips. The discrete solution may have more EMI issues and more charge injection.
The ripple compensation with the capacitive coupled inverted signal acts like an addional filter stage for the PMW part, but not adding much phase shift to the loop. So the ripple reduction is likely a good idea.
Another possible option would be 3 phase PWM, so 3 stages with 120 deg. phase shift, which would cancel the main ripple at 1/3 and 2/3 PWM setting and still reduced ripple close to those values.
More switches make discrete switches less attractive.
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I did my work and later wrote about a discrete implementation of a precision PWM output stage. My schematic in the LM399 thread shows a circuit i built and tested to some degree. As always there may be other solutions and i will be interested in other substantial proposals.
In my prototype i used a FDC6561AN dual mosfet with Rdson = 0.1 Ohm and about 0.03 Ohm match. If somebody wants to propose an IC switch with lower Rdson and/or better Rdson matching, you are welcome. In a totem-pole circuit with optimized dead time you can forget about charge injection.
Regards, Dieter
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The FDC6561AN are interesting in that they allow a relatively high gate voltage. Many other modern FETs have the protection start engaging at some 10-15 V already.
However I don't think these rather large FETs would be a good choice for precision PWM. The gate capacitance is quite large and this leads to quite some current spikes at the driving side and the supply / gate driver. CMOS switch chips have a much smaller current spike to the input side driver. The charge injection would be a measure for the current pulse to expect. I am afraid that by avoiding the switch resistance effect by all costs, the larger spike towards the driver would cause trouble that is difficult to solve.
It also helps to have the supply separate from the switch connection. So some of the gate driver chips are not a good option, if they have a common supply for the output stage and internal driver.
Instead of the brute force very low resistance way, I would prefer compensation of the switch resistance, like in the Fluke 57xx series. So have a 2nd pair of switches with a slightly higher votlage to provide essentially all the current. With integrated switches using 2 channels is no that bad.
Anyway for just a 7 to 10 V stage I would not worry so much about a little offset / nonlinearity. One can usually adjust the PWM ratio to get extact 10 V. There is no need to get it from the upfront calculated ratio.
My current favorite for a 7 to 10 V step is 3 phase PMW with 3 x DG419LE (vishay - maxim seems higher resistance) going to a common filter with relatively high resistance (e.g. 100 K range each). R_on is at some 15 ohms with matching in the 1 Ohms range. Over some 10 C temperature change this would change by something like 50 mOhms or some 2 ppm change relative to the 100 K filter resistance.
For capacitor leakage and amplifier bias the 3 channels are in parallel so that the overall resistance is not that high.
With relativly low charge injection the spikes effecting the drive should not be that bad. With a PWM ratio close to 2/3 the residual ripple to start with would be small, similar to better than with the coupled opposite phase compensation (which act like a separate filter stage).
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No, don't agree. My experience tells me that absolute accuracy is THE criterion. If you start fine tuning and adjusting at the 10 ppm level, you will never realize a circuit better than 0.02 ppm as required with a LTZ1000 (let's assume 200 nV). Just read the PWM discussion of branadic and Andreas in the LM399 thread.
The discrete PWM output stage i proposed recently is one way of solving the Mosfet drive problem you were speculating about. Your discussion does not replace a demonstration. Then you will learn to solve problems instead of inventing objections.
Regards, Dieter
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Recently there was a discussion about "reference oven gain". Today i have a result for our LTFLU ovens (image above).
These ovens have two temperature sensors: One NTC that is kept at constant temperature by the heater. Another temperature sensor is on-chip, derived from the TC of the transistor Ube. The setup also includes a JVR in a similar oven, with a diode as DUT temperature sensor (not on chip but close). The setup also includes two ambient temperature sensors (SHT = humidity and temperature, BMP = pressure and temperature). The first plot shows a log of the two ambient sensors (left y axis) and of the three DUT temperatures (righ y axis = deviation from average). The other diagram shows the correlation. The three line fits give values of about 0.02. That means the three oven gains are about 50. This is the suppression of ambient temperature changes by the oven.
Next step is fine control of the set temperature of each oven as a function of ambient temperature (pre) and DUT temperature (loop).
Regards, Dieter
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To get a better understanding I have redrawn the schematics of the Fluke 5440 voltage reference.
I also made a list of devices with SZA263 inside.
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MegaVolt newer devices like DMM7510, 5730/5720B, 732C, 85x8, etc do not use SZA263, but FLU1 chip. It's not the same thing.
Also some older 8508A use Datron LTZ module, newer ones use FLU1.
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Yes. I guess I need to be more specific.
I cheated off you, by the way :)))))))))))) https://xdevs.com/article/ltflu_ref/
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Don't forget about the Fluke 8800A DMM. Probably the lowest spec device that contained an SZA263. Whether it was binned differently is anyone's guess.
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That LTFLU report of branadic linked above certainly wins the prize for the most sophisticated way to build a reference worse than a LTZ1000. There is a LTFLU involved, but that may be misleading. Seems like real voltnuts prefer bad setups "for the adventure".
I have seen very good results from various LTFLU references, see above. I did not use "nearby" temperature sensors, but on-chip temperature sensors (similar to LTZ1000, schematic above).
And i used more zener current. In the ADR1000 datasheet they estimate that noise is inversely proportional to the square root of zener current. For an array like the LTFLU i found that each of the four zeners should get at least 2 mA, so 8 mA total. That worked well.
Recently i thought: What could be the performance and the price of an array of 20 000 burried zeners? You know that number is the array size of those superconducting Josephson standards. Power consumption would be about 500 W, i guess less than a Josephson standard. The LTFLU is the only zener array we have. Meanwhile i started making arrays of LM399s, also with promising results.
Regards, Dieter
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Another valuable comment by dieter1 with the absence of social skills, brilliant. :-+
I don't see why you think the reference implementation is "most sophisticated", it's simply a copy of the implementation done in several Fluke gear, but with an additional oven around it, similar to what is realized in Fluke 57xx calibrators and instead of selected ref amp resistors it's using a resistor network. So what is your point?
-branadic-
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MegaVolt newer devices like DMM7510, 5730/5720B, 732C, 85x8, etc do not use SZA263, but FLU1 chip. It's not the same thing.
Also some older 8508A use Datron LTZ module, newer ones use FLU1.
Corrected.
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I made a model of one stage of the reference source from Fluke 5440. R4 coarse current adjustment. R5 is an exact substring of the maximum of temperature.
R4 differs from the original circuit because a different zener diode is taken.
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Sorry but the voltage divider R4/R1 hardly reaches the zener voltage, so zener current will very small. I can't understand how you got that diagram.
Regards, Dieter
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Sorry but the voltage divider R4/R1 hardly reaches the zener voltage, so zener current will very small. I can't understand how you got that diagram.
Hmm.... Indeed the result is not correct.
For other values of R4, it is not possible to obtain a parabotlic curve. Only linearly increasing. Perhaps this zener diode is not suitable.
I need to fix something...
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(https://www.richis-lab.de/images/REF01/25x01.jpg)
Finally, a SZA263! 8)
I got it from a really dead FLuke 8842A.
(https://www.richis-lab.de/images/REF01/25x02.jpg)
Let´s see if I got everything right:
The servicemanual of the Fluke 8842A shows the structure of the SZA263 (U701). The base-emitter path of the transistor provides a negative temperature coefficient. The Z-diode offers a positive temperature coefficient. Suitably designed, there is a current at which the temperature coefficients balance out and the voltage between base and anode exhibits a vanishing temperature drift.
If the temperature drift of a Z-diode is compensated with a pn junction operated in "forward mode" this is just a temperature compensated Z-diode, as for example the 1N829A is (https://www.richis-lab.de/REF22.htm (https://www.richis-lab.de/REF22.htm)). A ref-amp contains a transistor instead of a simple diode for temperature compensation. The transistor in addition drives the first control loop.
In steady state the opamp U702A ensures that a voltage of -7V is present at the testpoint TP701. In this state voltage divider Z701 sets a base current in SZA263 via base resistor Z702_8-7 that is exactly the current at which the Ref-Amp temperature coefficient becomes minimum. The basic reference voltage is applied between the base and anode of U702. Voltage divider Z701 scales this voltage to the desired output voltage of -7V. If the voltage deviates from -7V, more or less current flows through SZA263, which changes the voltage drop across resistor R701 via the collector of SZA263. The opamp U702A then readjusts the -7V reference voltage accordingly.
I´m not 100% sure with Z702_1-2 and Z702_2-3. Apparently they do some biasing. But they shouldn´t be necessary. Perhaps they compensate the fluctuation of the remaining temperature coefficient a little.
U702B ultimately generates a +7V reference voltage from the -7V reference voltage. Z702_9-10 is the bias resistor of the opamp U702A. Diode CR701 ensures that the voltage at testpoint TP701 does not become positive when the circuit starts up.
(https://www.richis-lab.de/images/REF01/25x25.jpg)
Resistor R701 defines the current flowing through the SZA263 and must be set to hit the point of minimum temperature coefficient. The voltage divider Z701 defines the reference voltage. The exact values of the resistors are correspondingly critical. R701 is a precision resistor from Dale. Z701 is a special balanced resistor network.
Accordingly, the Fluke 8842 servicemanual does not individually identify Ref-Amp U701, resistor R701, and resistor network Z701 in the part list. If one of these components is needed, one has to order a "REF AMP SET" which contains all three components and all of them have to be exchanged. The resistors are adapted to just the one SZ263 so that the temperature drift is minimal and the reference voltage is -7V or +7V.
(https://www.richis-lab.de/images/REF01/25x03.jpg)
On the bottom of the SZA263 it can be seen that the case would have two more pins in addition to the four that are used. The additional pins have been shortened.
(https://www.richis-lab.de/images/REF01/25x04.jpg)
The additional pins are not contacted in the package.
(https://www.richis-lab.de/images/REF01/25x05.jpg)
(https://www.richis-lab.de/images/REF01/25x08.jpg)
The Z-diode and the transistor are located on a ceramic substrate, which is fixed in a recess of the housing. The package itself is not connected to any of the potentials.
(https://www.richis-lab.de/images/REF01/25x06.jpg)
(https://www.richis-lab.de/images/REF01/25x07.jpg)
Two metal surfaces are applied to the ceramic substrate. The short lead residues leading to the edges indicate that a larger ceramic element was electroplated with the desired structures during production and then got separated.
(https://www.richis-lab.de/images/REF01/25x10.jpg)
(https://www.richis-lab.de/images/REF01/25x11.jpg)
The edgelength of the diode is 0,59mm. The uneven surface in the center of the metal layer is reminiscent of the Z-diodes in the reference voltage source VRE305A (https://www.richis-lab.de/REF21.htm#ZD (https://www.richis-lab.de/REF21.htm#ZD)) and in the digital-to-analog converter DAC80 (https://www.richis-lab.de/DAC02.htm#ZD (https://www.richis-lab.de/DAC02.htm#ZD)).
(https://www.richis-lab.de/images/REF01/25x09.jpg)
(https://www.richis-lab.de/images/REF01/25x12.jpg)
The dimensions of the transistor are 0,48mm x 0,38mm. The ring-shaped emitter is striking. The underlying base area is contacted inside and around the ring. The area of the base-emitter junction is a critical point when matching the temperature coefficient of the transistor to the temperature coefficient of the Z-diode.
(https://www.richis-lab.de/images/REF01/25x13.jpg)
Here you can see the SZA263 in the Fluke 8842A bench multimeter. Below the SZA263 is the precision resistor R701 from Dale (411.62kΩ). Below that, the two resistor networks Z701 and Z702 are placed right next to each other. Most likely, the proximity was deliberately set up to keep the temperatures of the resistors as equal as possible. In the lower left area of the picture the operational amplifier U702 can be found.
(https://www.richis-lab.de/images/REF01/25x14.jpg)
The resistor network Z701 that sets the ideal current through the SZA263 is listed as part number 756031. It is noticeable that the resistor network originally had four connection pins, one of which has been cut off.
The resistors are placed on a ceramic substrate and are protected with a glass cover. The setup allows laser alignment at any time. For high-precision circuits, such a resistor network can be incorporated into an assembly and laser alignment can be performed as a final process on the completed circuit. In this way, it is also possible to compensate for disturbances that occur during a soldering or aging process.
(https://www.richis-lab.de/images/REF01/25x15.jpg)
The leads on the ceramic carrier appear to have been gold plated. Each resistor consists of many different geometries, which make it easier to exactly set the desired resistance value.
A closer look reveals that it is not just two resistors. There is a third resistor on pin 2 that acts as an additional base resistor for the SZA263. Pin 4 allows to contact the common node of the three resistors, which makes the adjustment easier. The following resistor values can be determined (referring to the schematic above):
Voltage divider "1-2": 10,582kΩ
Voltage divider "2-3": 202,28Ω
Additional base resistor: 643,67Ω
(https://www.richis-lab.de/images/REF01/25x26.jpg)
At the bottom left of the ceramic substrate, three masks can be seen. Mask 10A defines the structures of the conductive paths. Mask 12B forms the resistors. The B suggests that this mask was reworked once. Between the masks 10A and 12B you can guess the characters 11A. It seems that this is the mask that defines the transition points between traces and resistors. Apparently, a special treatment of these areas was necessary.
(https://www.richis-lab.de/images/REF01/25x24.jpg)
The labeling on the lower right corner of the ceramic carrier shows that the original designation of the resistor network is not 756031 but 755991.
(https://www.eevblog.com/forum/projects/the-ltflu-(aka-sza263)-reference-zener-diode-circuit/?action=dlattach;attach=136434;image)
Here we have seen that in the 8842A (with SZA263) Fluke used also a different resistor network. This one actually has just three pins. No additional base resistor can be seen. Apparently, it was advantageous to make the base resistor adjustable in the Ref-Amp package. Maybe it was also possible to use another already existing resistor network.
(https://www.richis-lab.de/images/REF01/25x16.jpg)
At the upper contact you can see that the second mask is relevant for the contact areas between trace and resistor. In the right area you can see that a large part of the resistor material has been prepared to contact a trace. Probably the resistor network 755991 can represent two very different resistor values here. Once the right area is just contacted at the upper edge, another time an extended trace bridges the right two resistor strips.
(https://www.richis-lab.de/images/REF01/25x17.jpg)
Not only the resistor surfaces, but also the conductor paths were cut with a laser. The labeling most likely serves the traceability of the alignment process.
(https://www.richis-lab.de/images/REF01/25x18.jpg)
With the inscription in the left area, one can see that the adjustment was made after the ceramic carrier was sealed. The remains of the inscription process have settled on the glass.
(https://www.richis-lab.de/images/REF01/25x19.jpg)
(https://www.richis-lab.de/images/REF01/25x20.jpg)
The second resistor network is called 756080. According to the optical appearance, the conductors are made of bare copper. In any case, it is not gold.
(https://www.richis-lab.de/images/REF01/25x21.jpg)
The individual resistors can be clearly seen. The resistance values are:
1-2: 1,7783kΩ
2-3: 4,4461kΩ
4-5: 20,005kΩ
5-6: 20,005kΩ
7-8: 3,2007kΩ
9-10: 3,3008kΩ
(https://www.richis-lab.de/images/REF01/25x23.jpg)
Here, too, a different number can be found on the ceramic carrier: 756049.
A mask for connecting the resistor surfaces with the conductor tracks cannot be seen.
(https://www.richis-lab.de/images/REF01/25x22.jpg)
Laser tuning alignment.
https://www.richis-lab.de/REF24.htm (https://www.richis-lab.de/REF24.htm)
:-/O
-
I wasn´t perfectly happy with my old LTFLU report. At first, I modified some pictures:
(https://www.richis-lab.de/images/REF01/03_14.jpg)
(https://www.richis-lab.de/images/REF01/03_13.jpg)
In my view here you can see more clearly what is happening on the die.
The next "problem" was that I wasn´t able to contact the die to let the four zener diodes glow. :(
So I bought some Alibaba-LTFLUs... ;D
(https://www.richis-lab.de/images/REF01/26x07.jpg)
(https://www.richis-lab.de/images/REF01/26x06.jpg)
20mA / 50mA
Looks nice! A very even current distribution. :-+
(https://www.richis-lab.de/images/REF01/26x08.jpg)
(https://www.richis-lab.de/images/REF01/26x09.jpg)
10mA / 20mA
You can see the individual areas glowing and growing with current. 8)
But that is not the end of the story! "I bought some LTFLUs from Alibaba." ...very interesting LTFLUs...
(https://www.richis-lab.de/images/REF01/26x01.jpg)
I bought the LTFLUs from "EC Mart Trading Limited" the package is labeled by "Jeking Electronic Corp.". :-//
(https://www.richis-lab.de/images/REF01/26x02.jpg)
(https://www.richis-lab.de/images/REF01/26x03.jpg)
(https://www.richis-lab.de/images/REF01/26x04.jpg)
(https://www.richis-lab.de/images/REF01/26x05.jpg)
The package doesn´t look particularly bad but the letters seem to be a little wider and lower in contrast than normal.
(https://www.richis-lab.de/images/REF01/26x10.jpg)
(https://www.richis-lab.de/images/REF01/26x11.jpg)
Surprise! That is a LTFLU but it´s not working. The bondwires look quite bad (I didn´t touch them) and one is even snapped.
I assume that is a scrapped LTFLU that had been bonded in a garage to sell it and make a lot of money.
That reminds me of the AD587 with the dirt on the die: https://www.richis-lab.de/REF06.htm (https://www.richis-lab.de/REF06.htm)
Probably you are lucky if the LTFLU is dead like this one because who knows what strange behaviour it might have had...
(https://www.richis-lab.de/images/REF01/26x12.jpg)
Since it sits on a second die I assume it was bonded to the package by Linear.
(https://www.richis-lab.de/images/REF01/26x13.jpg)
(https://www.richis-lab.de/images/REF01/26x14.jpg)
The die looks quite normal. It has been tuned with the metal fuses. They use just very few of the transistor areas like in the other LTFLUs we had.
(https://www.richis-lab.de/images/REF01/27x01.jpg)
The second one looks similar to the first one.
(https://www.richis-lab.de/images/REF01/27x02.jpg)
A little better but...
(https://www.richis-lab.de/images/REF01/27x03.jpg)
...here the upper left bondwire is cut.
(https://www.richis-lab.de/images/REF01/27x04.jpg)
Nothing interesting on the die...
(https://www.richis-lab.de/images/REF01/28x01.jpg)
And the third one.
(https://www.richis-lab.de/images/REF01/28x02.jpg)
(https://www.richis-lab.de/images/REF01/28x03.jpg)
Now that is really bad... :o
(https://www.richis-lab.de/images/REF01/28x04.jpg)
Nothing special to see on the die.
Here we have the updated LTFLU page: https://www.richis-lab.de/REF04.htm (https://www.richis-lab.de/REF04.htm)
And here we have the LTFLU (Alibaba) page: https://www.richis-lab.de/REF25.htm (https://www.richis-lab.de/REF25.htm)
:-/O
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Hello all,
Following Link seems to be broken.
...
Well, and JJ arrays on liq. Nitrogen temperature also have been demonstrated.. about 1e-8 uncertainty..See here:
http://juwel.fz-juelich.de:8080/dspace/bitstream/2128/2069/1/19406.pdf (http://juwel.fz-juelich.de:8080/dspace/bitstream/2128/2069/1/19406.pdf)
...
current correct link: Accurate measurements of quantum voltage steps on arrays of bicrystal
Josephson junctions (https://juser.fz-juelich.de/record/26808/files/19406.pdf)
General interesting database for public sience articles: JuSER (https://juser.fz-juelich.de/?ln=en)
Guido