Buffer LTZ1000 DR. Frank

[Kleinstein]

Voltage sensing from the output terminals absolutely makes sense. However the capacitor (C16) in feedback would still be better local on the board, possibly even from before the transistor.

@floobydust:
With the extra capacitor C2 there are two issues. One could be during turn on - here a lot more current could flow through the zener with C2 present. However it depends on the rest.

The other point is loop stability: with just C2 added loop stability suffers, up to the point of possible oscillation. The extra RC at the OP counteract the possible problem due to C2.

The capacitor at the collector of Q1 is part of the normal compensation in the LTZ circuit. It is needed to limit the extra gain of the transistor to low frequencies, to make the OPs loop stable despite of the extra gain. One might argue a little series resistor (e.g. 100 Ohms range) to C1 might be better, but not that much change.

[Andreas]

The very first thing you do on a sensitive precision analog circuit is keep it well shielded.

agreed.

even a standard drawn steel electrical box can work well.

Really?

Let's have a look on a state of the art device like Fluke 732B:

A housing from the picture book.
Fluke really knows how to shield and guard.
30 kg for the device, mostly metal for shielding the poor little RefAmp.

And obviously in the mean time they have learned to spec the EMI behavior.
From the handbook:

Electromagnetic Compatibility
This instrument is designed to operate in Standards Laboratory environments where the
radio frequency (RF) environment is highly controlled. If used in environments with
field strengths >0.18 V/m, there could be errors in measurements.

0.18V/m really? For all that metal surrounding? That is .... very .... low.
A 1.5V AA-cell connected to a metal plate on the floor with one pin and the other to the metal plate
on the ceiling in 3 m (10 feet) height already exceeds this value by a factor of nearly 3.
(ok for a DC-field a normal cirquit is not sensible. But what happens if you exchange polarity fast enough?)

Even cheap household appliances have to withstand at least 3V/m.
Devices for industrial environment at least 10V/m.
Critical automotive cirquits often above 50, 100 or even 200V/m since you do not want
that your mobile phone which can generate these field strengths in the near field triggers the airbag while driving.

The 0.18V/m and the 30 kg are facts.

My conclusion: a housing alone is not enough for a voltage standard which can be used in normal household environment.

with best regards

Andreas

[Dr. Frank]

Instead of getting into the previous discussion about how many angels can stand on the head of a pin, I have what I consider to be some practical observations about the Dr Frank LTZ1000A board I’m assembling and testing that I would like to share-for what it’s worth. To recap, I have most of the necessary parts to complete the board soldered in their proper locations and I have enough of the parts so I can do some testing. One resistor I need is still on order so I got the value I needed by putting two lower spec-ed resistors in series with the leads just poked into the holes on the pc board which is far from ideal but allows me to continue testing.

One problem I had was the 20R trimpot used as the adjustment for the 10V voltage divider was driving me nuts. The ‘granularity’ was such that the smallest adjustment I could get would cause the output to jump in steps of about .00005 volts. This was caused by the limited number of turns of resistance wire on what is essentially a one-turn pot with mechanical gearing. As the wiper would move from one wire turn to the next there was a much larger resistance change than desired. Also with the fixed 20R resistor across the 20R trimpot, that combination gave from 10R-20R which resulted in far too much adjustment range for my liking.

I calculated I needed 4K822 for the upper half of the voltage divider so I ordered a 4K750 RNC90Z 2ppm resistor like the other resistor I’ve used and found I had a nice salvaged 72R14 WW resistor to put in series with the 4K750 to give me the 4K822. I also had a small 5K thirty turn pot I put in series with a 5K resistor to put across the 72R14 resistor that gave me a smaller adjustment range (.5R) and excellent adjustability. Once I get the 4K750 resistor and check its exact value I’ll remove the 20R and 20R trimpot and jumper/put different resistors in that location.

Once I got the output adjusted to 10V I decided to check how loading affected the output with the 2N3904 emitter follower I had put inside the amplifier loop to get better drive and protection for the rest of the circuit. I soldered a couple of 9” #24 wires to the output pads and had a decade resistor I could use to adjust the load. I was quite disappointed in the variations caused by the load and after a lot of checking and head scratching concluded that by measuring the output voltage at the end of the two wires I was actually measuring the output through two low value resistors which had a small voltage drop. It doesn’t pay to worry about the voltage drop on a couple of mm of runs on the pc board and then put about 500 mm of wire in series with the output.

What I did to correct this was connect the negative lead from the power supply to what would be the negative binding post and connect the end of the negative wire from the board to the negative binding post so no load current was going through the wire going to the board. On the positive lead I connected it so only the emitter of the 2N3904 was going to what would be the positive binding post and ran a ‘sense’ wire back to the top of the 10V divider resistors. This kept the output current carrying wires only going to what would be the binding posts and the sensing points going directly to the binding posts and not seeing any load current. I did measure the drop across just the current carrying wires and at a 1K load the drop totaled about .0004 volts so the changes I made took that possible drop out of the circuit. Once I made these corrections I could drop the resistance load from 10M down to 500R and the output looked stable. Granted, you probably will never have such a low load on the voltage reference but it is worth making such a simple correction to make sure it isn’t a problem.   

For that 20 Ohm pot you should have used a 20 turn type. Thought this would be intuitively clear from the design.
I used two different sets of divider resistors, one was 5k6 over 15k, the other was 4k over 10k.
That slightly different ratio was selected for the 5 different LTZ1000 I had, so the trim resistors were optimized for lowest additional trim range.

Frank

[ArthurDent]

Quote - "For that 20 Ohm pot you should have used a 20 turn type. Thought this would be intuitively clear from the design."

As I clearly said, I used a multi-turn pot, actually 23 turn. If you've ever taken one apart you will see that they are a single turn pot with gearing as I said. 

[Dr. Frank]

Quote - "For that 20 Ohm pot you should have used a 20 turn type. Thought this would be intuitively clear from the design."

As I clearly said, I used a multi-turn pot, actually 23 turn. If you've ever taken one apart you will see that they are a single turn pot with gearing as I said.

I was confused by your description, that your pot contained a resistance wire.

These trimmers from Vishay or Bourns instead have a smooth CERMET resistance film inside, so that these coarse steps of 50µV or 5ppm should not appear.. I have no problem to trim the 10V outputs to below 1ppm, or below 10µV increments, even when using the full 20 Ohm range.

Which brand and type of trimmer did you use?

[try]

Hello MisterDiodes,

People are sure getting downright nasty, disrespectful and rude here.  I'm done and outta here.

are we talking double standards?
When push comes to shove MisterDiodes turns from Dr. Jekyll into Mr. Hyde:

https://www.eevblog.com/forum/metrology/want-to-measure-resistance-with-high-precision-what-would-be-best-to-get/msg1442177/#msg1442177

Who is downright nasty, disrespectful and rude here?
You are either unable to understand the few lines I wrote or you wrote what you did on purpose. Either way, that is sad.

[...
 I'm trying to leave at least a few bread crumbs for you guys from the real world beyond EEVblog - I'm trying to give you at least a clue as to what you want to look at to make your LTZ age well.  Use the information or not - up to you.
...]

The master of the imaginary 732A and 3458A army that has never reached the light of this forum is not able to quantify the effects on LTZ aging. This is as good as saying "I always put a towel over my multimeter connectors" without stating how much this affects the circuit in question.
Who else should be able to make such an informative statement than you, assuming your industry background you always talk about?
You unfortunately did not. 

Source:
https://www.eevblog.com/forum/metrology/buffer-ltz1000-dr-frank/msg1507591/#msg1507591

[...I'm trying to leave at least a few bread crumbs for you guys from the real world beyond EEVblog...].

Thank you, Sir.

Source: Same as above.

[...Not saying that's exactly what you've encountered, but that is a real "trap for young players"...]

Thank you again, Sir!

Source: Same as above.

[...
"A friend of a friend had the old eBay meter checked at Maker Fair 5 years ago and it was OK, and these relative measures will be perfectly fine to ppB"...]

Content-wise, this is the same projection towards my statement linked above. Except for the difference that is pointed at Dr. Frank  this time who never mentioned the maker fair. That is your invention/imagination. Now comes the fictitious 732A army of absolute correct measurement and heals it all and MisterDiodes is always right.
Just as a hint: I am not Dr. Frank.

From what I learned as a hobbyist is that it is necessary to TRY, build and fail with electronics stuff especially when chasing PPMs.
I've never seen any of your builds, any calculation or measurement.

So far, I can only consider your impressive-sounding contributions as pure infotainment.

(Young player)
try

[CalMachine]

Hello MisterDiodes,

People are sure getting downright nasty, disrespectful and rude here.  I'm done and outta here.

are we talking double standards?
When push comes to shove MisterDiodes turns from Dr. Jekyll into Mr. Hyde:

https://www.eevblog.com/forum/metrology/want-to-measure-resistance-with-high-precision-what-would-be-best-to-get/msg1442177/#msg1442177

Who is downright nasty, disrespectful and rude here?
You are either unable to understand the few lines I wrote or you wrote what you did on purpose. Either way, that is sad.

[...
 I'm trying to leave at least a few bread crumbs for you guys from the real world beyond EEVblog - I'm trying to give you at least a clue as to what you want to look at to make your LTZ age well.  Use the information or not - up to you.
...]

The master of the imaginary 732A and 3458A army that has never reached the light of this forum is not able to quantify the effects on LTZ aging. This is as good as saying "I always put a towel over my multimeter connectors" without stating how much this affects the circuit in question.
Who else should be able to make such an informative statement than you, assuming your industry background you always talk about?
You unfortunately did not. 

Source:
https://www.eevblog.com/forum/metrology/buffer-ltz1000-dr-frank/msg1507591/#msg1507591

[...I'm trying to leave at least a few bread crumbs for you guys from the real world beyond EEVblog...].

Thank you, Sir.

Source: Same as above.

[...Not saying that's exactly what you've encountered, but that is a real "trap for young players"...]

Thank you again, Sir!

Source: Same as above.

[...
"A friend of a friend had the old eBay meter checked at Maker Fair 5 years ago and it was OK, and these relative measures will be perfectly fine to ppB"...]

Content-wise, this is the same projection towards my statement linked above. Except for the difference that is pointed at Dr. Frank  this time who never mentioned the maker fair. That is your invention/imagination. Now comes the fictitious 732A army of absolute correct measurement and heals it all and MisterDiodes is always right.
Just as a hint: I am not Dr. Frank.

From what I learned as a hobbyist is that it is necessary to TRY, build and fail with electronics stuff especially when chasing PPMs.
I've never seen any of your builds, any calculation or measurement.

So far, I can only consider your impressive-sounding contributions as pure infotainment.

(Young player)
try

Edwin,

initially I did not intend to spread the word about it you but with you bragging about your measurement abilities while simultaneously failing to meet the specs I think others should be aware of my negative purchase experience with you.

This is what I wrote to you 2/27/2018 20:19:

Hello Edwin,

I hope you are doing well!

Unfortunately I have an issue with your resistors, they are off specification.

In your email dated on Christmas Eve (12/24/2016 18:30) you stated:

...The two 10Ks were about -2 PPM and -6 PPM from nominal....

When measuring them with PTB at the maker fair on 8/27/2017 they were 10,000 441 and 10,000 397 kOhm.
As the 3458A they used this time hadn't been calibrated/adjusted* for some time I waited for a arrival of a friend.

Last Friday we both went to the university department he had worked for.
We had a 3458A in there, 5 years old and never calibrated since.
We measured 10,000 485 kOhm and 10,000 491 kOhm.
By comparing to a Vishay VHP in 10k my friend had measured at work with his annually calibrated 3458A we were able to determine that the 3458A at university was reading roughly 16ppm too high.

But even then, the real value of your resistors would rather measure an adjusted 10,000 325 kOhm and 10,000 331 kOhm.
I just measured them on my 34401A that I adjusted to PTB specs at the maker fair and they were both at 10,000 34 kOhm.

I have to assume that the two candidates have either never exhibited the resistances you indicated or that they drifted roughly 35 ppm in just nine months.


The following pictures shows one of the two 10k resistors specified 10ppm (your writing!) that I purchased from you at the  end of 2016. I just measured it. They are both way off spec.

Now I have to read in this thread that you have a 3456A on your bench which you could have used to verify the resistor you produced for me what you obviously not bothered to do for a client order in the 10ppm precision range.

Conclusion:
Please do not claim high precision. You are probably a good source for PWW resistors in the 0,1% or 0,01% class and are delivering a good service that you do not charge for by producing PWW resistors with resistances as per customer specification.
I always enjoyed reading your contributions and still do with a grain of salt but the former confidence is gone.

I hope that the bunch of knowledge you acquired will show up in your PWW products sometime in the future.
And as opposed to my last email to you where I hinted to you that I ordered and paid 10ppm class and received two classes below:
You can continue to ignore it - no problem.

Greetings from Germany
try

* The 3458A had been calibrated in the voltage ranges shortly before the fair but not in the Ohm ranges.

[eurofox]

Hi guy's,

I'm the OP (original Poster) and you guy's change this thread in a battlefield 

There are several threads for the moment that go in this direction (Capacitor Leaking tester from Mr. Carlson, LTZ1000 .....) 

This is a place to share our experience, to learn from the people with experience, experience build up sometime by hobby or as professional. 

We have all different opinions, we can motivate our opinions and ideas and maybe the true is sometime between the two different opinions 

Most of us live in free country's and if we don't like the discussion anymore we can leave the discussion like MisterDiode did and no reason to start different pollution threads begging him to come back, this is a Metrology and not the Saturday market. 

This was just my two cents and I whish all reader a nice Sunday.

[aronake]

On the Dr Frank board, what is the purpose of the 2.2K resistor (R11) which is not in the reference design in the LTZ1000 data sheet? Changing this up or down, how would that effect output of the circuit?

[Kleinstein]

The extra 2.2 K resistor towards the 12 V provides part of the current for the reference. It is there for start up and in parts to reduce the current to drive by the LT1013 OP-amp.  Changing this up or down somewhat would not have very much effect. Considerably less current is sufficient for start up. With 2.2 K and some 5 V over the resistor this is only something like 1/2 the current needed - so still quite some load to the LT1013. The output impedance of the reference circuit is rather low, so there should be little voltage change from changing the resistor, especially if the 12 V supply is reasonable stable.

[dietert1]

As i have some LTZ1000 in a drawer since several years, i am reading these threads in order to learn something. But except Andreas' EMC study i found very little that can be called engineering. People exchange opinions based on what? A matter of taste?
E.g. if i want to determine what is a good value for that 2K2 resistor, i need to specify the 12 V supply and then build several references with different resistors and try to confirm and characterize differences. Obviously we cannot expect that from anybody here.
I would copy the datasheet or the DATRON circuit, maybe adding EMC measures as shown by Andreas.

Regards, Dieter

[aronake]

Here are my tests:

All according to component specification except for temperature setpoint where i have a ratio of 12.7 with slightly lower resistance for each of the resistors.

Sweep from 10 degrees Celsius to 52 degrees. Start at 10 degrees, step of 3 degrees, 18 min for each step.
Two graphs below

LTZ no 2.2K : Keep pretty much within 10 uV except for dip to 10V where heater was collapsed. I have fans in my TEC cooler box and no cover on the LTZ, so makes sense. I think this is a very good result.


LTZ with 2.2K: TC turns into a disaster.

Pretty strange.

[Andreas]

Pretty strange.

No. You are acting against the data sheet.
12.7 Ratio and 50/52 deg C environment do not fit together.
You need at least 10 deg C headroom for the heater.
So at 50 deg C minimum 60 deg C setpoint temperature (13:1) for the non A-Version.

These diy cirquits are usually optimized for 18-28 deg C environment operation to optimize ageing drift.

Which supply voltage do you use for the cirquit?

with best regards

Andreas

[cellularmitosis]

Pretty strange.

No. You are acting against the data sheet.
12.7 Ratio and 50/52 deg C environment do not fit together.
You need at least 10 deg C headroom for the heater.
So at 50 deg C minimum 60 deg C setpoint temperature (13:1) for the non A-Version.

These diy cirquits are usually optimized for 18-28 deg C environment operation to optimize ageing drift.

Ahh, so the deep V in the second image was the heater falling out of regulation, looks like it started at 37C.

[Kleinstein]

At first the curves look like a bit noisy, but there is a surprise to it: the 2 curves look pretty similar for the part where the oven works.  It looks like it is a kind of reproducible effect, e.g. of temperature gradients.
For some reason the  temperature steps at different temperature look a bit different at different points, but are quite similar between the runs.

Anyway when directly measuring an LTZ1000, chances are that the noise / stability of the meter can be a relevant part of the curves. So much of the 10 µV range could actually be the DMM and not the LTZ reference.

[dietert1]

So we learned that injecting more current (power) into the zener reduces the oven temperature margin, as the chip temperature rises.
Another test would be for supply voltage suppression/feedthrough from 12 V to the reference output.

By the way, in the HP 3456A reference they used a zener instead of that resistor to init the bootstrap. Another good solution proposed in this forum is a n-channel JFET with its gate connected to Gnd and its source connected to the zener. It injects current at low voltages only, similar to the zener.

Regards, Dieter

[KT88]

The 2.2k resistor doesn’t increase the zener current - it just off-loads the Opamp output…

[Dr. Frank]

Here are my tests:

All according to component specification except for temperature setpoint where i have a ratio of 12.7 with slightly lower resistance for each of the resistors.

Sweep from 10 degrees Celsius to 52 degrees. Start at 10 degrees, step of 3 degrees, 18 min for each step.
Two graphs below

LTZ no 2.2K : Keep pretty much within 10 uV except for dip to 10V where heater was collapsed. I have fans in my TEC cooler box and no cover on the LTZ, so makes sense. I think this is a very good result.


LTZ with 2.2K: TC turns into a disaster.

Pretty strange.

Hello,
I cannot see a "disaster" with the 2k2 assembled. As Andreas already wrote:
In both cases, the oven regulation does not work any more, I estimate from 40°C onwards, these are the dips decreases of -3 and -5ppm  in the middle of both diagrams.

Usually the reference voltage will go up in this case, not down, due to its +50ppm/K unregulated T.C. so that's kinda strange.

The noise is very big with about 1ppm peak-peak, might arise from low NPLC and lack of filtering. And I see several negative dips of many ppm, which definitely arise from external noise.

I simply copied Andreas design, removing some unnecessary angst capacitors, but this circuit, assembled into a tuner box is very immune to external noise, I've never again seen such dips. So maybe you are lacking this external shield, and you might have some EMC stinkers in your lab.

The 2k2 is meant for starting the reference regulation towards positive and to relieve the OpAmp from driving current, as Kleinstein already stated.
Some AZ types show much lower open loop gain, when they are loaded, or even have higher Offsets (not sure about latter). Another effect is less heat dissipation inside the OpAmp, which might reduce offsets for non chopper types.

Its value depends on the supply voltage, zener voltage, the zener current and a bit on the oven setting temperature, as this also changes the zener current.
The reference circuit w/o OpAmps and oven supply consumes less than 5mA in most cases. I have chosen 48..55°C, 12V supply, so I have used 1k5, if I remember right, leaving about 1.5mA for the OpAmp.  2k2 is good for 15V_max.. If you further increase the supply voltage, like in the 3458A, you must increase this resistor also, because the regulation will fail otherwise.

Changes in this resistor or change of external temperature won't give rise to any increase of the zener current, neither does it influence the reference circuit at all, as long as the oven works correctly.
The external temperature rise will decrease the heating power on the other hand.
I think Dieter Teuchert himself does not understand the function and influence of this resistor, and of the design modifications we have done.
But I also did not expect that from him, I have to retort.

A few other engineering hints:
This circuit was also designed for lowest cost, i.e. using a single sided PCB, lesser parts, less expensive PWW resistors. Another advantage is that all solder joints are on the bottom side, intended to have them all on the same temperature if the PCB is assembled horizontally, and if an appropriate heat isolation is used. The idea was to reduce or compensate any e.m.f. by creating an isothermal layer that way, as this is one of the greatest source of error in this circuit, see hints in the LTZ datasheet.

Meanwhile I replaced the LTC1052 by the AD4522, which is much less noisy.

Frank

Edit 14.8.:
I added an equivalent diagram, T_env. and V_ref for my LTZ #7.
This has an 'A' chip, requiring at least 12°C lower room than oven temperature.
It's set to about 52°C T_oven, 12k/1k resistors for 13:1 ratio, and you can clearly see in both diagrams, that the oven fails at about 40°C, with +50ppm/K.
The decrease of V_ref to the right, between 8400 .. 10200sec arises from too fast cooling down, i.e. that's probably caused by component temperature imbalance.
Compared to your measurement, you can also see how low the noise of my measurement is, typ. 0.15ppm_pp, and that there are no dips originating from the reference.