New LTZ1000 10V Reference build (WAS: Agilent Keysight 03458-66509...)

[Rax]

Here's a roughly 18hrs plot with R10 = 400.1k. I seem to be getting .694ppm/C for this R10 value. I not quite ure what is to be expected from this at this level of build, but running different values will certainly tell me.

Please note the Prema is currently running with upgrades that negate its calibration (so measurement values are probably about 7ppm high). The kit is not in an enclosure, but in free air. I think I'm OK with both those determinations, as they stretch a bit the environmental conditions which is conducive to the test.

[Smokey]

I'll continue posting here as I find more similar data.

My intention with this "new" LTZ1000 10V reference build thread is to make it as useful and as simple and as I can, organize as much as possible information accrued trough the efforts here (which should be for the benefit of others down the line from me), and to introduce new information where possible.

For instance, I don't think information on these "long term rated" PRC resistors was presented here before (anywhere on the Metrology section).

It would be great if you could keep an "index" in the first post of where important/interesting information is in this thread.  As you already found in the original LTZ1000 thread, it's a pain to try to find anything once a thread gets over a certain size.

[Rax]

It would be great if you could keep an "index" in the first post of where important/interesting information is in this thread.

That's a great point. I was planning to add the findings to the initial post - didn't think to have a second placeholder post as I maybe should have - or if I really get motivated write it up in a separate document, but links would probably work better (more true to the form of a forum and how these threads grow).

[Kleinstein]

The curve shown does not look like it can make a good test for the TC. The variations look rather fast and thus unsure if the temperature is stable.
Anyway the reference should be mounted in a case, as the TC correcton with R10 depends on the thermal setup. It uses the heater current to correct for the residual temp effect. Additional insulation can change things and the most suitable value for R10.

[Rax]

As you already found in the original LTZ1000 thread, it's a pain to try to find anything once a thread gets over a certain size.

I'm finding it just about impossible to find anything using the forum's search capability (within some limits). There was a recommendation somewhere to use google to search inside the forum instead (to bypass its own lackluster search engine), which some people have had success with. But I'm not having much more luck with that either.

The more massive the thread, the less chances awesome nuggets harbored deep inside the thread will emerge, is my experience. Or, really, any (useful) nuggets.

[Rax]

Before I put together a suitable tempco plotting rig (involving "beer boxes," per Frank... Which - yeah, go ahead and judge me!  - I have plenty of in different sizes. The only difficulty is which will I sacrifice?...) I'm going ahead with a LTZ1000A variety build.

There's one pick I'd appreciate some input on. R2 and R3 only have in the other of 1/1000 to 1/3000 contribution to the overall tempco, so not nearly as demanding as R1, R4, R5, R12, R13. It also seems the value is not critical, but I'm not sure where things get stretched too much. The datasheet seems to show in different places 30k, in (most others) 70k.

What I have at hand are:

• 50k PTF56 (10ppm TC)
• 40k VHS102 (2ppm TC, I think)

I didn't find any examples in the original thread of anyone going below 50k, but maybe there is and/or I missed it (but if the case, I'll be happy to use those VHS102s, which I have plenty of at hand).

[Kleinstein]

The value for the 70 K resistors is not that critical. However there is still a balance: a lower value gives marginally less noise (more current at the transistor), but also a slight larger TC without regulation and thus more importance for the set point. I both cases the difference should not be very large.

The 50 K PTF50 should be OK, good enough from the TC and drift, though there may be better choice in SMD form factor.
The VHS102 would be overkill and no real need for these in this application.

[Rax]

Please see modified first post - I restructured it to index/organize the information contained in this thread. A wiki of sorts.

[Rax]

After some aging of my current two kits:

• LTZ1000 (7.159V) kit: R12 = 10k; R13 = 3.95k
• LTZ1000A (7.187V) kit: R12 = 10.25k; R13 = 4k

Hence assuming the builder uses TCC to source custom fully specified resistors, given some calculations - once the stable output of the LTZ is known - ordering R12 and R13 discriminately removes the need for R14. Both the kits above can comfortably adjust to less than 1ppm of 10V without any need for a trim by R14. For the current modest price of about $40 for the two resistors, one can almost dial their LTZ kit to 8.5 digits worth of 10V, precisely. Honestly, one less anal than I am would be perfectly happy with just populating R12/R13, shunting everything else, and calling it a day.

I am using 20 ohms for R18 and a 20 ohm adjustable R15. The last fraction of the ppm becomes uber fine 1/4 or 1/16th of a rotation on R15.

[Rax]

For the current modest price of about $40 for the two resistors, one can almost dial their LTZ kit to 8.5 digits worth of 10V, precisely.

This is in Z201 flavor, which - hedging some criticism this spec is actually real - should provide .2ppm of TCC, which is obviously top notch. Third best to Fluke's legendary 000 TCC resistors and an "arm and a leg" VHP.

[Rax]

Slowly progressing with this while I let the two kits I built already age a bit.

I should probably put the kit using the LTZ1000A - which is my "primary" - in an insulated box (see pics for my following points). I'm planning to use copper tape for shielding. Not quite sure what would be the correct connecting of this shield - GND_PWR? Then the 10V output would use the gold-plated bananas, and the power supply would connect to the basic Pomona bananas.

Welcoming comments on these choices.

[Kleinstein]

Thermal isolation is a 2 sided thing.  It can attenuate fast temperature variations and with the active heater also reduce the overall temperature variations also for outside the regulated part. However it also leads to an high overall temperature and thus way more aging for parts like the resistors. The oven temperature at the LTZ1000 also needs a slightly higher value to still get regulation.  A temperature somewhat (e.g. 10 K to 20 K)  higher than room temperature can be good as this also reduces the humidity (the RH goes down by a factor or 2 for about every 10 K of temperature rise).

For the referenes the variable heat sources like the voltage regulators should ideally be outside the termal isolation, at least well away from the actual reference circuit to not effect gradients very much.

[Andreas]

Not quite sure what would be the correct connecting of this shield - GND_PWR?

Hello,

ask 3 different people and you will get 4 different opinions.
Effectively also the power supply and the wiring play a big role.
And without additional measures (filtering) the foil will not bring much improvement.

It would be better to have a thick walled metal housing which would improve the thermal equalisation around the reference.

In my references (with batteries within the housing) the outer metal shield can be used as guard.

For the referenes the variable heat sources like the voltage regulators should ideally be outside the termal isolation, at least well away from the actual reference circuit to not effect gradients very much.

This is true (at least for battery supplied equipment where the input voltage changes with charge state).

But I would not fear falling out of regulation.
I only had one LTZ out of 9 (with extremely low output voltage of 7.11V) where above 32 deg C environment there was a non linearity in T.C.
I simply set the temperature setpoint somewhat higher to fix it for my standard temperature range of 10-40 deg C

with best regards

Andreas

[Rax]

ask 3 different people and you will get 4 different opinions.
Effectively also the power supply and the wiring play a big role
Andreas

Good point, Andreas. I haven't explained my plan for supply this with power.

The second, nickel-plated set of banana posts would be for a simple two wire connection to one of my HP power supplies. I don't think I want this to have its own provisions (regulator inside its case, etc.), given it will be mostly off and tucked away, traveling occasionally, and if I wanted on for a long time, the HP supplies will be fine doing it.

Unless there a strong recommendation to provide regulation (and maybe further filtering, etc.) inside the case.

And without additional measures (filtering) the foil will not bring much improvement.

It would be better to have a thick walled metal housing which would improve the thermal equalisation around the reference.

Andreas

If the only advantage to the thick walled metal housing is thermal equalization (as it sounds like you're suggesting shielding is difficult to achieve without additional measures), I assume a similar result would be achieved by insulating the plastic box I was planning for this - wouldn't it?

I simply set the temperature setpoint somewhat higher to fix it for my standard temperature range of 10-40 deg C

with best regards

Andreas

This is something I also have to account for, Andreas, as though I don't quite have your fluctuations, I don't have metrology lab conditions either.

[Rax]

After about 3.5 months of aging, it seems the LTZ1000 kit (not properly temp compensated - btw, I see no useful path here, given the difficulties, additional work, and investment in additional gear to accomplish the tweaking of R10) has gone up by about 1.5ppm, while the LTZ1000A kit by about 4ppm. This doesn't factor in temperature conditions data, etc. - it's essentially ballpark. I don't think I care for logging this "pre-aging" phase of these kits; I think the useful logging starts about now.

So I'm readjusting them to as close to 10V as I can bring them (per Prema 6048), and then they soon may travel to a bench that can "17025 them."

[Dr. Frank]

The effort to trim R10 to T.C. is surprisingly low, but worth the time, and in your case of a lab with strong temperature changes, urgently necessary.
Otherwise, you will - again - get no fix point in your lab.

You only need a short term stable other reference, which you compare differentially.
The temperature on the DUT has to be changed rapidly, relative to the slow and lower temperature change of the room temperature for your other reference.

Then you can easily separate the T.C. of DUT vs. the other reference.

Frank

[Rax]

Maybe there's a finer point you're making here, Frank, but I thought the LTZ1000A equals or exceeds the thermal performance of the compensated LTZ1000, because it doesn't need the TC trimming resistor?

[...] 200k resistor , which should NOT be used on the A version of the LTZ1000, as it creates an unwanted T.C. of about 0.3ppm/K. W/o that resistor, it probably would have near zero T.C.

Or your samples and tests showed that not to be the case?

To state that differently, is the LTZ1000 w/ proper temperature compensation (by selected R10) more temperature-stable than the LTZ1000A (obviously) without?

You made the point elsewhere that one of the advantages of using the basic LTZ1000 is it can be run at lower temperatures, and therefore has lesser long term drift. I took note of that, but that's completely separate from this tempco consideration.

[Andreas]

Hello,

its all in the LTZ1000 thread.
T.C. depends on the 400K* resistor and lead length of the reference.
And most probably also on thermal isolation and temperature setpoint.

with best regards

Andreas

[Dr. Frank]

Maybe there's a finer point you're making here, Frank, but I thought the LTZ1000A equals or exceeds the thermal performance of the compensated LTZ1000, because it doesn't need the TC trimming resistor?

[...] 200k resistor , which should NOT be used on the A version of the LTZ1000, as it creates an unwanted T.C. of about 0.3ppm/K. W/o that resistor, it probably would have near zero T.C.

Or your samples and tests showed that not to be the case?

To state that differently, is the LTZ1000 w/ proper temperature compensation (by selected R10) more temperature-stable than the LTZ1000A (obviously) without?

You made the point elsewhere that one of the advantages of using the basic LTZ1000 is it can be run at lower temperatures, and therefore has lesser long term drift. I took note of that, but that's completely separate from this tempco consideration.

I only have two samples of A version references, and both were near zero TC w/o that R10.
I have no clue, why or if it is really a general characteristics of the A version.. no such detailed info from others, to my knowledge.

Anyhow, you need to check the individual TC, to make sure that you really got a reliable reference, over temperature.

As the LTZ1000A heats itself 10°C more than the non-A version, oven temperature has to be 10°C higher, under same environmental boundary conditions.
That means, if you run your reference between 18...28°C normal lab range, then you can as well use 50..55°C oven temperature for your A version.
I have tested my LTZ1000A assembly to work up to 32°C (or so), until the oven regulation freaks out.

The calculation is: max. environmental temperature + 7°C (enclosure heating) +10°C (self heating A version) +5°C (regulation margin) = minimum oven temperature.

This R10 does not affect the regulation stability, nor the overall stability, to my experience.

This check for zero TC can easily be done with a 2 temperature point measurement, i.e. you might cool, or heat the DUT by +/- 5°C, and then track the behaviour over the slow temperature relaxation.

Frank

[Rax]

Hello,

its all in the LTZ1000 thread.
T.C. depends on the 400K* resistor and lead length of the reference.
And most probably also on thermal isolation and temperature setpoint.

with best regards

Andreas

Andreas,
Thank you, but that wasn't at all my question.

I am not planning to use LTZ1000 parts, only LTZ1000A. So the whole conversation over the 400k resistor is N/A, except to the extent of whether the properly compensated part (LTZ1000) is more temperature-stable than the un-compensated (because it doesn't need temperature compensation) LTZ1000A.

[Rax]

As the LTZ1000A heats itself 10°C more than the non-A version, oven temperature has to be 10°C higher, under same environmental boundary conditions.
That means, if you run your reference between 18...28°C normal lab range, then you can as well use 50..55°C oven temperature for your A version.

This is another reason I prefer the LTZ1000A. Having it working at a higher temperature than the other part I think should make it a bit more tolerant to the occasionally higher temperature of my environment. Even so, keep in mind that, even if my lab is not temperature controlled 24/7, I do have heating and cooling in it which I can perform for sensitive measurements. These not being kits needing to be run 24/7 - such as a 732A/B would be - I can regulate my lab at a certain temperature, then turn on the kits and use them. Probably just make sure to build enough time in doing that for the kits to acclimate themselves.

I am using a 1k/12.5k ratio for the oven resistors, which should make the temperature about 55-60C per Frank. Per formula presented, that should provide sufficient margin for environmental conditions up to 33-38C, which is plenty more than I need.

[Kleinstein]

Well trimmed with R10 the non A version can reach near zero linear TC. Over a small temperature range it can this way be more temperature stable than the A version. However this can only trim the linear TC and not higher order terms (specially T² part). The compensation also adds additional 2nd oder TC as the compensation adds something proportion to about the square root of the heater power.

[dietert1]

Once more: The A-Version has less temperature margin for its oven due to more self heating of its chip inside the metal case. For a given ambient temperature span its oven temperature must be configured higher - a disadvantage, when low long term aging is desired. Then again, in this case ambient includes some plastic covers and so on.

At a given more or less constant heat resistance defined by the mechanical setup and LTZ variant selected the final oven power is about linear with ambient temperature (with a negative slope). So R10 is a way to add a nonlinear compensation (square root). The way to add a near linear compensation is a small resistor in series with the zener. This tunes the Ube temperature compensation by adding some gain. Usually a 15 to 20 Ohm resistor serves well. One can combine both methods and tune linear and first nonlinear term of TC(T). As far as i remember the F7000 schematic demonstrates that.

Regards, Dieter

[Kleinstein]

The resistor in series to the Zener adds some liner TC, however this is linear TC for the part inside the oven, not for external temperature changes.
The compensation with R10 is compensation for changes in the external temperature  or heater power that is related.

For a 10 V reference there can also be temperature effect from the 7 to 10 V gain stage, if this part is not termally stabilized. This would also apply the LTZ1000A version of cause.
One may get away with a single compensation for the 10 V TC, not necessay via R10 at the LTZ circuit, but with something like selecting suitable TC resistors with a fine trim.

[Rax]

For a given ambient temperature span its oven temperature must be configured higher - a disadvantage, when low long term aging is desired.
Regards, Dieter

I've started figuring out, I think, some ways to have yearly, seasonally, or more frequent traceable checks, so I think low long terms aging is less of a concern. I'm looking for a feasible, reasonable project with this, and so I think I just have to strike the LTZ1000 temp comp side project. In the "grand" scheme of things, that one alone is a pretty invested little project.