Electronics > Metrology

Lowest drift, lowest noise voltage reference (ADR1000AHZ)

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MaxTesla:

--- Quote from: Dr. Frank on August 27, 2021, 05:54:21 pm ---Hello,
Thanks to ScoobyDoo and branadic, I got 5 samples and some PCBs.
Here's the mockup board, where I made the first tests.

The lower zener voltage compared to the LTZ1000 now allows this Reference Amplifier to be trimmed to zero T.C. for a given oven temperature, by individual selection of the collector resistor R2. Compare this to the +50ppm/K for the unregulated LTZ1000.

That trimming process is the same procedure as for the SZA263 and LTFLU in the different FLUKE instruments.

I measured Uref vs. temperature, which always gives a negative, flat parable. The zero T.C. point varies from 45 to 60°C for a fixed value of R2, and the T.C. varies from about +5 .. -5ppm/K over 24 ... 80°C only.

At a fixed divider ratio R4:R5 for around 52°C oven temperature for all five samples, the individual T.C.s are well below +/- 2ppm/K.
It follows that the attenuation factors for R3 and the most critical R4, R5 are at least 10 times greater than in the datasheet. These resistors now can have much relaxed stability specifications, only R1 and R2 should be PWW or BMF types.

I also measured the low frequency noise 0.1 .. 10Hz, like branadic and Castorp.
I use an LNA from Andreas, and always made "typical" measurements of 10sec long samples, so to have a direct comparison to my LTZ1000s, but also to the measurements done by Andreas.
100s measurement time or averaging give no essentially different results.
Other LNAs will give different absolute noise figures, so these results are not directly comparable .
 
The noise of the ADRs RefAmp evidently is about 50% of the LTZ1000, which several others also have confirmed.

ADR #1 would be the most quiet sample, but it suffers from  :popcorn: noise, or frequent dips, but with a very moderate amplitude of maybe 400nVpp.
The ADA4522 buffer contributes a lot to the overall noise, so I have to investigate if it's only this specific unit, or what can be done to improve this.

I will add the noise figures of these samples, when measured with my 3458A, maybe now I can see a pronounced difference.

As my 34465A showed much higher stability / lower noise when I replaced its LM399 reference by a LTZ1000 clone PCB, my idea is to replace the LTZ1000 in my 3458A with an ADR1000 (*).
Maybe its noise figures will also go down, best by a factor of 2.
Castorp found that in his DVM other components overwhelmed the noise budget, so let's see what happens.

I would like to remind that TiN some time ago made a similar experiment on one of his 3458A, by implementing 4 parallel LTZ references, which would also yield 50% noise of a single LTZ.
I'm not aware what his final result was.

Anyhow, I think this new device will require quite a big time slot during the Metrology Meeting 2021, and I'm excited to meet the volt-nuts community personally and we'll have some fruitful discussions and comparisons   ;)

Frank

(*) my 3458A already has a modified FW, with CAL? 2,5 = 6.50000V (7V reference lower limit).
Thanks to Poul-Henning Kamp for your advice how to correct the checksum.

--- End quote ---


Hello Frank,

in your post you mentioned that you modified your 3458a's firmware to accept the lower voltage value of the ADR.
Would you mind sharing the process? I like to try it as well...

BR,

Max

Dr. Frank:

--- Quote from: MaxTesla on December 06, 2021, 01:57:09 pm ---
--- Quote from: Dr. Frank on August 27, 2021, 05:54:21 pm ---...
 
The noise of the ADRs RefAmp evidently is about 50% of the LTZ1000, which several others also have confirmed.

...

As my 34465A showed much higher stability / lower noise when I replaced its LM399 reference by a LTZ1000 clone PCB, my idea is to replace the LTZ1000 in my 3458A with an ADR1000 (*).
Maybe its noise figures will also go down, best by a factor of 2.
....

I would like to remind that TiN some time ago made a similar experiment on one of his 3458A, by implementing 4 parallel LTZ references, which would also yield 50% noise of a single LTZ.
I'm not aware what his final result was.


Frank

(*) my 3458A already has a modified FW, with CAL? 2,5 = 6.50000V (7V reference lower limit).
Thanks to Poul-Henning Kamp for your advice how to correct the checksum.

--- End quote ---


Hello Frank,

in your post you mentioned that you modified your 3458a's firmware to accept the lower voltage value of the ADR.
Would you mind sharing the process? I like to try it as well...

BR,

Max

--- End quote ---

Hello Max,

It was not sufficient to only change the minimum limit for the CAL? 2,5 parameter, i.e. the minimum internal voltage reference, to 6.5000V, but it was also necessary to modify all those lower ACAL limits, which the lower reference voltage also affects.
This can be identified by comparing the auto-calibration constants of a 3458A with two different reference voltages, like a regular LTZ1000, and one with a different LTZ chip, or with an ADR1000 on board, and slightly faked calibration reference values.
TiN kindly helped me by testing my first version on one of his 3458As, and sending me dumps of all his calibration constants. With the ADR1000, he found out, that the ACAL procedure gave system errors, due to several  lower limit violations.

So the procedure is to search for the location of these additional, crucial constants CAL? XXX,5 inside the firmware (=> 13 EA), and then lower them by the approximate difference by the ADR1000 voltage, about 6.667V, and the lower limit of the LTZ1000, i.e. CAL? 2,5 = 7.000V, i.e. each new minimum value has be calculated to about -6% each.
The displayed minimum values CAL? XXX,5 are not represented directly in the firmware, instead they are a calculated product of several internal cal parameters, so that's a bit tricky to find them in the hex file.

For that you need an appropriate hex editor, which is also capable of calculating the DOUBLE numbers back and forth, by taking care for the correct endianess and correct byte-order inside the single or 6 fold EPROMs, so that you can now determine the new lower limits.
I've done that successfully on my 2nd firmware version, which I also run in my own 3458A.

TiN was again so kind to test it and he confirmed that now the basic calibration processes, CAL 0, CAL 10 and CAL 10000 are successful, as well as the ACAL ALL procedure.
Next step would be to optimize the ADR1000 reference board for lowest noise performance, maybe also using less noisy OpAmps inside the 3458A, and then check against another external ADR1000 reference, or another 4x averaged zener reference, if the 3458A then shows a lower noise performance as well.

I don't know, how much further hints or spoilers you want to receive, just let me know.

Frank   

MaxTesla:

--- Quote from: Dr. Frank on December 06, 2021, 05:05:26 pm ---
--- Quote from: MaxTesla on December 06, 2021, 01:57:09 pm ---
--- Quote from: Dr. Frank on August 27, 2021, 05:54:21 pm ---...
 
The noise of the ADRs RefAmp evidently is about 50% of the LTZ1000, which several others also have confirmed.

...

As my 34465A showed much higher stability / lower noise when I replaced its LM399 reference by a LTZ1000 clone PCB, my idea is to replace the LTZ1000 in my 3458A with an ADR1000 (*).
Maybe its noise figures will also go down, best by a factor of 2.
....

I would like to remind that TiN some time ago made a similar experiment on one of his 3458A, by implementing 4 parallel LTZ references, which would also yield 50% noise of a single LTZ.
I'm not aware what his final result was.


Frank

(*) my 3458A already has a modified FW, with CAL? 2,5 = 6.50000V (7V reference lower limit).
Thanks to Poul-Henning Kamp for your advice how to correct the checksum.

--- End quote ---


Hello Frank,

in your post you mentioned that you modified your 3458a's firmware to accept the lower voltage value of the ADR.
Would you mind sharing the process? I like to try it as well...

BR,

Max

--- End quote ---

Hello Max,

It was not sufficient to only change the minimum limit for the CAL? 2,5 parameter, i.e. the minimum internal voltage reference, to 6.5000V, but it was also necessary to modify all those lower ACAL limits, which the lower reference voltage also affects.
This can be identified by comparing the auto-calibration constants of a 3458A with two different reference voltages, like a regular LTZ1000, and one with a different LTZ chip, or with an ADR1000 on board, and slightly faked calibration reference values.
TiN kindly helped me by testing my first version on one of his 3458As, and sending me dumps of all his calibration constants. With the ADR1000, he found out, that the ACAL procedure gave system errors, due to several  lower limit violations.

So the procedure is to search for the location of these additional, crucial constants CAL? XXX,5 inside the firmware (=> 13 EA), and then lower them by the approximate difference by the ADR1000 voltage, about 6.667V, and the lower limit of the LTZ1000, i.e. CAL? 2,5 = 7.000V, i.e. each new minimum value has be calculated to about -6% each.
The displayed minimum values CAL? XXX,5 are not represented directly in the firmware, instead they are a calculated product of several internal cal parameters, so that's a bit tricky to find them in the hex file.

For that you need an appropriate hex editor, which is also capable of calculating the DOUBLE numbers back and forth, by taking care for the correct endianess and correct byte-order inside the single or 6 fold EPROMs, so that you can now determine the new lower limits.
I've done that successfully on my 2nd firmware version, which I also run in my own 3458A.

TiN was again so kind to test it and he confirmed that now the basic calibration processes, CAL 0, CAL 10 and CAL 10000 are successful, as well as the ACAL ALL procedure.
Next step would be to optimize the ADR1000 reference board for lowest noise performance, maybe also using less noisy OpAmps inside the 3458A, and then check against another external ADR1000 reference, or another 4x averaged zener reference, if the 3458A then shows a lower noise performance as well.

I don't know, how much further hints or spoilers you want to receive, just let me know.

Frank   

--- End quote ---

Hey Frank,

thanks for your detailed answer. This sounds like a lot of changes to the firmware (especially for my limit skillset regarding this kind of stuff). I will definitly have a look into it.
THB I was more on the look for a higher stability spec then lower noise, but for that I first need to finish some boards to test and select some ADRs.

For me it would seem a lot simpler to redesign the A9 board with a small (stable) amp which boosts the ADR output just enough to get in the range of working (i.e. 7 Volts if i remember it correctly).
Obviously noise is concerne in this regard, but this should be manageable (at least from my perspective today) with the right selcetion of ADR/Circuit and would be a "upgrade" path for others without touching any firmware.
To compare agiant I would use another ADR reference, as I am currently designing my take on a 10V reference.

Opions on this "plan" are obviously welcome :)

BR,

Max




Kleinstein:
Adding some 6% of gain the reference circuit would still require very stable resistors, not to increase the drift. Added noise would be the least problem.
It it is just about getting a long term stable reference the old type LTZ1000 reference may be the better choice, maybe with a reduced temperature.

So far the limited reports on the ADR1000 are showing very low noise, but also still some drift. There is still some hope to improve on the drift with burn in or annealing, but I would not absolutely count on this. The advantage of the ADR1000 is more that it can get away with lesser grade resistors and still get good stability. With really good resistors the LTZ1000 may still be more stable.

branadic:
Speaking of drift, I reached 3000 h but I'm not happy with the result. Nevertheless, I guess it is good advice to share such results anyway.

-branadic-

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