Electronics > Metrology
Lowest drift, lowest noise voltage reference (ADR1000AHZ)
MaxTesla:
--- Quote from: branadic on December 06, 2021, 07:40:45 pm ---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-
--- End quote ---
Wow nice work :)
What temperature setpoint do you use on your ADRs?
Dr. Frank:
--- Quote from: MaxTesla on December 06, 2021, 08:04:20 pm ---
--- Quote from: Kleinstein on December 06, 2021, 06:19:08 pm ---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.
--- End quote ---
Yes you might be right. But with the limited dataset I still plane on experimenting. I have treated myself with a set of VHP101 for a LTZ (70k,120,13k/1k) and the ADR (acc to the Datasheet). Plane is to put the LTZ in the mix with the ADR in with the same layout and test them side by side. This will take some time though as my ADR resistors are scheduled for march…goal is to compare drift and tempco. This will be done against my well aged 732a so may in a year from now I will have somewhat meaningful results, after that I will have a look at the A9 board project again I think.
--- End quote ---
Hello Max,
unfortunately, reading every speculation from him, always the complement seems to be correct. No own experience from his own experiments, as far as one can deduce.
A divider being a few % off from "1" is extremely stable.. simply calculate the total differential! A 6% amplification should attenuate all drifts of the implemented resistors by about these 6%. I.e. a resistors T.C. of 2ppm/K will be attenuated to 0.1ppm/K of amplification drift. 5ppm/yr. => 0.3ppm/yr.
Added noise by an OpAmp IS the most severe problem for the ADR1000, as I have demonstrated upwards in this thread, in conjunction with the ADA4522 @ x1 amplification!! Any additional buffer amp might spoil the superior noise performance of the ADR1000. Therefore the easy replacement of the FW is the best option.. if the 3458A circuit really benefits from the low noise ADR.
The long-term stability of the ADR1000 MIGHT initially be worse compared to the LTZ1000. Only branadic has presented so far a single drift result @10V.
My single sample experiment, measured directly at the raw ADR1000 output, w/o to 10V amplification, shows no remarkable drift so far after 500h. That's also no reliable result at all. Others in the background reported that the ADR needs a longer burn-in than the LTZ. So the speculation which chip is more stable, is at the moment completely baseless and speculative.
Spoiler alert: CERN obviously uses the ADR based circuit as an improved 10mA precision source!
Lower oven-temperature is always a good way to improve timely stability, see P.J. Spreadbury publication from 1990. Chose about 55 .. 60°C in any case. Don't use the ADRs datasheet values for best performance of the ADR. See hints in my post above.
The T.C. of the circuit for both chips have to be trimmed to near zero. There's a lot of potential on the 3458A's LTZ1000A board. Mine has about 0.2ppm/K, half of the overall temperature drift of my instrument.
Use that infamous 200k resistor for T.C. trimming. (or not at all).
The LTZ1000A inside my 3458A drifted -2ppm over about 9 years only, due to the reduced oven temperature, but mainly because the 3458A is running only sparsely. Again, see Spreadbury.
My ADR #3 runs at about 55°C, just at its determined zero TC point of the Reference Amplifier. Simple thing.
And no, the LTZ1000 does not necessarily need more stable resistors. The timely stability of the circuit is mainly determined by the chip itself, the T.C. drift can be trimmed to near zero. Exactly the same goes for the ADR. So, please, no Vodoo stuff here, again. The ADR has much better noise performance, definitely, and that's the main reason to go for it.
The 3458A is no voltage reference, by all means. Therefore, I might exchange its very stable LTZ1000A by the ADR1000 some day, using the very same PCB with different resistors, and adding Andreas capacitors, for higher noise immunity.
Frank
MaxTesla:
--- Quote from: Dr. Frank on December 06, 2021, 10:04:44 pm ---
--- Quote from: MaxTesla on December 06, 2021, 08:04:20 pm ---
--- Quote from: Kleinstein on December 06, 2021, 06:19:08 pm ---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.
--- End quote ---
Yes you might be right. But with the limited dataset I still plane on experimenting. I have treated myself with a set of VHP101 for a LTZ (70k,120,13k/1k) and the ADR (acc to the Datasheet). Plane is to put the LTZ in the mix with the ADR in with the same layout and test them side by side. This will take some time though as my ADR resistors are scheduled for march…goal is to compare drift and tempco. This will be done against my well aged 732a so may in a year from now I will have somewhat meaningful results, after that I will have a look at the A9 board project again I think.
--- End quote ---
Hello Max,
unfortunately, reading every speculation from him, always the complement seems to be correct. No own experience from his own experiments, as far as one can deduce.
A divider being a few % off from "1" is extremely stable.. simply calculate the total differential! A 6% amplification should attenuate all drifts of the implemented resistors by about these 6%. I.e. a resistors T.C. of 2ppm/K will be attenuated to 0.1ppm/K of amplification drift. 5ppm/yr. => 0.3ppm/yr.
Added noise by an OpAmp IS the most severe problem for the ADR1000, as I have demonstrated upwards in this thread, in conjunction with the ADA4522 @ x1 amplification!! Any additional buffer amp might spoil the superior noise performance of the ADR1000. Therefore the easy replacement of the FW is the best option.. if the 3458A circuit really benefits from the low noise ADR.
The long-term stability of the ADR1000 MIGHT initially be worse compared to the LTZ1000. Only branadic has presented so far a single drift result @10V.
My single sample experiment, measured directly at the raw ADR1000 output, w/o to 10V amplification, shows no remarkable drift so far after 500h. That's also no reliable result at all. Others in the background reported that the ADR needs a longer burn-in than the LTZ. So the speculation which chip is more stable, is at the moment completely baseless and speculative.
Spoiler alert: CERN obviously uses the ADR based circuit as an improved 10mA precision source!
Lower oven-temperature is always a good way to improve timely stability, see P.J. Spreadbury publication from 1990. Chose about 55 .. 60°C in any case. Don't use the ADRs datasheet values for best performance of the ADR. See hints in my post above.
The T.C. of the circuit for both chips have to be trimmed to near zero. There's a lot of potential on the 3458A's LTZ1000A board. Mine has about 0.2ppm/K, half of the overall temperature drift of my instrument.
Use that infamous 200k resistor for T.C. trimming. (or not at all).
The LTZ1000A inside my 3458A drifted -2ppm over about 9 years only, due to the reduced oven temperature, but mainly because the 3458A is running only sparsely. Again, see Spreadbury.
My ADR #3 runs at about 55°C, just at its determined zero TC point of the Reference Amplifier. Simple thing.
And no, the LTZ1000 does not necessarily need more stable resistors. The timely stability of the circuit is mainly determined by the chip itself, the T.C. drift can be trimmed to near zero. Exactly the same goes for the ADR. So, please, no Vodoo stuff here, again. The ADR has much better noise performance, definitely, and that's the main reason to go for it.
The 3458A is no voltage reference, by all means. Therefore, I might exchange its very stable LTZ1000A by the ADR1000 some day, using the very same PCB with different resistors, and adding Andreas capacitors, for higher noise immunity.
Frank
--- End quote ---
Hello Frank,
thanks a lot for the insight and the paper, didn't knew that one.
This is highly interesting, out of my guts i would have suspected stabiltiy to be more of an topic, but as the amplifer would be one rougly 1:1.06 you are right of course.
My idea was to implement the amp with a VHD200, so tempco/stability impact is very small.
Regarding the noise and your buffer setup: This suppries me a lot tbh, I mean the ADA4522 with the right kind of input lowpass should not contribute that much, with the assumption that the datasheet noise values are correct, but as you discribed your samples are much lower noise than the spec. Will be interresting to see what my samples do, I have enought of them to get a little bit more "meat at the bone" so to speak.
Thanks for the tipp with the temp setpoint, I don`t have the appropriate means of measuring it myself (yet). So revised plan for tempco/stability test is the following: 6 ADRs in total, 3 with 12K/1K divider (~60°C) and 3 with 11k6/1k (~53°C) setpoint to get a span, each mounted in a housing with the same board, no preconditioning and just accquiring data form the start. This should give me/us a somewhat good idea what is going on, as three is the statistical minimum. Other resitors are 61.9k and 95.3Ohms. Monitoring ambienten contiditions of course to see any kind of correlation during normal weather shifts, all done automaticly every day with my 1281 and 7168 scanner. One channel reserved for a short, one for the 732a. If all goes well we can see a difference in drift with setpoint temperature in two points.
Thats at least the plan so far :)
After the results comes the tuning...
BR,
Max
Dr. Frank:
--- Quote from: MaxTesla on December 07, 2021, 11:31:10 am ---Hello Frank,
thanks a lot for the insight and the paper, didn't knew that one.
This is highly interesting, out of my guts i would have suspected stability to be more of an topic, but as the amplifier would be one roughly 1:1.06 you are right of course.
My idea was to implement the amp with a VHD200, so tempco/stability impact is very small.
--- End quote ---
Well, a VHD200 is technical overkill, expensive, and has a long leadtime.
Use two appropriate cheaper single resistors from stock, WW or MBF ones, like 6k8 + 470 Ohm, or so.
--- Quote from: MaxTesla on December 07, 2021, 11:31:10 am ---Regarding the noise and your buffer setup: This surprises me a lot tbh, I mean the ADA4522 with the right kind of input lowpass should not contribute that much, with the assumption that the datasheet noise values are correct, but as you described your samples are much lower noise than the spec. Will be interesting to see what my samples do, I have enough of them to get a little bit more "meat at the bone" so to speak.
--- End quote ---
I was also surprised, that the MAX4522 (of course with input LPF) adds so much noise. The ADA4522 has typically 115nV pp noise only, which should not affect the ADR1000 noise performance.
This combined noise level is about the same as an LTZ1000 w/o buffer. Maybe I have to look for a better sample of the ADA4522.
It's not only my 5 samples, which have 50% noise of that of the LTZ1000. Other volt-nuts also reported the same.
Inside the 3458A, both LT1001 on A5 board, U160 and U165, which generate the +/- 12V reference for the A/D will definitely spoil the game.
These have 300..600nV pp noise, i.e. at the same level what I've measured for the ADR1000.
These might have to be replaced also.
--- Quote from: MaxTesla on December 07, 2021, 11:31:10 am ---Thanks for the tipp with the temp setpoint, I don`t have the appropriate means of measuring it myself (yet).
--- End quote ---
Probably you already HAVE the possibility to measure the oven setpoint by yourself. A faster bench DMM is sufficient, to catch the initial diode voltage @ R.T., with oven disabled, and then extrapolate by using -2.2m/K for the temperature dependency. A 34401A is sufficent for that, and your 1281 will definitely do the job.
You may first test, at which collector current of the Reference Amplifier you get zero T.C. for the (not ovenized) RefAmp at your intended oven temperature.
--- Quote from: MaxTesla on December 07, 2021, 11:31:10 am ---So revised plan for tempco/stability test is the following: 6 ADRs in total, 3 with 12K/1K divider (~60°C) and 3 with 11k6/1k (~53°C) setpoint to get a span, each mounted in a housing with the same board, no preconditioning and just acquiring data form the start. This should give me/us a somewhat good idea what is going on, as three is the statistical minimum. Other resistors are 61.9k and 95.3Ohms. Monitoring ambient conditions of course to see any kind of correlation during normal weather shifts, all done automatically every day with my 1281 and 7168 scanner. One channel reserved for a short, one for the 732a. If all goes well we can see a difference in drift with setpoint temperature in two points.
That's at least the plan so far :)
After the results comes the tuning...
BR,
Max
--- End quote ---
Setpoint of the oven can also be trimmed by changing the collector resistor of the temperature sensing transistor inside the LTZ/ADR circuit.
So you may always use a 12k/1k divider and select the collector resistor instead. The stability requirement on latter is relaxed, especially if you run your circuit exactly at the sweet spot, i.e. at zero T.C.
I do not recommend 11k6/1k, that may be too low a temperature.
Select the zener resistor for a bit more than 5mA @ oven temperature, and select both collector resistors as described above.
Your monitoring capabilities sound pretty good.
Frank
Kleinstein:
The resistors for the 6.6 to 7 V gain would still be the most critical one when it comes to drift. They enter with a 6% factor, or a bout a facto 17 attenuation. The resistors at the ADR have a larger attenuation, like 200 times and higher.
For the gain stage one may just get away with an LT5400-7 as a maybe better available resistor.
Az OPs can react to the input impedance and details in the supply decoupling. So there is a potenetial for them to show more than the datasheet noise. They are also a possible source for EMI so that other parts than give the extra voltage.
As a lower noise OP at the ADC I would consider the OPA202 as a possible candidate.
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