Electronics > Metrology

ADR1001 - Ovenized Voltage Reference System

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iMo:
After I had spent a day with experimenting I wired the output buffer as you may see below.
I replaced the OPA189 with an ADA4523. On the first glance I do not see any difference between them, both behave the same way, output voltage the same, the noise needs longer observation (but I doubt I would see some difference in my noisy setup).
I also put a 100n foil in the feedback with 1k resistor in series.
The output voltage is the same as before, we will se how the new ADA4523 will do.

iMo:
The output buffer - after a series of experiments I settled with the V5 - see below. I have not seen a difference using the ADA4523 with compensation there thus I continue with the OPA189.

Below also the latest aging report.

The measurements are already challenging the capabilities of my 34401A pretty much. Without the TC compensation and having the DMM 24x7 on for at least 2-3 days after a week powered off the results will be much hairy, imho.

What I've learned in the last 10 months:

1. the TC of my meter is aprox +0.5ppm/C in 38-46C internal DMM temperature (I have the LM35 sensor glued over the ADC)

2. the humidity (?) after the meter is powered off affects the values +2..+3ppm and slowly settles down after 2-10 days after the meter is powered on

3. there is a random walk which is typically <1.5ppm peak-peak within an overnight measurement, the running STD (over last 100 samples, 100NPLC) shows typically 0.7-1.2uV (that includes temperature changes), in quiet periods 0.6-0.8uV

4. the ADR1001#1's voltage dropped down by some 2-2.5ppm in last 10 months, and its TC looks like -0.14ppm/C

5. the difference between a battery vs an ac_mains powering the Vref is not visible in my case (here the reference pcb sandwich is isolated by ~1.5cm thick foam inside the box, and its internal temperature measured by an LM35 typically rises up by 1.2C when powered off the transformer placed in the same plastic box with air vent slots)

6. I messed with the R1 values from 1k to 47k. I saw none difference in the measurement noisiness except a small drop of the output voltage, perhaps 1-1.5ppm with the 47k.

Also, it would be great if our better equipped contributors here would be kind enough to analyze in detail the performance of the AZ opamp buffer as depicted below, for the stability and noise (and will suggest improvements).. In the next Vref I would put an 1k into Q2 base, and not sure whether to filter out the AZ switching at the opamp's output..).

Andreas:
Hello,

I would add a small frequency compensation capacitor across output and negative Input of OP-Amp.

with best regards

Andreas

iMo:
Below is the LTspice Stability Analysis of the V5 output buffer without a comp cap and with a 1nF and 10nF comp cap.

It looks to me the best stability is without a comp cap..

Made with the ADA4523 as the OPA189 model does not work here properly..

Also attached is the .asc file for the simulation..

PS: set the Gmin=1e-16 in the sim settings..

iMo:
A hundred times faster sim version below..
The above slow math came from switching power supply stability analysis by ADI, afaik..

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