I just want to share and discuss an idea with you.
Lets assume we have:
- a calibrated adc, we use a calibrated LTC2400, means, compensated for linearity etc.
- a calibrated high resolution dac with >20bit, means, compensated for linearity etc.
- three references (ref1, ref2 and ref3) with good short-term stability, we use three off-the-shelf LT1236* in LS8 for the first assumption, because they output 5V nominal
- an opto isolated µC that controls the adc, dac and the required switches
- in front of the adc input is a 2:1 divider (e.g. LTC1043) to measure at half of the full scale (2.5V nominal)
Ref1 is connected to the dac that outputs let's say for the moment 1V, that is measured against a Josephson standard and adjusted to the exact value of 1.000000V. The correction factor is saved (initial calibration and adjustment).
We now connect ref1 directly to the ref input of the adc and measure ref2, afterwards ref3 and save the difference ref2 - ref3.
We connect ref2 directly to the ref input of the adc and we measure ref1, afterwards ref3 and save the difference ref1 - ref3.
We connect ref3 directly to the ref input of the adc and measure ref1 and afterwards ref2 and save the difference ref1 - ref2.
Now the drift value for each reference is calulated and saved and the ouput of the dac with ref1 connected to it is corrected to the new value for 1.000000V based on the drift info.
We expect, all three references are uncorrelated, so their noise and drift is different. After the initial calibration the system is monitoring itself. Any difference in trace length is static and therefore systematic, means compensated when calculating the three differences. If one of the three references dies, you will notice by an extrem change in the calculated difference, it needs to be replaced and the system to be calibrated and adjusted.
Beside the additional power for driving ref2 and ref3, is this arrangement able to output a very, a high (not to say ultra) stabile output voltage of 1V? What are the traps I've overlooked in this is experiment?
Remember the magical number 3, suggesting ultra high stability (three legs are more stable than two or four --> see tripod, you need three sheets of granite grinding against each other to get high precision surface...). I'm curios about your opinion.
* The reference can be replaced by any other reference of you choise, I used it because it outputs 5V nominal.