Thanks for the explanation on the way it works, agree that is not Constant Current as I would have thought where the voltage is modulated to keep the current constant within a max voltage level.
Wasn't thinking about LTZ1000 or LM399 performance levels, but if your seeing effects using this as a supply for the LTZ1000 that's something to consider, as well as if it's not good enough for just a quick DMM validation where it just needs to maintain a voltage at a fixed level and not drift or have too much noise. I recall some old lab power supplies we had while I was working that could be dialed to levels of 10uv out of 10 volts, were very low noise and stable, so was hoping for something along these performance levels, maybe at the mv level.
The specs don't say much about short or long term drift, or temperature performance, so don't have much to go on. The use of the mentioned 24 bit ADC and a good reference one would think should produce a stable output. 1mv out of 32V is only 15 bits resolution, well within the capability of the ADC.
Best,