Bob wrote: >10 PPM is stretching my measurement capabilities, The Easy way to get relative measurements down to the sub ppm level is go differential. To do that, consider building 3 or 4 of the basic voltage references into the same package. Then by comparing the voltage difference you be able to see sub PPM noise and drifts between outputs. Another good reason to start with something KISS simple and cheap. Need to watch out for thermos if you are going to use Heated parts. (Likely that is what is causing the turn over voltage change.) ws **************** Andreas's results imply that, at least at the 10 ppm level, that hysteresis may not be an issue. It is stretching my measurement capabilities, but I will try some experiments after I have some data on several potential Traveling Standards. Thanks,