I'd like to know how voltage reference stability is obtained in products like 6.5 digit multimeters that use LM399 as a voltage reference in spite of using two gain resistors. Let's assume an ideal LM399 and ideal op-amp and ideal current source (so ignore the 200k and 5k resistors.) It would seem to me that even at 5ppm, the output would be very sensitive to temperature (relative to a real LM399.) So then how can this work? Is it the case that the resistor temperatures do not fluctuate much because they are near the temperature controlled heater? Or are they measuring the temperature with something like an LM35 (and matched tempcos on the resistors) and calibrating in software?
Even at 5ppm/C and only 0.5C change in temperature (leaving out the trimpot), I'm calculating a range of 10.170 to 10.167 (gain ranging from 1.4634 to 1.4629) which would seem horrible for such an instrument. Do resistors effect gain in the way that I think they do?
I've looked at schematics for a couple such multimeters now and they all use at least two or three discrete resistors and usually aim for an stable output voltage around +/-10V or maybe +/-12V, so is the "Portable Calibrator" a reasonable approximation of a real application?