A very cheap 3458A

[TiN]

Well, I agree on vacuum part, but since we mere mortals usually don't have vacuum gear hanging around, oven came to mind first...

I'd think +/-12V references on A/D are used to couple easy with input range (which is 1.2x overrange?). But I might be wrong.

[Kleinstein]

The relatively high reference voltage helps to have not that much contribution of the switching FETs. With a reference of about the maximum input range one could use the same size FET and thus get easy matching - though less important with the custom switch chip.  For me the question is more why they used only 12 V and not 16 V or so, the maximum they could get from the supply available.

Using only a +-7 V ref.  for the ADC would only eliminate one resistors pair (that's only effecting something like  40%), but there is still the neg ref. Resistor pair (for about half the reference) and the integrator resistors and the FETs R_on anyway.  so they need the direct measurement of the 7 V reference (during ACAL) anyway, and it does not really matter correcting the low drift of one more resistor pair.  In a fully working unit the FETs might be drifting more than the resistors, so smaller Fets might be preferred over one pair of resistors less.

Besides drift of the resistor inside U180, there might be also drift due to increasing leakage. Though a different type of ceramics and higher temperatures, I once had trouble with contamination at a ceramic surface causing increasing leakage. It's not that much needed to add 10 ppm to a 50 K resistor.