Frank
I was wondering why they(Fluke and I know in the EDC calibrators) they use these resistor strings as feedback resistors rather than using the reference to create a stable 10V reverence, then feed it through a voltage divider to either an output buffer/amplifier to get the desired voltage. This way you could easily account for voltage reference drift/ divide drift seperately.
That's an interesting argument.
Here are some easy answers:
1. It makes no difference if you chose one topology, or the other.. due to error chain calculus, the error sum stays the same.. as all errors add up, the lesser complicated topology (only one resistor chain or divider) is more stable.
2. KV dividers are much more complicated, i.e. due to switching and resistor manufacturing, compared to simple linear resistor chains (decade box type)
3. A topology with fixed reference, divided by KV and again amplified by a resistive divider would a) imply an additional source of error which would add up more than the simpler topology, and b) would maybe require a non-inverting amplifier. Latter one is worse compared to the non-inverting type, because you have common-mode errors of the OpAmp, and null trimming is not so easy.
The range and sample string resistors in the 33xD are very interestingly matched:
The range resistor consists of two resistors, with exactly inverse T.C.: They are stamped 749.25k .. P1.0 and N1.0 or 74.925k .. P2.0 and N1.5.
So they have a maximum T.C. of 0.5ppm/K.
Same goes for the sample string, in the first three decades.. The resistors are combined in groups of 1, 2 or 4 resistors in series or in parallel.
The single resistors have 0.5ppm/K only, but the groups of 2 or 4 are all matched to virtually zero T.C. (P1.0 + P1.0 + N1.0 + N1.0)
Therefore, the combined inverting amplifier has a maximum T.C. of 1ppm/K, or even less, if range and sample string resistors are also matched to less than 0.5ppm/K.
That's still valid as per today.. I've briefly checked that just recently
Therefore, these resistors, plus the switches, are still much more worth than those lousy 35$..

Frank