[volt-nuts] HP 3458A DC current accuracy

[ed breya]

Yes, the effect can be estimated quite easily. Also, keep in mind 
that the 0.13 nV is the RMS noise, so the peak to peak excursions can 
be around six times that, or almost 1 nV p-p. If the FS is 1 mA, then 
it's about 1 ppm - one count on a six digit DVM, or ten times more 
with each additional digit. In reality, one ohm would be commensurate 
with a higher range like hundreds of mA, so the noise voltage would 
be much further down as a percentage of FS.

Also, it's hard to amplify up from a very small FS value up to the 
native range of high resolution DVMs without adding even more noise 
and thermal effects, so it makes sense to use more resistance - and 
undesirable burden - to get a decent FS signal that needs less 
amplification, but the resistor noise would be higher. It's part of 
the tradeoffs needed in design.

When comparing current range performance to that of the voltage 
ranges, you should compare to the voltage range that is closest to 
the FS burden in order to take into account the noise and error 
contribution of the required amplification. Also, the specs should 
include the effect of amplifier bias current, which introduces 
additional error on high sensitivity (low I-FS) ranges.

In the pursuit of ever-higher DC resolution, once you get down to 
where individual to tens of nV are significant to your signal, 
realistic resistance values contribute significant noise. If your 
signals are big, then it's insignificant. That's one of the reasons 
why the native range of the DVM is made as big as possible.

Ed