[volt-nuts] HP 3458A DC current accuracy

[ed breya]

That which is more fundamental to the problem is the unavoidable (at 
room temperature) noise from the resistors. Even a "perfect" resistor 
with zero tempco has noise, so if you use resistors to measure 
current with a high-precision voltmeter, eventually you reach a 
resolution where the noise becomes dominant. If you reduce the 
bandwidth by averaging, the precision should reach the basic 
stability of the resistor - but the (in)stability itself may be 
viewed as noise of very low frequency - always too low to filter out 
unless you have a very long time.

The resistors chosen for DMM current ranges may just be rational 
choices and compromises for the types of performance to be expected 
under normal usage and conditions, and considering the noise limits 
to resolution, versus the cost of extremely low tempco sample resistors.

Ed