[volt-nuts] HP 3458A DC current accuracy

[Tony Holt]

Frank,

Thanks for taking to trouble to respond. Its interesting that the Datron 
1281 has exactly the same issue - best 24hr uncertainty:

  DC V:      .5ppm + .3,
  Resistance: 1    + .3,
  DC A:       10   +  2

So its not a HP specific design trade-off. Perhaps there's something 
more fundamental such as the difficulty arranging the self-calibrating 
circuitry to include the shunt resistors. Perhaps your suggestion that 
current measurements are seen to be the poor relations to voltage and 
resistance has some merit, but I find it hard to believe the designers 
of these high-end instruments would compromise the current measurement 
accuracy unless it was very hard and/or expensive to avoid it.

Having said that, the voltage burden when measuring current is extremely 
poor for almost any multimeter you care to look at, making them useless 
for current measurements in many low voltage situations. Eg. measuring 
the short-circuit current of a .55V solar cell.

I've never understood why relatively expensive and sophisticated 
instruments don't have significantly lower resistance shunts in 
conjunction with appropriate amplification (at least as an option). The 
resulting loss of accuracy would be more than compensated by the reduced 
impact of the shunt resistor on the circuit under test.

I can't count the number of times I've had to use a 10 or 20A range to 
measure a few tens or hundreds of milliamps to prevent the shunt 
resistor badly affecting the measurement or even stopping the circuit 
working altogether. If you've only got a 3 1/2 digit meter you're not 
left with much resolution!

Tony