[volt-nuts] HP 3458A DC current accuracy
vnuts at toneh.demon.co.uk
Tue Jul 10 22:09:24 UTC 2012
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!
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