Hello Tony, With the normal DMM current shunt arrangement, the tradeoff is lower burden equals lower signal voltage. Basically, the DMM uses a shunt and measures voltage on its 100mV range.
But the DMM design is optimized for voltage, and the current range switching introduces additional relay contacts etc, so the 100mV signal is degraded by thermal voltages etc. The shunt, even an expensive good one, will get a bit warm, thus generating thermal voltages. My experience with Datron 1271 at 1 Amp or the Fluke 8508A at 10 Amps is that the thermal offset is the greatest error source. E.g. zero the range exactly, and then supply full scale current. The readings drift away for a long time. Remove the current, and the zero has now offset, by nearly exactly the same amount as the difference of full scale readings from cold to hot. After a while, the zero will come back to the original exact null. Thus, practically all the error is from thermally generated voltages. I bet that this effect would be hard to specify closely, so the manufacturer probably has no choice but to give a wide tolerance. If the shunt is not expensive/good enough, it might also have a poor temperature coefficient. You can see this if applying full scale makes the reading drift, but the zero point remains stable. Of course, a particular DMM could have both problems. Making the shunt value lower would decrease the burden voltage, but make the thermal offset voltages proportionately larger, and they are already a big enough problem. Amplification will not help with this. To measure low currents, at least up to 20 mA, you could use a feedback type ammeter. For example, Keithley Picoammeters and Electrometers have burden voltages of <20uV to <5mV depending on model and range. Regards, Laurence Motteram -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Tony Holt Sent: Wednesday, 11 July 2012 8:09 AM To: [email protected] Subject: Re: [volt-nuts] HP 3458A DC current accuracy 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 _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there. _______________________________________________ volt-nuts mailing list -- [email protected] To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts and follow the instructions there.
