Multimeter current measurement: internal resistance or input impedance specification

Question

As the internal resistance (or input impedance) of a multimeter influences the circuit you are measuring, and therefore influences the accuracy of your measurement, it would make sense to me to if this internal resistance were mentioned somewhere in the multimeter spec sheet.

When multimeters used physical d'Arsonval movement "galvanometer" where a typical industry standard developed around 50 µA for full-scale deflection, that was all the user needed to know. Regardless of current range, 50µA would flow through the galvanometer and the remainder through the shunt resistors.

For example, when measuring DC current through a low resistance load, a couple of Ohms internal resistance can make a big difference, so it would be helpful to know at least its order of magnitude.

However, although some manufacturers (e.g. Fluke) specify the input impedance for voltage measurements, I have not been able to find similar specifications for the current measurement part (e.g. the ammeter internal resistance or shunt resistor values for different measurement ranges).

I know how to measure the ammeter resistance using a load in series and resistors (or decade box) in parallel with the multimeter, but I don't understand why manufacturers do not provide these values in their spec sheets.

Does anyone know? Am I missing something obvious? Am I looking at the wrong specification perhaps?

As electronic DMMs replaced d'Arsonval galvanometer, it took time for the industry to determine how to represent the equivalent effect. Eventually they settled on the term 'burden voltage' to convey equivalent information.

Answer 1

Burden Voltage

What is Burden Voltage? One thing we rarely look at is the effect we have on a circuit when we attempt to measure the current flowing through it. In this case, the act of inserting the dmm 'A' or 'mA' input shunt in series with a current loop can significantly reduce the actual current flowing in the circuit. If the source voltage in the loop is very high, or the loop is current regulated, then the effect is minimized, but when you are operating in a circuit using a pair of AA batteries in series as a power source, you have only 3 volts to play with, and the shunt of a dmm's mA input can present a significant circuit loading error if you fail to account for it's effect.

So how much effect does the current shunt present? There is a little used specification in the user manual that can help you figure that out – it's called Burden Voltage, and once you know how to use the information, you can make corrections to your measurements to compensate for the effect.

- Fluke 77IV user manual