The reason for multimeters to have a high input impedance

Question

My question applies not only to multimeters, but all devices which require a low or a high impedance at the input or output.

Based on my understanding, a device would require a high input impedance, so that the input signal does not get affected by it. In other words, if the circuit was like shown below, then at point A, the effective input signal would be like an output at the voltage divider, hence Rinput / (Rinput + R_source), but since R_input is large, this value will likely become close to unity.

^{simulate this circuit – Schematic created using CircuitLab}

Am I correct in understanding this? Also, is it the same reason multimeters have a high impedance? I.e., so the measurement is not affected and is correct?

Answer 1

It's not that multimeters have a high impedance - just voltmeters do.

When measuring voltage, you connect the voltmeter (a multimeter in voltmeter mode) to the nodes between which you want to measure: it is a parallel connection. In order to measure correctly, you need the impedance of the voltmeter to be as high as possible so that very little current goes through it, so that it doesn't disturb the circuit (the current that was going through the, for example, resistance that you are measuring voltage on will continue to be almost exactly the same, thus the voltage drop will as well).

When measuring current, on the other hand, you connect the ammeter in series. This time, to prevent the device from disturbing the measuring, its impedance should be as low as possible, so that the circuit keeps behaving the same way (no voltage drops in the ammeter and the current flowing in that branch of the circuit stays the same).

Answer 2

Yes you are right, a voltmeter have a high impedance to not disturb the system you are monitoring. If the impedance is low, the voltmeter will draw too much current from the circuit and then give you a false voltage value.

It's the exact opposite for amp measurement, the internal impedance has to be as low as possible, otherwise, the global current of the branch you are monitoring will increase.

I made a post few weeks ago about the impact measurement device on circuit if you want to some more lecture : Schrödinger and electrical measurement issue