Electronic – how to measure the internal resistance of a voltmeter

multimetervoltage measurement

How to find out what is the internal resistance of a voltmeter? I imagine the higher the better – it'll have less influence on the measured circuit.

What is the approximate internal resistance of cheap DMM on the voltmeter setting? Do more expensive multimeters have better (higher) internal resistance? Is there a significant difference in internal resistance of the voltmeter between a Fluke and a 5$ DMM?

What is the best method to measure the internal resistance of a voltmeter?

Best Answer

Apply a known voltage over a series resistor. This resistor in combination with the internal resistance will form a voltage divider. Say you apply 5V over a 1M series resistor, and the DMM shows it as 2.5V, then the internal resistance is 1M.

Now that I reread it, I guess it's not completely unambiguous. By "applying a voltage over a series resistor" I meant you connect the + to the resistor and the - to the ref. input of the DMM.