How Does a differential Voltmeter Work?

Question

How Does a differential Voltmeter Work?

Answer 1

Any hand held voltmeter is a differential voltmeter since the measurement circuit is floating and can be connected to any two points to measure the voltage between them.

In instrumentation where a ground referenced output is required, a differential voltmeter (also called an instrumentation amplifier) consists of two high quality opamps connected as high input impedance followers followed by a differential op amp followed by an A-D converter.

Answer 2

Hiya,

How deep an answer do you want?  

Basically, it comprises of a precise voltage generator coupled to a very sensitive ammeter. It is connected to the circuit to be measured with the same polarity. With no voltage being generated there will be a current passed through the voltmeter which will be indicated by the ammeter. By slowly increasing the generated voltage of the generator the current flowing will gradually drop to zero.  This is the point at which the generated voltage is equal to the voltage to be measured.  Read the generated voltage off the dial, and you've measured your circuit's voltage.

So... what's the point?  Quite simply that it has a virtually infinite input impedance.

Regards,

Tony