What is the common mode voltage on the differential probe?

What is common mode voltage?

Imagine that in a differential measurement, you have two test points, which we call the positive side and the negative side.

Differential signal: This is the useful signal you really want to measure. It is the difference between the positive terminal voltage and the negative terminal voltage.

Common mode signal: This is the same (or very close) voltage that appears on the positive and negative terminals at the same time. It is a "common" pattern, usually you don't want noise or DC bias. Its value is the average of the two test point voltages.

A vivid analogy: Think of a differential measurement as two people talking in a noisy room (common mode noise).

Differential signal = what one person is talking to another person (useful information).

Common mode signal = background noise in the room (air conditioning sound, other people's voices), these noises enter the ears of two people at the same time, interfere with their hearing each other.

The role of the differential probe is to suppress this huge background noise (common mode voltage), and only amplify and extract the content of the dialogue between them (differential signal).

Why is common mode voltage so important in differential measurements?

There is universality: in power electronics (such as motor drive, inverter, switching power supply), communication bus (such as RS485, CAN) and other scenarios, the signal often "floats" on a very high DC voltage or high frequency noise. This "base" is the common mode voltage.

Safety risk: High common-mode voltages may far exceed the withstand voltage limit between the oscilloscope channel and the earth (usually only a few tens of volts). If the wrong use of ordinary passive probe direct measurement, will immediately damage the oscilloscope, probe, and even endanger personal safety.

Measurement accuracy: Even if there is no safety problem, high common-mode voltage will cause measurement error, distort the waveform, and cannot see the real differential signal.