Why is common-mode noise on a power line bad?
Why is common-mode noise on a power line bad?


Say you have 100 mV p-p at 100 kHz common mode voltage on the output of a power supply that feeds a target circuit. If that circuit was totally isolated from earth in all its nodes then there would be no problem.

However, just think about what a CM voltage is - it's the same voltage on both lines but with respect to earth. The device connected to the power supply might be quite sensitive and even though it may not be intentionally connected to earth, different nodes of that circuit will have capacitance to earth and if all nodes have exactly the same capacitance to earth then again, there will be no problem.

What are the chances that the target circuit has all its nodes isolated from earth and all its nodes have the same capacitive impedance to earth?