The idea of having a common ground is an idealization. The best common ground you have, when you look at two points sitting close to each other on a solid ground plane and no relevant current is flowing along the solid ground plane. Now assume that you have two separated circuits sitting in separate enclosures and being several meters apart from each other. Then, the two signal grounds of both devices can drastically differ.

Instead to hope that there's no common mode noise, the better idea is to accept it and to expect it to be present and finally to design your circuit so that it can easily withstand and handle common monde noise.

A major cause of common mode noise is ground loops. Try to avoid them. But sometimes, for saftey reasons, you cannot avoid them and then your circuit must be able to withstand the resulting common mode noise.


Kai