#033 EMI Suppression ~Mechanism of common mode noise generation (2) Sequel~

Continuing from the previous article, I’ll talk about differential circuits.

A square wave is input to a positive phase signal terminal of a differential circuit. In addition, a 180-degree phase reversed signal is input to a negative phase signal terminal, and since the two input signals have an inverse phase relationship, no common mode current is generated.



What happens if a signal that rises 10ns earlier is inputted to the negative phase signal terminal?

A common-mode current (noise) is generated as a 10ns difference between the two signal lines on the input side.



Next, you can calculate the radiated electric field using the following model.

Both the normal mode current and the common mode current are assumed to be 2 mA, and the radiated electric field of the normal mode and common mode is shown at a distance of 3 m from the input line of the differential circuit (30 cm in length).



It can be seen that the electric field strength due to the common mode current is much larger than that of the normal mode. When EMI test results exceed a standard criterion, EMI suppression measures must be taken.

In such a case, optimization of an input signal line impedance or a use of EMI filters with common mode effect can be considered as a solution.


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