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

In this article, I’ll talk about differential circuits and common mode noise.

USB communication and LVDS (MIPI, etc.), which is a serial interface for communication between the CPU and LCD driver, are composed of differential circuits.



The differential circuit has two input terminals, one with a positive phase signal and the other with a negative phase signal.

There is no common mode noise between the two signal lines because they are in reverse phase with each other.

Since it is a premise that the impedances of the signal lines on the positive and negative phase are in balance, if there is a difference in line impedance (unbalance), there is a difference in the rise and fall of the two differential input signals, there is a difference in signal amplitude, there is a difference in signal duty, etc., common mode noise will be generated because the magnetic field cannot cancel out when the current balance that should be in an opposite phase relationship is disturbed.


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