#039 EMI Suppression ~Common Mode Noise Suppression~

Continuing from #33, this section shows you a common mode filter to use for common mode noise suppression.

This filter is characterized by having high impedance in common mode, but low impedance in normal mode. As a result, a quality of an intended signal to be transferred (normal mode) is not deteriorated and only common mode noise can be suppressed. This mechanism is illustrated in the following figure.





If a sending line and a return line are winded in the same direction on a ferrite core, the impedance will be lower in normal mode because the magnetic field generated by each line will cancel each other out. On the other hand, in common mode, the impedance (inductance component) becomes higher because the magnetic fields are same direction and strengthen each other. Therefore, if a common mode noise is superimposed on a differential signal line, the noise can be suppressed by inserting a common mode filter.


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