EMI Filter Design, 3rd Edition

3Common Mode and Differential Mode: Definition, Cause, and Elimination

There is a wide-ranging set of opinions about the definition, cause, and elimination of both common-mode and differential-mode noise. This chapter attempts to cover most of these concepts.

3.1 Definition of Common and Differential Modes

Normal-mode noise is simply a voltage differential that appears briefly between the power line and its accompanying neutral or return line. The neutral line may not be power ground. As the name implies, these two lines represent the normal path of power through electric circuits, which gives any normal-mode transient a direct path into sensitive circuits and therefore the opportunity to degrade system performance. Normal mode is most often ...