AppNotes - Understanding EMC Stands and Specifications Page 8 (I



Interference Types
To understand the problems associated with conducted EMI it is the first necessary to understand the two modes of conducted noise: differential mode (or symmetrical mode) and common mode (or asymmetrical mode).
Differential mode interference crates a voltage between the phases of the system and is independent of earth; the differential mode currents flow along one phase and returns along another phase (Fig.10).
Common mode noise creates a voltage between each phase and the earth. The Common mode currents flow from the noise source to the earth (usually via parasitic capacitance) along the earth path and returns along the phases (fig.11).
A power line filter must be designed to attenuate both common and differential mode interference.