Common-Mode Interference
Interference that appears on all conductors of a cable bundle in the same direction and phase, returning through the chassis or ground. Because it does not change the difference between two conductors it is invisible to differential measurement, but it causes faults by shifting the circuit's reference and creates emission problems.
Common-mode current flows in the same direction in both the outgoing and return conductors. As a result, the cable's magnetic fields do not cancel each other and the cable becomes an efficient emitting antenna. Its source is usually fast-switching power stages and brush arcing.
- Suppression: using a common-mode choke at the power input. The choke presents a high impedance to current flowing in the same direction while hardly affecting the normal operating current.
- Y capacitor: a small capacitor placed between each line and the chassis offers common-mode current a short return path close to the source.
- Ferrite ring: passing the entire cable bundle through the ring adds series impedance to the common-mode current.
The telltale sign is this: the noise appears on both conductors of the signal pair at the same time, and improving the chassis connection changes the situation. In systems driving brushed DC motors, the range of a nearby radio receiver drops unless common-mode suppression is in place.
Context where this term is used: Installation, Redundancy and Field Continuity