In most electrical circuits the signal is transferred by a differential voltage between two conductors. If the voltages on these conductors are U1 and U2, the common-mode signal is the half-sum of the voltages:
When referenced to the local common or ground, a common-mode signal appears on both lines of a two-wire cable, in phase and with equal amplitudes. Technically, a common-mode voltage is one-half the vector sum of the voltages from each conductor of a balanced circuit to local ground or common. Such signals can arise from one or more of the following sources:
- Radiated signals coupled equally to both lines,
- An offset from signal common created in the driver circuit, or
- A ground differential between the transmitting and receiving locations.
Noise induced into a cable, or transmitted from a cable, usually occurs in the common mode, as the same signal tends to be picked up by both conductors in a two-wire cable. Likewise, RF noise transmitted from a cable tends to emanate from both conductors. Elimination of common-mode signals on cables entering or leaving electronic equipment is important to ensure electromagnetic compatibility. Unless the intention is to transmit or receive radio signals, an electronic designer generally designs electronic circuits to minimise or eliminate common-mode effects.
Methods of eliminating common mode signals
- Differential amplifiers or receivers that respond only to voltage differences, e.g., those between the wires that constitute a pair. This method is particularly suited for instrumentation where signals are transmitted through DC bias.
- An inductor where a pair of signalling wires follow the same path through the inductor. E.g. in a bifilar winding configuration such as used in Ethernet magnetics. Useful for AC and DC signals, but will filter only higher frequency common mode signals.
- A transformer, which is useful for AC signals only, and will filter any form of common mode noise, but may be used in combination with a bifilar wound coil to eliminate capacitive coupling of higher frequency common mode signals across the transformer. Used in twisted pair Ethernet.
Common mode filtering may also be used to prevent egress of noise for electromagnetic compatibility purposes.
High frequency common mode signals, for example, RF noise from a computing circuit, may be blocked using a ferrite bead clamped to the outside of a cable. These are often observable on laptop computer power supplies near the jack socket, and good quality mouse or printer USB cables and HDMI cables.
Common-mode rejection ratio is a measure of how well a circuit eliminates common-mode interference.