commonmode

Common-mode describes a signal or disturbance that is present identically on multiple conductors with respect to a common reference, usually ground. It is distinguished from differential mode, where signals appear as opposite values on the conductors. In a two-conductor system, a common-mode voltage is the average of the two conductor voltages relative to ground; a differential voltage is their difference.

Common-mode noise refers to interference that couples equally into both conductors. It can arise from EMI, power-line harmonics, ground loops, or radiated fields. Such noise may be rejected by differential front ends, shielded and twisted pair cabling, or proper grounding.

Devices and techniques: Differential amplifiers and instrumentation amplifiers are designed to react to the difference between inputs while rejecting common-mode signals. The ability to reject common-mode signals is quantified by the common-mode rejection ratio (CMRR), defined as Ad/Ac, the ratio of differential gain to common-mode gain, often expressed in decibels as 20 log10(|Ad/Ac|). A higher CMRR indicates better suppression of common-mode noise.

Mitigation methods include using balanced lines, proper shielding, isolation, and common-mode chokes. In power systems, common-mode currents can flow along shields or enclosures, producing EMI; measures include proper grounding and isolation barriers.

In practice, the term also appears in op-amp specifications as the allowable common-mode input range, which is the range of input voltages that the amplifier can handle with acceptable performance when both inputs move together.