Common-mode signal is the component of an analog signal

Analog signal

An analog or analogue signal is any continuous signal for which the time varying feature of the signal is a representation of some other time varying quantity, i.e., analogous to another time varying signal. It differs from a digital signal in terms of small fluctuations in the signal which are...

 which is present with one sign on all considered conclusions. In electronics

Electronics

Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

where the signal is transferred with differential voltage use, the common-mode signal is called a half-sum of voltages:


When referenced to the local common or ground, a common-mode signal appears on both lines of a 2-wire cable, in-phase and with equal amplitudes. Clearly, a common-mode signal cannot be present if one of the lines is connected to local common. 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.