Common-mode signal

In <a href="/facts/Electrical_engineering/YYXW1KLW">electrical engineering</a>, a common-mode signal is the identical component of <a href="/facts/Voltage/EN2po6N2">voltage</a> present at both input <a href="/facts/Terminal_(electronics)/CwbKGSuB">terminals</a> of an electrical device. In <a href="/facts/Telecommunication/vxoj1cxR">telecommunication</a>, the common-mode signal on a <a href="/facts/Transmission_line/cYlqaPo3">transmission line</a> is also known as longitudinal voltage.
Common-mode interference (CMI) is a type of common-mode signal. Common-mode interference is interference that appears on both signal leads, or coherent interference that affects two or more elements of a network.
In most <a href="/facts/Electrical_circuit/zt94kTW0">electrical circuits</a>, desired signals are transferred by a differential voltage between two <a href="/facts/Electrical_conductor/BSuyupLP">conductors</a>. If the voltages on these conductors are U1 and U2, the common-mode signal is the <a href="/facts/Arithmetic_mean/L3Pv333o">average</a> of the voltages:

 
 
 
 
 U
 
 cm
 
 
 =
 
 
 
 
 U
 
 1
 
 
 +
 
 U
 
 2
 
 
 
 2
 
 
 
 
 {\displaystyle U_{\text{cm}}={\frac {U_{1}+U_{2}}{2}}}

When referenced to the local common or <a href="/facts/Ground_(electricity)/F6TEMIVv">ground</a>, a common-mode signal appears on both lines of a two-wire cable, <a href="/facts/In_phase/cpGtuVll">in phase</a> and with equal amplitudes. Technically, a common-mode voltage is one-half the <a href="/facts/Vector_sum/bCLrdksy">vector sum</a> of the voltages from each conductor of a <a href="/facts/Balanced_circuit/sMCd065d">balanced circuit</a> to local ground or common. Such signals can arise from one or more of the following sources:

<ul><li>Radiated signals coupled equally to both lines,</li>
<li>An offset from signal common created in the driver circuit, or</li>
<li>A ground differential between the transmitting and receiving locations.</li></ul>
<a href="/facts/Noise_(electronics)/Izbv38Rr">Noise</a> 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 <a href="/facts/Electromagnetic_compatibility/vRBTnls5">electromagnetic compatibility</a>. Unless the intention is to transmit or receive radio signals, an electronic designer generally designs electronic circuits to minimise or eliminate common-mode effects.

Common-mode signal open-in-new

Common-mode signal