electrical engineering Electrical engineering is an engineering discipline concerned with the study, design, and application of equipment, devices, and systems that use electricity, electronics, and electromagnetism. It emerged as an identifiable occupation in the l ...

, a common-mode signal is the identical component of

voltage Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...

present at both input terminals of an electrical device. In

telecommunication Telecommunication, often used in its plural form or abbreviated as telecom, is the transmission of information over a distance using electronic means, typically through cables, radio waves, or other communication technologies. These means of ...

, the common-mode signal on a

transmission line In electrical engineering, a transmission line is a specialized cable or other structure designed to conduct electromagnetic waves in a contained manner. The term applies when the conductors are long enough that the wave nature of the transmis ...

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

electrical circuit An electrical network is an interconnection of electrical components (e.g., battery (electricity), batteries, resistors, inductors, capacitors, switches, transistors) or a model of such an interconnection, consisting of electrical elements (e. ...

s, desired signals are transferred by a differential voltage between two conductors. If the voltages on these conductors are and , the common-mode signal is the

average In colloquial, ordinary language, an average is a single number or value that best represents a set of data. The type of average taken as most typically representative of a list of numbers is the arithmetic mean the sum of the numbers divided by ...

of the voltages:

U_\text = \frac

When referenced to the local common or ground, a common-mode signal appears on both lines of a two-wire cable,

in phase In physics and mathematics, the phase (symbol φ or ϕ) of a wave or other periodic function F of some real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is expressed in such a s ...

and with equal amplitudes. Technically, a common-mode voltage is one-half the

vector sum In mathematics, physics, and engineering, a Euclidean vector or simply a vector (sometimes called a geometric vector or spatial vector) is a geometric object that has magnitude (or length) and direction. Euclidean vectors can be added and scal ...

of the voltages from each conductor of a

balanced circuit In electrical engineering, a balanced circuit is electronic circuitry for use with a balanced line, or the balanced line itself. Balanced lines are a common method of Signal transmission, transmitting many types of electrical Signal (electronics ...

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 Noise is sound, chiefly unwanted, unintentional, or harmful sound considered unpleasant, loud, or disruptive to mental or hearing faculties. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrat ...

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 amplifier A differential amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs. It is an analog circuit with two inputs V_\text^- and V_\text^+ and one outp ...

s 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 Instrumentation is a collective term for measuring instruments, used for indicating, measuring, and recording physical quantities. It is also a field of study about the art and science about making measurement instruments, involving the related ...

where signals are transmitted through DC bias. For

sensors A sensor is often defined as a device that receives and responds to a signal or stimulus. The stimulus is the quantity, property, or condition that is sensed and converted into electrical signal. In the broadest definition, a sensor is a devi ...

with very high

output impedance In electrical engineering, the output impedance of an electrical network is the measure of the opposition to current flow ( impedance), both static ( resistance) and dynamic ( reactance), into the load network being connected that is ''internal ...

that require very high

common-mode rejection ratio In electronics, the common mode rejection ratio (CMRR) of a differential amplifier (or other device) is a metric used to quantify the ability of the device to reject common-mode signals, i.e. those that appear simultaneously and in-phase on both ...

, a differential amplifier is combined with input buffers to form an

instrumentation amplifier An instrumentation amplifier (sometimes shorthanded as in-amp or InAmp) is a type of differential amplifier that has been outfitted with input buffer amplifiers, which eliminate the need for input impedance matching and thus make the amplifier ...

. * An inductor where a pair of signaling 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 In electrical engineering, a transformer is a passive component that transfers electrical energy from one electrical circuit to another circuit, or multiple Electrical network, circuits. A varying current in any coil of the transformer produces ...

, 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 Capacitive coupling (electronics), coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) node (circuits) , nodes, induced by the electric field. This coup ...

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 (e.g., RF noise from a computing circuit) may be blocked using a

ferrite bead A ferrite beadalso called a ferrite block, ferrite core, ferrite ring, EMI filter, or ferrite chokeis a type of choke (electronics), choke that suppresses high-frequency electronic noise in electronic circuits. Ferrite beads employ high-frequen ...

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. * Switch mode power supplies include common and differential mode filtering inductors to block the switching signal noise returning into mains wiring.Understanding And Controlling Common-Mode Emissions In High-Power Electronics

Common-mode rejection ratio In electronics, the common mode rejection ratio (CMRR) of a differential amplifier (or other device) is a metric used to quantify the ability of the device to reject common-mode signals, i.e. those that appear simultaneously and in-phase on both ...

is a measure of how well a circuit eliminates common-mode interference.

References

{{FS1037C Electrical parameters Telecommunications