In
electronics
The field of electronics is a branch of physics and electrical engineering that deals with the emission, behaviour and effects of electrons using electronic devices. Electronics uses active devices to control electron flow by amplification ...
, the common mode rejection ratio (CMRR) of a
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 outpu ...
(or other device) is a metric used to quantify the ability of the device to reject
common-mode signal Common-mode signal is the voltage common to both input terminals of an electrical device. In telecommunication, the common-mode signal on a transmission line is also known as longitudinal voltage.
In most electrical circuits the signal is transferr ...
s, i.e. those that appear simultaneously and
in-phase
In physics and mathematics, the phase of a periodic function F of some real number, real variable t (such as time) is an angle-like quantity representing the fraction of the cycle covered up to t. It is denoted \phi(t) and expressed in such a sca ...
on both inputs. An
ideal
Ideal may refer to:
Philosophy
* Ideal (ethics), values that one actively pursues as goals
* Platonic ideal, a philosophical idea of trueness of form, associated with Plato
Mathematics
* Ideal (ring theory), special subsets of a ring considere ...
differential amplifier would have infinite CMRR, however this is not achievable in practice. A high CMRR is required when a differential signal must be amplified in the presence of a possibly large common-mode input, such as strong
electromagnetic interference (EMI). An example is audio transmission over
balanced line in
sound reinforcement
A sound reinforcement system is the combination of microphones, signal processors, amplifiers, and loudspeakers in enclosures all controlled by a mixing console that makes live or pre-recorded sounds louder and may also distribute those sound ...
or
recording
A record, recording or records may refer to:
An item or collection of data Computing
* Record (computer science), a data structure
** Record, or row (database), a set of fields in a database related to one entity
** Boot sector or boot record, r ...
.
Theory
Ideally, a differential amplifier takes the voltages,
and
on its two inputs and produces an output voltage
, where
is the differential gain. However, the output of a real differential amplifier is better described as :
:
where
is the "common-mode gain", which is typically much smaller than the
differential gain
Differential gain is a kind of linearity distortion which affects the color saturation in TV broadcasting.
Composite color video signal
Composite color video signal (CCVS) consists of three terms:
*Luminance (monochrome) signal
*Auxiliary s ...
.
The CMRR is defined as the ratio of the
powers of the differential gain over the common-mode gain, measured in positive
decibels (thus using the
20 log rule):
:
As differential gain should exceed common-mode gain, this will be a positive number, and the higher the better.
The CMRR is a very important specification, as it indicates how much of the
common-mode signal Common-mode signal is the voltage common to both input terminals of an electrical device. In telecommunication, the common-mode signal on a transmission line is also known as longitudinal voltage.
In most electrical circuits the signal is transferr ...
will appear in your measurement. The value of the CMRR often depends on signal
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
as well, and must be specified as a
function
Function or functionality may refer to:
Computing
* Function key, a type of key on computer keyboards
* Function model, a structured representation of processes in a system
* Function object or functor or functionoid, a concept of object-oriente ...
thereof.
It is often important in reducing noise on transmission lines.{{Citation needed, date=October 2014 For example, when measuring the resistance of a
thermocouple
A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of th ...
in a noisy environment, the
noise
Noise is unwanted sound considered unpleasant, loud or disruptive to hearing. From a physics standpoint, there is no distinction between noise and desired sound, as both are vibrations through a medium, such as air or water. The difference aris ...
from the environment appears as an offset on both input leads, making it a common-mode
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to m ...
signal. The CMRR of the measurement instrument determines the
attenuation
In physics, attenuation (in some contexts, extinction) is the gradual loss of flux intensity through a medium. For instance, dark glasses attenuate sunlight, lead attenuates X-rays, and water and air attenuate both light and sound at variabl ...
applied to the offset or noise.
Amplifier Design
CMRR is an important feature of operational amplifiers, difference amplifiers and instrumentation amplifiers, and can be found in the datasheet. The CMRR often varies depending upon the frequency of the common-mode signal. CMRR is often much higher at higher gain settings. The key to achieving a high CMRR is usually the use of very precisely matched resistors (better than 0.1%) to minimise any difference in the amplification of the negative and positive sides of the signal. Single-chip instrumentation amplifiers typically have laser-trimmed resistors to achieve a CMRR in excess of 100 dB, sometimes even 130 dB.
See also
*
Balanced line
*
XLR connector
The XLR connector is a type of electrical connector primarily used in professional audio, video, and stage lighting equipment. XLR connectors are cylindical in design, and have three to seven connector pins, and are often employed for analog b ...
*
Tip ring sleeve
External links
Powerpoint presentation on audio connectors
Electrical parameters
Engineering ratios