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Opamp common mode rejection ratio
One of the most important sources of error in an opamp is the offset error.
The values listed for the offset voltage are the typical values
at 25
degrees C at a fixed common mode voltage. As the common mode
voltage varies,
there
will be an inevitable change in offset voltage, which must be included
as part of the error budget in precision
applications. Precision devices will have a high common mode rejection
ration (CMRR), to minimise this contribution.
Typical applications that require a high CMRR are difference
amplifiers and instrumentation amplifiers. Many high speed applications
such as video amplifiers require only a modest CMRR.
The CMRR is specified by the device manufacturer over a limited input
common mode voltage range. This is actually one of the more relevant
ways of specifying the input voltage range. Some devices are specified
to meet a reduced CMRR over an extended input voltage range.
Some opamps contain two input sections, one of which handles
input signals near the positive supply whilst the other handles input
signals near the negative supply. In the crossover region, the offset
voltage may change quite rapidly. This means the CMRR is low in the crossover region and high elsewhere.
Some opamps include nulling terminals which
allow
the offset voltage to be trimmed out, usually using a trim pot, but
this will not improve the CMRR.