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Opamp input voltage offset
An opamp should amplify the difference in voltage between the input
pins, so that Vout = Gain * (Vin+ - Vin-). Real devices a more accurate
description includes an offset voltage error, so that Vout = Gain *
(Vin+ - Vin- + Voffset). Any application concerned with DC
accuracy will need to factor the offset voltage in to the error budget.
The offset voltage is dependant on temperature, time and common
mode voltage. Some opamps include nulling terminals which allow
the offset voltage to be trimmed out, usually using a trim pot, but
this will only null the offset at one temperature and one common mode
voltage.
The values listed for the offset voltage are the typical values at 25
degrees C. This parameter is used in preference to either the
maximum offset voltage at 25 deg C or the maximum offset voltage over
temperature because it is quoted for nearly all devices. Not all
applications are subjected to wide temperature ranges.
The opamp will only meet its quoted offset voltage over a range of
common mode voltages. As the common mode voltage varies, there
will be an inevitable change in offset voltage, although precision
devices will have a high common mode rejection ration (CMRR).
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.