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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.