1.2.6 Input Offset Voltage
The ideal op-amp produces zero volts out for zero volts in. In a practical op-amp,
however, a small dc voltage. VOUT(error), appears at the output when no differential
input voltage is applied. Its primary cause is a slight mismatch of the base-emitter
voltages of the differential amplifier input stage of an op-amp.
As specified on an op-amp data sheet, the input offset voltage, Vos, is the differential
de-voltage required between the inputs to force the output to zero volts. Typical
values of input offset voltage are in the range of 2 mV or less. In the ideal case,
it is 0 V. The input offset voltage drift is a parameter related to V os that specifies
how much change occurs in the input offset voltage for each degree change in
temperature. Typical values range anywhere from about 5 μV per degree centigrade to
about 50 μV per degree centigrade. Usually, an op-amp with a higher nominal value
of input offset voltage exhibits a higher drift.