Input offset voltage/bias current noise contribution in operational amplifiers
In noise analysis of operational amplifiers, the following is considered:
- Noise generated by the external components (resistors in feedback network etc.)
- Noise generated by the operational amplifier itself
Manufacturers provide "input-referred" voltage and current noise specifications for most operational amplifiers. This figure includes shot, thermal, and flicker noise (Texas Instruments). Op-amps also have an input offset voltage and input bias current specifications.
Does the input offset/bias current contribute to the noise of the operational amplifier? If yes, is it included in the input-referred noise figures or does it have to be accounted for separately?
1 answer
While input offset voltage is technically noise in the sense that it is something unwanted added to the signal, it is not included in opamp noise specs as far as I have seen.
Think of the "noise" spec as the AC part of the noise, with the offset voltage and current telling you about the DC part of the noise. Note that "noise" is usually given as a function of the overall bandwidth used, like nanovolts per sqrt(Hz). Since this indicates 0 noise at 0 Hz (DC), it makes it clear that DC errors are specified separately.
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