Op amp offset measurement/digital compensation

[daqq]

Hi guys,

Assuming you have a simple non inverting amplifier and a source with a large enough impedance (10kOhm+) and low voltage output (<1mV, such as a infra thermocouple) that doesn't mind short circuits, could you measure the offset voltage by shorting the source periodically with a MOSFET transistor? In theory you'd only get the offset voltage amplified on the output, which you could measure with your ADC and then simply subtract it from the normal measurement. If you did this often enough, you'd compensate against both the actual offset voltage and its drift.

In theory and simulation it works, how about in practice? Would this be a bad idea?

Thanks,

David

[Harvs]

This is sort of commonly done in DMMs.  Often at some point in the analog signal chain, an analog mux will switch from the signal source to GND, then the perform a conversion cycle and subtract it from the result.

For auto-zeroing bench meters like the HP 3456A, this is done every conversion cycle by default, or you can switch it off and double your sample rate.

As to whether you should short your sensor out, about the only issue I can think of is the leakage current through the MOSFET.  You'd need to check the specs of the FET's off-state leakage and see how much error this would introduce to your measurement. Be aware this is unlikely to be linear.