I was trying to simulate ac-coupled instrumentation amplifier for my EEG amplifier, to get rid of dc offset from my signal. The inputs have 300mV of differential dc offset and 0.1 mV signal. So, I was expecting output as :
Vout = gain * 0.1mV
without the dc offset in output, as the integrator feedback acts as a high pass filter.

But when I set gain of 20, I get 20 mV of signal with 2.20 V of dc offset. How is this dc offset coming here?

Can someone describe what is wrong with this circuit? I want to have maximum gain for the signal as possible from the inst. amplifier so that noise from further circuit doesnt have much effect on signal.

 

Are you running what I think of as transient analysis in spice. That filter need some time to settle. Try to run the simulations for at least some seconds.

 

Try to run the simulations for at least some seconds.

I did, for 5 sec. But still same result. Here is my output for gain of 20.

 

300mV of offset is too much for the ref input to compensate for.

300mV times 20 is 6 ref volts, & the ref pin range is +Vs -1.6V & -Vs +1.6V.

Reduce the offset to 30mV and it will work. You could also increase the supply to ±10V or so.

Regards,
Ifixit

 

Reduce the offset to 30mV and it will work.

I reduced the offest to 30mV and set the gain to 10. But now I get amplification of only 2.
I checked changing the gain, but the actual gain is 5 times smaller now.

Thank you