AC-coupling of instrumentation amplifier for EEG

Discussion in 'The Projects Forum' started by sudil99, Mar 20, 2012.

  1. sudil99

    Thread Starter New Member

    Mar 9, 2012
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    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.
     
  2. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    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.
     
  3. sudil99

    Thread Starter New Member

    Mar 9, 2012
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    I did, for 5 sec. But still same result. Here is my output for gain of 20.
     
  4. ifixit

    Distinguished Member

    Nov 20, 2008
    638
    108
    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
     
    Last edited: Mar 20, 2012
  5. sudil99

    Thread Starter New Member

    Mar 9, 2012
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