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AC coupling for bio-potential measurements

Jeffrey Asare
Prodigy 60 points

Hi there, 

I have been trying to analyse the difference between these two circuits. I have been able to work out, mathematically, the merits of the right-hand schematic but I am finding it difficult to work out the left-hand schematic. I run both schematics on TINA, using a 1V p-p sinusoidal waveform to the inputs and measuring the outputs. In both cases, I had similar results but there was a slight difference in the gain vs frequency plot but I can't explain. 

I need some help with the above. 

Regards, 

Jeffrey. 

  • 0
    TI__Mastermind 22825 points

    Hi Jeffrey,

    I think that the RHS schematic carries the input CM voltage before the AC coupling caps to the instrumentation amplifier OPA's but the one on the left biases these OPA's close to ground (or to a fixed reference voltage if you lift R2 from ground and instead tie it to a Vref). So, most likely the circuit on the right left (corrected 3/1/17) only works if you use dual supplies (e.g. +/-15V, etc.).

    The gain vs. frequency plot differences you mentioned could be due to the difference in "equivalent resistance" that the AC coupling caps see which would affect the high pass characteristics low frequency roll-off. The lower this resistance, the higher frequency this low frequency roll-off point would be.

    However, I've not tested these circuits myself and I'm only guesstimating this.

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Prodigy 60 points

    Hi Hooman, 

    Thanks very much for your reply. I'm very grateful. 

    I think I understand what you mean by the equivalent resistance being the slight difference between the two schematics. That makes a lot of sense. 

    Regards, 
    Jeffrey.