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Measuring Skin Response

Discussion in 'General Electronics Discussion' started by Johnny101, Nov 6, 2015.

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  1. Johnny101


    Jul 13, 2014
    Hi everyone!

    I am trying to measure the skin conductance, for that I found this circuit online.
    So from it what I have understood so far:

    - There is a low pass active filter at first stage with cut off of 1.6 Hz and variable gain.
    - The second stage is also a filter with unity gain and is biased at half the voltage supply and the first stage is biased at the output of second.

    What I don't understand:

    - Why is the gain variable at first stage as the Rskin chages the gain would change?
    - Isn't the first stage enough?
    - Can't understand the biasing of the op-amps as well.
  2. Johnny101


    Jul 13, 2014
  3. duke37


    Jan 9, 2011
    I do not understand the circuit in detail but, OA1 is not a variable gain amp, it is a virtual earth low pass amplifier with variable input current derived from the skin resistance. The output voltage is input current times 1M.

    OA2 takes a constant voltage and provides OA1 with an offset bias to keep it on track.

    Try simulating it with spice.
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