Measuring Skin Response

[Johnny101]

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.

 

[Johnny101]

Here is the link to the image if you can't see it
http://obrazki.elektroda.pl/1838370700_1446817605.png

 

[duke37]

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.