# Independent voltage sources

Discussion in 'The Projects Forum' started by cagnarrogna, Jan 10, 2013.

1. ### cagnarrogna Thread Starter New Member

Jan 10, 2013
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Hi everybody! I've been a long time reader, but that's just my first time writing!

I'm doing a instrumental amplifier (ia) for a galvanic skin response (gsr) circuit. I found the schematics here http://www.chris3000.com/wp-content/uploads/2010/02/Gsr02.jpg
The issue is that I need to have a circuit which measures gsr from 4 different subject. According from the schematic, electrode 1 gets current from a battery voltage supply. Can I just use that same supply for the others 3 electrodes if I have the 1M pot and resistance for the 2d electrode for each instrument amplifier, or each value at the input of each ia would be influenced by the others? Is there some other solution for avoiding to have 4 different batteries?

thanks!

2. ### wayneh Expert

Sep 9, 2010
12,394
3,246
As I understand it, yes, the battery voltage and the power supply could be shared for all 4. The circuit you supplied is for each subject, and each person will be connected to an electrode 1 and 2, right?

3. ### cagnarrogna Thread Starter New Member

Jan 10, 2013
4
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Yes. So this will work as long as I have a different pot and resistor that go from electrode 2 to ground for each ia circuit right?

thanks!

4. ### DickCappels Moderator

Aug 21, 2008
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674
I might be mistaken, but this circuit is a "voltmeter" (diffamp) in series with a battery and some high resistances. If the diffamp has input resistances in the gigohms and the high value resistors are in the meagohms range, then the output will be the battery voltage and have little to do with the galvanic skin response.

A better arrangement is to apply voltage to the skin contacts through a large resistor, can the a pot, then look at the voltage across the skin sensors. That voltage will be a function of skin resistance.

5. ### cagnarrogna Thread Starter New Member

Jan 10, 2013
4
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Hi Dick, thanks for your reply. Do you mean to put a large value resistor in series with the battery before the electrode 1?

6. ### DickCappels Moderator

Aug 21, 2008
2,777
674
Yes, and then you can remove the two resistors in series with electrode 2 since the opamp input resistance is so much greater than the values of those resistors.

7. ### cagnarrogna Thread Starter New Member

Jan 10, 2013
4
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Thank you very much Dick, but I have another issue too, I tried the circuit connecting the two electrodes, but instead of getting 0 v, at the output there is a voltage of 0.6v, but measuring tension at first stages' outputs i get the same voltage of 1.96 v. That's not related with battery supply I suppose, since putting the resistor you were telling, apart from different values I keep having the same problem.

thanks!