Re: electrical problems with Galvanic Skin Response circuit ( GSR SCL )

Hi Steffen

Here are a few things to consider:

1. A high skin resistance gives a low output voltage, a low skin resistance
value gives a high output voltage.

2. The above point can be proven by inspection - if skin resistance is
infinitely high the opamp acts as a simple unity gain amplifier and the
same 0.5V Vref will be applied to the + and - inputs of the INA, the
differential voltage is thus zero and the INA's output is zero.

3. If the skin resistance is 0 ohms, the opamp's gain will be infinitely
high and it will saturate at it's Vcc.  The gain of the INA should be
adjusted to then give a FSD value compatible with your ADC's maximum input
voltage (also FSD).

4. High skin resistance will produce low(ish) opamp outputs, but for the
circuit to behave properly at low skin resistances, it is important to use
an opamp that has a rail-to-rail output.

5.  There are many instrumentation amplifiers on the market.  But the AD623
is very unique.  It is pritty much the only one that has a rail-to-rail
output swing, which is crucial for this circuit.  It is very odd that TI,
and other semiconductor manufacturers don't have a similar part one could
use as a drop-in replacement.  The AD623 is cheap but much more noisy than
most INAs.  But noise here is totally irrelevant, you are not building an
EEG amplifier here.

6. Do not randomly guess resistance values.  Choose a combination of
resistors, calculate the expected voltages at the opamp and instrumentation
amplifier's outputs and then verify with a multimeter or scope if the
measure values correspond with the theoretical values.  First ensure that
your opamp works as expected and then check correct operation of your INA.

7. To do point (6) consider the opamp as just a simple non-inverting
amplifier.  The gain is set by  R24 and SCL.  The "input" voltage is simply
Vref, which I have fixed in my design at 500mV.

8. The INA is simply a difference amplifier.  You take the output voltage
of the opamp, subtract Vref (0.5V) and multiply by the INAs gain.

9. Although the skin resistance always tends to be high, the circuit was
designed to measure down to nearly zero.  This is also what the lightstone
does.  If you don't like that operation, you would need to shift the INA's
DC offset by applying a low impedance source, DC offset voltage to pin 5
(Vref of the INA).

Regards
Stefan

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