Can't interprete this "adaptation circuit"

[MrAl]

I really appreciated what you two did here. Despite confirming what I lately assumed about this circuit's purpose, both of you gave me hints about the GSR_IN variable.
THANK YOU!

Hello again,

Oh so the goal is to measure skin resistance. That makes this a different problem because the thing to be measured is not voltage or current but is an actual resistance. In this case i have to propose the following....

First, with the measurement actually being a resistance at the input, a current source could be used to measure the resistance as is done in a regular everyday Ohm meter. Ohm meters set a constant current on the output such as 1ma and then measure the voltage, then calculate the resistance. If it is out of range, they decrease the current to a lower level and try again. So there could be several constant current sources in play.

The output response then becomes:
Vout=-(I1*R1*R2-j*w*C1*E1*R1*R2-E1*R2-E1*R1)/((j*w*C1*R1+1)*R2)

where
R1 is 100k,
C1 is 100nf,
R2 is the skin resistance at the time of the measurement,
I1 is a constant current source chosen to have a reasonable value based on the expected resistances,
E1 is 3v,
w=2*pi*f,
f is frequency in Hertz.

The output amplitude is then:
Vout(w)=sqrt((I1*R1*R2-E1*R2-E1*R1)^2+w^2*C1^2*E1^2*R1^2*R2^2)/(sqrt(w^2*C1^2*R1^2+1)*R2)

Now with the values set except for R2 and with the test current I1=1e-6 we get the DC output component:
Vout=(2.9*R2+3.0e5)/R2

Solving this for R2 we get:
R2=3000000/(10*v1-29)

In full it looks like this:
R2=300000/(100000*I1+v1-3)

and because of the negative sign in the denominator we have to choose the value of I1 appropriately or else we may have to take the absolute value of R2.

So here we can see that measuring resistance is different than measuring voltage or current.

The animal skin resistance R2 is found using the formula for R2, and the AC response is found using the formula for Vout(w). The cutoff frequency will vary based on the choice of I1 and what R2 is at the time although R2 may be more limited in range.

 

[Ratchit]

Hello everyone, I'm working on my thesis and came to a point where I can't interprete a part of a circuit I'm using.
This should be an adaptation circuit, so I am guessing it's filtering my signal in some kind of way, but cannot say with certainty which kind of filter should it be.
View attachment 108804
I could say that the most right part is a low pass filter, but then, if I try to think through frequencies, I can't understand nothing more.
Could PLEASE any of you help me?
Thank you, Danke, Grazie <3View attachment 108804

What you have there is a unity gain noninverting voltage amplifier. No significant current will exist in either the positive or negative input leads of the op amp, so it will not be a current amplifier or a transrestance amplifier. The resistor and capacitor in the feedback loop will not filter out anything because there is no connection to the source input. The break frequency of the RC time constant in the feedback path is 16 hertz. Below 16 hertz, the cap can be disregarded and the feedback voltage will be from the resistor. Above 16 hertz, the feedback voltage will be from the capacitor. This means the output voltage will be out phase with the input by 90° at frequencies much above 16 hertz. I don't know what the phase shift will do for you, but the voltage amplifier gain will be unity throughout the bandwidth of the op amp.

Ratch