I was looking at the AD620 instrumentation amplifier data sheet. I'm looking at the ECG practical application on there (circuit shown below). Pins 1 and 8 of the instrumentation amp go through a resistor circuit that allows us to use the common mode signal. The voltage at the node between R2 and R3 are fed into some sort of phase shifter/amplifier. This phase shifter is supposed to shift the common mode signal out of phase so it can be used as a feedback into the body.
My question is: How does this phase shifter work? What does each resister do, and what is the point of the capacitor? I tried simulating this in pspice, but changing the value of the capacitor didn't seem to affect the phase shift at all.
So in short, how does that phase shifter/amplifier work? What components control what exactly?
Thanks in advance!