Biopotential measurement circuit

Question

I have read the following paper (the link at the end), and I have not understand the schematic, which is included in the picture below. To be more specific, there is a feedback integrator, denoted by AMP2, which is said that:

1. to increase CMRR.
2. to be low pass filter.

The second point is clear, but the first point I have not understood well. Another thing: how does AMP1 work, if the negative feedback has a integrator circuit? I understand if it has a resistor which is given that it works as an amplifier, but how it will work, if it configured as "an amplifier with integrator circuit feedback"!

This is the link of the paper: http://www.biosemi.com/publications/artikel7.htm

Answer 1

Just think of AMP1 as a summing amplifier; its two inputs are the outputs of AMP0 and AMP2. If AMP2 is functioning as an inverting integrator or low-pass filter, this makes the output of AMP1 the difference between the original signal and the low-pass version, which leaves the high-pass signal.

Answer 2

This is a Two-op-amp instrumentation amplifier with driven leg (DRL) circuit added to it. As Dave says, the low frequency information is calculated by the AMP2 circuit and subtracted out.

I suspect that the caps in the feedback path of that amplifier as well as the DRL circuit will prove problematic in practice. Those are integrators, and integrators drift and accumulate errors. There is no negative feedback on those amps at low frequency. I would not expect this circuit to function robustly without adding resistors to those feedback paths. Further, you probably don't want the DRL to function this way. You want it broad band.

A last point is that I'm not sure that the DRL circuit is actually feeding back the common mode voltage, which is (V+ + V-)/2.