Modified the design to make stage three passive again and moved stage 5 (addition of offset) to stage 4. PFA the circuit schematic.
Also attached are LISO analysis of the circuit.
The first file is the transfer function of the feedback circuit alone with its input impedance.
The second file is the open loop transfer function of the feedback circuit with AOM and cavity, the input referred noise and the opamps' stability in closed loop operation.
P.S. The unstable region shown in figure one of the second file is calculated as the region where |KP + 1|<1 as per the phase of calculated KP, where KP is the open loop gain function.
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By making stage 3 active you may be adding a lot of OpAmp noise, since you're amplifying it by a factor of 100 at low frequencies. It sits behind the first two gain stages so it might not matter for the input referred noise? But I don't think it's good practice. What was wrong with having that stage passive? A gain stage like this, which doesn't provide gains higher than 1 and actually attenuates over a broad range is usually better kept passive because of amplifier noise.
Did you check the circuit for stability? The AD829 in stage 3 is driving a pretty low resistance as load. Not sure if LTSpice can identify such problems.
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