I am a student trying to reverse engineer a rather interesting PCB, trying to learn from it but have come across some circuits that I am struggling to identify the roles of.
"ref" that I have circled inputs a reference signal, which at first I thought was the GND. The negative input pin is connected to other op amps, serving as a ground, and also to the Ref pin of an instrumentation amplifier. The role is pretty clear that it is providing some kind of a stable reference for all the other amps, but I don't know exactly what it is doing.
This is the second part of the circuit which takes in a signal filter from an instrumentation amp. OA3's positive input is connected to OA4's negative terminal. I am pretty sure that it is a rectifier of some sort, judging by the function of the board, but I have never seen one like this. The simulation results show that OA3 has no effect on the signal, the entire this just inverts the signal. Maybe I have my schematics wrong..?
This is the last part of the question. OA5 takes in the signal from OA2(shown above) but weirdly it halves the signal, then amplifies it by 2. The negative termainal is connected to the negative terminal of OA4 from Q1. Why on earth would this kind of a circuit be here? Impedance matching doesn't seem probable because it is between cascade of op amps.