First off, let’s find out how that last single op-amp circuit works in practice. The chip used is an LMC6484 quad CMOS op-amp with extremely high input impedance. Since it’s also rail-to-rail on both inputs and outputs, the Vs+ positive supply can be 5V and the negative Vs- supply can be -5V, as generated by the TC7660S negative voltage generator (it’s a small DIP-8 package which only needs two 10 µF capacitors to work).
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