And here it is, modelled using LTSpice. It is the same circuit as the classic simple precision rectifier (set to pass the negative half-sine), but with the non-inverting input of the op-amp connected ...
The charge pump's output feeds IC 1 's V– terminal, pulling it below 0V, thus permitting an output swing to and below ground. In Figure 2, the amplifier's V– pin (Trace B) initially rises at supply ...