Turning the output of an opamp into a square wave
I'd use a MAX999 comparator. Feed the signal into one pin via a series capacitor and biasing resistor to mid-rail then on the other pin have a resistor feeding a capacitor so that the average level can be followed - it's called a data slicer: -



With the waveform in the OP's picture looking like it could undulate its average level a fair bit, using a data slicer probably makes sense.

The MAX999 has a 4ns propogation delay and can work from 5V or 3V3 rails. It would directly interface with logic as I have done several times on previous occasions.