And in the end, you ended up where you started. I hope it was an entertaining detour at least. The best design is the one that does what you need. What you came up with is pretty clean. As far as parts availability- it will be a cold dark day after the Zombie apocalypse. Use any low voltage dual op-amp, there are thousands and they all share a pin out. I generically put an LM358 in SO-8 for my layout but the actual part can be anything. Well done- enjoyed it from my end.
Oh, no, actually I ended up in a much better place than I started, thanks to the help I got for all of you
sorry if I gave the wrong impression
I now have 2 working versions, one using the ADC and one using the comparator. I just posted the code sample for the timer/comparator one because there was no example and I used techniques that don't have many examples in the Nordic SDK. I also think it's a relatively elegant use of the nRF52 functions done purely in hardware, and hopefully it will help other people. I didn't post the ADC/DSP solution which incidentally is more portable to other processors, because capturing a signal with an ADC is a trivial piece of coding and the necessary DSP code was already posted here and working well.
I learned a ton, and have 2 working solutions from which to choose, depending on how the sensor will be used in future boards/devices. I couldn't be happier