This is also why open source wearables can't have the best PPG heart rate/SPO2 tracking.

There are only about 4 PPG analog front ends still in production that don't require an NDA and the Max86141 is the only one worth anything it seems.

You can't do analog front ends with any small FPGAs that I know of and it is simply to large to do it (well, at least) on a small wearable with discrete components.