> The problem is I can't see the unamplified
> signal alone as it's in the uA range.

Get the professional gear to see it. If you want to make billions
people happy, you have to get one.

> however there's absolutely no apparent
> modulation due to blood flow. =A0It's really
> frustrating because I don't have a commercial
> pre-amplifier to amplify the output of the first stage.

See above.

> Conversely using a sin wave through the IR led
> essentially as a signal generator has verified
> the gain stages and filter, so I'm not sure as to
> the root of the problem.

Start with the root, the rest makes sense after you solve it.

-- =

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