Russell McMahon wrote:
> I wish to measure human pulse rate in an exercise environment.
>
> A
> major factor is the wide variation in optical transmission through
> the ear lobe, time to time, and user to user, and in session.

I've never done this, but I always thought this was done by measuring the
ratio of absorption at two or more wavelengths.  In theory that cancells out
the DC bias.  In practise it should at least make it better.

> The received signal must be reasonably aggressively low pass filtered
> to remove 50 (or 60) Hz hum artefacts. The low frequency of the
> desired signal leads to large capacitor and resistor values (around
> the 1 uF and 1 to 10 Mohm range) which makes DC operating point
> susceptible to capacitor leakage (so electrolytics no good) and
> offset bias currents.

This sounds like a good case for less electronics and more firmware.  Filter
just enough in analog to eliminate aliasing, then do the rest in firmware.
No capacitor leakage problem there ;-)


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