You wrote...

>My problem is in understanding the function those components
>in the diagram have from the PWM output to the op-amp.  Specifically
>what the relationship is between the PWM coming out from the PIC
>and what is going on with the resistors and caps.

That network of resistors and capacitors functions as a low-pass
filter: it takes the high-frequency PWM signal and removes the AC
components (or most of them, anyway, except for a little bit of
ripple) from it, leaving only the DC component which represents
the average value of the PWM waveform.

Dave D.

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