> I tried a simple passive RC filter on the A/D input in simulation - this
> changes the square wave to a triangular wave (as I remember it should
> from analog electronics).

Each piece is actually an exponential.  The waveform approaches a triangle
wave when the RC time constant is large compared to the signal period.

> I guess this would in itself improve the
> averages because of the rising and falling slopes of the triangular
> wave being hit more times than the low (or high) state.  I only need to
> sample the current about once a second.
>
> What is your idea for using 2 resistors and two caps ?

Two RC filters in series.  Filtering 5KHz PWM down to .5Hz average is
easy.  With a 10,000:1 frequency ratio a single R-C will do very nicely.
However, the capacitor would need to be around 30uF with a single 10Kohm
resistor.  You could use R-C-R-C where R is 4.7Kohm and C is 100nF, then
sample every millisecond and low pass filter the rest of the way in
software.  This has the advantage of a "live" value always available in
software without the need to kick off an A/D reading when you need it.


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