Hi Ian,

A simple passive RC lowpass filter on the A/D input should do the trick.
 Put it between whatever amplifier you use to amplify the current sense
signal and the A/D input.  This is roughly equivalent to your software
averaging approach.

Later,

Eric

>Hi all,
>
>An interesting observation arose for me just now whilst cutting the code on
>an f877 to do the A/D for current sensing from a device being controlled
>from a PWM voltage source.
>
>My PWM is at 5kHz.  I discovered that the current readings were not
>averages, because the A/D acquisition time is much faster that the PWM
>frequency, so the current sense readings were sometimes coming back as
>maximum, and sometimes as zero !  It is obviously catching either the low or
>high state of the PWM output.  Only occasionally were they coming back
>somewhere in between - obviously when the A/D acquisition happened to catch
>the PWM on a rising or falling edge.
>
>My simple solution was to take a number of readings and average them.  In
>this case, I took 20 readings, which seems now to give me a good "average"
>current output, which is what I was after.  Any comments on better ways to
>do this would be appreciated.
>
>Rgs
>Ian.
>
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