piclist 1998\10\01\200547a >
Thread: Possibility of using the PWM output as a proportional DC output??
picon face BY : Harold Hallikainen email (remove spam text)

On Thu, 1 Oct 1998 16:29:55 -0600 Craig Lee <cleeEraseMEspamATTCANADA.NET> writes:
>I am attempting to use the PWM output on a '73A through a
>capacitor to ground to get a proportional DC voltage out.
>The PWM frequency is 3090 Hz and the duty cycle is varied.
>However, I get too much ripple on the DC output.
>Any brilliant ideas?  Is my frequency too low? Too high? How
>do I select the proper capacitor? Do I need a diode?

       You need to add a resistor between the output pin and the
capacitor.  This forms a low pass filter with a cutoff frequency of
1/(2piRC) .  You can juggle cutoff frequency and PWM frequency to get the
ripple lower, generally at the expense of response time.  You can also go
to a higher order filter to attenuate the PWM while keeping the DC.
       I successfully did one project where the PIC PWM output drove a
resistor which then drove a 4051 demux.  Each demux output had a
capacitor.  I sent about 20 cycles of PWM to a capacitor, disabled the
mux, moved on to the next channel, set up the PWM, set up the demux, then
enabled the mux.
       In another project (the one I'm working on right now), I also had
a ripple problem that could not be solved without intolerable delays in
response.  I went to an external serial D/A.


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See also: www.piclist.com/techref/io/pwm/index.htm?key=pwm
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