AVR-Chat

At 12:46 PM 1/7/06 -0800, James Washer wrote:
>Finally, I would assume one needs to worry about the "ratio" of R to 
>C.  If one chooses a LOW resistance and a large capacitance, we might 
>still have a reasonable RC compared to the PWM frequency, however, we'll 
>also have large currents on the AVR pin. Is this correct?
>
>Further is one chooses a high resistance with small capacitance, the 
>circuit becomes more sensitive to it's output load.

Good summary overall.  You can eliminate the load sensitivity by following 
the RC with an op-amp.  You can also use more complex filters (both active 
and passive).

There are additional sensitivities in the filter at the extremes.  At too 
low a resistance the source impedance of the output pin will dominate and 
the R will be effectively irrelevant.  At very high R values you become 
sensitive to noise pickup.

Also there is a limit to how far it makes sense to filter the output.  If 
you only have an 8 bit PWM it 'probably' doesn't make sense to try to 
reduce the ripple to the 12'th bit

Robert

" 'Freedom' has no meaning of itself.  There are always restrictions,   be 
they legal, genetic, or physical.  If you don't believe me, try to chew a 
radio signal. "  -- Kelvin Throop, III
http://www.aeolusdevelopment.com/

Good point on the 8bit/12bit... thanks

 - jim

On Sat, 07 Jan 2006 18:02:19 -0500
Robert Adsett <subscriptions@aeolusdevelopment.com> wrote:

> At 12:46 PM 1/7/06 -0800, James Washer wrote:
> >Finally, I would assume one needs to worry about the "ratio" of R to 
> >C.  If one chooses a LOW resistance and a large capacitance, we might 
> >still have a reasonable RC compared to the PWM frequency, however, we'll 
> >also have large currents on the AVR pin. Is this correct?
> >
> >Further is one chooses a high resistance with small capacitance, the 
> >circuit becomes more sensitive to it's output load.
> 
> Good summary overall.  You can eliminate the load sensitivity by following 
> the RC with an op-amp.  You can also use more complex filters (both active 
> and passive).
> 
> There are additional sensitivities in the filter at the extremes.  At too 
> low a resistance the source impedance of the output pin will dominate and 
> the R will be effectively irrelevant.  At very high R values you become 
> sensitive to noise pickup.
> 
> Also there is a limit to how far it makes sense to filter the output.  If 
> you only have an 8 bit PWM it 'probably' doesn't make sense to try to 
> reduce the ripple to the 12'th bit
> 
> Robert
> 
> " 'Freedom' has no meaning of itself.  There are always restrictions,   be 
> they legal, genetic, or physical.  If you don't believe me, try to chew a 
> radio signal. "  -- Kelvin Throop, III
> http://www.aeolusdevelopment.com/
> 
> 
> 
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