AVR-Chat

dlc Wrote
>If the cap is across it then the ADC might not get the correct voltage 
>either.  The ADC relies upon charging a cap inside the module.  That 
>relies upon a certain target RC charge rate, that charge rate also must 
>not exceed the current capacity of the pin.  

Say what!?

Yes, the sample and hold does rely on a cap but its only requirement is
that the (internal) cap be charged faster than a certain rate.  That's the
reason behind the upper bound on source impedance.  This s/h cap is small,
typically on the order of pf when directly specified.  The current into
this cap is limited by the resistance of the A/D multiplexer's switch.

>You are playing a finicky 
>game by using a cap to boost the inrush current to offset the RC time 
>constant.  It might work under some circumstances but since the ADC 
>modules specifically states that you should only be using slowly 
>changing signals on the circuit with low-pass filters that would tend to 
>contra indicate using a bypass cap I think.  

That's a caution that the s/h cannot track a rapidly changing signal.  It,
after all, forms a low pass filter as well.  Also the cap will form a low
pass filter with the divider with quite a low frequency given the resistor
values.  The signal simply will not change quickly enough to effect the low
pass filter.  And the cap is large enough the voltage it won't be disturbed
by the delta Q from the sample and hold cap.

>Have you used such a circuit successfully before?

Many times.  I use a simple RC to provide Nyquist filtering quite
frequently, even or maybe especially on bandwidth limited signals.

Robert


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