AVR-Chat

1) The concept here is that the 0.1uF cap is way larger than the input cap
in the A/D and will present a low impedance source to the input.

2) What do you mean by "a certain RC charge rate?" AFAIK, the slope has
nothing to do with the A/D measurement, other than to say that you want a
steady input voltage during the actual measurement.

3) How in the world could adding a cap across the A/D input cause the input
current to exceed the capacity of the pin? We're not talking about a voltage
doubler - just a simple low pass filter.

4) I have used such a circuit very successfully before.

Phillip
________________________________

From: AVR-Chat@yahoogroups.com [mailto:AVR-Chat@yahoogroups.com] On Behalf
Of dlc
Sent: Tuesday, June 19, 2007 1:01 AM
To: AVR-Chat@yahoogroups.com
Subject: Re: [AVR-Chat] Using the ADC to measure the battery supply?



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. 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. Have you used such a 
circuit successfully before?

curious,
DLC