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.

  True, a very low impedance.

> 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.

  I wasn't sure how you were using the cap, as a shunt around the resistor
or as a resevoir cap, I see now it is the latter.

> 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.

  Current = dV/dT through a resistor - if it is a sufficient dV and no
resistor then you can get a very high, very short duration current flow.
 This is always a concern when _driving_ a MOSFET with a micro, I'm not
just sure what kind of an issue it might be when taking an input for an
ADC, I don't know what the internal circuit is.  It could be (seems
likely in hindsight) that this current would be internally limited in
some fashion.  Your only delay now is how long it takes to charge that
cap through the resistor network - but since this is a slowly changing
signal (one hopes!) that shouldn't matter.

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

  You have certainly convinced me to try this method on future projects!

DLC
--
Dennis Clark

> 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
>
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