AVR-Chat

Firstly, the ADC uses 3 ADC clock cycles to perform it's sample and
hold. You can calculate the sample and hold time by multiplying your
ADC clock period by 3. 

Secondly, the ADC must be fed from a low impedance source - have a
look at the input with your scope when your feeding it from a high
impedance source, I had my voltage dropping 200mV on a 3V signal by
feeding it from a too high impedance source. (Keep the loop impdance
low!!)

Generally when I want to measure the supply voltage in a AVR, I
connect the high side of the voltage divider to a output pin, connect
the center to the ADC and connect the low side to ground. That means
you can control very easily without any external parts when the bulk
of the current flows through your "low" impedance voltage divider, I
usually use 2 * 10k resistors for the divider, but you should be able
to up these values into the 100s of k without any problems. I
personally wouldnt use MOHM resistors as they cause lots of problems
with temperature and humidity. You change the value of a 3M3 resistor
by blowing on it! <- caused a problem in one of my projects.

This is an interesting topic, but in the end you need to decide what
is the easiest method, generally for measuring the supply voltage you
probably dont need to be VERY accurate anyway! You are usually just
trying to find out whether the battery is good or low.

Cheers
Hein B
Auckland, New Zealand. 


--- In AVR-Chat@yahoogroups.com, "Jim Wagner" <jim_d_wagner@...> wrote:
>
> On Tue, 19 Jun 2007 19:35:43 -0000
>  "Dave VanHorn" <microbrix@...> wrote:
> > 
> > > I'm still curious enough to experiment with this idea
> > compared to my
> > > typical OpAmp solution.
> > 
> > It may "work" on one unit, but if you can't handle the
> > leakage allowed 
> > in the spec without significant error, then you're in
> > trouble.
> > 
> > Remember, you have to add this error to all the other
> > sources of error 
> > in the ADC measurement.
> > 
> > 
> 
> 
> And Op-Amps have their own input "leakage".
> 
> Jim
> 
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