AVR-Chat

At 02:16 PM 11/3/04 +1100, you wrote:

> >>> dave_mucha@yahoo.com 03/11/2004 1:48:24 pm >>>
> >I figured the 4.096v was to generate an exact integer for a 12 bit
> >ADC.
>
>Yep. It'll do the same for any number of bits actually.
>
> >Guess that means a 0-5 requires a voltge divider to get to the 4.096
> >value at 5 volts.
>
>Yes, although you are probably better off using a 5V reference if you
>want 0-5V range. Otherwise if you use 4.096V you have to use say 0.1%
>precision resistors for the divider. Extra components, extra cost,
>greater error.

2 resistors at 0.1%, a 0.1% reference, the ATmega8535 has a 10bit converter 
so that's a best case accuracy of a little under 0.1% (the data sheet 
suggests it's actually probably closer to 0.3% depending on what kind of 
accuracy you are looking for).  That doesn't leave much of an error budget 
for sensor errors layout or other considerations.

I am assuming that it's unlikely for the source signal level range to match 
the A/D input range and that it will either have to be increased or reduced 
(and will thus likely have at least 2 resistors involved in the input).

Some of that can be calibrated out but that will mean some sort of 
precision source (better certainly than 0.1%) and additional care in 
testing/calibration.

If I was presented with a request for any particular accuracy my first 
question would be to ask what the accuracy of the source transducer 
was.  To get 0.5% using the ATmega I would at first blush estimate the 
source would have to 0.1% accurate or better.  A lot of sources don't 
approach that, a fair number have trouble even being 8 bits accurate.


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