AVR-Chat

>>> 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.
Regardless of whether you use 4.096 with a divider, or 5V without a
divider, you will still have an oddball value per bit. i.e for a 5V
input and 12bit ADC you will have 1.22mV per bit. Can't escape that
fact.

>I assume that 4.096 volts is exactly 12 bits also and not some 80% of

>the range.  If I am not mistaken, the whole idea of the voltage ref 
>is that with that specifc voltage, the input will be exactly 12.000 
>bits so even if it were a 2 volt ref, 2 volts in on an ADC pin would 
>be exactly 12 bits.

Correct.
Use a 5V reference you get a 0-5V range, use a 2.5V ref you get 0-2.5V
range etc. All of them will still give you 12 bits of resolution.
Although the lower the voltage reference gets, the greater impact you'll
have from noise. i.e. a 2.5V ref gives 610uV per bit. So if you have 1mV
of noise in your system, that is almost 2 least significant bits worth
of noise.

Dave :)