AVR-Chat

--- In AVR-Chat@yahoogroups.com, "David Jones" <david.jones@s...> 
wrote:
> >>> dave_mucha@y... 03/11/2004 12:20:07 pm >>>
> --- In AVR-Chat@yahoogroups.com, Dave VanHorn <dvanhorn@d...> wrote:
> > 
> > >
> > >any suggestions on what to use for an external ref ?
> > 
> > Any convenient band-gap chip will work. In my case, I used the 
> 4.096V version.
> 
> >Can you offer any type of link to how to use one ?
> >I down loaded the TI data sheet for an LM-385, 3 pin TO-92 package 
> >unit and the unit is shown as a zener with the voltage after a 
> >resistor.
> 
> That's exactly what it is, a precision zener. You need a resistor in
> series. The datasheet should have typical application notes and 
tell you
> how to pick the correct resistor value. Some other references like 
the
> REF02 do not need the external resistor.
> If the TI datasheet isn't helpful go to another manufacturer, e.g
> National Semi.
> 
> >The idea of using 4.096 is interesting, but it would seem that 
4.096 
> >would be the maximum voltage it could read ?
> 
> That is correct.
> 4.096 is convienient because it means you will have a nice round 
number
> per bit. In the case of a 10bit ADC, a 4.096V reference would give 
you
> exactly 4mV per bit. A 5V reference on the other hand gives 4.883mV 
per
> bit. These oddball values may require more memory to process if you
> don't want rounding errors.
> 
> Dave :)

I figured the 4.096v was to generate an exact integer for a 12 bit 
ADC.

Guess that means a 0-5 requires a voltge divider to get to the 4.096 
value at 5 volts.

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.

Dave