AVR-Chat

>>> dave_mucha@yahoo.com 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 :)

--- 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