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While I'm a moderately competent digital electronics hobbyist, I avoid
delving into the analog realm whenever possible. I have what is probably a
simple problem here, but I don't know the best approach to solving it.

I'd like to use an LM20 to measure temperatures from -55C to +50C. The LM20
has a mostly linear voltage output in response to temperature. At -55C, the
output is 2485mV. At 80C, the output will be 919mV.  (The datasheet doesn't
give a specific number for 50C and the chart is too small to interpolate
with any accuracy so let's play with the 80C value.) The LM20 is relatively
inexpensive ($1 in onesies) and is usable for the temperature range in which
I'm interested. 

The LM20 and a PIC18F2xK20 are going to be powered from a 3.3V power supply.
The PIC chip has VREF- and VREF+ inputs to the ADC. I believe that if I can
set VREF- at 0.9V and VREF+ at 2.5V, I can greatly increase the resolution
of the ADC over the input range that the LM20 will deliver. 

How do I approach building a voltage divider to provide me with these
voltage references? How do I select the best order-of-magnitude for the
resistors? What other considerations do I need to pay attention to?

(Now... I am aware that this may be a bit extreme. The PIC has a 10bit ADC,
and even if I leave VREF- and GND and VREF+ at 3.3V, I can probably get +/-
1 degree C resolution, but this still makes for a nice topic of discussion,
IMHO, and I'm bound to learn something new from the discussion.)

Another potential topic: what are the pro's and con's of the LM20 versus a
NTC thermistor for this application? The thermistor seems to require more
claibration and interpolation to correct for nonlinearities.

Thanks,

-Matthew

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