I designed the M700 and M750 "cow thermometers" at http://www.gla-ag.com .
We used a 20 bit LTC ADC driving a PIC over SPI. The ADC has an internal
reference, though not super precise. However, as others have pointed out,
the ADC is ratiometric, so as long as the sample varies with the
reference, errors cancel. I use a 0.1% resistor between the reference
output of the chip and the top of the thermistor (with the bottom of the
thermistor going to ground). I used the resistance tables supplied by the
thermistor manufacturer. I put them in a spreadsheet and calculated what
the ADC output would be at each temperature they specified. I then did a
lookup table in the PIC with linear interpolation between the two points
where the ADC value landed. Accuracy is better than 0.1degree C or
0.1degree F in the temperature range of interest (90 to 110 degrees F).

Harold



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