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Would like to know the best approach to using a thermistor instead of a digital sensor. Would I use an op-amp or could I just use thermistor straight into the AD? With Dallas/Maxim having problems with their DS18xx lines I have to look to an alternative source.
Sensirion's SHT11 is a good (if expensive) Temperature sensor, and there's plenty of code for it floating about (as I recall) in the MeLabs archives. Plus it also gives you Humidity and from that along with the Temperature you can then calculate the Dewpoint too.
Meanwhile, if you go the Thermistor route, you'll discover it needs either a constant voltage or constant current. Buffer the supply to the Thermistor with an Op-Amp... it's tempting to use the +5v from your PCB, but it's bad practice to send that off your board because any noise picked up will come straight back to your PIC. Thermistors are NOT linear, using a constant current method will attempt to straighten the curve, but you will still need either a lookup-table or some math to get any accuracy. Saying that, I have achieved 0.25C in the range -20C thru +40C using a Thermistor and the PIC's internal 10-bit ADC using just a small lookup-table to get within 5C and then interpolating the remainder with a bit of math.