I actually started with a Maxim ADC/DAC/RTC, but deliveries got unreliable, so I moved to the LTC2420 and the PIC18LF2520. Harold > What PIC and LTC ADC did you use, Harold? > > ----- Original Message ----- > From: "Harold Hallikainen" > To: "Microcontroller discussion list - Public." > Sent: Wednesday, July 25, 2007 11:42 PM > Subject: Re: [PIC] Linearizing a Thermistor and Measuring Resistance > > >>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 >> >> >> >> -- >> FCC Rules Updated Daily at http://www.hallikainen.com - Advertising >> opportunities available! >> -- >> http://www.piclist.com PIC/SX FAQ & list archive >> View/change your membership options at >> http://mailman.mit.edu/mailman/listinfo/piclist > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- FCC Rules Updated Daily at http://www.hallikainen.com - Advertising opportunities available! -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist