I had 1 thermistor set up and working great. When I added the 2nd thermistor the temperature went up 5 degrees on the first. I am pretty sure my wiring is correct, but am looking for any thoughts as to what is going on here.
I have attached a drawing of my wiring.
Your schematic looks fine. Try putting a short delay between readings (one millisecond should be plenty) or take each reading twice and ignore the 1st reading.
...There is only one ADC in the ATmega chip and it uses multiplexing to switch between inputs. It may take a short time for the voltage to settle/stablize when you switch pins.
Thanks for the response. I've tried that, but no luck. That was my first thought this morning. This has been wearing me out for a couple days now.
If both sensors are plugged in I can calibrate and it works great. As soon as I unplug one of the sensors the other drops 5 degrees. Very frustrating.
You need to add 100nF capacitors from each analog pin to ground, otherwise the ~1M
impedance of your circuit is far too high for the ADC sample hold circuit. Reading the
same pin several times in a row is also a fix, because it gives a much longer time to
charge the sample/hold capacitor up.
The datasheet says 10k or less impedance to drive the analog pins for full accuracy at
standard ADC speed. There are 1.5 analog clocks to settle the sample/hold on each change
of analog pin, which is 12us, or if you read again on the same pin, 104us.
Thanks Mark, that helps a bunch. I'll post results when I get some capacitors.