Comments:ADC reading on AN0, with a 33K resistor, and calculations according to post #60.
Rich (BB code):x=adc_raw_ch0; a3=0.00000018624250; a2=-0.00025641150597; a1=0.19058077255347; a0=-55.86182880225911; temperature1 = a3*pow(x,3)+a2*pow(x,2)+a1*x+a0;
ADC reading | T0 | Commercial
770 | 24.0 | 25.8|
633 | 9.2 | 13.1|
575 | 4.0 | 9.4|
564 | 3.5 | 6.7|
515 | -0.3 | -0.3|
510 | -0.6 | -0.6|
496 | -1.7 | -2.6|
495 | -1.8 | -2.6|
1. There is a typo error in a3. It should be
a3 = 0.00000018524250;
You should avoid typing in numbers in order to avoid mistakes. Use copy and paste.
2. The commercial (reference) reading vs ADC reading don't fall on a nice smooth curve. There is sufficient variability to suggest that something is not quite right.
How much time are you waiting before taking a measurements?
How stable are the ADC counts? What is the observable variation?
How stable is the commercial (reference) reading?
What are you using for a power supply? How many volts?
Do you have proper power supply decoupling at the MCU and at the thermistor? 0.1μF and 10μF at both VDD and thermocouple VDD.
What is your mcu clock source and frequency?
What is your ADC clock frequency?
The maximum source resistance suggested is 10kΩ. I don't know what effect this should have with regards the readings not falling on the curve.