INTERNAL: an built-in reference, equal to 1.1 volts on the ATmega168 or ATmega328P and 2.56 volts on the ATmega8
Normally the reference for the ADC is the power supply rail (+5v) but you can select an internal analogReference()
Which is not a wise move if you use a ratiometric sensor like a voltage divider with thermistor.Leo..
There are two types of sensors.1) the ones that output a voltage.An example is an LM35/TMP36.The produce a certain output voltage depending on what they sense, but not depending on their supply.2) the ones that (also) output a ratio of their supply voltage.An example is a voltage divider (thermistor/resistor), or a load cell.A/D results depends on two things. Input voltage and reference voltage.If input voltage goes up, A/D value goes up, and if Aref goes up, A/D value goes down.It is obvious that you want a stable/fixed Aref for constant voltage sensors, so only sensor output voltage will change the A/D result.1.1volt Aref or 2.56volt Aref or even the 3.3volt supply as Aref is great for those sensors.For ratiometric sensors, you want their supply linked to Aref.So the output of the sensor stays a ratio of the A/D.A 10k:10k divider connected to 5volt, outputs 2.5volt, but read with an A/D with a reference of 5volt will read 512A 10k:10k divider connected to 4volt, outputs 2volt, but read with an A/D with a reference of 4volt wil also read 512.Hope you understand my weird explanation.Leo.
I want to use an Arduino Due for this project, and from what I understand from reading the analogReference() docs is that my only option is 3.3v
You should use a special amplifier+A/D breakout board for this sensor.
I hope you can find a good formula to calculate that crooked PT200 derived ratio into a reasonably accurate temp.
where did the 1024 come from in the 0.28/1.1 * 1024 equations?
about the pull up resistor part