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Puzzle: Analog inputs work with E3D PT100 but cannot read normal thermistor

Posted by pyrophreek 
Puzzle: Analog inputs work with E3D PT100 but cannot read normal thermistor
December 24, 2015 10:45PM
I have been pulling my hair out with this problem, and I am looking for a good explanation of what the hell is going on. I just upgraded my electronics from a Sanguinololu to a RAMB0, and in the process I think I screwed something up (not sure how, but I am hoping that with a better understanding of what is going on, I can determine that).

Problem:

Normal thermistor inserted into ANY of the four thermistor slots (I am using one of two 100K thermistors, which work fine on the sanguinololu) results in a reading of 600-1000C (depended on a few things, but was generally reproducible) and more puzzling, heating the thermistor resulted in a reproducible REDUCTION in the measured temperature (and before you ask, I checked that I was using the right thermistor table....several times and in several ways).
The thermistor slots have a 4.7k trim resistor attached to 5V, and a 20V ESD protection varistor attached to ground. I tested all of these components, and they all tested within expected parameters for voltage, resistance, etc.(except for the ESD ones, which I wasn't entirely sure how to properly test). I even went so far as to remove one set and set up my own 4.7k resistor to verify (without ESD varistor), and got the same readings (not just on the thermistor slots, but also on the broken out analog ports, so 0-8 as well).

Now for the puzzling part. As part of the upgrade, I switched my hotend thermistor with the new E3D PT100 sensor and amp. When I used this on any of the analog pins, it worked fine, however if I plugged in the thermistor, the reading would either be the previously mentioned error, or the reading would mirror the PT100 reading.

So I want to know, does anyone know what is going on? have I fried the analog ports on the board (even though they work fine with the PT100...) or is there something else going on. Please feel free to get as technical as possible, I want to understand EXACTLY why I am seeing this behaviour.

Thanks for reading my rant,
K
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