Thermistor readings incorrect

[bobc]

There appear to be two problems:

1. AREF is an input to Due, not an output, and it is not connected
   Fix is to revert to 3.3V as reference voltage, as previous revision.
2. The OVREF voltage is too low at about 2.35V. The resistors R1002, R1003 and R1004 result in a reference of about 0.9V on U1001.
   Values of 6k8, 1k8 and 1k should give better output, approx 3.125V.

[blacslam]

Hi. I have a problem with thermistor reading issue, and searched over the internet, and didnt found much useful info, so may be if you have a time, you can help me.
I made a RAMPS-FD v2 rev B myself, and all works great, but 100k thermistor shows about 100-110 degree celsius all the time. When i try to hot it, i see some rising temp reading, but only for 120 degrees, when real temp is about 300-400 degrees. When i unplug RAMPS from DUE, temp reading rise to 230 celsius. May be such case nown to you, and yo can give me some advise?

[bobc]

Are you sure it is not a firmware problem?

[blacslam]

I checked this several times, using different firmwares. For example i tried latest Repitier for Ramps-FD (with DUE) and for Ramps 1.4 (with Mega) with the same settings, and on mega thermistor readings were correct. And i have a lack of expirience to correctly debug this issue.