Tramflug: If you want anything resembling accurate temp out of a thermistor, then yes. Difference between heated bed on and off (current flowing through the ground) can, in some instances, alter the temp reading by 10 (or more) degrees with 1k bias and a standard 100k thermistor if the current flow is high enough (based on a report from the RAMPS-FD thread). That can lead to quite a bit of overshoot in the wrong situation.
Specifically, think of this scenario:
You've got a hot end with a max temp of ~240, and you're printing ABS at 235 because you have a small nozzle (say 0.3). Heated bed comes on and your reading plunges to 225, so it turns on the heat till it gets to 235. Heated bed comes off, you're now at 245. Not going to do the print any good, nor is it going to do the hot end any good.
That's a lot of hysteresis caused by bad analog readings.
I've seen this issue with 2-3 degree fluctuations with RAMPS/Arduino Mega (5V) if there is a bit of a ground loop with the PC. With more electronics going to 3.3V electronics, this only gets worse.
Specifically, think of this scenario:
You've got a hot end with a max temp of ~240, and you're printing ABS at 235 because you have a small nozzle (say 0.3). Heated bed comes on and your reading plunges to 225, so it turns on the heat till it gets to 235. Heated bed comes off, you're now at 245. Not going to do the print any good, nor is it going to do the hot end any good.
That's a lot of hysteresis caused by bad analog readings.
I've seen this issue with 2-3 degree fluctuations with RAMPS/Arduino Mega (5V) if there is a bit of a ground loop with the PC. With more electronics going to 3.3V electronics, this only gets worse.
