Issues with MESA 7976eu analog-in

Hey guys,
I want to measure the internal temp of my jianken ATC spindle. I have a NTC with a classic voltage divider setup, and at room temperature the V_out of this divider carries 12V. (Measured against GND with a multimeter).

However, as soon as I put this cable into analog_in1 of my Mesa 7i76eu, the voltage drops to 8v, and thous the calculated temperature is garbage...

Any ideas why this is happening? 

regards
Sebastian

 

 

The 7I76EU inputs have and ~11K input impedance which needs to be accounted for
when driven by a high impedance source (it looks like your source impedance is around 5K Ohms) 

Thx Peter.
How would you “handle” this? Could I just subtract a constant value from the calculated temp?

If the NTC is in the positive side of the divider, its the equivalent
of lowering the negative side resistor value to its current value
in parallel with 11K Ohms. If it is setup this way, you could raise
the value of the negative side resistor so the parallel combination
on this new resistor and the 11K is the same as the old resistor.
 

Thx, this is how my setup looks like... 



So if I understood your answer correctly - I replace R1 (currently 10k) with 110k, which gives me 10k (110k & 11k in parallel). 
Will try that tomorrow....

thx!!

Yes, that should be correct.