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Noise problem with NTC thermistor connection

eem2am

Aug 3, 2009
429
Joined
Aug 3, 2009
Messages
429
Hello,

Please could you help me in making a noise-free (as far as possible) connection to a NTC Thermistor?

We are continuously detecting temperature by using the ADC of a microcontroller to read the voltage across a divider ……one resistor of which is the NTC Thermistor.



-before reading the voltage with the ADC , we use an op-amp (LMC6035) to buffer the thermistor signal, and also to expand the signal range, so that we read the temperature with a higher resolution.

CIRCUIT DIAGRAM:
http://i56.tinypic.com/125qqs9.jpg

Anyway, the thermistor is not on the PCB that contains the opamp and the microcontroller, but it is about 10cms away from this pcb, -and in fact the thermistor is actually mounted on a water pipe.

-so the thermistor connects to the pcb via a 10cm pair of wires, which are not twisted together, but do run pretty closely to each other.

-these wires are connected to the pcb by a push-in connector (i.e. the typical connector with shrouded-housing soldered to the pcb, and the crimp-housing connected to the thermistor wires).

Anyway, unfortunately we could not get the op-amp to be located on the part of the pcb which is near the thermistor wire connector.
-in fact, the opamp is about 10cms away from this connector.

-therefore we had to run a 10cm pcb track from the pcb thermistor connector to the opamp.

…I am worried that such long runs of wire, and also of pcb track, will cause noise to be picked up, and corrupt the thermistor signal. ?

Do you think that this is possible?

Anyway, I have brought the thermistor signal to the opamp via a pair of tracks which run alongside each other…from the pcb thermistor connector..
……I needn’t have done this since one side of the thermistor is connected to Vcc (5V), and there is a Vcc node available at the op-amp (Vcc actually powers the op-amp and the microcontroller)

…..however, I just assumed that running the thermistor signal as a pair of pcb tracks , side-by-side, would mean less noise corruption………do you know if this is true?

Do you know of any other measures that I can take to reduce noise corrupting this temperature measurement.?

The thermistor is around 10K at 25C.
 

davenn

Moderator
Sep 5, 2009
14,125
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Sep 5, 2009
Messages
14,125
use screened lead between the thermister and the main PCB
preferably a screened 2 core lead (2 wires +screen) some stereo audio hookup lead would be ideal
Connect the screen to the 0V rail


Dave
 

Resqueline

Jul 31, 2009
2,848
Joined
Jul 31, 2009
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Yes, running it as a close coupled pair from the source is a very good way to reduce induced noise, + what Dave said. Capacitor(s) between the lines may also help.
 
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