Using a thermistor with the FRDM board

[steve_fae]

Trying to accurately measure the temperature of a heat sink using the KL25 FRDM board with an attached daughter card.  The thermistor is a 10K series from Vishay and so I bias it to 3v and use a pull down resistor recommended by Vishay.  I need to measure within a few degrees of accuracy from 70deg C to 100deg C.  The problem is the ADC bounces all over the place.  I have tried averaging, throwing out an errant reading, etc.  but I still do not have a stable solution.  I capture 16 bits from the ADC then throw out the least 4 bits.  I even tried to look at just 8 bits of accuracy.  Is there something I am missing?

Also I only need to capture 32 steps along that range. 

[steve_fae]

To add, I am using console printf's to send out what the ADC is reading, every 3 seconds or so.  I have played with that value quite a bit as well.

[DavidS]

Hi Steve,

Which ADC channel/pin are you using and how are you configuring the ADC module and sample rate?

Have you tried connecting to another source? Also testing to VCC and GND.

Are you using other ADC channel/pin combinations too?  If yes try only sampling your temp sensor and not the other stuff.

Any wiring issues?

Regards,

David

[steve_fae · Accepted solution]

Here is the configuration.  The core is running at 20 Mhz btw.

I think that the folks at Vishay let me astray.  I had asked if I needed any additional caps/resistors on the thermistor and they said no.  I found this diagram at the Arduino Playground showing some nice bulk caps on VCC as well as at the center point of the thermistor connection.  Hooking up the caps really calmed things down.

[DavidS]

Hi Steve,

Great news.  Good digging into the issue and thanks for letting us know the solution.

Regards,

David