Thermistor with nRF 52840

[Sundar_santhanam@1998]

Hi guys ,

I am trying to connect thermistor with the ADC pins of nRF.

I use the steinhart formula to calculate the temperature in degree celcius. I used a voltage divider bias circuit where R1 is my thermistor and R2 is 10K ohms and I attach the output to the NRF adc pin.

Now , I use this formula Rt =R0 * ((adcMax / adcVal) - 1)

where Rt is the resistance of thermistor at particular temperature 

R0 is 10K ohms 

adcMax is maximum adc values

adcVal is the output adc value

I also want to convert Rt into its logarithm (ln in math) thus ,

logRt = log(R0 * ((adcMax / adcVal) - 1))

after which steinhart is 1/T = A + B*ln(R) + C*(ln(R))^3

After I tested this I get an accuracy issue of +20 DEGREE celcius in my output.

But in this case,

Vs is source voltage = 5V

Vo is the output voltage (depends on the output)

assuming Vref = Vs ( vref is reference adc voltage)

the above formula is applicable. But Iam giving Vs as 5V and I dont know how change my Vref to the same.

What can I do in this issue ?

PLease tell me how can I correct my +20 DEGREES error?

thanks

 .

[Simon]

Check out the section Reference voltage and gain settings in the infocenter on how to change the reference voltage for SAADC. If you want me to go more into details about anything please ask.

These cases may also be helpful:

• https://devzone.nordicsemi.com/f/nordic-q-a/27/what-voltage-range-can-be-measured-with-the-adc
• https://devzone.nordicsemi.com/f/nordic-q-a/38613/is-it-possible-a-biggest-voltage-in-the-adc-input-than-the-voltage-supply

Best regards,

Simon

[Sundar_santhanam@1998]

Is it possible to have a reference voltage of 5 Volts and how? Because as of now the ADC reference voltage is 3.6V and the source voltage to the thermistor is 5V ? Even if I give 5V to the Vdd to make it as a reference voltage , it may damage the board. So my source voltage to the sensor and my adc reference voltage are not the same . what can I do in this case?

[Simon]

I am sorry for the delay. Lately, a huge portion of the support team has been on Christmas vacation. Most of the staff will be back 6 of January 2020 and you can be sure to get an answer shortly after that.

Best regards,

Simon

[Sundar_santhanam@1998]

My idea is just to connect thermistor with nrf. I just wanted to know how to get accurate results in degree celcius , the formula to calculate thermistor resistance involves adc values , my biggest problem is my adc reference voltage and my source voltage to the thermistor are different . Pls help me solve this issue

[Simon]

Here is one solution:

• Set the reference to VDD/4
• Set the gain to 1/4
• The input range will then be (VDD/4)/(1/4) = VDD, as described in the infocenter
• Then set Vs to VDD, which will make it equal to adcMax (=1024 if 10 bit resolution)
• Then you can read the input on Vo, where a reading of 1024 corresponds to VDD, 1024/2 corresponds to VDD/2 and so on...

Best regards,

Simon