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ADS112C04: Temperature measurement using thermistor

Vineeth N
Expert 1180 points
Part Number: ADS112C04
Other Parts Discussed in Thread: ADS1119,

Hi Team,

I am using ADS112C04 for temperature measurement. Normally I use thermocouple with this IC. But my question is can I replace the thermocouple with an NTC thermistor in the same thermocouple circuit? . Also my reference for the IC in therrmocouple measurement is at 1V will it affect my thermistor measurement reading, if it is possible to use a thermistor. Since I came across something like 'the reference should be same for thermistor measurement and ADC'. If thermistor reading is possible can I use the equation in ADS1119's datasheet to get temperature or resistance value back.

Thank you,

Vineeth N

  • 0
    TI__Guru** 113765 points

    Hi Vineeth,

    A thermocouple (TC) is self-powered whereas the thermistor requires some form of excitation.  So you cannot simply drop the thermistor at the same location of the TC and be able to measure the temperature without proper excitation. 

    If you are using pull-ups and pull-downs on the TC inputs, these resistors will provide a current path, but the range of the measurement will be very small.  Let's say the pull-up and pull-down resistors are 1M Ohm and the resistance of the thermistor ranges from 2k to 10k for your temperature measurement range.  Using a 3.3V supply the maximum current would be 1.65uA and will become even lower with increased thermistor resistance.  Also, any drift of the pull-up and pull-down will affect the measurement results.  Obviously if the thermistor resistance is much higher, the voltage drop across the thermistor will also be higher.  Because you are using large values of resistance the thermal noise will also be higher.

    The quotation you used is based on a ratiometric measurement to lower noise and drift effects of the measurement.  If you excite the thermistor with the supply voltage, then to make the measurement ratiometric you would use the supply as the reference.  The analog supply is a reference selection option in configuration register 1.

    The equation in the ADS1119 datasheet that you are referring to uses the same supply for both the reference and the excitation of the thermistor.  Also, as you have noted in a different post, the equation has an error.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Expert 1180 points

    Hi Bob,

    As always your response was quick, thank you. I am using ADS112C04 in my current design with TC for temperature measurement. So, I just thought if I can put thermistor instead of TC in the circuit, I don`t need to make a large modification on the circuit which I already have. Anyway I will go with your suggestions on ratiometric measurement. If  it turns out good I will use it or else I can go for ADS1119.

    Glad that you have noticed my other post on equation error.

    Thank you,

    Vineeth N