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ADS1118: K-type thermocouple EMF

Masayoshi Takahashi
Intellectual 1530 points
Part Number: ADS1118

Hi team,

My customer would like to know what is the value of EMF(electromotive force) output of PGA when they use 40uV/℃ K-type thermocouple.

Is this value depend on the gain? (If the Gain is 16, the value is 0.64mV/℃?)

Regards,

Yoshi

  • 0
    TI__Guru* 92715 points

    Takahashi-san,

    First, if you are supporting someone that is using the ADS1118 with a thermocouple I want to make sure you have the following collateral:

    A Basic Guide to Thermocouples:
    http://www.ti.com/lit/an/sbaa274/sbaa274.pdf

    Cookbook using the ADS1118 for thermocouples:
    http://www.ti.com/lit/an/sbaa355/sbaa355.pdf

    TI Design Using the ADS1118 for thermcouples
    https://www.ti.com/lit/ug/slau509/slau509.pdf

    Now, I'm not entirely sure I understand your question. Let me explain a bit about the thermocouple measurement. This information will be in the basic guide to thermocouples in the first link above.

    The output of the PGA is dependent of the input of the ADC, which will be the thermocouple measurement. If the thermocouple is about 40uV/C, then the PGA measurement will be approximately 0.64mV/C.

    However, to make a precise thermocouple measurement, you need an accurate cold-junction temperature measurement. The thermocouple temperature to output voltage is a non-linear relationship. You must take the following steps:

    1. Determine the cold-junction temperature and convert it to the equivalent thermoelectric voltage.

    2. Add the cold-junction voltage to the measured thermocouple voltage.

    3. Convert the summed cold-junction voltage and thermocouple voltage to the thermocouple temperature.

    You can then use the tables found on the NIST site to calculate the temperature or voltages.

    http://srdata.nist.gov/its90/menu/menu.html

    Following the steps is very important to get a proper temperature measurement. Without this, you may be several degrees off from the expected value.

    Joseph Wu