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ADS1118: how to convert thermocouple input millivolt to degree celcius ????

Sungho Kim
Prodigy 20 points
Part Number: ADS1118

Hello~

I'm testing ADS1118 with arduino mega and ADS1118-master library.

Test circuit is referenced from Fig.50. Two-Channel Thermocouple Measurement System in ADS1118 datasheet.

K-type thermocouple, FSR = ±0.256V (7.8125 uV resolution), data rate=8 SPS.

But I changed VDD to +5V like below.

I got about 28.33 degree celcius from On-chip temperature sensor.

1. changing VDD to +5V can cause the wrong measurement ?

2. How can I get VCJ from TCJ ???  28.33 degree celcius to Voltage

 - it just need to find out thermoelectric voltage in millivolts from  K thermocouple table?  

  • 0
    TI__Expert 8175 points

    Hello,

    Welcome to the forum! 

    It looks like your design is very similar to this reference design we have: https://www.ti.com/lit/ug/slau509/slau509.pdf. You may find the design discussion and theory helpful for your design.

    Sungho Kim said:
    1. changing VDD to +5V can cause the wrong measurement ?

    The RPU and RPD resistors in this design are only used to bias the measurement voltage up to the mid-supply level of the ADC, so changing VDD will not change the circuit operation.

    Sungho Kim said:

    2. How can I get VCJ from TCJ ???  28.33 degree celcius to Voltage

     - it just need to find out thermoelectric voltage in millivolts from  K thermocouple table?  

    Yes you are correct, you simply need to translate the temperature from the ADC to the equivalent voltage you would see on the thermocouple at this temperature. This voltage is then added to the measured voltage from the thermocouple and converted back into a temperature.

    Let me know if you have any more questions,

    Levi DeVries