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Original question:

ADS1118: Input Buffer

ADS1118: Themocouple Measurement Discrepancy.

Pang
Expert 2415 points
Part Number: ADS1118

Hi Ti,

Coding has been completely done and it's able to read/write the configuration registers. but when it is in temperature measurement mode, the return value given after my conversion according to Type K formula it was approximately 10 Degree Celcius only.
However, I did measured the voltage output from the thermocouple with oscilloscope and it show fluactuate around 39x to 42x uV, which standard show Type K are approx 39uV/degree celcius, can I assume what I got are correct?

But the Thermocouple are not given the correct voltage output?

Please help to advise.

  • 0
    TI__Guru* 97775 points

    Electrobud,


    Thanks for your post. Before I go too far into your circuit and method, there are a few resources that you might want to look through (if you haven't already).

    First there's a basic guide to thermocouple measurement:
    http://www.ti.com/lit/an/sbaa274/sbaa274.pdf

    And then there are a couple of basic circuit guides using the ADS1118 for measuring thermocouples:
    https://www.ti.com/lit/an/sbaa355/sbaa355.pdf
    https://www.ti.com/lit/an/sbaa189/sbaa189.pdf

    For your results, I would want to know how you have the circuit set up. I also need to know how you've done your cold-junction compensation. My guess is that you haven't factored that into your measurement. The basic guide has a section on cold-junction compensation and you can read through the methodology.

    However if you have added this in, I'll need to know more about what values the ADC is putting out, how you measure the cold-junction, and how you've factored the cold-junction in the resulting measurement. Just take an example measurement and report back your calculation for each step.


    Joseph Wu

  • 0 in reply to Joseph Wu
    Expert 2415 points

    Hi Joseph,

    I've gone through the basic guide but may I know how the cold-junction compensation is done? It's mentioned that thermocouple generates a voltage related to the temperature difference between the thermocouple junction and the leads to attached to the cold junction at the isothermal block. How can I actually connect to the isothermal block?

  • 0 in reply to Pang
    TI__Guru* 97775 points

    Electrobud,



    For the thermocouple measurement to be accurate, the two leads of the thermocouple and the cold-junction measurement must all be at the same temperature. Generally, the PCB is designed so that all three are close together, so the temperature should be the same. In the ADS1118EVM which shows this as an application, the device is placed between the two contacts for the thermocouple. It's a little hard to see, but it's the black device near the "OMEGA" lettering as you can see in the following picture:



    You can find more information about the EVM here: http://www.ti.com/tool/ADS1118EVM

    For the measurement, the thermocouple will give a small voltage dependent on the difference from the thermocouple temperature and the junction temperature. Here are a few thermocouple responses for when the cold-junction is 0°C.




    However, you can see that this voltage is non-linear. The problem is that the thermocouple voltage is also dependent on the junction temperature.

    To calculate the thermocouple voltage, you follow these steps:

    1. Determine the cold-junction temperature and convert the cold-junction temperature(TCJ) to a voltage (VCJ)
    2. Add the cold-junction voltage to the measured thermocouple voltage (VCJ + VTC)
    3. Convert the summed cold-junction voltage and thermocouple voltage to the thermocouple temperature (TTC)

    As a figure, it looks like this:



    For determining the temperature to voltage for the cold junction and the voltage to temperature calculation for the thermocouple, you could use the polynomial calculation, but often, people use a lookup table. Both can be found here for a variety of thermocouples:

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



    Joseph Wu