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ADS122C04EVM: Correct settings and interpreting output for thermocouple and analog sensor

ANDREY POLONSKY
Prodigy 30 points
Part Number: ADS122C04EVM
Other Parts Discussed in Thread: ADS122C04

Hello,

I'm hoping you could help in setting and/or interpreting the output of the ADS122C04EVM when using a thermocouple with and without an analog sensor (LM35CAZ/NOPB). Looking at just the thermocouple on its own, the jumpers set as below and starting with the default register settings, at room temperature I'm seeing -0.3degC (original count of -56 converted to -0.0137mV and then to -0.3degC) and also at an elevated (breath) temperature I'm seeing 10degC (original count of 1720 converted to 0.4199mV and then to 10degC). It would make sense if this was with respect to the cold junction temperature, but from what I understand, that is not performed automatically especially with default setting of "TS=Disables Temperature Sensor." 

So the questions are:

1) Is there a way to avoid having to convert from count to voltage by exporting the values graphed on the "Time Domain" tab's graph? I can't find a way to see those values other than as dots on the graph.

2) Why is the temperature so low?

3) In order to connect the output of LM35CAZ/NOPB to IN3 so as to measure an accurate cold junction, are the only necessary register changes "MUX->AINP=AIN3, AINN=AVSS" and "PGA_EN->PGA disabled and bypassed (Gain=1 to 4)"?

4) When providing a precision Vref=3.3V, are the only changes "VREF->External reference using the REFP and REFN inputs" and EVM Parameter "VRef (V)->3.3V"?

        

Thank you very much for your time,

Andrey

  • +1
    TI__Guru** 113765 points

    Hi Andrey,

    First of all I would suggest you read through A Basic Guide to Thermocouple Measurements.  In the document you see an explanation on how the thermocouple tables are derived.  All of the tables are based on a junction temperature of 0 deg C.  With the EVM the junction temperature is room temperature.  So you need to change the ADC result to reflect a value to compensate for the different temperature.

    Second, I would suggest connecting the thermocouple in the way it was intended for the EVM.  If you look and the ADS122C04EVM user's guide in section 5.1.2 you will see a basic connection diagram and what the jumper positions should be.

    ANDREY POLONSKY said:
    1) Is there a way to avoid having to convert from count to voltage by exporting the values graphed on the "Time Domain" tab's graph? I can't find a way to see those values other than as dots on the graph.

    Unfortunately there is no direct way to read a particular point as a voltage in this GUI.  Generally I just take an average value based on the graph results.  Otherwise you can convert the counts to voltage manually using the Data Inspector.

    ANDREY POLONSKY said:
    2) Why is the temperature so low?

    This is due to the cold junction temperature.  Compensation is explained in the document reference above.

    ANDREY POLONSKY said:
    3) In order to connect the output of LM35CAZ/NOPB to IN3 so as to measure an accurate cold junction, are the only necessary register changes "MUX->AINP=AIN3, AINN=AVSS" and "PGA_EN->PGA disabled and bypassed (Gain=1 to 4)"?

    The EVM is not setup for the LM35CAZ, although it should be able to be used.  You could connect to AIN3.  I would refer to the diagram for better details on how you might be able to accomplish this, but my concern is that the placement is not going to be in a good thermally isolated area for the thermocouple connection.  Instead I think it would be easier to use the onboard temperature sensor in the ADS122C04 to make your compensation.  This way you don't have to make any other changes to the EVM and instead just read the temperature sensor output.

    ANDREY POLONSKY said:
    4) When providing a precision Vref=3.3V, are the only changes "VREF->External reference using the REFP and REFN inputs" and EVM Parameter "VRef (V)->3.3V"?

    I'm not sure why you would need an external reference.  This will reduce the dynamic range. As far as connection refer to the user's guide.

    Best regards,

    Bob B

  • +1 in reply to Bob Benjamin
    Prodigy 30 points

    Thanks as always Bob!

    Andrey