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ADS124S08EVM: EvalBoard connectet with 100 Ohm resistor gets wrong temperature results

user6355302
Prodigy 30 points
Part Number: ADS124S08EVM

Hello,

I want to start measurements with the ADS124S08EMV evaluation board. At the moment I have connected a 100 Ohm resistor like on the picture 15. However, I get a value between -45 and -50 °C in the software when reading out. What can be the reason for this ?

I connected the 100 Ohm resistor on J7 Pin 1 and 4
Normally the value should fluctuate around 0°C.

Maybe someone here has already had the same problem and can help me here.

Thank you very much.

  • 0
    TI__Guru** 113765 points

    Hi user6355302,

    Welcome to the E2E forum! The Data Inspector display you are referring to is not temperature.  It is the number of codes output by the ADC from the conversion.  So what you are seeing are output codes slightly below 0V. 

    It would also appear that the IDAC is not turned on or is at a very low current output.  Did you use the script to setup your registers?  You need to route IDAC current through AIN5 and also enable the REF1 as the reference input.  You will also need to turn on the internal reference even though it is not used for the measurement.  the internal reference establishes the reference for the current sources.

    I would highly recommend reviewing the document:

    A Basic Guide to RTD Measurements

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 30 points

    Hello Bob B,

    thank you for the answer and the document. Unfortunately I didn't have time to test the board in the last days. That's why my question comes a little delayed.

    As mentioned, I connected a resistor of 100 Ohm (PT100) to input J7 at PIN 1 and PIN4. The other components are already integrated on the board, so only the inputs have to be enabled accordingly. I set the current to 1000µ, that is 1mA. Also the other settings should fit. But I still do not get the "correct" results. See picture number 1_DEVICE which shows the results using the DEVICE program.

    Under the program point SCRIPTS I used the script 2-wire RTD written by you. Here other values come out, but I also have the problem that I cannot change these values to °C so that I can read the temperature of the sensor directly.

    Maybe you can briefly explain me what I am doing wrong or have a document that explains this.

    Thanks a lot.

    Best Regards

    Daniel

  • 0 in reply to user6355302
    TI__Guru** 113765 points

    Hi Daniel,

    If you follow the script commands you should be able to see what the differences are between your initial setup and the one I setup in the script.

    First of all, for the IDACs to actually deliver current there must be a reference source to establish the correct output value of the current selected.  The reference used is the internal reference, so you must enable the internal reference even when using the external reference as the measurement for the RTD.  So REF1 is selected as the reference source and the internal reference is set to always on.  The reference buffers are both enabled as the reference is a high-side reference.

    The next command sets the PGA to a gain of 4 with the PGA enabled.

    You must select the correct input combination or you may see negative values instead of positive values.  So the input combination should be set to AINP = AIN1 and AINN = AIN0.

    You will need to set the IDAC magnitude to the desired value of 1000uA.  This appears to be set correctly in your initial settings.  However, you need to route the current out one of the pins, and for this configuration it is AIN5.

    Now the calculation of the codes to resistance is relatively straightforward.  If I take one of the values I see in the result, 3342092 and convert this to resistance by using the ratiometric formula Rrtd = Rref * output code / (Total Positive full-scale codes * PGA Gain) = 1k (R68) * 3342092 / (2^23 * 4) = 99.6 Ohms.  To convert to temperature you would need to use polynomial equations or a look-up table.

    As the EVM can be used in a variety of ways the GUI implementation does not have the capability of direct conversion to temperature.  You would need to export the data and then read the data into a spreadsheet with conversions done there.  You will not be able to directly read the temperature from the EVM GUI.  The calculation would be similar to what I have described above.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 30 points

    Hello Bob B,

    thank you so much for the help. I have now compared the script and the manual data of mine and found my mistake. The code that is generated is what I thought. I have now written a script that calculates the resistance and the temperature directly from the code.

    If there are any further questions during my tests, I would contact you again.

    Thanks.

    Regards and a nice day

    Daniel