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ADS124S08: Design check of ADS124S08IRHBR

Gopika Raj V S
Intellectual 360 points
Part Number: ADS124S08

Hi Team,

We are planning to use the RTD to digital converter IC, ADS124S08IRHBR for our new design. The application should work in a refrigerator cum oven setup (Temperature range). Our mandatory requirements are listed below. We need to know weather the design here can achieve all this requirements. PFA the design file with this thread. Kindly, have a design check on this and please share TI's recommendations and comments for the same. The major requirements are:

  • RTD should achieve 0.1 degree celcius of accuracy in the measurement.
  • We are using 4 wire RTD (PT1000) sensor. Can we use same connector for both RTD and thermocouple, as both are supported with this converter IC?

RTD CUM THERMOCOUPLE DESIGN.pdf

  • 0
    TI__Guru** 113765 points

    Hi Gopika Raj V S,

    It is not totally clear as to your intentions.  Are you planning on using two ADS124S08 devices?  There are actually enough inputs where you can connect both the RTD and TC (thermocouple) to a single ADS124S08.  In either case I would recommend not using a ferrite between the analog and digital grounds.  We have seen where large transients can create enough difference voltage between the grounds where the ADC can be damaged.  One thought is to keep the grounds separate for routing purposes to keep the analog portion and digital portions from crossing over into each other's domain, and then tie the grounds together with a 0 Ohm resistor near the ADC.  However, there should be good success using a single ground plane area which can minimize any ground loops.

    The RTD circuit appears to be pretty straight-forward.  The only thing I would suggest in addition to what you are doing is to add some input protection to the ADS124S08 IDAC output on AIN0.  This could be in the form of a resistor or a diode.  I would recommend connecting the REFN0 connection directly to analog ground, otherwise you will need to connect through the AVSS-SW which will add some additional analog settling delay for the measurement. 

    The TC input circuit does not require excitation as the TC creates it's own voltage by Seebeck effect.  I would suggest reading A Basic Guide to Thermocouple Measurements which also includes several circuits you can use with the ADS124S08.  Most common is the use of the internal reference for the conversion and some form of bias to keep the TC within the measurement range.

    Is the plan to use the TC for the actual temperature measurement and the RTD to measure the cold-junction?  As I said at the beginning, it is not clear as to your intentions.  For the TC measurement you will need to measure the cold-junction to correctly compensate the ADC voltage relative to the cold-junction.  This is all explained in the TC guide.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Intellectual 360 points

    Hi Bob,

    Thanks for the reply. 

    Our intention is to measure the temperature with an accuracy less than 1degree celsius. Here we are in evaluation phase where we have to check both RTD and Thermocouple and evaluate its accuracy individually. Our final product will have either of them. As thermocouple is cost effective most probably we will choose this. We prefer an operating range of -50degree celsius to +300degree celsius and the accuracy over 120degree celsius.

  • 0 in reply to Gopika Raj V S
    TI__Guru** 113765 points

    Hi Gopika Raj V S,

    Again I would emphasize reading the following guides for information:

    This is not a trivial task.  Noise can be easily picked up and affect the accuracy of the measurement.  Also, the accuracy of the thermocouple measurements will be directly proportional to the measurement of the cold-junction of where the thermocouple enters the PCB.

    TI also has a temperature sensing library of 'C' code that you can use to compute the temperature;

    Best regards,

    Bob B