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ADS114S08B: Question on RTD / Thermocouple setup

Darren (FAE)
Expert 5483 points
Part Number: ADS114S08B
I have a customer asking about some implementations. 1) 3-wire RTD. Can 1 ADS114S08B handle 4x 3-wire RTDs? I found some reference material showing 4 channels used per RTD for up to 3x (12 ADC channels) with the mux to switch excitation currents...any way to wire it up for 4x RTDs? 2) Any problems/concerns with using 1 ADS114S08B for 4x thermocouples? (J or K) 2 channels per thermocouple seems to indicate the 12 channels available should be more than enough, but I just want to see if there are any points to consider. Thanks!
  • 0
    TI__Guru** 113765 points

    Hi Darren,

    The RTD design you are describing uses 2 IDAC currents to cancel lead resistance.  This method requires 4 channels for each 3-wire RTD, so you max out using this approach at 3, 3-wire RTDs.  However there is another method (apart from using an external mux) that requires 2 measurements.  The first measurement is the RTD measurement, and the second measurement is for determining the RTD lead resistance.  An example of this circuit (for a single 3-wire RTD) is shown in A Basic Guide to RTD Measurements (SBAA275) in section 2.4 on page 17.  To make 4, 3-wire RTD measurements you would need 4 inputs to use as the current excitation (one for each RTD) and connecting to lead 1 of each RTD.  Lead 1 would also connect to an RTD measurement channel input, as would lead 2.  Lead 3 would be a common for all four RTDs and would connect to AINCOM and the reference resistor.

    So for this configuration you would need to use AIN0-AIN11 and AINCOM.  The downside of this method is you need to make multiple measurements for each RTD.  If this sounds doable, I can draw up something to show more accurately what this would look like.

    As for the TC, there should be no issue measuring the TC.  The bigger issue is in accurately measuring the cold-junction (CJ) for cold-junction compensation and if the ADS114S08B will be used for making the CJ measurement. There is also A Basic Guide to Thermocouple Measurements (SBAA274) with additional information.

    Best regards,

    Bob B