This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

ADS124S06: Combination of 2 4-wire RTDs and one 2-wire RTD

Part Number: ADS124S06
Other Parts Discussed in Thread: ADS124S08


I am trying to do RTD measurements using the ADS124S06. Can you verify this scheme for me please.

The resistance range of the RTDs are:

  • RTD1 - 4wire measurement: 900 - 1400 Ohm
  • RTD2 - 4wire measurement: 600 - 1400 Ohm (This is a PT1000 with P/N: 32208707 - 0 degree C to 100 degree C)
  • RTD3 - 2wire measurement: 100 - 500 Ohm

The measuring scenario is:

  1. Device bootup -> RTD3 is measured once.
  2. After that, RTD1 and RTD2 are continuously measured throughout the process: I also have some questions at this point:
    1. Can I drive IDAC1 and IDAC2 simultaneously?
    2. I don't need the PGA gain for the RTD1 and RTD2 measurement, right? Because the measurement voltage at AIN is about ~2,2V max.

Sorry for the bad drawing :) 

I am referring to this 4-wire design and 2-wire design guideline by TI.

Thank you very much in advance.



  • Oh one more note:

    • IDAC1 = 0.3mA
    • IDAC2 = 0.3mA (for the PT1000)

    Thank you in advance :) 

  • Hi Thai,

    Welcome to the E2E forum!  Let me address a couple of the easy things first.  Verify that the current you want to use is available.  There is a 250uA setting and a 500uA setting.  There is no 300uA setting.

    The next thing to consider is the availability of IDAC source pins.  The IDAC cannot be sourced out the pins designated as GPIO on the ADS124S06.  You would need to use the ADS124S08 instead.

    As far as the circuit design, you may want to review information in A Basic Guide to RTD Measurements.  Also one of my colleagues put together an RTD design calculator tool that you may find helpful.  This tool will help make sure that your design will work with the desired resistance ranges and analog supply voltage.

    Can I drive IDAC1 and IDAC2 simultaneously?

    Yes they can be driven at the same time, but they cannot be individually set to different current output values.  Also keep in mind that the reference voltage will be 2 * IDAC * Rref.  Normally you would only activate one IDAC path at a time to keep the measurement fully ratiometric.  Another reason is the input uses a mux and there is only one ADC so you can only measure one at a time and would need to switch back and forth between the two sets of input channels.

    I don't need the PGA gain for the RTD1 and RTD2 measurement, right? Because the measurement voltage at AIN is about ~2,2V max.

    Whether or not you need gain will depend on the desired measurement resolution.  This is where the calculator tool may be of some benefit.

    Best regards,

    Bob B

  • Hello Bob,

    Thank you very much for your prompt response.

    The ADS124S08 is not available on the market at the moment so I guess I have to stick with the ADS124S06.

    According to our datasheet, the RTD can be driven with 250uA.

    In which case, is it feasible to mux the IDAC1 to AINCOM and then have another mux to drive each RTD?

    For example, this TS5A3359DCUR seems like a good candidate for the mux.

    I'm relatively new to this RTD measurement so please be patient :).

    Thank you very much in advance.



  • Hi Thai,

    Using an external mux to distribute the current to the various RTDs is an appropriate solution when the number of delivery options are reduced between the S06 and S08 devices.

    Best regards,

    Bob B