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ADS1248 thermocouple measurement with 2-wire RTD

Ryuji
Guru 20090 points
Other Parts Discussed in Thread: ADS1248

Hello,

Please let me know the ADS1248 thermocouple measurement with 2-wire RTD.

(1) Settling time of Refference
We are using internal reference for thermocouple and using VREFP1-VREFN1 via Rbias voltage for 2-wire RTD.
Could please let me know the settling time from internal to VREFP1-VREFN1 when we change the REFSELT bit?

And for thermocouple measurement with 2-wire RTD, internal reference is always on.
In this case , should we consider the internal reference  settling time when we change the REFSELT bit from REF1 input pair to onboard reference ?

(2)Rbias and IDAC setting
Are there any methodology for deciding the Rbias and IDAC setting?


Best Regards,
Ryuji Asaka

  • 0
    TI__Guru* 97585 points

    Ryuji-san,


    If the internal reference is already on and settled, it should not take any time for the reference value to be settled at the change of the REFSELT bit. Since the reference is already to the final value, settling time is not required for a change in reference selection.

    However, if you are using the IDAC to drive a reference resistor, it may take time to settle after turning the IDAC on, or switching from different outputs. The reference resistor may have capacitance, and will have some RC time constant to settle.

    For the Rbias value selection, I assume that you are referring to the reference resistor. I would start by selecting the Rbias based on the value of the RTD that you are using so that the RTD maximum value is less than the Rbias value. After that, the IDAC setting is chosen so that the reference is as large as possible, but the voltage of across the RTD+Rbias is less than AVDD-0.7V so that the IDAC upper voltage compliance is met. A good value would be 2V for the reference and 2V for the max RTD value.


    Joseph Wu