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Dual ADS1248 4-Wire RTD options

ka88
Intellectual 275 points
Other Parts Discussed in Thread: ADS1248

I intend to measure two 4-wire PT100 RTD's on a single ADS1248.  Please advise on the best connection method to do this. I will refer to the four RTD wires as 1+, 2+, 3-, 4-.  My intention was to connect as follows:

RTDA_1+ connected to IEXC1

RTDA_2+ connected to AIN0

RTDA_3- connected to AIN1

RTDA_4- connected to REFP0 through 1.65k resistor.

 

RTDB_1+ connected to IEXC2

RTDB_2+ connected to AIN2

RTDB_3- connected to AIN3

RTDB_4- connected to REFN0 through 1.65k resistor.

The advantages to this option I believe are using the dedicated IEXC sources, which doesn't require waiting for the IDAC to settle as I switch between the two measurements... which would be the case if I used the AINx pins for excitation.  I expect to sample at 160SPS. AVDD is 3.3V.

Is there any advantage/disadvantage to using a shared reference connection (both connected to REFP)?  Any advantage/disadvantage for using the IEXC versus AINx for excitation?

Thank you.

  • 0
    Intellectual 275 points

    Mistake in my above post:

    RTDB_4- connected to REFP1 through 1.65k resistor.

    Is there any advantage/disadvantage to using a shared reference connection (both connected to REFP0)?  Any advantage/disadvantage for using the IEXC versus AINx for excitation?

     

  • 0 in reply to ka88
    TI__Guru* 97535 points

    Kevin,



    I think you've hit on the major reason not to route the current back and forth, which is because of the settling required for switching the current sources from one input to the next. This is especially important if there are capacitances that need to be charged when the IDAC is turned on.

    Using one of the AINx pins as the output of the IDAC shouldn't be any issue. It won't change the IDAC output compared to using one of the IEXC pins.

    One possibility in using a single reference would be to chain up the RTDs so that a single IDAC current goes through both RTDs and then to the reference. This way, it's one uninterrupted current path. Note that the reference and RTD voltages may need to be even smaller that what you have now, and that the IDAC current source output voltage should be lower than AVDD-0.7V (or in your case 2.6V).

    Also, if you haven't seen these, check out the ADS1248 write ups that we have here:

    http://www.ti.com/lit/an/sbaa180/sbaa180.pdf

    http://www.ti.com/lit/an/sbaa201/sbaa201.pdf



    Joseph Wu