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ADS124S08: connecting to 4-wire PT100 RTD

Sai Loo
Expert 4660 points
Part Number: ADS124S08
Other Parts Discussed in Thread: ADS1248, ADS1220

Hi team,

My customer is designing multiple RTD and will be using 4-wire connection PT100. Do we have any reference design and calculation for 4-wire connection? And the max number of RTD be connected in single ADS124S08? (they need 9 channels so will use either 2 or 3 ICs).

In our datasheet, it only shows 3 wire connection and calculation.

Referring to TIDA491, it is using ADS1248. Will the 4-wire connection (and calculation) be used in ADS124S08?

I'm also a little confuse on the sensor burn-out detection. How could we implement this in the RTD application?

  • 0
    TI__Prodigy 330 points

    Hello Sai,

    The ADS1220 datasheet include sections about 2-3-4 wire RTD measurements. (4-wire RTD on page 56)

    The picture shows how the circuit would basically look with 4 RTDs but i recommend you to following the design tips of the datasheet.

    As you can see has each RTD to connect to 3 channels. 1 channel to force the current from the IDAC and two channels for the differential signal. The max number of RTDs will be 4 for the ADS124S08.

    Instead of working with the internal reference we using the same current to generate a VREF to compensate the noise and ripple from the IDAC.

    The code represent the relationship between V_diff against VREF, which is because of the same current equal to R_rtd to R_bias.

    Calculation:

    Code/2^24 = VRDT / (2* VREF)
    Code/2^24 = (IDAC * RRTD) / (2 * IDAC * RREF)
    Code/2^24 = RRTD / (2  * RREF)

    RRTD = RREF * Code/2^23