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Original question:

RE: ADS124S08EVM: ADS124S08EVM How to make a voltage measurement across a resistor

ADS124S08: Four-Wire PT100 RTD Measurement C

Sadashiva Kamath
Expert 1135 points
Part Number: ADS124S08

Hi,

I have configured for ADS124S08 for Four-Wire PT100 RTD Measurement. RREF is 1K,1%.

I got below formula for RRTD from  "Four-wire PT100 RTD measurement circuit with low-side reference"

RRTD = RREF • [Output Code / (Gain • 2^23)]

Couple of questions -

1) Is RRTD is independent of IDAC current? Output Code remains same for 1mA/2mA IDAC current?

2) I see that RREF in the RRTD calculation formula. Is tolerance of RREF (1%, 0.1%) really matters for calculated value of RRTD ? As tolerance will vary from part to part resistors.

  • +1
    TI__Guru** 112895 points

    Hi Sadashiva,

    Sadashiva Kamath said:
    1) Is RRTD is independent of IDAC current? Output Code remains same for 1mA/2mA IDAC current?

    With a ratiometric measurement where the IDAC excites both the sensor and the reference, the value of current drops out of the transfer function as you have shown.  So as long as the current source is capable of providing the minimum reference voltage and the sensor/reference maintains the IDAC compliance voltage the result should be the same.

    Sadashiva Kamath said:
    2) I see that RREF in the RRTD calculation formula. Is tolerance of RREF (1%, 0.1%) really matters for calculated value of RRTD ? As tolerance will vary from part to part resistors.

    The initial value of the reference resistor can be calibrated.  Using a better tolerance resistors may reduce or eliminate the need for calibration.  However the temperature coefficient is really the problem as the resistor can drift considerably.  Usually the drift performance improves with better tolerance resistors.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Expert 1135 points

    Thanks Bob, I have done ADS124S08 functional simulation which incorporates IDAC, RRTD, RREF, GAIN, ADC output code and output code to Resistance conversion. It works as expected. I thought of deleting my queries at tech forum when I completed simulation. I saw later that you are already replied with useful details on calibration, drift etc.,

    Able to visualize the following from simulations -

    1) Change in IDAC changes full-scale voltage (I mean Vref). ADC's output code remains same irrespective of change in IDAC

    2) To avoid calibration (part to part variation), use better tolerance resistor, Rref.