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Original question:

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

ADS124S08EVM: ADS124S08EVM: 2-wire RTD measurement

Sadashiva Kamath
Expert 1135 points
Part Number: ADS124S08EVM

Hi,

I'm using ADS124S08EVM for 2-wire RTD measurements. I have connected approx. 98.86 ohm (value to be measured) across pin1-4 of J7. 

In time domain data analysis of evaluation software, I see 0.24715V for ADC0. How ADC0 voltage will be 0.24715V?

I'm fine with ADC codes of 3317177  (Since, 2^24-1 corresponds to 2V and Gain 4 used)

  • +1
    TI__Guru** 112725 points

    Hi Sadashiva,

    As the measurement is ratiometric you should make the resistance conversion relative to the reference resistor.  As far as the voltage is concerned, note that if you are using a 1mA IDAC current and 1k reference resistor the reference is 1V.  The Data Analysis window must also reflect the same voltage as you are using as there is no way for the GUI to know exactly what value of reference you are using unless the internal reference is selected.  The default is 2.5V (internal reference) and you must change the value to use the correct reference voltage for the calculation by changing the value in the VRef box to 1.

    Also note that if you divide the 0.24715 by 2.5 you will get 0.09886V.  Dividing this voltage by 1mA you will get a resistance of 98.86 Ohms.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Expert 1135 points

    Thanks Bob,

    Software is knowing Vref. Since, Rref of 1K used in eval kit. and IDAC/IMAG, REFSEL etc., known to software.

    Also, REFP1 & REFN1 connected to ADC's input mux.

    Additional option of Vref auto-detect is a good feature in software as default option?