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ADS1220: ADS1220

Sisto Zanier
Prodigy 70 points
Part Number: ADS1220

Hi to all,

for a battery powered thermometer, with PT100 RTD sensor, I'm currently using a ratiometric ADC MAX31865 (15bit) which is not good for me because its 50/60Hz internal filter cannot be excluded and this penalizes the performance because, in "Single Shot" mode (every two seconds), the conversion time at 50Hz, is 60mSec.

Since the RTD is excited with a 4mA current, this means an important higher consumption and therefore shorter battery life.
For this reason, now I would like to use ADS1220 which has very similar characteristics, but it has the possibility to be configured to exclude the 50/60Hz filter.
For RTD to T°C conversion I used Callendar-Van Dusen and/or polynomial formulas.
My question is: can I use the same formulas for the conversion with ADS1220?

/*-------------- Polynomial ------------------


            R(c1+R(c2+R(c3+c4R))
  TC = c0+ --------------------------------
            1+R(c5+R(c6+c7R))

R = RTD_VALUE calculated using the formula: (float)RTD_VALUE = (float)ADC_VALUE * MAX_RREF/MAX_ADC; // (MAX_ADC: 32768 with MAX31865)

*/

#Define c0 -245.19
#Define c1 2.5293
#Define c2 -0.066046
#define c3 4.0422e-3
#define c4 -2.0697e-6
#Define c5 -0.025422
#define c6 1.6883e-3
#Define c7 -1.3601e-6
----------------------------------------------------

------------ Callendar Van Dusen --------

                   ------------------------------------
                  /                   /  RTD_VALUE \ 
      -RTD_A+ \  / 2*RTD_A -4*RTD_B* | 1- --------  |
               \/                     \        RT0 /
  TC = ------------------------------------------------
                        2*RTD_B
*/

// Correction constants according to IEC EN60751

#Define RTD_A 3.9083e-3
#Define RTD_B -5.775e-7
#Define RTD_C -4.183e-12
#define RT0 100           // Ohm value of PT100 at 0°C

Thanks for your any replay

Sisto

  • 0
    TI__Guru** 112725 points

    Hi Sisto,

    Th polynomial is for calculation of the RTD resistance to temperature.  This would be the same for both ADCs.  The calculation for the RTD resistance would be similar, but could be slightly different with respect to the number of IDACs used, the measurement type and if gain is applied.  I would suggest reviewing the RTD application circuits in the ADS1220 datasheet.  Another source of very useful information regarding the various different measurement types that can be used is A Basic Guide to RTD Measurements.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    Prodigy 70 points

    Hi Bob,
    thank you for your answer.

    I agree with you about the small differences depending on the number of IDACs used that with the MAX31865 is unique, while in the ADS1220, used as RTD converter in three-wire configuration (my case) I will use two IDACs both set at 500uA. In both cases the voltage at the ends of the RREF is measured.
    Already before deciding to use ADS1220, I downloaded and read carefully the AN SBAA275 you suggest and from it I obtained the electrical circuit for my application.
    The ideal would be to buy the EVB for this component, but the cost seems to me excessive and so I will experiment directly on the prototype of my PCB board. (I attach the part of diagram concerning the ADS1220)

    Best regards,

    Sisto Zànier