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ADS1220: settings to eliminate the error vs ambient temperature?

Viktorija Cecil
Mastermind 18491 points
Part Number: ADS1220
Other Parts Discussed in Thread: TPS22810

We’re testing the ADS1220 circuit with a 100 ohm test resistor, which simulates a platinum RTD probe.  In our circuit, this corresponds to an indicated temperature of 0 °C.

With the resistance outside the test chamber (to keep the value constant), we change the ambient temperature of the ADS1220 circuit, with the following results:

          When the ambient temperature is between 22 °C and 40 °C, the indicated temperature remains 0 °C, as expected.

 However, at – 30 °C ambient, the indication is -0.3 °C instead of 0 °C.

We’re using the ADS1220 in the 4-wire RTD circuit configuration described in the datasheet on Page 56  (ADS1220 SBAS501C).  Rref = 500 ohms, ± 0.2 ppm / °C.  In the test, we took care to emulate the probe we use in the field (proper shielding, etc).  Our circuit also has the TPS22810, to ensure the power-on voltage ramp rate meets ADS1220 requirements. 

What settings should we examine to eliminate the error vs ambient temperature?

 Thanks

Viktorija

  • 0
    TI__Guru** 113765 points

    Hi Viktorija,

    It would be helpful to know a few things such as the ADS1220 register configuration settings, and the actual raw output codes at the various temperatures.  To reduce the effects of excitation drift, it is important that the measurement/calculation is made ratiometrically.  The resistance calculation should be based on the ratio RTD/Rref = (2*Output Code)/(Total Number of Codes * Gain).

    A 4-wire solution requires a Kelvin connection to the external resistor.  In the end, there will be some drift of the reference resistor,  as well as gain and offset drift of the ADS1220. The offset of the ADS1220 can be adjusted by using the internal short of the ADS1220 to correct for initial offset and any drift.

    Best regards,

    Bob B 

  • 0 in reply to Bob Benjamin
    TI__Mastermind 18491 points

    Bob, thank you for your inputs. Customer is reviewing them. 

    They also noted that this behavior is limited to one of four boards. Does that give any clues what could be happening?

    Thanks

    Viktorija

  • 0 in reply to Viktorija Cecil
    TI__Guru** 113765 points

    Hi Viktorija,

    It is difficult to speculate on why they are seeing a difference.  I cannot determine the amount of ADC error from just a temperature difference as I do not know how they are calculating the RTD resistance and conversion to temperature.  I really need more information, such as the schematic, register settings used, the calculations being used and the raw data.  I would also like to see data in what is considered a good case.  Again the data should be in codes and not temperature. 

    It would also be helpful to know if any calibration is being done.  The ADS1220 will have some inherent offset along with offset drift.  That said, the characteristics of the ADS1220 trends pretty much flat at temperatures below room temperature.  If we assume that the offset is flat, then there is also the possibility of gain error, or error in the method of calculation.  The accuracy of the measurement can diminish if the measurement/calculation is non-ratiometric due to variation of the IDAC accuracy and drift.

    Best regards,

    Bob B