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ADS122C04: Max tolerance and long term drift

Christoph Guntermann
Prodigy 10 points
Part Number: ADS122C04

Hello

I would like to calculate the long term drift of ADS122C04IPW device.
Assuming the connected PT100 provides always the right resistance value: the question is, how big is the max tolerance and how could the ADS122C04IPW drift over a time of 7-8 years?
How can I calculate such a drift? Output vs Input voltage/R_PT100 (2 wired).


The idea would be that the device with the PT100 would be once calibrated at the beginning of the lifetime and that then the device works accuaratly enough for 7-8 year (continous operation).

I already had a look at the docs:
- https://www.ti.com/lit/ug/tidude4/tidude4.pdf?ts=1615900215012&ref_url=https%253A%252F%252Fwww.ti.com%252Ftool%252FTIDA-01526
- https://www.ti.com/lit/ds/symlink/ads122c04.pdf?ts=1615897715749

Regards Christoph

  • +1
    TI__Guru** 112895 points

    Hi Christoph,

    The maximum values in the datasheet are shown in the electrical characteristics table for the ADS122C04.  If the measurement is ratiometric, then any long term drift of the IDAC source will cancel out of the measurement (section 2.3 of TIDUDE4).  Offset error and drift can be canceled by taking 2 measurements and swapping the analog input between measurements and averaging (section 3.2.1 of TIDUDE4).

    The dominate error will come from gain error/drift.  There will be the gain error of the ADC which is listed in the electrical characteristics table, and gain error caused by the drift of the reference resistor.  With the RTD ratiometric measurement, the output code is proportional to the reference resistor.  So you don't think in terms of voltage with this type of measurement but instead resistance.  It is the resistor used that will have the most dramatic effect on the conversion result as the drift and aging effects of this resistor usually dominates (depending on the quality of resistor used).  For example, a typical resistor may have 200ppm drift where as a higher quality resistor may be 25ppm or even better.

    I would suggest reviewing the material in A Basic Guide to RTD Measurements for additional information.

    Best regards,

    Bob B