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ADS1220: ADS1220 input impedance

li yu
Prodigy 40 points
Part Number: ADS1220


Tool/software:

Hi

  As for the input impedance of ADS1220, I did not find this parameter in the specification. Now I need to use ADS1220 to detect PT1000. I am going to add a follower in the input side, which I think is necessary,thanks!

  • 0
    TI__Expert 8130 points

    Hi Li Yu,

    It is difficult to specify a specific value for the input impedance as the input current to the ADS1220 depends on a number of factors including whether or not the PGA is enabled, as well as the operational temperature of the device. 

    Impedance implies some kind of dynamic affect that can change over various operating conditions. In other words, the impedance can change and is not linear like a true resistance. 

    For the ADS1220 it was decided that the input current could be better described graphically showing the effects of the change of impedance over temperature and input voltage. 

    These graphs are in the Typical Characteristics curves starting with Figure 15 on page 12 of the ADS1220 datasheet and continue through Figure 26:

    Note Figures 19 and 20 in the Typical Characteristics plots for when the PGA is enabled.  Here the information is much clearer as to how the differential input current changes over operating conditions.  Figures 25 and 26 show PGA disabled and bypassed.  The impedance will be quite high for the differential case as the current is shown to be quite low unless the device is operating at very high temperatures.

    There should be no issue with the input impedance of the ADS1220 for a PT1000 application.

    You might also find the information found in following app note useful: A Basic Guide to RTD Measurements (Rev. A) (ti.com)

    Best Regards,

    Angel