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ADS1220: ADC input filter impedance

Part Number: ADS1220

Hello Team,

in the datasheet it is recommended for the Rf resistors not to be higher than 1K.

If I can accept:

  • reduction of the dynamic due to the input current flowing through the Rf
  • increased differential error due to Rf mismatch

I'm assuming I can use Rf higher then 1Kohm

  • What would be a limit I shell use as "no go"?

Currently the plan would be to test 5-10Kohm.

  • Would that work for the ADC1220 by accepting the limitations?

From settling time/sampling frequency I'm assuming some limitations as well.

  • Would the input current for the ADC be guaranteed by using the higher resistors?

The plan would be to use PT1000 with a ratiometric configuration.



  • Hi SunSet,

    I would recommend taking a look at A Basic Guide to RTD Measurements and also the new RTD calculator as helpful reference material.  As to the ADS1220 input current there are two main considerations.  One is the leakage current of the device over temperature and the second is input bias current.  The leakage is greatest on AIN3.  The input bias current will be greatest when the PGA is disabled and bypassed.  I would suggest taking a look at the input current graphs on page 12 of the ADS1220 datasheet.  In the end what may happen is a voltage drop across the input filter resistor which will be seen as a measurement error.

    The value of 1k is primarily a suggestion based on highest leakage and PGA disabled.  You should be able to see acceptable results with higher filter resistance.  I would suggest staying lower than 10k if possible.  If you have a 3-wire RTD and use AIN2 and AIN3 to deliver the current for excitation this would be the best scenario as the leakage would not have a large impact to the measurement.

    As you can see from the graphs, the input current is not static and can change with voltage applied and temperature.  Also, as you mentioned, analog settling time behavior must  be considered.  This would also include the reference filter as well and the analog input from the RTD.

    Best regards,

    Bob B