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ADS114S08B: Input impedance with PGA bypassed

Adam Daluga
Expert 5770 points
Part Number: ADS114S08B

Hello,

what is the input impedance with the PGA bypassed? I don’t see it in the datasheet.

best,

adam

  • 0
    TI__Guru** 113765 points

    Hi Adam,

    It was decided not to specify an impedance as the actual input current is dynamic over input voltage.  Instead, absolute and differential input current are given in the Electrical Characteristics table.  Absolute input current would include leakage from ESD diodes.  The Typical Characteristics graphs in section 7.8 show the input current over input voltage and temperature.  See Figures 7 and 8 for PGA bypassed for absolute input current and Figures 11 and 12 for differential current.

    Best regards,

    Bob B

  • 0 in reply to Bob Benjamin
    TI__Expert 5770 points

    Thanks Bob,

    Is it possible to get a plot across input voltage? the current is not what my customer is really looking for, more the complex impedance.

  • 0 in reply to Adam Daluga
    TI__Guru** 113765 points

    Hi Adam,

    Please put the customer name and project information in the Notes Section of the Thread Tracking Toolkit at the top of the post.

    As the impedance is complex I cannot give you a specific number as the impedance is dynamic over temperature, input voltage, data output rate and whether or not the PGA is enabled or disabled.  Also, the plots take into account the total measured current which includes leakage.  This is what the sensor will see not just an R-L-C value making up the total impedance.  The PGA, when enabled, is an INA structure so this will appear as 10's of Gig Ohms.  When the PGA is disabled and bypassed, the input limits current by using a pre-charge buffer to limit current from the sensor.

    Normally this impedance question gets asked when the customer has a high-impedance sensor or wants to use extremely large input filter resistors.  In both of these cases the error can be derived by current analysis.  Is there a specific reason why the customer wants a 'value' of impedance?  Maybe I can explain it better if I know why this is so important.  But I would need to know the specific use case of the ADC.

    Best regards,

    Bob B