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ADS8866: 55 pF sampling capacitor charge

Jonathan Geronga
Expert 7930 points
Part Number: ADS8866

Hi Team,

Our customer would like to confirm if the 55 pF sampling capacitor is discharged after each conversion or does it still hold charge from the previous sample when the new sample begins for the ADS8866? 

Please let me know if you have any questions with the customer.

Thanks,

Jonathan

  • 0
    TI__Guru 62085 points

    Hello Jonathan,

    The sampling capacitor is often reset to a different value than the previous conversion in most SAR ADCs.  This was often VREF, GND, or VREF/2 in original SAR designs.  Modern SARs use more advanced resetting and CDAC topologies than the original devices so it's very possibly a combination of some different levels. 

    Note that if you're asking about this to help determine your input driver requirements then we typically recommend to use the worst-case analysis that discharges the sampling capacitance to GND or VREF/2 between conversions.  This will provide a conservative estimate if the sampling capacitance is reset to a closer value to the previous conversion, but will be a robust design.

    For more information on SAR ADC drive theory please take a look at the following TI Precision Labs Videos:  https://training.ti.com/node/1139106?context=1139747-1140267-1128375-1139106