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ADS8353-Q1: Input Impedance of Analog IO pins.

Part Number: ADS8353-Q1
Other Parts Discussed in Thread: TINA-TI

Hai, 

How do we calculate input impedance for Analog Input Output pins of ADS8353QPWRQ1? 

  • Hi Rony,

    Thank you for your post. 

    Both the ADC inputs and the reference I/O pins have a dynamic switch-capacitor input structure. As a result, the input impedance is relatively low, on average a few kOhms. The key design criteria fall on the input and reference drive circuits, which must have adequate bandwidth to charge the internal ADC sampling capacitors on the input and reference pins during the ADC acquisition phase. The application section of the data sheet gives you some examples of the external circuits required to drive these pins.

    For more information, you can also view our TI - Precision Labs topics on driving SAR ADCs.

    Best regards,

    Ryan

  • Thank you Ryan for the quick replay.

    I was actually performing a worst case analysis for a circuit containing above IC.is it possible to find the worst case impedance of that pin using max applied voltage/ input current (10nA) of analog io pin?

  • The worst-case would be to assume that the input capacitors begin the acquisition phase completely discharged and need to charge to full-scale within 1/2 LSB of error before the end of the acquisition phase. This is covered in great detail in some of the videos I mentioned above. We also explain how to simulate this using TINA-TI so that you can verify your analog front-end meets the requirements.

    The Analog Engineer's Calculator is another way to get a good first-order approximation of the amplifier and R-C component values needed at the ADC input.

    Regards,

    Ryan