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AFE5832: Input impedance

Danilo A.
Mastermind 25855 points
Part Number: AFE5832

Hi Team,

Can you please us help with this inquiry?

I am working with the AFE5832. I understand from the datasheet that when the attenuator is enabled, it essentially acts as a voltage divider and therefore an internal resistor is implemented.

When the attenuator is disabled, what is the input impedance of the AFE? The datasheet mentions that the input impedance is given by the input capacitance alone (35pF), but certainly there must also be a (very) high resistance to ground when the attenuator is 'open'? What would be a reasonable value for that resistance?

Also, why is the input impedance at low frequencies (<500kHz) not following the capacitive impedance model? (see attached figure). If this model were true, the impedance would be hyperbolic (1/Cw), but this model breaks down at around 700kHz

Regards,

Danilo

  • 0
    TI__Expert 6790 points

    Hi Danilo,

    Thanks for reaching out.

    I am currently out of office due to health.

    I will be back to office either tomorrow or next monday.

    Kindly allow me to come back and have a detailed look at your query.

    Thanks & regards,

    Abhishek

  • 0 in reply to Abhishek Vishwa1
    TI__Mastermind 25855 points

    Hi Abhishek,

    Thank you for your response. Our customer would like to request an update to his inquiry.

    Regards,

    Danilo

  • 0 in reply to Danilo A.
    TI__Expert 6790 points

    Hi Danilo,

    Sorry for the delay in the response as I was not out of office due to health issues.

    The Figure 4 in the datasheet illustrates the magnitude of the input impedance versus frequency with the attenuator disabled.

    The roll-off in the impedance with frequency comes from the capacitive parasitic at the input pin.

    With the attenuator enabled, the input impedance can be calculated as the parallel combination of the attenuator’s resistance and the ‘attenuator-open’ input impedance shown in Figure 4.

    I hope this helps. Let me know if there are any further questions.

    Thanks & regards,

    Abhishek