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AFE4404: AFE4404 Rf, Cf Features

Part Number: AFE4404


I have a question while I was picking ppg with afe4404.

I understand OFFDAC, Gain.

But I don't know the function of the CF.

Thank you.

  • Hi,

    Cf, Rf together form complex impedance that allow user to set complex gain for the Transimpedance amplifier for AC signal.

    And, from datasheet "Cf is adjusted to keep the TIA time constant at 1/10th of the sampling duration"

    for further detail, please refer to data sheet section TIA Bandwidth Settings.


  • Thank you for your reply.

    See datasheet 16p TIA Bandwidth Settings.
    The product of Rf and Cf gives the time constant of the TIA and must be set approximately 1/5th (or less) of the LED or sampling pulse durations.

    What exactly is the time constant for TIA?
    LED, Sampling pulse duration 2 are related to Timing Controls shown in the picture below?

    I don't know which value to compare the product of Rf and Cf.

    Thank you.

    AFE4404: AFE4404 EVM Configuration - Sensors forum - Sensors - TI E2E  support forums

  • Hi,

    For "What exactly is the time constant for TIA?"

    RC time constant or RC cutoff frequency can be simply googled online - 

    e.g. RC time constant tau = 1/RC



    • τ is the time constant in s
    • R is the resistance in Ω
    • C is the capacitance in F

    And try to set the RC such that the time constant is equal or less than the LED ON time OR Sample LED time duration.