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Multiple photodiodes with AFE4490

ccent
Intellectual 340 points
Other Parts Discussed in Thread: AFE4490

Hi,

I would like to be able to connect one of three photodiodes to the AFE4490. Is it possible to use an analog switch/multiplexer to bring the signals into the TIA? Or will this compromise the accuracy of the system? Are there recommended switch/mux parts for this type of application?

Thanks!

  • 0
    TI__Mastermind 49906 points
    Hello Ccent,

    Thank you for your enquiry regarding the application of AFE4490.
    I am checking with the Systems team and will get back to you as soon as possible.
  • 0
    TI__Mastermind 49906 points
    Ccent,

    Here is the response from the Systems team.

    The bias for the PD is set by the TIA through negative feedback. For AFE4490, the TIA sets the PD to a zero differential bias. This bias is essential to ensure the PD operates with a predictable sensitivity.

    When you disconnect a PD from the TIA through an external switch, there is no longer any mechanism of setting the PD bias. So the PD bias can drift.

    When it is reconnected to the TIA, there will be a transient behavior associated with the TIA trying to get the PD back to zero bias. The time constant associated with this transient would be 2*Rf*Cin where Rf is the TIA gain and Cin is the differential capacitance of the PD.

    So there are two things to analyze before doing something like this:
    1. Make sure the PD voltage in open circuit does not drift to too high a value that goes out of bounds of the AFE4490 input pins.
    2. Give sufficient time for the PD bias to settle after reconnecting it before the sampling is done.

    We do not have any recommended switch parts.