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Original question:

ONET8551T: Exact Pad Locations and Adapting for Flip Chipping

ONET8551T: Input Impedance into ONET8551T

Nathan Abrams
Prodigy 40 points
Part Number: ONET8551T

Hello,

I was hoping to learn more about the input impedance looking into the ONET8551T from the PD (so IN/FILTER). We are intending to first design a circuit to emulate the PD current for initial tests prior to connecting the ONET8551T to the PD, and would like to ensure impedance matching. My understanding is typically TIAs are expecting the current source of the PD to look as a high impedance source. I also see from the datasheet that the recommended operating conditions are 200 fF for the PD and 0.3 nH for the wirebonds. We could use those values to get an estimate of ideal input impedance looking the photodiode, but we would appreciate any additional information that could be provided related to this, such as input impedance into the TIA and range of acceptable impedances looking from the TIA back to the PD.

Thank you,

Nathan Abrams

  • 0
    TI__Expert 8400 points
    Hi Nathan,

    I will have to check for you on the specific input impedance for this device, but it is generally a high impedance. You are correct that it generally expects a high impedance source from a photodiode so you will need to be careful about how you are biasing any input signal. I will see if I have any information on the suggested way to electrically connect the input for evaluation.

    Regards,
  • 0
    TI__Genius 13935 points

    Hello Nathan,

    I wrote an article on this topic.  This may help:

    I would add that there is no expectation of impedance matching when you connect a photo diode to a trans impedance amplifier (TIA).  The diode acts as a  current source and the TIA input is typically fairly low impedance.  This forms a symbiotic relationship where the current from the diode has minimal impact on the diode bias voltage.  The key to this interface is using a short bond wire.

    Matching will be more important with an electrical interface, but the TIA input may be close enough to 50 Ohms such that matching is good enough.  The only way to be sure is to build up a board and test it.  The board trace geometry will influence the overall impedance of the TIA.

    Regards,

    Loren