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  • TI Thinks Resolved

TLC5947: Input impedance of SPI pins

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Replies: 7

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Part Number: TLC5947

I have a client that thinks the signal traces have to be 50 ohms and the SPI source has to have a series termination resistor of 50 ohms.  This approach is only valid if the input impedance of the TLC5947 is also 50 ohms.  How can I find out what the input impedance really is?

  • Hi,

    We also don't have the specific input impedance data. As you can see from the figure 12, since input is connected to gate of the FETs, the input impedance should be quite large.

    Can you provide more details about the question why there is the requirement of 50 ohms?

    Thanks.

    Regards,

    Kenneth

    Thanks and best regards,

    Kenneth

  • In reply to Kenneth Du:

    Thanks Kenneth.  Your assessment is right in line with mine.  I am doing contract work for a company and they are wanting me to reuse previous work from within their organization that seemed to think that the SPI related signals needed to be 50 ohms.  I am doing due diligence to make sure I am not missing something in the previous approach.  Any other input is most welcome.

    Sean

  • In reply to Sean Kelly84:

    Hi, Sean,

    Do you know why there is 50 ohms requirement for the SPI? I don't see such kind limitation before.

    Thanks.
    Regards,
    Kenneth

    Thanks and best regards,

    Kenneth

  • In reply to Kenneth Du:

    HI Kenneth,
    No I do not and that is why I am asking the question in the the forum. I never saw such a need before so I am wondering if I missed something in my understanding of the circuit.
    Thanks,
    Sean
  • In reply to Sean Kelly84:

    Hi, Sean,

    Do you use long SPI wire? How much frequency do you use?

    Regards,
    Kenneth

    Thanks and best regards,

    Kenneth

  • In reply to Kenneth Du:

    No. Just the normal 30MHz speed.
  • In reply to Sean Kelly84:

    Hi, Sean,

    There is no other input from my side.

    Thanks.

    Regards,
    Kenneth

    Thanks and best regards,

    Kenneth

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