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DS90UB983-Q1: The reason why AC capacitor should place very close to transmitter pin

Fei Jiang
Prodigy 10 points
Part Number: DS90UB983-Q1

Tool/software:

Hi Ti experts,

Why AC should be very close to transmitter pin? 

  • 0
    TI__Genius 13250 points

    Hi Fei,

    The reason why we recommend our customers to place the AC-coupling capacitor close to the IC pins is because the AC capacitor landing pads and the IC landing pads can each be seen as seperate sources of Impedance mismatch. IF you place the AC capacitors close to the IC then the impedance drop cause by both components could approximatley be seen as one impedance discontinuity point which is better for Signal Integrity.

    Best Regards,

    Gil Abarca

  • 0
    Prodigy 10 points

    Hi Gil，

    Thanks for reply. I still have little doubt. if two impedance discontinuity were put closer, the total capacitance of the two point are not decreased. I change the distance of the two impedance discontinuity points(parasitic capacitance of pin and AC capacitor ) by simulation tool and found that as the distance goes closer, the sink wave become worse, source wave become better.

  • 0 in reply to Fei Jiang
    TI__Genius 13250 points

    Hi Fei,

    This is mostly a general recommendation. What is most important about our devices is that you met our Channel Specifications and TDR requirements. If your simulations follow them the devices specs you should be good with either putting the Cap closer to the pin or a little further.

    Best Regards,

    Gil Abarca