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LMK00105: Questions about ac coupling cap

Kazuhiko Hirano
Genius 11613 points
Part Number: LMK00105

Hello,

Looking at datasheet figure 6, there are two 0.1uF caps before and after 50ohms transmission line.
My customer thinks either one is good enough for ac coupling.
Any specific reasons to recommend two caps for ac coupling?
If only one cap is also good, which one is better before the transmission line or after, and why?

Best regards,

K.Hirano

  • +1
    TI__Genius 10125 points

    Hi Hirano,

    It helps to visualize the results with the IBIS simulations. As you can see, there is not much change between having both caps (before and after) and one cap after the transmission line. Also not much difference between having a cap before the t-line and no caps at all. The cap placement affects the VCM level.

    From the excerpt below, the CLKin input already has an internal bias voltage, so to properly AC-couple CLKIN, there must be at least the coupling cap after the transmission line (the one before is optional).

    Regards,

    Jennifer

  • 0 in reply to Jennifer Bernal
    TI__Genius 11613 points

    Jennifer,

    Thank you for your confirmation!

    Best regards,

    K.Hirano