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CDCLVD1208: The single ended character impedance and differetial character impedance

Xiang Maverick
Intellectual 750 points
Part Number: CDCLVD1208

I have read the datasheet of CDCLVD1208, I see the recommended input and output differential clock routing is two lines with character impedance of 50ohm to ground and 100ohm termination across the differential lines.

I think can I design the differential impedance of 100ohm when routing, instead of single ended 50ohm to ground. Which is better?

I read some document that tells me when the two lines with impedance to ground of R are seperated with a significant distance, the differential impedance is 2R. I think design differental impedance of 100 ohm is the better choice, becasuse the input and output is diferential signal.

  • 0
    TI__Genius 16845 points

    In general, I prefer to design transmission lines like matched 50ohm single-ended lines. As you mentioned, it is possible to design for 50ohm single-ended impedance while achieving 100ohm differential impedance.  This routing provides best signal integrity if the clock signal is change from differential to single-ended; this is a common use case when evaluating with a clock buffer evaluation board, for example. 

    Kind regards,
    Lane

  • 0 in reply to Lane Boyd
    Intellectual 750 points

    Thank you.

    "it is possible to design for 50ohm single-ended impedance while achieving 100ohm differential impedance. "

    How to achieve them at the same time?

    Do you think the differential impedance of 100ohm is the rule with higher priority?

  • 0 in reply to Xiang Maverick
    TI__Genius 16845 points

    Yes, 100ohm differential impedance is more important. You can achieve this with differential routing depending on the spacing between the traces; as the separation is increased, the coupling will reduce, and the impedance will approach 2x the single ended impedance. You could also check it with an impedance calculator tool.

    Kind regards,
    Lane

  • 0
    TI__Intellectual 770 points

    Hi,

        For any differential signalling, particularly LVDS, it is important to keep the signal/clock routing close to each other since this will greatly increase the common-mode noise rejection i.e, we want any external noise to couple uniformly to both the cables as this noise will be rejected as common-mode noise at the differential input of the receiver. Having said this, the termination resistance in the LVDS scheme should be equal to the characteristic differential impedance of the routing. Therefore, i would recommend designing the differential impedance of the routing to 100 ohms and given that the routing should be close to each other, this may not translate to single-ended characteristic impedance of exactly 50 ohms which is OKAY. 

    Thank you,

  • 0 in reply to Badarish
    Intellectual 750 points

    Thank you, the question is well answered.