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LMK04828: LMK04828 IBIS simulation for clock output

Yarn Guo
Intellectual 1440 points
Part Number: LMK04828
Other Parts Discussed in Thread: TEST2

Hi, Team:

I am helping customer to simulate the SI of SDCLKout waveform with LVDS output format for a signal output at 122.88MHz.

 I have done two tests:

1. First one, I connect 100nF ac coupling cap between LMK04828 SDCLKout output and differential100 Ohm termination. I found the waveform at the two ends of 100 Ohm load resistor is wired as below:

 

2. The second, I remove the AC coupling caps and do the simualtion again. I found the waveform is more reasonable which has common mode of 1.2V, while it shows a lot of ringing. Do you think this is correct? 

May I know why I need to remove the AC coupling cap between the transimitter and load? Could you please help me to check whether the simulation in my second test is correct? I don't know why there is a lot of ringing. 

My Hyperlynx simulation file is below:

LMK04828 DCLKout2 LVDS.zip

  • 0
    TI__Expert 8865 points

    Hello Yarn,

    If you want to ac couple the LVDS outputs directly, please put a 560 ohm resister between P/N output like shown below. The LVDS stages in LMK0482x needs a dc path before ac coupling.

    Best regards

    Puneet

  • 0 in reply to Puneet Sareen
    TI__Intellectual 1440 points

    Hi, Puneet:

    Thanks for your prompt reply.

    By adding the 560 Ohm DC biasing resistor before ac coupling, I still found the same problem, while removing the cap still help.

    Can you please double check with my simulation as below?

    1. Test1: add 560 Ohm for DC biasing at the output of LMK04828

    2. Test2: removing the AC cap still helps the simulation

    Thank you so much for your time to look at this.

    Yarn

  • 0 in reply to Yarn Guo
    TI__Expert 8865 points
    Hi Yarn,
    Sorry for the late reply. I tried to simulate that and i see the same with the ac coupling. I think it is a convergence issue with the IBIS model. When you attach weak dc biasing to the capacitors and it is working fine. I will have to forward this to the IBIS modelling team.
    Best regards
    Puneet