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SN65LVDS101: Seeing unbalanced differential signals when using the ibis model

Eric Nitz
Prodigy 30 points
Part Number: SN65LVDS101

I am using the sllc125a.ibs model for simulating the SN65LVDS101, and am seeing unexpected behavior in the signals. The positive and negative signals both seem to have skewed duty cycles, so that high pulses are very wide and low pulses are very narrow as seen in the attached image. I am expecting to see a normal, balanced LVPECL signal here and can't find anything on my end that is causing the skew.

My simulation is being done in Hyperlynx LineSim @100MHz.

The issues I'm seeing with the signals are:

  • Expecting 800mV swing, only seeing ~600mV
  • Duty cycle issue

Looking for any assistance in getting this model properly set up, or reasoning behind this behavior.

  • 0
    TI__Genius 12866 points
    Hello Eric,
    Would it be possible for you to provide your setup?


    Best Regards,
    Charley Cai
  • 0 in reply to Charley Cai
    Prodigy 30 points

    I have attached a zip with the linesim .ffs file and both necessary models. I am running this at 100MHz.

    Thanks!

    Interface.zip

  • 0 in reply to Eric Nitz
    TI__Genius 10250 points
    Hello Eric,

    The duty cycle issues may be due to the fact that rising and falling waveform begins at different time: At 6n s for rising and 8 ns for falling waveform. This might be fixable using Hyperlynx IBIS editor (I don't have access to one to make the change, unfortunately).
    I am not sure about the cause of the swing not being as expected.

    Regards,
    Yaser
  • 0 in reply to Yaser Ibrahim
    Prodigy 30 points

    Hi Yaser,

    I was able to correct the rising/falling waveform issue and am seeing proper signals now.

    However, the swing issue is still present. For a standard LVPECL signal I would expect to see 800mV swing centered around 2V, but with this model I am only seeing approximately 500mV swing. I have removed as many variables as possible and am still seeing the issue, so I think this is something incorrect in the ibis model itself. Looking at the model data with Hyperlynx IBIS viewer, I can see that the rising and falling waveforms seem to only be set up to have ~500mV swing. This matches what I'm seeing in simulation, but does not follow the expected pattern for LVPECL signals.

    It seems the model needs to be fixed, or there is something I'm missing here. Please let me know if you have thoughts on how to resolve this. I have attached images of the signals and the model's rising waveform data.

    -Eric

  • 0 in reply to Eric Nitz
    TI__Genius 10250 points
    Hello Eric,

    This does seem to be a problem with the IBIS model, and I don't think it can be fixed without generating a new model. Unfortunately, we are unable to generate a new IBIS model for this device.

    Regards,
    Yaser