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SN74LVCH16T245: No monotonic signal rise at the thresholds when SN74LVCH16T245 is used as source IBIS.

Vilas Kumar
Prodigy 170 points
Part Number: SN74LVCH16T245
Other Parts Discussed in Thread: SN74AVCH16T245, SN74AXC8T245

Hello Team,

In my design i am using a total of 36 voltage translators. Whenever the signal is driven from the voltage translator to the FPGA we are observing non monotonic signal rise at the thresholds. By adding series termination resistor we can overcome this issue, but doing so will increase the resistors count and complexity(density) of the design. So kindly provide a solution on this issue. Also we suspect the model kindly have a through check on the same.

Regards,

Vilas Kumar S

  • 0
    TI__Guru 62240 points
    Hi Vilas,

    Non-monotonic behavior could be limited by impedance matching on the layout as well. Reflections caused due to driving into hi-z loads or mismatched impedance can lead to non-monotonic behavior as well.
    usually, the receiver can detect the signals without causing false switching, but is this an issue here?
    Please send the schematic and the scope shots for review.

    For the model, I will contact the modeling team for further information.
  • 0 in reply to ShreyasRao
    Prodigy 170 points

    Hello Shreyas,

    Thanks for your reply. Kindly find the attached report for your reference.

    Regards,

    Vilas Kumar STI_FAE.rar

  • 0 in reply to Vilas Kumar
    TI__Guru 62240 points
    Hi Vilas,

    Thank you for the detailed report from the simulations.

    I do note here a few points:
    1) the sn74LVCH16T245 is being operated at its lower end of the voltage range (1.8V) and I would prefer considering using the AVCH16T device is optimized for 1.8V operation.
    2) sn74LVCH16T245 device is sending out 100Mhz which is also a stretch when being operated at 1.8V and sn74AVCH16T245 may be more suitable for this.
    4) Although I see non-monotonic behavior in most of the simulations, the signal integrity is linear near the threshold value in most of the cases except in page 6.
    5)Having the series 25ohm termination resistor helps in reducing the non-monotonic behavior by matching trace impedance, reduces the overshoot, although it reduces the peak value.

    We also came up with SN74AXC8T245 device which can be considered as well for 1.8V operation.
  • 0 in reply to ShreyasRao
    Prodigy 170 points
    Hello Shreyas,

    Thanks for your response.

    IBIS model for the alternate device that you have suggested doesn't contain 1.8 V logic buffer models. Kindly provide the needed.

    Thanks,
    Vilas Kumar S
  • 0 in reply to Vilas Kumar
    TI__Guru 62240 points
    Vilas,

    I checked the SN74AVCH16T245 IBIS model to confirm the 1.8V model existing on the IO ports.
    www.ti.com/.../litabsmultiplefilelist.tsp

    The AXC8T245 is the newly released device and model which will support down to 0.65V, so a lower voltage performance can be the worst case scenario which will support the 1.8V operation.