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SN74LVC1G14: Signal Integrity Analysis - Direct device to device connection has large ringing

Tharaka Goonetilleke
Prodigy 20 points
Part Number: SN74LVC1G14

Tool/software:

Hello,

I am doing some signal integrity analysis. When I connect an output of 74LVC series to an input of a device from the same series (or any other device with high input impedance), I see a large ringing at the receiver. It has been the general practice to connect them direct in the past, as well as I haven't ever observed this king of ringing in practice (monitoring via an oscilloscope). I am wondering if this is an issue with the IBIS file, a tool issue or I am doing something wrong with simulation setup. I had to have a 49 ohm source termination to get the signal quieted down (see below). I appreciate any help on this regard. If I were to use source termination, I have to add 86 resistors to my design (yes, I have 86 outputs driving other circuits directly). Below is the schematic (without and with termination) and Waveforms (without and with termination).

Thank you,

Tharaka.

  • 0
    Prodigy 20 points

    I forgot to mention. I am using HyperLynx line sim for simulation.

  • 0
    Guru 265470 points

    The output impedance of LVC devices indeed is much lower than 50 Ω, so when the trace is long enough that you get transmission line effects, you should consider source termination. (The output impedance is not zero, so something like 22 Ω or 33 Ω would actually raise the total output impedance to about 50 Ω.)

    I do not think that an actual board would have that much ringing for a 3" trace; the simulation might not be accurate.

    If you do not want to add source termination resistors, consider logic families with weaker outputs, i.e., AHC or AUP.