Re: Rising Waveform Loading Effects

The current crop of IBIS models are a far cry from being accurate enough
to really predict the kinds of things Jon is talking about, but many of
them are good enough to ensure that I don't drastically mis-terminate
the transmission lines on my boards.

        One almost does not need any models for that, IBIS or otherwise.
        If a simulator will not provide 200 mV and 500 psec accuracy
        while simulating a situation complex enough to be non-intuitive
        then it's hardly worth the trouble.

Another question to think about: do Ibis models derived from Spice models
really have the kind of accuracy that Jon needs?

        I don't know. How do we know how accurate such IBIS models
        are? That uncertainty is why I'm not rushing to embrace IBIS
        as a substitute for SPICE or measurement-derived I-V curves
        in simulation. When a vendor is willing to make direct
        comparisions between simulations (using their own nominal
        SPICE models, measured I-V curves and IBIS models) and
        measurements of actual circuits, and demonstrate at least
        200 mV/500 psec accuracy, then I'll start to have some
        confidence. Such simulations must address realistic situations
        in multiple cases (heavy vs light loading, lumped vs
        transmission line environments), and not merely 50 pF - 500
        ohm test cases. Until then I'll continue to press vendors
        for SPICE models or I-V curves, or generate my own.

        Richard Schumacher

        I do not speak in any capacity for the Convex Division of the
        Hewlett Packard Company.
Received on Tue Oct 29 06:34:53 1996