RE: IBIS in simulation

Larry Forsythe wrote:

>I'm confused about how IBIS is used in actual simulations. I haven't seen
>any articles about this (please indicate if there are some).

And you might not find any that go into much detail. The actual algorithm
used by each simulator, especially as to how it handles the V-T data, might
be a guarded secret.

> What I'm
>confused about is how can a few static I-V curves and some V-T curves done
>at specific loads be used to simulate other loads. The I-V curves involve
>fully on transistors. The V-T curves involve partially turned-on
transistors
>driving a specific load.

Typically, an output buffer interacts with its load over a period of several
nanoseconds while the signal rattles around and eventually settles. Yet the
time it spends actually changing -- the time for the drive to its output
transistors to switch -- is much less, perhaps a few hundred picoseconds.
It is a fair approximation to treat the output buffer as if it were static
(defined by one I-V curve or the other) almost all of the time. In between
the two, it makes some sort of transition from one I-V curve to another, in
a relatively short interval, the effects of which might be swamped by the
interaction with the load.

> Is it valid to scale the I-V curves using the V-T
>curves in order to derive the response to driving other loads?

Here's where you get into the proprietary details of each simulator's
vendor.

If you're lucky, the load for the V-T curves is close to the trace
characteristic impedance.

> Can the I-V
>curves and V-T curves be used to determine the characteristics of a
>partially turned on transistor driving a different load-I guess that is the
>source of my confusion.

To varying degrees, apparently yes.

Regards,
Andy Ingraham
Received on Tue Aug 31 19:35:53 1999