[IBIS-Users] RE: [IBIS] Re: hspice vs IBIS model correlation at different capacitive loads

Andy,

You are absolutely correct, there are different
types of feedbacks, and I should have defined
which one I was referring to in my posting...

I was thinking of one that adjusts the drive
strength real time as the buffer is switching.
Legacy IBIS can't do that.  (Although one could
model these too using the *-AMS extensions of
IBIS).

On the other hand, buffers which adjust their
own drive strength only once and a while to
compensate for temperature variations, for
example, can be modeled in IBIS as if they were
normal buffers, because they don't change skins
while driving signals onto the bus.  You may
need the [Model Selector] keyword to make a
few different driver models available for the
various strengths, but that's all.

Arpad
------------------------------------------------

-----Original Message-----
From: owner-ibis@eda.org [mailto:owner-ibis@eda.org] On Behalf Of Andrew Ingraham
Sent: Wednesday, January 04, 2006 9:31 AM
To: ibis-users@eda.org; ibis@eda.org
Subject: [IBIS] Re: hspice vs IBIS model correlation at different capacitive loads

> I think he is saying that the oscillations happen
> because of the feedback in the buffer is trying to
> settle to a steady state condition.
>
> If so, legacy IBIS can not model your buffer, since
> there are no provisions for describing feedback.

Yes.

Although it depends on the feedback.

Linear negative feedback, such as used on op-amps to reduce output
impedance, should work OK, I think.  It's probably also possible that some
forms of feedback to reduce variations in drive strength (for example),
ought to be somewhat compatible with conventional IBIS.  But things like
positive feedback to give hysteresis, don't play well in IBIS.

Regards,
Andy

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