[SI-LIST] Re: IBIS question!!
- From: "Lynne Green" <lgreen22@xxxxxxxxxxxxxx>
- To: <a.ingraham@xxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 3 Jun 2004 17:28:06 -0700
Thanks for the important additions. :-)
- Lynne
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] On
Behalf Of Ingraham, Andrew
Sent: Thursday, June 03, 2004 11:46 AM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] Re: IBIS question!!
(This is a re-send because my first attempt bounced after a couple days in
the queue.)
> 1. Does the IBIS model is better when DC voltage sweep of VI table is
> smaller??
>
> -- Yes. Better yet, a table has more points where the slope is changing
> fastest, and fewer where is changing slowly. (Generally, too many points
> slows simulation; too few points reduces accuracy.)
I fear this needs some explanation. The answer above might not apply to the
intended question (voltage sweep vs. voltage step).
Smaller voltage *steps* tend to be better, providing more detail and better
accuracy to the simulator if the VI curve isn't a straight line.
A smaller voltage *sweep* range is not better, and could be worse, if it
omits data on the ends needed for overshoots.
> 2. If the output voltage swing of the pad is 0~3.3V, does it means the
> voltage range(0V~3.3V) of the VI table is enough.
No.
The simple reason is that IBIS *requires* VI data from -Vdd to +2*Vdd. Just
do it.
Even if you haven't seen waveforms going below 0V or above 3.3V in your
application, someone else using the IBIS model in a different simulation,
might.
Furthermore, simulators frequently try points that they do not converge on.
Even if the simulated voltage never goes negative, the simulator may
attempt intermediate solutions at those points. If VI data is missing from
those points, the simulator might either fail to converge (abort the
simulation), or give you incorrect waveforms.
In my opinion: don't withhold information from the simulator.
Regards,
Andy
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