RE: [IBIS-Users] Requesting for some help / information regarding the use of IBIS in SI simulation.

hello Scott,

I don't agree with you. Such simulators (or model extractors which write out
such "postprocessed" model data) are no more assured accurate than simply
applying or writing out the base RLC data.

Most package extractors, especially ones you may refer to as applicable
"from the beginning", are based on static RLC simulation in which a single L
and C are extracted at exactly DC from two distinct simulations;
magnetostatic and electrostatic respectively. One has NO CLUE how to form
the equivalent circuit given only the static lumped L and C computed
independently. Of course, as asked and discussed by Ray, if the user has
insight from the physical structure then a more broadband model MIGHT be
generated, but there is no guarantee!
One person assuming a PI circuit (with the C split equally in half - or even
unequally - and the L in the middle) and another person assuming a TEE
circuit (with the L split and the shunt C in the middle) are no more correct
than the other. One has NO IDEA of the high frequency behavior of the
circuit (either just above the quasistatic limit where the likely spectral
content is relevant or higher where it is less likely relevant) without some
form of AC high-frequency simulation or measurement. The behavior at
"infinite" frequency in meaningless and to argue that a uniformly
distributed multi-stage ladder network is valid has no more justification
than another person arguing it should me a lowpass network with all the
inductance on the die side because of wirebond inductance. This argument has
blindly been accepted and argued for many years. It has been unjustified and
wrong, as you would say, "from the beginning".

Want a valid model at higher frequencies, then extract AC information from
which to generate it. One's guess or intuitive argument without AC
information to back it up is only a guess, as Andy stated. The classical
uniformly distributed ladder network is a deceptively simple and intuitively
pleasing assumption that is easy to understand and unfortunately accepted by
way too many people in the industry. This supposed more accurate guess has
failed in too many cases and can even be worse than a simple totally
unbalanced low pass filter assumption over the frequency spectrum of
relevance.

Many commercially available RLC extractors continue to make this assumption
today. Users are placed at significant risk because they falsely believe the
model is in some sense "broadband" or "wideband". It is a guess with no
guarantee to be any better than any other assumed more distributed
equivalent circuit for which there is no theoretical or physical
justification.

If one starts with AC data, assumes an equivalent circuit and *then* selects
element values in such to fit the data then there is reason to argue this is
a more valid model at higher frequencies. One cannot do this from only DC
terminal-based data. Not that it's difficult, it is impossible.

best regards,
 -Brad

> -----Original Message-----
> From: owner-ibis-users@eda.org
> [mailto:owner-ibis-users@eda.org] On Behalf Of Scott McMorrow
> Sent: Tuesday, May 31, 2011 5:04 PM
> To: Ray Anderson
> Cc: ibis-users; Ray Anderson
> Subject: Re: [IBIS-Users] Requesting for some help /
> information regarding the use of IBIS in SI simulation.
>
> Since the beginning of IBIS, the most accurate simulators
> would turn the RLC package parasitics into a transmission
> line equivalent. In lieu of any additional information it is
> the most reasonable thing to do, since packages can be
> approximated as transmission lines.
>
>
> Scott McMorrow
> Teraspeed Consulting Group LLC
> 121 North River Drive
> Narragansett, RI 02882
> (401) 284-1827 Business
> (401) 284-1840 Fax
>
> http://www.teraspeed.com
>
> TeraspeedR is the registered service mark of Teraspeed
> Consulting Group LLC
>
>
> On 5/31/2011 7:59 PM, Ray Anderson wrote:
> > If you are using extracted RLC data to manually set up the package
> > model you can certainly make intelligent decisions on the
> topology of
> > the model and the distribution of the RLC parasitics.
> However if you
> > are depending on a simulator to read the data from a IBIS
> .pkg format
> > file (or the [PIN] data) and set up a model for you then
> your results
> > may differ from simulator to simulator.
> >
> > -Ray
> > Xilinx Inc.
> >
> >
> > -----Original Message-----
> > From: owner-ibis-users@eda.org [mailto:owner-ibis-users@eda.org] On
> > Behalf Of Andrew Ingraham
> > Sent: Tuesday, May 31, 2011 4:48 PM
> > To: ibis-users
> > Subject: Re: [IBIS-Users] Requesting for some help / information
> > regarding the use of IBIS in SI simulation.
> >
> > Yes.
> >
> > I am usually tempted to start with an evenly distributed
> RLC model ...
> > unless I have reason to think that the distribution is
> otherwise. For
> > example, wirebonds might have most of the inductance and
> resistance,
> > whereas the leadframe may have more capacitance, which
> might influence
> > one to imbalance those factors unequally between die end
> and pin end.
> > But without hard data to back it up, that is just guessing.
> >
> > Andy
> >
>
> --
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