RE: [IBIS-Users] Open drain buffer : Discrepency in IBIS model vs SPICE model simulations

Sandeep,

This is not too surprising, although the amount of
mismatch is larger than what I have seen in other
cases.  The mismatch happens because the IBIS model
cannot "extrapolate" to other load impedances if
your model only has data obtained at one value of
R_fixture.  This is why the specification allows
multiple sets of V-t curves at different R_fixture
values, although this capability is not supported
by many tools.

Think about it this way.  The model is supposed to
reproduce waveforms.  The data that is put into the
model is generated under certain conditions.  The
IBIS model will only be able to reproduce the waveforms
exactly if the conditions in your simulations are
the same as when the data was generated.  The rest
is guesswork...

Arpad
====================================================

-----Original Message-----
From: owner-ibis-users@server.eda.org [mailto:owner-ibis-users@server.eda.org] On Behalf Of Sandeep S K
Sent: Friday, September 26, 2008 8:04 AM
To: ibis-users@server.eda.org
Subject: [IBIS-Users] Open drain buffer : Discrepency in IBIS model vs SPICE model simulations

Hi
    I am generating an IBIS model for an open drain buffer (I2C). The 
simulated waveforms match when I use a 50 ohm (which is what was used 
for V-t extraction) load connected to VCC, Whereas when a different load 
is used I see a mismatch in the waveforms. Attached are the ibis model 
and a snapshot of waveforms when 100 ohm load was used. Can you please 
suggest why.

Thanks and Regards
Sandeep

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