[SI-LIST] What do you do when the manufacturer's model doesn't match the measurements?

Hi all, please can you advise!

Our aim is to prove the vendors' IBIS models on a dev board so we can have 
confidence that simulations of our own 400MHz DDR2 design (using the same 
parts) will be accurate.  We're simulating with Hyperlynx 8.0 and have taken 
the most up-to-date IBIS models from the vendors' websites.

We've made a pretty good free-form schematic model of the dev board with the 
target CPU and memories (plus 'scope probe at the receiver's pin).  The initial 
simulations were horrible with a massively non-monotonic rising edge crossing 
the Vil and Vih levels of the receiver twice.  We used a 6GHz LeCroy 'scope 
with 1GHz probes to measure the actual dev board's signal integrity and the 
edges were clean.

In my mind, that proves that I can't trust the model.  The catch, however, is 
that the vendor then told us to put the simulation probes at the die instead of 
at the pin.  We re-simulated and the non-monotonic edges went away - they 
actually looked like the measurement!

Would you trust the results of a simulation when the measured case only 
correlates with a simulation having probes in a different place?  Can we use 
the wrong (but matching) model to guide us to the best routing topology?  I'm 
tempted to just use standard good SI practises and ignore the simulations.

Best regards,

Iain Waugh

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