[SI-LIST] Re: local and global ground

I have done what you are trying to do ... use a local non-ground
reference in HSPICE ... and it can work if done right.

Sometimes you run into problems where some vendor's model has hidden
connections to ground, either because it is encrypted so you can't tell
what the heck is in it, or because they did something incredibly
un-user-friendly like having .GLOBAL node statements within their
subcircuits.  I have spent hours un-doing this kind of junk in vendor
models.

But in your case the daughter card side is so simple, that I suspect the
connector models are at fault.

Are you sure the connector models were created to include what one might
call "common mode" effects, when the two sides are unhinged?  If it only
models local effects between neighboring pins, or if it was intended to
have reference node 0 on both sides, it might have completely missed the
effects between the two boards.  Getting a totally "correct" model that
really behaves just like the real connector, under all conditions, isn't
something that just automatically pops out of the modeling process.
Just like SPICE's transmission line model ignores the common-mode
(unless we explicitly include it, which takes some effort), the
connector models might have done the same.

What you are seeing is probably not "numerical noise", but rather "real"
noise from the point of view of the simulator, but due to poor modeling.
There's a difference.

I am puzzled why the second vendor's model gives you an
"inductor/voltage loop" error message, which is fixed by connecting the
daughter card side to node 0.  Usually that error means there is a
connection that shouldn't be there, like maybe you unintentionally
re-used a node number.  I might have expected a "floating node" or "no
DC path to ground" error message, in your case.  Track down exactly
where that inductor/voltage loop is, it might tell you something.

Andy


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