[SI-LIST] Re: Ground nodes in spice
- From: "Fabrizio Zanella" <fzanella@xxxxxxxxxxxx>
- To: "Scott McMorrow" <scott@xxxxxxxxxxxxx>, <rhaller@xxxxxxxxxx>
- Date: Thu, 27 Feb 2003 17:03:47 -0500
Thanks for your valuable comments, Bob and Scott. I'm using Hspice, the
connector is the HSD6, and I'm using one of the differential pair two
line models, 'hsd6ab.cir'. This model is made up of 23 cascaded
pi-sections, and contains five nodes in the following arrangement
.SUBCKT HSD6AB
+ 100 101 102
+ 2401 2402
I've found that using the 6x6 style connector models makes the
simulation times much too long, especially when running for 200ns or
more for eye pattern simulations. So with this 2 line model there's
only one ground which is common for every ground pin, on both ends of
the connector.
So I'd like to refer to the accuracy issue you brought up regarding
tying both ends of a connector model to the same ground node - using
this model is this not an issue? With this connector model, how does
the ground pass from the daughtercard to the backplane if I were to use
separate reference nodes - for example, node 10 for the card, node 20
for the backplane, etc?
Is it advisable to connect these reference nodes to each other through
an LR circuit, and if so what would be reasonable values to use?
Thanks, Fabrizio
-----Original Message-----
From: Scott McMorrow [mailto:scott@xxxxxxxxxxxxx]
Sent: Thursday, February 27, 2003 12:58 PM
To: rhaller@xxxxxxxxxx
Cc: Fabrizio Zanella; si-list@xxxxxxxxxxxxx
Subject: Re: [SI-LIST] Re: Ground nodes in spice
Fabrizio,
I would agree with everything that Bob has suggested, and emphatically
reinforce item 3. Every backplane connector model that I have had
experience with has been modeled with partial mutual inductance for
signal and power pins. If both sides of the connector are tied to the
same ground node, (whether node 0 or some other node), then mutual
inductances are shorted out, changing the characteristics of the model.
For some connectors this change is huge and can amount to large
differences in delay, impedance and coupling.
Many board-level transmission line SI simulators utilize node 0 for
simulation across all boards and connectors. This is, of course,
incorrect and will lead to significantly incorrect results. There is
nothing that can be done but to move to another nodal simulator like
Spice. When you do this, and separate the grounds from each section of
the design, then you do have to adhere to Bob's item #2. The reference
for displaying your signals must be the local reference on the card,
chip, or thing that you are measuring. Typically, all waveform displays
will reference all signals to node 0. In order to utilize the remote
reference you will either have to use some math with your waveform
viewer and view the difference of the signal to the remote reference, or
use a level shifting circuit element in the net list (my preference.).
Finally, there are some connector models that are poorly constructed,
having a large dynamic range between the largest and smallest inductive
elements. This wrecks havoc in simulation of connectors with large
mutual inductance matrices and will cause convergence issues or
significant slow down in Spice performance (10 to 100:1) when compared
with models from competitors. A particular connector I am thinking
about was specifically designed for 2 to 3 Gig backplanes. When the
grounds on either side of the connector are isolated (as they should be)
then huge convergence problems occur, since this requires that a large
number of extremely small partial mutual inductance elements be
simulated. Tying both sides of the connector to the same ground will
eliminate the issue, but with a severe loss in accuracy, and some still
greatly increased simulation times.
I hope this helps.
regards,
scott
Robert Haller wrote:
Fabrizio,
I have a couple of points you should be aware of.
1. You do not HAVE to tie all of the ground nodes set to zero, but if
you don't
be prepared to accurately model your interconnect, connectors,
decoupling
and signal to return ratio's.
2. When you are displaying an Eye pattern on a remote board not tied to
zero,
use the remote reference for your signals reference.
3. If you are using a connector model that contains partial inductances
you will violate
the assumptions of the field solver used to create that model if you
ground (tie to zero)
both sides of the connector.
Regards,
Bob
Robert J. Haller (rhaller@xxxxxxxxxx)
Principal Consultant
Signal Integrity Software Inc.
6 Clock Tower Place, Suite 250
Maynard, MA 01754
Phone: (978) 461-0449, ext 15
Fabrizio Zanella wrote:
I have a question regarding backplane simulations of serial
8b/10b
signals, running 2.5Gbs and above. Does it matter whether the
ground
nodes of the daughtercards and backplane are all set to 'zero'
or to a
floating node for eye pattern results?
Thanks,
Fabrizio Zanella
Principal Hardware Design Engineer
Broadbus Technologies
fzanella@xxxxxxxxxxxx
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--
Scott McMorrow
Teraspeed Consulting Group LLC
2926 SE Yamhill St.
Portland, OR 97214
(503) 239-5536
http://www.teraspeed.com
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