[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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