[SI-LIST] Re: SSN Vs Load cap

  • From: "Larry Smith" <LSMITH@xxxxxxxxxx>
  • To: <starsilic@xxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
  • Date: Fri, 2 Feb 2007 09:36:08 -0800

Canes - Do you have a transmission line between your driver and the load
cap?  I assume that you are simulating a bunch of simultaneously
switching drivers that pump current into some kind of inductance matrix
attached to transmission lines and load capacitors at the far end. =20

In today's technology, the driver rise and fall time is likely to be
shorter than the time of flight of the transmission line.  Most drivers
make their transitions in less than 500pSec and the transmission line is
likely to be 1000pSec (6 inches long).  The driver does not feel the
load cap until 2 time of flights (2000pSec) have passed, long after the
SSN event is over. =20

The load cap will have no effect on the SSN waveform at the driver
during the initial event but will have a huge effect in reducing the SSN
seen at the far end of the transmission line because of it's filtering
properties.  You essentially have a 50 Ohm transmission line driving
into a capacitance which implies an RC time constant.  The fast edge of
the SSN spike gets filtered.=20

It makes perfect sense for load capacitors at the far end of
transmission lines to reduce the amount of SSN measured in that
location.

Regards,
Larry Smith
Altera Corporation

-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]
On Behalf Of Canes Venatici
Sent: Wednesday, January 31, 2007 11:30 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] SSN Vs Load cap

Hi all,
I couldn't understand why if load capacitance is increased, SSN is
reduced (as the recommendation is increased SSO index, more signal pads
can be accommodated by power pads).
As di/dt is directly proportional to load cap, increasing the load
causes more drop in parasitic inductance and SSN should increase.
Could anyone clarify.
Thanks,
Canes






=20
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