Re: SSO and load capacitance
Jason - the comment is correct: the worst case SSN waveforms will be found with minimum load capacitance.
But some explanation is required.
First, SSN can be broken down into two components: inductive coupling and PDN. Steve is referring to the PDN
part in his response. But usually the greatest SSN noise amplitude measured at the far end of a signal
transmission line comes from inductive coupling, not PDN.
Inductive coupling is related to mutual inductance between aggressor signals and the victim signal. It
only happens during the rise (fall) time of the driver because that is when the di/dt takes place. To a first
approximation, the voltage noise that gets launched into a victim transmission line (under the BGA that
makes the SSN) is proportional to m*di/dt where m is the sum of the mutual inductance from all the
aggressors to the victim and i is the current in the aggressors. Mutual inductance occurs in the wire
bonds, package vias, balls and PCB vias and to a first approximation is proportional to the length of these structures.
These days, the aggressor rise time is on the order of 200pSec, which is the time that it takes signals to
travel about an inch down a transmission line. The capacitance load in question is down at the far end of the
transmission line, let's assume 6 inches. The 200pSec rise time aggressors launch an SSN noise pulse into
the victim signal net that is approximately 200pSec wide and it arrives at the capacitance load about
1000pSec later. The load capacitance at the far end will have no effect on the SSN event that launches the
SSN glitch into the victim transmission line.
When the SSN glitch arrives at the far end of the transmission line, it often finds a 50 ohm termination. The
noise measured at the far end is identical to the glitch launched into the near end, assuming lossless
lines. Now if there is any capacitance load at the far end, glitch energy goes into charging up the load. The
measured SSN glitch voltage amplitude will be less with more load capacitance.