[SI-LIST] Re: SSN Vs Load cap

One study of a complex SSN model, which I am intimately familiar with, showed 
that DECREASING the load capacitance from 10pF down to about 2-3pF actually 
reduces the SSN noise observed at the load. Below 2-3pF, it's flat. However, 
this was true for the loads being about 3 inches away from the component (i.e. 
about 500ps of flight time). For this length of the t-lines, separating the 
loads from the drivers,  the rise time is  somewhat comparable with the flight 
time.  Thus, it's a mixed picture. If the lines are much longer or much shorter 
than Tr/Tf the results could be quite different. However, the same model showed 
that stretching the lines to 9" inches did not materially affect the amount of 
noise at the load. If the lines are long, the driver does not feel the presence 
of the load - only the t-lines (the point made by Larry Smith earlier on this 
thread.) 
It would be interesting to see results of an experiment - rather than 
simulation.
-Vadim Heyfitch
Altera


Canes Venatici <starsilic@xxxxxxxxx> wrote: 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






 
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