[SI-LIST] Common-mode noise on LVD Drivers

Hi All,
There was a thread some time back about differential to common-mode 
conversion.  My problem is also in this area.  I have two similar ASICs 
with multiple LVD drivers.  On one ASIC, I measure common-mode noise about 
20 dB higher than the other.  Even within this ASIC, some bits show about 
6 dB more common-mode compared to the quietest bits.  The two ASICs are 
from different vendors, and the one with higher CM noise uses faster 11 uM 
technology.  I don't know the technology of the other ASIC, but its die 
size is considerably larger.  This high common-mode noise is causing EMI 
problem, which I am trying to fix.  I have tried changes in the card 
decoupling scheme but it does not make any difference.  I am convinced 
that the common-mode noise is coming out of the ASIC, and the PCB 
improvements may not help.  Is there any way to come to a definite 
conclusion about this, and is there anything I can do besides placing 
common-mode ferrite beads on every driver pair.
The ASIC uses a core voltage of 1.8V, and has on-board regulators to 
regulate 3.3V input to 1.8V. 
I will really appreciate your help in solving this puzzle.
Regards, Ravinder
Server PCB Development
Hitachi Global Storage Technologies

Email: Ravinder.Ajmani@xxxxxxxx

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