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

Ravinder,

There are a couple of ASIC suppliers who have package design problems that
lead to common mode problems in the form of Vcc and ground bounce.  If you
have one of those, there is nothing you can do at the board level to fix
the problem.

Lee


> [Original Message]
> From: <Ravinder.Ajmani@xxxxxxxx>
> To: <si-list@xxxxxxxxxxxxx>
> Date: 3/29/2004 9:13:16 AM
> Subject: [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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