[SI-LIST] Re: Shield ground isolation

I did some EMI measurements on InfiniBand and my results show that isolating
the two grounds is a really bad thing. It significantly increases the EMI
risk.  If you do not have a continuous ground connection, the common mode
current does not have a continuous path (i.e., has to jump from connector
shield to digital ground) and causes EMI problem.

If you really neead DC isolation between these two grounds, you need to
provide a really good AC low impedance connection between them to reduce
EMI. The following paper provides detailed information, and also a solution
to achieve the AC low impedance connection.

X. Ye, D. Miller, and A. Tripathi, "EMI reduction techniques for InfiniBand
multi-GHz interconnect," Proc. of the 2002 IEEE International Symposium on
Electromagnetic Compatibility, Minneapolis, August 2002, pp. 430-433.

Xiaoning Ye
Intel Corp.


-----Original Message-----
From: Val Mandrusov [mailto:valman@xxxxxxxxx] 
Sent: Monday, February 24, 2003 9:05 AM
To: Anand.Kuriakose@xxxxxxx
Cc: 'si-list@xxxxxxxxxxxxx'
Subject: [SI-LIST] Re: Shield ground isolation



Anand,

May be you have a special case but, I am not familiar with this role - 
"shield ground of the connectors should be isolated from the digital
ground".

If PCB has two grounds (Digital and Analog) you might want to separate 
Digital (high noise) connector area from an Analog (low noise) PCB area 
through a moat. But in any case, the metal connector still should be 
tighten to a metal chassis.
If PCB has only one Digital ground I will connect connector shield ground 
to a PCB ground and to the metal I/O plate.

Ferrite beads very often used to separate a power islands from the rest of 
power distribution system.

Isolation of two grounds creates an EM Field which is the source of EMI 
radiation - especially through a Capacitor which has a pretty narrow 
resonance characteristic.

Best Regards,  Val


At 22:37 22/02/2003 +0530, Kuriakose, Anand wrote:

>Hi All,
>
>On high speed boards with I/O connections running to outside world, the
>shield ground of the connectors should be isolated from the digital ground
>if the board has to meet the requirements of EMI/EMC compliance standards.
>
>Are there any side-effects of isolating the shield ground from the digital
>ground?
>
>
>I have seen on some boards, that the isolation is achieved by using ferrite
>beads. On few others i have seen capacitors being used b/n shield and
>digital ground.
>
>How are capacitors useful in acheiving isolation? Or are these caps used
for
>some other purpose, which i am not able to understand?
>
>What is the best method to achieve this isolation?
>
>Thanks in advance.
>
>Regards,
>Anand.
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Val Mandrusov
Sr. Compliance Engineer

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