[SI-LIST] : Best System Grounding Scheme for ESD, and RF emission

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From: sarmad Albanna (samii111@hotmail.com)
Date: Mon Oct 30 2000 - 07:03:54 PST


Hello group,

Here are my questions:

We have a module that has DGND and AGND isolated, however they could be
connected in the middle of the module and at the end as well. Then, there
is a metal plate that supports the PCB with 10 screws. The screw points are
completely isolated from the DGND and AGND layers. So, non of the gnds are
connected to that metal plate.

DGND and AGND are only connected to one point at the backplane ( after the
Connector). The metal plate slides into the chassis that holds number of
those modules. The module/s has face plate that is connected only to that
metal plate and not to any of the PCBs grounds. Then the face plate make
Contact with the Chassis.

Q1. Is it better to extend the GND planes in the PCB( Digital and Analog)
to the screw points and let the screws make the contact between those PCB
gnds and the metal plate? For Emission purposes, will that reduce Ground
loop currents? However is also bad to have DGND and AGND connected at the
metal plate in many points because of the Analog Circuit sensitivity?

Q2. For ESD protection purposes, Is it better to create a wide Cu strip
around the Periphery of the board on both ( top and bottom layers) and have
the metal screw make the contact between the copper strip to the DGND and
AGND layers into the metal plate? IS that a practice.

Q3 Should I follow the above approach or keep that metal plate isolated from
those PCB grounds for ESD and for RF emission purposes.

Note: The board has Clks that run at very high frequencies ( 3Ghz).

We are seeing emission at those frequencies and it's harmonics 2nd and 3rd.

Any input on that regards will be highly appreciated.

Thanks again.

Sarmad Albanna
Compliance Engineering
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