[SI-LIST] Re: Chassis GND and Digital GND shorting on daughterboard
- From: "Shailendra Shukla" <shuklas@xxxxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Tue, 29 Jun 2010 12:39:44 +0530
facing problem with hyperlynx as attached
>>> Ignas Mikulevicius <mikulevi@xxxxxxxxx> 6/29/2010 7:20 AM >>>
Hello everyone,
Yes, the topic's back. I know this has been discussed ad nauseum in various
previous posts, but I felt none of them conclusively established a solution.
First of all, from previous discussions it seems to me like there are some
trade-offs between ESD and EMI protection in terms of chassis GND - digital
GND connections. The way I understand it, for ESD, we ideally want to
connect chassis ground and digital ground with a large resistor (say, 1
MOhm), to allow any ESD charge to be bled off to chassis without causing any
excessive currents and correspondingly dangerous voltages across components.
On the other hand, for EMI, I understand it may be advantageous to short
digital ground and chassis ground at multiple locations in the vicinity of
the I/O connector to divert any problematic currents to chassis ground
before they can radiate out on the cable. So is there any way to reconcile
the two and get the best of both worlds?
And how should these recommendations be applied when designing a standard
PCI or PCI Express add-in card?
The specific scenario I am interested in is this:
Say I have a PCI Express daughterboard (add-in card) that plugs into a
motherboard in a standard PC (let's call this System A). The PCB bracket is
screwed on to the I/O connector of the PCB and, of course, also contacts the
computer chassis. The board sends differential signals across a long,
shielded cable to another system (say, System B).
First of all, in the case of differential signals inside a shielded cable,
how much potential for EMI can there be? I would assume the differential
nature of the signals would greatly minimize radiation.
The cable shield is shorted to chassis GND. Each differential pair in the
cable has its own ground wire, and, finally, there is a wire that is called
"Frame Ground", tied to chassis ground at system B. On the daughterboard,
there is a "chassis GND" island underneath the connector in the layer below.
The connector posts as well as the "Frame GND" signal are shorted here. So
basically we have the chassis GND of system A and system B grounded and this
also grounds the cable shield. Now clearly I can't leave this floating as it
would seem that would create an EMI nightmare.
What would be the ideal way to connect this island and digital GND? A
capacitor, dead shorting, a resistor?
It seems whatever method I choose I can find at least one article to endorse
it.
Any thoughts or insight would be greatly appreciated. I am not expecting
someone to remedy my dillemma for me, but would really like to understand
the theory and thinking behind this once and for all. If someone could
either elucidate me or point me to some definitive, conclusive articles or
books on this subject I would be very grateful.
Thank you very much,
Ignas M.
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