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Copper Fills as shielding?

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turjohn

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I've been designing boards for a system that inherently generates A LOT of EMI (very high pulsed power application). We always have issues with noise coupling onto our small signals, so on the boards I've been designing I have been putting copper pours on the top and bottom layers after all the routing is finished and connecting those pours to the chassis of our system. My initial thought was that those pours would provide a some level of shielding to the signals on the board, but I wonder now if maybe they are doing more harm than good as we sometimes experience 'ground bounce' during our high energy pulsing. I wonder if maybe I'm inducing more noise onto my signals by placing this fill next to them. Any thoughts on this or pointers in general on placing planes or pours/fills?
 

KlausST

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Hi,

GND planes may be used as shielding.
GND is considered as the reference signal, so it has zero noise.
But as soon as currents flow through this plane there will be current coupled in nearby signal lines.

Also every inductivity and resistance of the GND plane combined with currrent causes a voltage drop. So although solid you might consider one star point as reference for critical signals.

***
For better assistance we need your scematic with the information where the high current pulses travel.

Klaus
 

marce

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I agree with Klaus, without a schematic and layout we can only guess.
I have done similar designs with high power switch mode power supplies and similar circuits where you get very high switching currents. There will be two main methods of noise coupling inductive and capacitive... The inductive is combatted by trying to avoid any sensitive routes within the switching loop area or near it (not always possible) but distance is your friend here. Capacitive can be made worse by using copper pours as they can actually provide capacitive coupling where there was non and make the matter worse...
Running supplies as planar bus bars can help as well, broadside coupled wide routes on adjacent layers.
 

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