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I am asking a question on behave of a colleague who's struggling with the following. Does the relocation of the ground bonding point on a CAN bus ground shield make a difference? In the photo below the scenario of the before and after. The bonding point was 3" away from the connector, it will be now 10" away. enter image description here I did some research on the topic which is completely new to me. My thoughts are that it comes down to a factor called the shielding performance of a cable which is indicated by its transfer impedance Zt. The transfer impedance relates a current on the surface of the shield to the voltage drop generated by this current on the opposite surface of the shield: Zt = U2 / (I1 x L)  where L is the cable length.

The lower the transfer impedance value, the more effective the shielding enter image description here Is the equation above enough to prove that moving the bonding location will not affect the shielding performance?

It says impedance but I don't see the frequency factor in that equation. I am really not sure about this and any help is appreciated.

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The coax picture and equations are refered to unbalanced source/load and coax. With CAN you have balanced, twisted pair configuration, where noise is picked up equaly in both conductors, therefore noise voltage is eliminating each other. No current is passing in the shield, the shield is just a Faraday cage screening twisted pairs from noise. It has to be on earth potential in one point. You have best results if the shield is tied on the ground on both sides, but usually we tend not to connect shiled on both sides, as the ground loop current can flow trough shield - this if ground potentials of both sides is different.

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  • \$\begingroup\$ Thank you for your answer. That was my original thought and you just confirmed it. \$\endgroup\$ – Rocky79 May 20 '16 at 14:47
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Just kind-of winging an answer here, but the decrease in effectiveness will be determined by the additional impedance (especially inductance) in the increased length of the shield strap to ground and the increased impedance internal to the device in getting back to whichever ground it is seeking.

Edit: Or by 'bonding point' you mean the place where the shield stops? Considering that a lot of CAN is unshielded, it is fairly resilient in tough environments. If there is a nasty RF source nearby, then sure, it might make a difference, but otherwise... eh.. just try it.

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  • \$\begingroup\$ Thank you for the reply. By bonding point I mean where the shield is bonded to ground. The shield covers the whole cable. \$\endgroup\$ – Rocky79 May 20 '16 at 14:49

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