[SI-LIST] Re: Connection to cable shields
- From: steve weir <weirsp@xxxxxxxxxx>
- To: cchalmers@xxxxxxxxxxx, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 19 Aug 2004 09:26:33 -0700
Chris, you have multiple issues to deal with here. The first is that the
common mode range of the receivers is finite. You need to get the DGND
of both systems near the same potential, or you just can't signal. The
second is an issue of power. The DGND and chassis being joined is fine,
provided that the battery returns do not also join to the case. If they
do, you could have monstrous currents flowing through your cable
shields. Assuming that your battery returns are isolated, then the DGND
should join to the case often in both cases. The third issue is one of RF
currents. CM noise between the two chassis will circulating in the
shield. But that is the entire point, to circulate it within the shields
and not through the outside air. The only other thing that you probably
want to be aware of is that the shield is a wave guide and will exhibit 1/4
wave and 1/2 wave effects.
Steve.
At 04:54 PM 8/19/2004 +0100, Chris Chalmers wrote:
>Si list,
>Where to ground a cable. I always get mixed up on where. This a
> corner case which I am just not sure on and I wondered if someone
> could just give me a nudge in the right
> direction. The following case is
>
>
> [ 10m shielded twisted pair [
>source [ > -----------------------------------------> [ Load
> [ [
> [ [
> chassis chassis
>
>The signals traveling the cable are LVDS at ~400Mb/s.
>They all transmit (no receivers at the source end).
>The source's DGND is connected to the Chassis at the
> point of exit of the cable. The cable screen is connected
> to the chassis. I believe this is right.
>
>At the load end, I don't know whether to ground DGND
>to the chassis or not. Or even ground the cable shield
> to the chassis. The load end circuitry DOES NOT have
> its own GND. The returning current from the receivers at
> the load end flows back down the cable to the source. The
> chassis at the load end does not have a connection to GND.
>
>For high speed signals I want to create a magnetic
> shield so I think I need both ends of the cable shield connected
> to the chassis's. I also think I need the source end DGND
>connected to the source chassis. However, I am not convinced
> I need the load end DGND connected to the load end chassis.
>The circuitry at the load end works at 5V.
>
>Any help with this would be most appreciated.
>
>Thanks
>
>Chris
>
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>
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