Steve and SI list, Thanks for replying. This is the dumbest question I can ever remember asking. How do you identify a battery return from DGND? Are they not the same thing? The power for the receivers at the load end is transmitted up the cable along with a few grounds. So at the load end the chips see the +5V and ground. The power flows through the chip, and out the ground pin on to a common DGND. DGND would connect to the grounds in the cable and to the chassis (if you connected it to it). This would (as you say) lead to current travelling down the shield. When you say that the battery returns should flow down the GNDs in the cable do you mean the DC current? The only way I can see of splitting the battery returns from flowing down the shield is to capactively couple DGND to the chassis at the load end with a 0.1. Any help is greatly appreciated as always. Chris (confused) -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx]On Behalf Of steve weir Sent: 19 August 2004 17:27 To: cchalmers@xxxxxxxxxxx; si-list@xxxxxxxxxxxxx Subject: [SI-LIST] Re: Connection to cable shields 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 > > > > > > > > > > > > > > > >********************************************************************** >This communication contains information which is confidential >and may also be privileged. It is for the exclusive use of the >intended recipient(s). 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