Connecting the shield of an isolated twisted pair cable - Page 1
Forum section: electronics / projects

author: kristjanbp
author_rank: Newbie
timestamp: September 14, 2016, 05:45:45 pm
content: Hi.I'm designing a battery management system using the LTC6804-1 which uses an isolated SPI interface to communicate (see image below) and I'm trying to make it as noise immune as possible. The links between the ICs are isolated and use a shielded twisted pair cable. I've been trying to figure out what's the best way to connect the shield of the twisted pair cable since the signal on the twisted pair is not referenced to any point in the system. In an application note from Analog Devices (AN-347) it says that grounding a shield is useless unless the signal on the cable is ground referenced and that the shield should be connected to the reference potential at the signal-reference node. I'm not sure if I should ground it (chassis), leave it floating or somehow reference it to the differential signal on the twisted pair cable. Grounding and shielding can be tricky so I'm looking for some input on this.Thanks!

author: raptor1956
author_rank: Frequent Contributor
timestamp: September 14, 2016, 06:14:38 pm
content: The shield should only be tied to ground at one place and one place only. The worst thing to do would be to connect the shield to ground at both ends.Brian

author: dom0
author_rank: Super Contributor
timestamp: September 14, 2016, 06:38:56 pm
content: A shield doesn't have to be connected anywhere to be a shield, but it can improve things (and sometimes make things worse). I'd first try if that suffices here. You can also try a small coupling capacitor from shield to ground (of suitable voltage rating, and safety approvals if applicable). Often, but not always, a shield connected on the sending/source side is better than on the receiving end.Shielding is really one of these areas where you have some rules of thumbs, but ultimately you have to trial-and-error at least a few configurations to find the best. Often enoughanyshielding is already sufficient, though.

author: MagicSmoker
author_rank: Super Contributor
timestamp: September 14, 2016, 10:24:52 pm
content: Quote from: raptor1956 on September 14, 2016, 06:14:38 pmThat thumb rule mainly applies to DC and audio applications; if the main goal is to reduce HF interference then it is best to ground the shield at both ends*. A good compromise - one that is still effective at HF while not introducing any DC ground loops is to directly connect one end of the shield to ground but connect the other end to ground through a ceramic/film/mica dielectric capacitor (optional: add a resistor in parallel for damping).* - and better still is to continuously ground the shield, but that is rarely practical for a cable.

author: David Hess
author_rank: Super Contributor
timestamp: September 15, 2016, 07:29:30 am
content: Ground the shield at the receiver end.

author: kristjanbp
author_rank: Newbie
timestamp: September 15, 2016, 10:16:37 am
content: Thanks for the input guys. Do the same "rules" still apply when the signal is completely isolated from ground and ground is just an arbitrary point for the signal? What makes me want to connect to ground?A discussion on floating shielding is in this thread:https://www.eevblog.com/forum/projects/shielding/and as I understand it grounding the shield does nothing if the signal is not referenced to it in any way. I want to avoid grounding the shield if it does not benefit the design since it will make the layout harder to implement (safety isolation is needed between the ground (chassis) and the HV battery system).

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 15, 2016, 01:03:20 pm
content: Quote from: raptor1956 on September 14, 2016, 06:14:38 pm...If by "ground" you mean protective earth, then yes, that would introduce a ground loop, which would make things worse.If by "ground" you mean local circuit ground, you're on the losing side ofI'll tell ya what.As for "ground" (in the local sense), in the presence of a transformer: there is unavoidable capacitance between windings. In fact, the capacitance is necessary and critical for signal transmission: if there were none, then the winding impedances would be infinite, and the bandwidth null! (You'd also have the strange case that the speed of light is, uh, zero in this case I think? Which obviously can't happen, so we're left with finite and nonzero capacitance AND inductance in the real world!)The key is balancing the coupling capacitance between the two winding terminals, so that the differential signal is passed without influence from the common mode. A common mode choke can be used to further increase the CM impedance (note that the CMC inductance will resonate with the winding capacitance at some frequency; this needs to be dampened, otherwise the CMRR can beworseat the resonant frequency!), and differential mode (center tapped) chokes can be used to provide a CM ground to the signal pairs.It's worthwhile to evaluate the Ethernet standard: how it's designed, and why. Isolation transformers, with CT primary and secondary, are used on all links, one at each end. The CT windings ensure balance and provide a grounding point, where CM interference can be shunted through, while the isolation prevents problems from ground loop. A common mode choke (usually on the connector side) prevents RFI problems.Tim

author: bicc1306
author_rank: Contributor
timestamp: September 29, 2016, 12:36:47 pm
content: Quote from: raptor1956 on September 14, 2016, 06:14:38 pmI agree. By grounding both end you create a physical loop that is essentially an antenna for stray magnetetic lines of flux and an inductor will soak up this noise and cause possible problems with communication wiring.