Isolated part - where to connect shielding? - Page 1
Forum section: electronics / projects

author: mrburnzie
author_rank: Regular Contributor
timestamp: March 31, 2023, 10:10:20 am
content: Hi!I would like to get an opinion on what kind of connection I should do here.It is all explained in the pictures but a bit of a context.- there is an isolated part, that is connected to a connector and a switch. Both of them have shielding connections.- the isolated part is connected to the internal part of the system with a power and data isolation part.How and where should I connect the outer shielding?Which connection is the best, or are they even the best option?The interface in question is an RS485 line that I would like to isolate from the rest of the electronics.

author: hanakp
author_rank: Regular Contributor
timestamp: March 31, 2023, 11:52:38 am
content: It depends why you put in the shield in the first place. Do you want to prevent emissions from your device radiating outwards or protect it from external fields? Do you want to protect your device as a whole or only the big green part with GND?In either case, there is a rule of thumb: for best results, the shielding must be connected to the protected ground at one point only. Otherwise ground currents could start flowing through it, reducing its effectivity. It's rather complicated to do it properly in the real world though, so compromises are usually made.

author: redkitedesign
author_rank: Regular Contributor
timestamp: March 31, 2023, 12:06:38 pm
content: Assuming your shielding is mostly for EMC, use the simplest option. Everything in one shield, and the shield connected to Earth.Having multiple zones only creates additional leaks. Any shield not connected to earth is an antenna, i.e. worse than useless.Your isolated part can share earth with the rest. Just keep power, ground and signals isolated. There should not be any functional current in the earth-connected parts anyhow (i.e.: both parts of the design should have power and ground isolated from earth)In some situations (i.e. battery operated stuff, car stuff) ground is already connected to the outer metal casing. That can be OK, as long as you don't use the casing as the ground wire (or know what you are doing). Or the thingy uses an isolated supply, with the low voltage ground side connected to Earth (e.g desktop PC's). That's OK too, but don't use the case as ground wire.As to the single ground connection: Forget it. Just make sure there is a ground wire (plane) next to each power or signal wire (plane/trace), and for any interesting frequency the path through the shield will have too high an impedance (since its loop is bigger) than the path through the wire.For DC (of significant amperage) you might want to ensure your intended path really has the lowest impedance... But high-amperage DC applications tend to have the + and - isolated from earth, if not for EMC then because a short can be really awful.

author: Terry Bites
author_rank: Super Contributor
timestamp: March 31, 2023, 02:59:25 pm
content: If you connect the shield to iso-gnd you'll add capacitance across the iso barrier.This will undo some of the benfits of isolation. Loss of iso withstand voltage and CM noise coupling in particular.All shields need to be tied to the same potential. Magnetic shields have eddy currents flowing in them that will be converted to unwanted emfs elsewhere. They need grounding too.