Grounding Chassis of USB Powered Device to USB Ground/ Shield? - Page 1
Forum section: electronics / projects

author: Par3689
author_rank: Contributor
timestamp: December 27, 2018, 07:15:45 pm
content: I am working on a project which is a USB powered device. Data lines and the power line of the USB are isolated from the main circuitry.I am using an anodized aluminium enclosure for the board. I am not sure what to do with the chassis/ enclosure.Should I keep the enclosure floating ie. without connecting to any groundOr should I connect it to USB ground/ shieldWhat is the standard way of doing this properly?More info: I am using hammond chassis similar to this one 1455N1201

author: soldar
author_rank: Super Contributor
timestamp: December 27, 2018, 07:27:20 pm
content: Unles there is a good reason not to do it I would connect the enclosure to the USB shield.

author: Par3689
author_rank: Contributor
timestamp: December 28, 2018, 02:45:57 am
content: And should the shield be directly connected to USB ground? Right now I am connecting USB shield to USB ground with RC parallel comboIf a charged individual touches the chassis,should 1M resistor to ground be used, like the ESD strap?or there should be low impedance path from chassis to shield to USB groundIs there any other scenario I have to take care of? I have put TVS diode for the data lines and Vbus.I am not able to decide on this

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2018, 05:02:56 am
content: search the forum, people literally fight about putting some kind of impedance between the shield and the cable (it gets pretty ridiculous).i would only put impedance on the shield if something was severe garbage and i was forced to do it. imo it means something is fucked up.

author: Par3689
author_rank: Contributor
timestamp: December 28, 2018, 05:20:28 am
content: While searching on the forum I found this post by free_electronhttps://www.eevblog.com/forum/projects/usb-device-circuit-design/msg235913/#msg235913He says to connect RC combination from system ground to system shield with R = 4.7k and C=27pFFor my device will this be sufficient?I am attaching his reference image

author: Par3689
author_rank: Contributor
timestamp: December 28, 2018, 05:23:31 am
content: Quote from: coppercone2 on December 28, 2018, 05:02:56 amThat's what I did first. Reading the answers made me repost the question as I was not sure what to doMany of the posts are very old too

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2018, 06:05:45 am
content: ok when you put a 4.7k in parallel with a capacitor your making a filter network on a shield.do you want a filter network on your shield? its going to be low impedance - high z filter - low impedance.is 4.7k matched to the shield some how? i think its nasty.

author: Par3689
author_rank: Contributor
timestamp: December 28, 2018, 11:46:04 am
content: Is it necessary to connect shield and ground of USB powered device?For USB powered device if I connect chassis to USB shield, is it necessary to then connect to USB ground?

author: coppercone2
author_rank: Super Contributor
timestamp: December 28, 2018, 12:27:10 pm
content: you don't want current flowing through your shield

author: Andreas
author_rank: Super Contributor
timestamp: December 28, 2018, 02:22:53 pm
content: Quote from: Par3689 on December 28, 2018, 11:46:04 amnoQuote from: Par3689 on December 28, 2018, 11:46:04 amno, usually ground and shield are connected within host at the power supply.a second connection would build a ground loop which makes the shield useless.Quote from: Par3689 on December 28, 2018, 05:20:28 amC=27 pf against the ESD modeling capacity of 150 pF -> completely useless you would need a capacitor which can withstand 10000 VThe best method is to isolate the cirquit against the metal housing so that isolation withstands 8-15 kVIn this case you need neither a capacitor nor a resistor.If you cannot isolate enough then you may need a (ESD resistant) capacitor but usually with more than 4700 pF.And geometrical placing of the capacitor is essential so that it gets not worse.with best regardsAndreas

author: ogden
author_rank: Super Contributor
timestamp: December 28, 2018, 04:22:36 pm
content: Quote from: Andreas on December 28, 2018, 02:22:53 pmUsually it just does not matter. For some devices good contact between USB shield and case is required to close "RF gaps" so EMI does not leak out of device. Look at PC's - power supply, motherboard and I/O shield all have connection to chassis and everything is fine. Obviously it requires proper design so internal currents do not find path through enclosure/case.

author: deBug
author_rank: Contributor
timestamp: December 31, 2018, 08:44:48 am
content: Connect them together, that's what I do.Also note that some USB cables connect them together internally so even if you don't connect them, inserting certain cables will do it for you.

author: deBug
author_rank: Contributor
timestamp: December 31, 2018, 09:16:58 am
content: QuoteWell.. no you wont.Putting one shield on top of another will improve shielding, not destroy it.If you have two grounding path in parallel they lower the resistance and the inductance so they can only improve the grounding not worsening it.I never understood all the fuss about avoiding ground loops, star grounding and other weird constructs.Just provide the lowest enough inductance possible for the ground.Inductance lowers if a wire is wide, that's why a ground plane work as a low inductance ground.I imagine that in a large factory if would be expensive to put down enough copper to make a low inductance ground over the entire plant so you will have to live with inductance and resistance in the cables and that will in turn create ground loops between different stations spread out on the plant. In that case it makes sense to create star grounding or use optical or magnetic isolation between stations.But on the desk, just provide the lowest possible inductance and resistance for ground, alternative path for ground in this case will be an improvement as it means you paralleled the resistance or inductance with another and that will bring the total resistance or inductance down - that is what we want!Shielding helps avoid emitting or picking up radiation due to the antenna effect (current in a wire will radiate, radiation in to a wire will create a current).This problem gets worse if the wire is long or if the impedance of the receiver/input end of amplifier is high impedance, that is the reson why wires are usually shielded - they are long.