[sdiy] semi-OT: USB cable shield

[Barry Klein]

I've seen the cap and resistor on firewire/1394 specifications and some products but haven't seen them in USB products.
I asked one of our USB design engineers and he said it was done in the past for ESD susceptibility on the host side - but now controllers are more tolerant and shields are typically grounded to the pcb/chassis ground.

Barry



-----Original Message-----
From: synth-diy-bounces at dropmix.xs4all.nl [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Ingo Debus
Sent: Tuesday, April 22, 2014 12:13 PM
To: synthdiy diy
Subject: [sdiy] semi-OT: USB cable shield

Hi all,

as far as I know, the shield of USB connectors usually is connected directly to ground. However I've seen designs where there's a smallish capacitor (10 nF) in parallel with an 1 megohm resistor between the housing of the USB receptacle and ground. Why is this? I understand this is done for MIDI connectors to break up ground loops (see? it's not completely OT), same for Ethernet, but USB? USB connectors have a ground pin anyway, so it seems pointless to me to break the ground shield connection for low frequencies while there's still a direct ground connection via the pin.

Any Ideas?

Ingo
_______________________________________________
Synth-diy mailing list
Synth-diy at dropmix.xs4all.nl
http://dropmix.xs4all.nl/mailman/listinfo/synth-diy