[sdiy] semi-OT: USB cable shield

[rburnett at richieburnett.co.uk]

The 10nF/1M parallel combination in series with the shield is done to 
break a potential earth loop at low frequencies.  The 10nF capacitor 
tends towards a short circuit at high frequencies so the shielding 
appears continuous from the perspective of unwanted RF signals trying to 
get in or out, but at low frequency the capacitor presents a very high 
impedance to any ground currents that try to flow between the two 
devices.  The 1Meg resistor is to bleed away any static charge.  This 
arrangement is common in other differential signalling standards like 
RS-485.  It is also a common way to "soft ground" circuitry that is 
inside an earthed metal box, so you sometimes see it between the 0v 
trace on an amplifier PCB and the chassis ground, or between the 0v 
terminal of a power supply and the mains earth.

It is true that the USB cable has a wire for the 0 volt line itself, 
however this line along with the D+, D- and +5V lines can be 
galvanically isolated to break earth loops using optical or magnetic 
coupling.  Connecting the chassis of two pieces of equipment together 
via the shield of a USB cable would defeat this galvanic isolation and 
could potentially complete an earth loop.  Cue 50/60Hz hum and USB whine 
(>.<)

-Richie,



On 2014-04-22 22:10, Barry Klein wrote:
> 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
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy