Dear John,
In case the device enclosure is non conducting, then where to connect
the USB cable shield conductor? Should we isolate the DC negative (the
digital ground of the circuitry) of the USB 5 volts power from the USB
cable shield?
K. BALA
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|John Woodgate <[email protected]>
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|[email protected]
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|01/06/2010 03:42 PM
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|Re: ESD Test Failure of Stainless USB Mouse
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In message
<ofaaf9b4a4.e676f095-on652576a3.0032e3a9-652576a3.00338...@scmmicro.co.in
>, dated Wed, 6 Jan 2010, [email protected] writes:
> But many times I realised that connecting the shield conductor of
>the USB cable to the digital ground of the circuitry in the PCB
>improved the ESD / EFT immunity. Leaving the USB cable shield conductor
>open (either at device end or at computer end) worsens the situation.
I entirely agree; shields left open at one end are antennas. But shields
should always be connected to enclosures, if conducting, and preferably
to the outside surface. Think of the shield as a pipe connecting two
water tanks. You need a full 360 degree seal at each end. No pigtails
more that 10 mm long at most.
If you connect the shield to the PC board, then all the hash on the
shield is dumped into input circuits, where it can cause most trouble.
> In the device introducing a ferrite bead between the digital ground of
>the circuitry and the shield conductor of the USB cable also worsened
>the ESD situation.
Yes, because that open-circuits the shield at high frequencies.
--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK
I should be disillusioned, but it's not worth the effort.
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This message is from the IEEE Product Safety Engineering Society emc-pstc
discussion list. To post a message to the list, send your e-mail to
<[email protected]>
All emc-pstc postings are archived and searchable on the web at:
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