[SI-LIST] Re: Silly question
- From: sdb@xxxxxxxxxx (Stuart Brorson)
- To: erturkk@xxxxxxxxxxx
- Date: Fri, 3 Sep 2004 07:34:11 -0400 (EDT)
> On my home project front-panel I/O board, I initially had 1394 and
> usb2.0 differential pairs going from connector to connector on a
> _single_ layer board. The signals come from the pc motherboard with
> a cable, and on this i/o board I connect them to appropriate
> connectors.
>
> After learning the impedance requirements must be met, I ended up
> adding a dummy plane so that the fab house can control the impedance.
>
> Do I have to connect that dummy plane to ground?
Yes, you should connect the plane to GND. Although diff-mode currents
flowing on the plane will cancel each other out, common-mode currents
still need the GND return. And although you think your signal is
differential, it will still have a common-mode component due to
non-idealities. For example, any rise time skew at the source
produces a common-mode component to the signal. Also, if the pos and
neg signal swings are not equal, you will get a common-mode
component.
Since these common-mode components will be there, you should provide a
conduction path via the GND plane.
Stuart
>
> If the signals were single-ended, I know the answer would be yes. =20
>
> What do you think?
>
> thanks,
>
> erturk
>
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