[SI-LIST] Re: LVDS
- From: "Ingraham, Andrew" <a.ingraham@xxxxxxxx>
- To: "si" <si-list@xxxxxxxxxxxxx>
- Date: Fri, 22 Aug 2003 09:48:17 -0400
Timo wrote:
> I am working on LVDS signals. And are wondering if
> the following configuration is correct or not:
>
> LVDS Output(+) -----o********o------ LVDS input(+)
> ()
> LVDS Output(-) -----o********o------ LVDS input(-)
>
> o = SMA connector.
> ****** = cable which is 50 Ohm.
> () = 100 Ohm.
>
> The LVDS is terminated with 100 Ohm differential.
> I am wondering does the cable ground shield matter at
> all? Since the termination is differential should we
> float the ground of SMA connector and cable, or it
> didn't matter at all?
The cable shield does indeed matter very much, since it conducts the return
current for the signal passing through the cable. Remember each cable is a
transmission line, and the transmission line's return current is in the
cable shield, regardless of what circuits you connect to its ends.
Ideally the two cable shield currents will be equal and opposite (so that
the net ground current between source and load is nearly zero), but they
must be allowed to flow around this loop. If they can't flow, then you'd
have two really lousy transmission lines between the SMA connectors.
Regards,
Andy
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