Re: XAUI AC coupling
Ali,
The common mode voltage is too high. Several vendors are doing XAUI in
0.25 um. Many are going to 0.18 or better. Some have announced SerDes at
0.13 um already. The direction of several key XAUI vendors will be to go
0.13 um or better for XAUI by the time the standard is published.
Therefore, a 0.75 V +/-0.25 V common mode spec is short sighted and will
be obsolete prior to publication. This is why I'm recommending no spec
for DC-coupling and no mandate for AC-coupling for XAUI.
Best Regards,
Rich
--
Ali Ghiasi wrote:
>
> Hi
>
> About 8 month ago we debated the same issue rigorously in another
> standard. The XCVR mfg
> wanted AC coupling and the system company wanted DC coupling. Can you
> imagine if you
> have a 64 ports 10 Gig switch how many capacitor you need.
>
> In that group I proposed the following satisfying the need of both AC
> and DC:
> - every transmitter must provide 0.75+/- 0.25 volts of common mode
> else it must be AC coupled.
> - Every receiver must be capable of operating with 0.75 V of common
> mode
> else it must be AC coupled.
> - All external link the receiver will be AC coupled.
>
> I believe there is genuine need for DC coupled link in the backplane and
> switch applications. The
> above implementation may add two capacitors one for each blade, assuming
> both serialize and
> de-serializer do not operate with the 0.75 V common mode. But the
> capacitors can be eliminated
> from the blades if you operate with common mode of 0.75 V.
>
> Why 0.75 V Common mode; it is half 1.5 Volts, support >500 mV on each
> leg, supports
> advance CMOS (Sub 0.18 um).
>
> AC coupling will be used often, but lets not force high density
> applications to use
> 1000's of caps. We need to define the common mode voltage, but many can
> just ignore it.
>
> Thanks,
>
> Ali Ghiasi
> Newport Communications
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