Re: [SI-LIST] : LVDS vs. CML

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From: DORIN OPREA (dorin.oprea@alcatel.com)
Date: Fri Mar 09 2001 - 09:22:53 PST


Not Found

The requested URL /~sessions/SLVS-400-pdf was not found on this server.

Can you help it ?

Thanks,
Dorin

"D. C. Sessions" wrote:

> On Monday 05 March 2001 21:32, John Lipsius wrote:
> # Hello,
> #
> # I'm interested in how one might judge one superior to the other due to
> # something inherent. Or, is it board layout, path discontinuities, and
> # environmental factors that are the only determinant.
> #
> # Assume identical 50 ohm and pcb environments, etcetera. And, the
> # same rcvr type as the driver (no level shifting).
> #
> # For example: "assuring no skew between wires of the pair is more
> # important than any factor inherent to CML vs. LVDS" might be one
> # answer.
> #
> # 1. An initial question is: why is CML offered when there's LVDS ?
> # (Just a switching capability difference?)
>
> For a few reasons:
> 1) CML is trivial in bipolar, LVDS is much less so since it requires both
> sourcing and sinking current along with crossover management.
> 2) LVDS is tricky at best to implement in advanced CMOS processes,
> since the 2.4 V common-mode range is lethal to small-geometry
> (read high-performance) transistors.
> 3) LVDS has problems with crossover between receiver sections. The
> common-mode range is wide enough that both P- and N-channel
> input differential receivers are required, with the two outputs mixed.
> Obviously, this has an impact on receiver performance.
> 4) LVDS floats the receiver termination, which makes for high CM
> reflection coefficients.
> 5) CML is easily implemented in minimum geometry NMOS transistors at
> core supply potentials, which means no added power supply just to run
> the I/O.
>
> All in all, the only advantages that LVDS has over CML are its superior
> tolerance for ground shift and independence of power-supply potential.
>
> # 3. Lastly, ANYONE KNOW OF SOME CLEAR REF. SOURCES ON CML regarding
> # both ac/dc specs and these issues for real links? Apparently, there's
> # no 'standard' version of CML, correct. The web appears completely dry.
>
> There's been some interest shown in creating a JEDEC standard CML specification.
>
> Personally, I prefer SLVS (http://www.primenet.com/~sessions/SLVS-400-pdf)
>
> --
> | The race is not always to the swift, nor the battle to the strong. |
> | Because the slow, feeble old codgers like me cheat. |
> +--------------- D. C. Sessions <dcs@lumbercartel.com> --------------+
>
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