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

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From: DORIN OPREA (dorin.oprea@alcatel.com)
Date: Fri Mar 09 2001 - 12:22:40 PST


Thank you very much for all the prompt responses.

Cheers,
Dorin

"D. C. Sessions" wrote:

> On Friday 09 March 2001 10:22, DORIN OPREA wrote:
>
> # # Not Found
> #
> # The requested URL /~sessions/SLVS-400-pdf was not found on this server.
> #
> # Can you help it ?
>
> That should have been http://www.primenet.com/~sessions/SLVS-400.pdf
>
> # "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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>
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> --
> | I'm old enough that I don't have to pretend to be grown up.|
> +----------- D. C. Sessions <dcs@lumbercartel.com> ----------+
>
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