[SI-LIST] : LVDS vs. CML

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From: John Lipsius (jlipsius@JasmineNetworks.com)
Date: Mon Mar 05 2001 - 20:32:16 PST


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?)

2. Would the following comparisons be valid to argue one or the
   other case? Note, I'm not comparing which can drive a faster edge,
   though that fact contributes to the faster having more dB loss at the
   rcvr.

   Assume CML Vcc=1.8V and LVDS Vcc=2.5V. Also assume the CML output
terminations
   are internal to driver device and matched perfectly, as is the LVDS diff.

   termination (external to rcvr and perfect).

   * CML and LVDS diff. swing of each wire are about the same...
     LVDS Vodiff < 400mV and CML Vodiff =400mV typ (not p-p) so, for
     identical imbalanced trace layouts, neither would be worse in the
     comm. mode noise area.

   * LVDS is superior due to its 1V Vcm range being larger, period.
     (if, in fact, true).

   * One can ac tap the differential termination of LVDS pair and
     thus "assure" common mode rejection at the rcvr. However,
     CML rcvrs have internal 50ohm cm termination to handle this --
     so no clear advantage here.
 
   * ... ?

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.

   So far, only info is the following from
   Zabinski, on thread dated Mon Aug 21 2000 - 05:20:59 PDT
   RE: [SI-LIST] : Why CML for high-speed interfaces?

   TITLE DC Analysis of Current Mode Logic
   AUTHOR Ronald L. Treadway
   SOURCE IEEE Circuits and Devices Magazine
   DATE March, 1989 PAGES 21-35

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John Lipsius 408-895-5058
Sr. Hardware Engineer
Jasmine Networks
3061 Zanker Rd.
San Jose, CA 95134
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