[SI-LIST] Re: Analog or Digital

On Thursday 20 September 2001 18:06, Bob Patel wrote:
> Hi! I had a question regarding analog signals
> i.e.analog signals are signals whose amplitude varies
> continuous with time.
> Now, my question is can I calssify LVPECL, LVDS
> signals as analog signals or as digital. I know what
> to classify it as according to my understanding but I
> needed some opinions on this.

There is no such thing as digital.  It's a fairy tale that
people tell to students, management, and other simple
gullible sorts.

> Now, why do I ask this because some engineers are
> trying to put lvpecl or lvds under analog and now try
> to connect these chips to agnd which I diagree and say
> it is much better to have a single gnd rather than
> having separate gnd and then worry about connecting
> them.

LVDS has utterly insane common-mode tolerance, which
voids any potential benefit from running it on a "quiet"
power system.  Although PECL *can* be made to operate
fairly quietly, it usually isn't -- so putting it on your analog
system will usually corrupt the analog system for little or
no benefit to the logic.

In both cases, running the LVDS/PECL/whatever signals
from analog ground domain over digital ground planes will
*really* screw up the signals while at the same time kicking
nasty transients into your "quiet" analog ground system.
If you run them over analog ground, you're going to either
going to put lots of valuable routing space off-limits for
digital signals or corrupt the analog ground system by
having digital stuff routed over analog ground -- once again
with the plane-crossing transients.

The ugly secret is that LVDS, LVPECL, SLVS etc. aren't
any less "analog" than LVCMOS is.  They're just designed to
work better in the face of analog effects that were ignored
in the design of LVCMOS.

-- 
| 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@xxxxxxxxxxxxxxxx> --------------+
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