Re: [SI-LIST] : PECL output buffer implementation in CMOS

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From: D. C. Sessions (dc.sessions@vlsi.com)
Date: Tue Nov 02 1999 - 08:37:07 PST


Jayarama Shenoy wrote:
>
> Hi All,
>
> Can someone provide insight into PECL output buffer
> implementation in CMOS tecnologies? It is being claimed
> that this cannot be done while at the same time retaining
> the power supply noise rejection of differential output
> drivers, which I find hard to understand.
>
> Any pointers to public literature on PECL (or similar diff-
> erential) output drivers in CMOS will be greatly appreciated.

Hey, Jay.

PECL can most certainly be done in CMOS. That, or the ones I'm
shipping use previously-unknown laws of physics. As for telling
YOU the details, you now need an NDA. Should've asked before you
left!

Seriously, what your sources were referring to was that since CMOS
doesn't make efficient low-offset pullup followers the way bipolar
does, you don't get the benefits of hanging the positive rail on
the high-impedance collector (or drain) node of the transistor.
The high impedance of the collector node means that voltage noise
on the positive rail doesn't show up as current noice on the output.

The hidden assumption here is that the CMOS output follower has to
run on the same supply as the predriver. Where bipolar thrives on
small base-emitter voltages, MOS devices need more voltage bias and
the voltages available in PECL aren't well-chosen for this. (Duh!)

On the other hand, nowhere is it written in stone that the positive
rail for PECL has to be +5 volts. 2.5 volts with a 3.3 volt predriver
gives an output common-mode point of about 1.2 volts, which is by
astonishing coincidence also (a) centered in the rails, and (b) the
common-mode point for LVDS. Astonishing. And along with this comes
a high-state Vgs of 1.5 volts, which is enough to make a reasonable
NMOS output device happy without desaturating it.

Another possibility is to run open-drain. If you absolutely need the
speed, this is nice because you don't have to coordinate the pullup
an pulldown devices.

The reference supply voltage for PECL is a system tradeoff. Personally,
the more I work with PECL the more unprintable things I find to say
about it. That acronym lends itself to some amazing abuse, let me
tell you. If you have *anything* resembling a choice, run (don't walk)
to a more sensible scheme like HSTL or GLVDS. A lot of the 'PECL'
applications I'm seeing actually don't require PECL and would work fine
with almost any low-swing differential scheme.

-- 
D. C. Sessions
dc.sessions@vlsi.com

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