From: Larry Miller (email@example.com)
Date: Wed Nov 03 1999 - 06:17:13 PST
Our 1 Gb Ethernet SERDES chips have gone over from bipolar to CMOS "PECL".
We had some trouble with early versions latching up.
Have you had to allow for this?
At 09:37 AM 11/2/99 -0700, you wrote:
>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.
>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
>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
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