Re: [SI-LIST] : Clean sheet of paper

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From: Jon Powell ([email protected])
Date: Wed Dec 22 1999 - 19:55:51 PST

I find your GMR technology very interesting.
As for the problem with temperature.
A favorite quote of mine is:

"Liquid Nitrogen is cheaper than beer"

(and cheap beer, at that).

Merry Christmas,

Howard Johnson wrote:

> Regarding new I/O technologies:
> My personal wish-list for Christmas includes new developments in the field
> of giant-magnetio-resistance (GMR) materials. I don't know if you've
> followed this lately, but some of the recent advances have been
> breathtaking. GMR materials may eventually be used to support some pretty
> neat signaling methods.
> Here's the deal: a GMR material changes its resistance in reaction to a
> small (very small) applied magnetic field. It's a bulk material effect -- it
> happens real fast. The guys I've talked to say they can't even measure how
> fast it is. The required magnetic fields as SO small that it can detect the
> current flowing in a signal PCB trace just by being NEAR it (no electrical
> contact required). What you do is put a dot of this material near the point
> of interest, hook up some terminals to the dot so you can measure its
> resistance, and Voila! you have a very sensitive, DC-active, current probe.
> At a distance of perhaps a few mils, as you might imagine in a backplane
> connector application, a typical GMR dot would contribute a load impedance
> on the line of only perhaps a few tenths of a pF. Think about it. A
> backplane receiver that had only a few tenths of a pF would significantly
> improve our ability to make multidrop backplanes.
> Rumor is that GMR dots are being designed into magnetic read heads for
> next-generation magnetic storage disks. B.T.W., does anybody have any
> specific information on these projects?
> OK, so that's the "good news" part, but what are the drawbacks? Well, at
> present, operating at room temperature, you only get about a 10% change in
> resistance from the best materials. That means your receiver has to work
> with a really tiny signal. BUT HERE'S THE EXCITING PART: researchers are
> reporting development of materials with GMR changes on the order of 1000:1
> at cryrogenic temperatures. If they can make these materials work at room
> temperature, the GMR material will actually represent a "new" device with
> susbstantial power gain, that accepts current in and controls current out,
> in a fundamentally new way. It's like getting a whole new kind of FET, only
> it's sensitive to magnetic fields, not electric ones. Who knows where this
> subject will lead?
> Anyway, I can imagine using the GMR material to make terrific differential
> isolators (suppose a current inside your chip controls the resistance of a
> GMR dot -- the GMR dot is connected to an external bias circuit that
> converts its change in resistance into a measureable signal). The advantage
> of this structure is that, if the GMR dot is fabricated inside the IC (oh
> yes, did I tell you that this can be done??), and if the bias circuit is
> done correctly, it will eliminate ground bounce. Cool.
> GMR dots embedded inside connectors might someday operate kind of like
> super-fast, and super-cheap, optical isolators -- they could eliminate
> circulating common-mode currents.
> Of course, all this theorizing depends on getting the material operating
> temperature up into a useful range--something over which we as engineers
> have very little control, but it's always nice to dream about what you
> want for Christmas.
> Best regards,
> Dr. Howard Johnson
> At 04:04 PM 12/21/99 -0700, you wrote:
> >With the year wrapping up and my inbox filling with
> >"Out of Office Autoresponse" messages, I thought I'd
> >kick off something more interesting than the joys of LVDS.
> >
> >In particular, what would we use for signaling if we could
> >start with a totally clean sheet of paper? Rather than
> >immediately jump to a solution, I'm looking for some criteria:
> >
> >* It has to be scalable. Given silicon technology trends, it
> > should migrate gracefully to lower-voltages and less
> > voltage-stress-tolerant semiconductors.
> >
> >* It has to be SI clean. Output impedance should be matched
> > (stringency variable) to the line across the switching range.
> > Inputs switchpoints should be symmetrical and well-defined
> > (ie differential receivers). Power plane proliferation
> > leads to bad SI and wasted money, so separate termination
> > supplies are a Bad Thing.
> >
> >* It has to be versatile. Single-ended, balanced single-ended, or
> > differential; multidrop or point-to-point; uni- or bidirectional;
> > all should be minor variations on the same system.
> >
> >* It should be economical. Wasted power is a Bad Thing, so low
> > swing is a must. Padrings are some of the most expensive real
> > estate around, so pincount should be minimized. Line termination
> > can dominate a PWB so KISS is the rule. Power supplies (esp.
> > ones that can both sink and source current) are expensive and
> > nasty to deal with, so do without (both for termination and
> > funny analog functions in the I/O circuits.)
> >
> >What can we add to the list? Remove? Priorities? (This is
> >engineering, we make tradeoffs.) Where does this take us?
> >
> >--
> >D. C. Sessions
> >[email protected]
> >
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> >
> >
> _________________________________________________
> Dr. Howard Johnson, Signal Consulting, Inc.
> tel 425.556.0800 // fax 425.881.6149 // email [email protected]
> -- High-Speed Digital Design books, tools, and
> workshops
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Jon Powell
Director of HSSD Consulting Services
Viewlogic Systems, INC.
805 988 8250

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