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

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From: Howard Johnson ([email protected])
Date: Wed Dec 22 1999 - 09:48:33 PST

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

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