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

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From: Douglas C. Smith ([email protected])
Date: Wed Dec 22 1999 - 12:02:01 PST

Hi All,

I love Howard's write-up below but there is one technical hurdle that
to be overcome before we can get there. Al Wallash of Quantum and
published at both the IEEE-EMC and EOS/ESD symposia data on the ESD
of GMR heads, which are used on the current generation of disk drives.

The fact that they are so sensitive, fast, and small is the root of
problem. I have successfully melted a GMR head by exposure to the
field from a 4 kV ESD event (not all that big) from several feet away!
only takes a few tens of milliamperes of current for a nanosecond to
a GMR head.

So equipment using this technology will need to be very well shielded
and require
special assembly techniques. For instance, it will not be good enough
to control
the electrical charge on personnel as we do now. The radiated field
from a spark at
a swithch contact (a light switch, for example) is also an issue. If
fact, GMR heads
have been damaged by just such a mechanism.

There is an opportunity here for some new form of internal ESD
protection for devices.


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]
> >
> >**** To unsubscribe from si-list: send e-mail to
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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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