volume39-number4(1).pdf

A forum for the exchange of circuits, systems, and software for real-world signal processing

Volume 39, Number 4, 2005

In This Issue

Editors’ Notes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

The Four Dees of Analog, circa 2025 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

i Coupler ® Digital Isolators Protect RS-232, RS-485, and CAN Buses

in Industrial, Instrumentation, and Computer Applications . . . . . . . . . . . . . . . . . . . . 5

Using Dual-Axis Accelerometers to Protect Hard Disk Drives . . . . . . . . . . . . . . . . . 9

A Reference Design for High-Performance, Low-Cost Weigh Scales . . . . . . . . . . . 13

Authors and Product Introductions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

www.analog.com/analogdialogue

Editors’ Notes

40 th ANNIverSArIeS

We’re winding down the celebration of the

40 th anniversary of Analog Devices, Inc.,

but inspection of your calendar will conirm

that—by the time you receive these printed

pages— Analog Dialogue will be launched well

into its 40 th year of existence (and eighth year

live online). As a inal reminder of ADI’s 40th,

if you’re interested in the high points of our

corporate history, the spread on pages 10-11

of the last issue,Volume 39, Number 3, depicts the contents page of

a timeline accessible on the Web (www.analog.com/timeline); in it

you can click on any year to access a brief audiovisual clip reviewing

some of that year’s major corporate events.

This publication has served its several generations of readers well—

offering in-depth articles on design, technology, and applications of

ADI’s many innovative products, brief introductions to hundreds of

signiicant new products, and a growing potpourri of links to useful

information of all kinds, from data-sheet revisions, book reviews,

application notes, and patent grants to articles in the trade press.

Interestingly, the arrival of the company’s 40 th anniversary showed

us an unheralded additional value of Analog Dialogue : its usefulness

to us as a cumulative history of our products and technologies—a

fount of information for nurturing the retrospective activities of the

celebration (for example, the above-mentioned timeline). Although

it also reminded us of the occasional lost opportunity and no-go

setback, it is a remarkable chronicle of progress. In the coming

year, part of our celebration will be to share the early parts of the

story with those of you who were too young to experience it and to

refresh the memory of those who enjoy nostalgia.

the QUeStION OF ANALOG

In this growingly “digital” age, the lay world has increasingly come

to believe that “analog is antique.” We sometimes are asked, “When

are you going to change your name to something more modern?”

In actuality, everything we make is an analog device ; analog is the

tangible world of time, space, matter, and energy—even a “digital”

processor has concerns about power supply, heat dissipation, speed,

and threshold levels. Digital signal processing is the processing of

analog signals by applying programs , logic , and symbols to analog

variables in a physical system (such as a radio, camera, or CAT

scanner) comprising such analog devices as ampliiers, converters,

and DSPs.

So, if the digital world grows, the future must also look bright for

analog. But don’t take our word for it. As a precursor to the Dialogue’s

fortieth year of celebration, we asked our sage and analog champion,

Dr. Barrie Gilbert, ADI Fellow, to look into his crystal ball and give

us a vision of the future. In the pages that follow, you will experience

a futuristic story he has woven, imagining the pervasiveness and

ever-bright possibilities of analog technologies and designs—and

their human designers.

IN thIS ISSUe

No signal chain would be complete without

sensors or actuators, and the generic signal

chains shown on our four 40 th anniversary

commemorative covers are no different.

The irst quarterly issue featured digital

signal processors; the second featured A/D

and D/A converters; and the third featured

ampliiers and linear circuits. This, the

fourth and inal issue, features the input-

and output circuitry.

Few circuits simply crunch numbers—instead they process

real-world signals, such as voice, music, or video; or data, such

as temperature, pressure, or acceleration. Furthermore, circuits

must also communicate with other circuits that may operate in

remote locations or hazardous environments. Thus, this issue

discusses system considerations encountered in industrial

measu rement a nd cont rol, laborator y i nst r u mentat ion,

communications, computers, and other signal-processing

applications.

The irst article in this issue shows how digital isolators can

enable safe transmission of serial data between systems that may

be separated by long distances. Galvanic isolation is often required

to break ground loops, protect the system from high-voltage

transients, reduce signal distortion, and enhance physical safety.

The digital isolation technology described here uses chip-scale

transformers made from CMOS metal layers, plus a gold layer that

is placed on top of the passivation. A high breakdown polyimide

layer underneath the gold layer insulates the top transformer coil

from the bottom. High-speed CMOS circuits connected to the top

and bottom coils provide the interface between each transformer

and its external signals.

The second article shows how a dual-axis accelerometer can sense

an impending crash and protect a personal media player from

being destroyed. It describes a novel technique that calculates

differential acceleration and—if excessive—parks the read/write

head of a hard disk drive, thus protecting the head and the

platter from damage. The acceleration is sensed by a polysilicon,

surface-micromachined structure built on top of a silicon wafer.

Polysilicon springs suspend the structure over the surface of the

wafer and provide resistance against acceleration forces. Delection

of the structure is measured using a differential capacitor formed

by ixed independent plates in relation to plates attached to the

moving mass. The complete single-chip accelerometer includes

the sensor and all of the signal-conditioning circuitry required

to measure acceleration.

The third article shows how implementing a precision weigh scale

may not be as easy as it irst seems. Typical weigh-scale resolution

is only 1:3,000 to 1:10,000, so a 12-bit to 14-bit ADC would

seemingly be adequate. A closer examination, however, shows

that a 20-bit ADC is really required. Designers must consider

weigh-scale system specifications including noise, dynamic

range, offset drift, gain drift, and iltering. The most common

weigh-scale implementation uses a bridge-type load-cell sensor,

with voltage output directly proportional to the weight placed on

it. A typical load-cell is a 4-resistor bridge circuit with at least

two variable arms, where the resistance changes with the weight

applied, creating a differential voltage at a common-mode level

of one-half the supply voltage.

Dan Sheingold [dan.sheingold@analog.com]

www.analog.com/analogdialogue dialogue.editor@analog.com

Analog Dialogue is the free technical magazine of Analog Devices, Inc., published

continuously for 39 years—starting in 1967. It discusses products, applications,

technology, and techniques for analog, digital, and mixed-signal processing. It is

currently published in two editions— online , monthly at the above URL, and quarterly

in print , as periodic retrospective collections of articles that have appeared online. In

addition to technical articles, the online edition has timely announcements, linking to

data sheets of newly released and pre-release products, and “Potpourri”—a universe

of links to important and rapidly proliferating sources of relevant information

and activity on the Analog Devices website and elsewhere. The Analog Dialogue

site is, in effect, a “high-pass-iltered” point of entry to the www.analog.com

site—the virtual world of Analog Devices . In addition to all its current information,

the Analog Dialogue site has archives with all recent editions, starting from Volume

29, Number 2 (1995), plus three special anniversary issues, containing useful articles

extracted from earlier editions, going all the way back to Volume 1, Number 1.

If you wish to subscribe to—or receive copies of—the print edition, please go to

www.analog.com/analogdialogue and click on <subscribe> . Your comments

are always welcome; please send messages to dialogue.editor@analog.com

or to these individuals: Dan Sheingold , Editor [dan.sheingold@analog.com]

or Scott Wayne , Managing Editor and Publisher [scott.wayne@analog.com].

COMING IN 2006

By the end of 2006 we plan to have every issue of Analog Dialogue

available in our online archives, including all of the rare and

long-out-of-print volumes from 1967 on. Also, be on the

lookout for a searchable index and perhaps some contests and

promotions for you, our loyal readers, to help us celebrate 40

years of Analog Dialogue . We welcome your suggestions.

Scott Wayne [scott.wayne@analog.com]

ISSN 0161-3626 ©Analog Devices, Inc. 2005 and 2006

the FOUr DeeS OF ANALOG, circa 2025

By Barrie Gilbert [ barrie.gilbert@analog.com ]

The young woman standing at the BlueBoard, wearing the

least-casual consumables she could ind on the cybermall for

this important occasion, is Niku Yeng. Aware that every aspect

of her interview is being net-vetted—via this fusion of the old

whiteboard, HDTV plasma display, two-way mirror, noncontact

stress monitor and data link—she is looking a little concerned;

but not because of this familiar tool. At the outset, she felt very

conident. Her curriculum at Nova Terra University had covered

all the major topics of the day. But after an hour of grilling,

she felt her hopes of employment at Analog Devices fading, as

question after probing question had been so unexpectedly related

to fundamental issues.

She’d been asked: “Why are resistors noisy?”; “State the noise-

spectral-density of a 50-V resistor.”; “Show me how the forward

voltage of a PN junction varies with temperature.”; “What are

the essential differences between bipolar transistors—such as

today’s 25 THz HBTs—and, say, a 10-nm MOS transistor?”;

“What causes shot noise?”; “Where do you think the designer of

this ampliier got the idea for its topology?” Rather than testing

her knowledge of advanced signal processing, modern tools for

the rapid realization of microsyms, or even the standard circuit

cells in the realm of continuous-value analog (CVA) techniques,

the questions seemed aimed at assessing her ability to invent

circuit solutions—on the spot!—from fundamental properties

of circuit elements.

This way of thinking about analog design differed greatly from what

she’d learned at Nova Terra. Advised of today’s extreme emphasis

on quick-turn delivery and totally reproducible performance,

Niku’s emphasis in her Ph.D. thesis was concerned with design

and routing tools for F-cell re-use, by developers of microsyms

called Fusers . (An F-cell is a simple functional unit of less than, say,

a thousand elements.) Nowadays, the word “transistor” is rarely

heard among the Fusers, since the mazes of devices in today’s

products are merged to the degree that individual elements are

indistinguishable. Few Fusers know—or need to know—anything

about the physical properties of an isolated element. But she was

aware that this cannot be said of the originators —those who design

basic cells from ground zero. It seemed that Analog Devices was

currently interested in hiring people of that sort.

Fortunately, her electives had covered the concepts of binary-

value analog (BVA—or “binanalog”) circuits quite well. She

was clear about how the broad class of sigmadel cells work, and

familiar with what used to be called “Class-D” ampliiers, using

fully switching output devices and duty-cycle modulation—the

basis of the sigmadel paradigm—to eficiently deliver generous

amounts of audio power. Nowadays, using highly elaborated

forms, almost indistinguishable from digital VLSI, binanalog

products are ubiquitous. Since about 2010, Class-D ampliiers

have been regarded, even by audiophiles, as the most accurate

load-driving elements. Sigmadel products using similar output

stages now power antennas well into the gigahertz range, thanks

to the extremely low inertia of modern transistors and the use of

low-loss resonators to convert all the power to the fundamental

carrier frequency.

The LEIF (Local E-net Interview Facilitator) turned to probing

her perception of how invention and innovation occur in modern

companies. Like her fellow students, Niku used the CyberLearn ™

system to access an abundance of information. But she’s learning

from Dr. Leif (coincidentally eponymous) that accumulating

information cannot be equated with acquiring knowledge, which

is far more profound. Knowledge is about knowing the value of

information, skimmed from numerous sources; then iltered,

distilled, adapted, extended, assimilated and inally sublimated

into a whole, with a set of contextual hooks, in each individual

mind, each holding a peculiar world view—a highly differentiated

and unique model of reality. “ Knowledge is information activated by

thought ,” he had told her.

Job interviews aren’t usually intended to be mentoring experiences;

but Niku was already seeing the prospect of a career in analog

design in a new and invigorating light. The old mantra “Analog

means antiquated” had emerged from subliminal messages in wave

after wave of ads, proclaiming the beneits of “going digital,” which

conveniently neglected to mention the continued importance of

challenges in the real world of analog processors.

“Analog design,” said Dr. Leif, “requires one to think in the many

dimensions that characterize physical elements and signals. It calls

for meticulous attention to minuscule details, always bound by

the Fundaments.” She’d learned that meant all of the physics of

materials and the ground rules governing what can and (probably)

cannot be realized in practice. “Analog design can be summed up

by the Four Dees, which I briely mentioned earlier.”

Although she’d forgotten what “Four Dees” referred to, the insights

gained during the past hour were now illuminating Niku’s vision

of an exciting future in analog, as an originator. As the interview

drew to a close, her eyes twinkling with genuine enthusiasm,

she said: “Dr. Leif, thank you for being so generous with your

time. I’ve learned a great deal, and it’s apparent you have a deep

knowledge of what you call ‘The Fundaments.’ I’m sure you also

have many recollections of the analog world at the beginning of

the century. If you’d allow me to buy you coffee at Galaxybux, I’d

love to continue this conversation just a little longer.”

Leif’s smile broadened. Her sincerity and evident zeal fueled his

impression that here was more than a talented young lady. She had

shown by her answers to many tough questions that she was one

of those rarities—a keen problem analyzer and an independent

thinker—who compels a manager to give forethought to which of

the incentives available to him in acquiring exceptionally talented

people he should pick.

“I’d be delighted! I promise not to order anything over $25 a cup!”

As they strolled across the campus, he chatted about the microsym

business. “That name was suggested by one of the old-timers at

ADI decades ago, to capture the notion that the ‘ICs’ of his time

were becoming little cities—microcosms of bustling electronic

activity,” Leif said. “Today’s microsyms have a lot in common with

the old silicon ICs, but they no longer depend on the exclusive use

of monolithic solutions—which we once thought to be the only way

to keep the cost down; that, and the belief that it was imperative

to use so-called ‘deep submicron CMOS,’ which used to mean

channel lengths of the order of 100 nm.

“Increasingly, these processes became so severely optimized for

binary applications that we needed to re-examine the wisdom of

relying on them for high-performance analog signal processing.

We realized the law in this popular dictate, and went a different

way. As you know, our products have for many years combined

the unique advantages of a variety of technologies spread out

over several smaller chips, each optimally suited to serve the local

processing objectives, pre-tested and assembled on tiny substrates

entirely automatically using microbots, as you saw earlier today,

endowed with a dexterity, speed, and autonomy unimaginable in

2000. Here we are,” he said, on reaching the coffee shop.

As they entered, the autowelcomer interrogated the 72-GHz

transponder in Leif’s pocket. After checking his biometrics, it

selected its U.S.-English female voice.

“Hello, Dr. Leif. I heard your team just released another microsym.

Congratulations! By the way, have you tried SplendoMix.......?”

Analog Dialogue volume 39 Number 4

Generated by fully analog neural networks, integrated with a local

database of customer proiles, these greetings varied intelligently;

but the adpops got tiresome after a while. Ordering a Galaxy

Express for Leif and a Hitchhiker for herself, charging them

via her own transponder, Niku said, “I’ve read that the price of

a coffee has increased 10% per year over the past 20 years, but

the price of microsyms continues to decline. Why is that?” Leif

smiled with secret pride.

“An astonishing aspect of our industry is that the products keep

getting cheaper, in relative terms; and that fact continues to be

the key driving force behind today’s endless electronic innovations

and all the high-tech products they enable. Some of the reasons for

this are to be found in the way today’s lat-world economy works.

But let’s just stick to technical matters for now.”

“Alright,” said Niku, “You briely mentioned the ‘Four Dees of

Analog.’ I’d like to hear more.”

Savoring his espresso, Leif explained, “The Dees are the

distinctive differences between the nature of analog and digital

design, the products, the components they utilize, and the

signals themselves.”

“The ‘Dees’ are the Four Differences, then?” she responded.

“Well, you could say that, but that’s not what I have in mind.”

He took a PDA from an inside pocket. “Let me show you a few

slides from a lecture I have on the NovaWeb ... here ... The Dees

are mnemonics for capturing these crucial distinctions in four

words starting with that letter. They’re equally important, but

let’s start with one that touches on that issue of economy.

“Analog microsyms are DURABLE. Very successful products can

have a lifetime of over thirty years.”

“I’m surprised!” she replied. “From what I learned at college, a

lifetime of a few years is generally the case for VLSI, before they’re

outdated by a new technology. Why doesn’t that impact analog

in the same way?”

“A good question, one not easy to answer. It’s partly because analog

functions still tend to be generic in spite of many examples of very

complex and specialized analog products designed for a speciic

service. But while the latter may include scores of ampliiers, there

are still uses for single units. A good all-rounder, well-designed

in an older technology, eventually becomes so cheap—a penny

or two—that it continues to ind applications in many places.

Unlike, say, a binary AND gate, which is far too primitive a

function to make as a standalone part, an ampliier actually

serves each application differently, and there’s no need for the

fastest technology in many of them. It’s also a relection of the

three other Dees.”

“This is fascinating! I’ve never thought about analog design at such

a basic level as you’ve presented. I thought it was all re-use, now,”

said Niku, with evident delight. “What’s the second Dee?”

Touching the screen of his PDA, Leif retrieved another slide.

Underneath a row of notes on a music staff, it said DIVERSE.

“What does that mean?”

It was Leif’s turn to show signs of delight. Speaking with boyish

glee, “Analog design is all about writing new tunes. You see this

row of just 16 notes, equal in duration? Suppose each has one

of 20 allowable pitches. How many tunes can you write?”

“Hundreds, I imagine!”

“How about 655 billion-billion? The notes are your components;

but today’s tunes—that is, the cell topologies—use many more

than 16 components, and each can have far more than 20

‘pitches’—the parametric variations. Not all these combinations

are useful, of course, just like the tunes; but in practice there are

still endless opportunities for invention.”

“That’s a wonderful metaphor, Dr. Leif.” Suddenly, the coffee-

shop lights lickered and died. “Well!” she laughed, “ that’s a

digital problem!”

“You’re probably right—I’ve heard that the control systems

for these latest white emitters have some software bugs. But

you can still see my PDA, right?” The screen showed the word

DIMENSIONAL, above which was a list of several signal

parameters—voltage, current, frequency and so on; and below,

component parameters, including resistance, capacitance, and

inductance. All were deined in terms of just four dimensions in

the modiied MKS system: mass, time, length, and charge. “Now,

this is the most profound difference.” Then teasingly, Leif said

“Tell me: What is the dimension of a logical variable?” Niku was

not to be caught off-guard.

“Well, it doesn’t have any dimension!”

“Right! So given that latitude, why are we still using very similar

elements for digital processors as we use for analog?”

“Well, they are very cheap, very tiny, and very fast.”

“Right again! But why haven’t all those other nanodevices stepped

in, to replace silicon?”

“Economics?”

“Exactly! Even though digital processors are only incidentally

electronic, silicon is a very serviceable medium. On the other hand,

analog circuits are fundamentally dimensional: their performance

depends on the quality, actual physical size, and absolute value of

the components, so the designer must be constantly aware of the

consequences of misjudging such factors.”

“Well, component matching is important too.”

“Yes, that’s true; but matching like against like is a dimensionless

proposition. You see, there are two kinds of sensitivity in an analog

circuit: aspects of performance that are dependent on absolute

parameter values, and those that are tolerant of variations in absolute

values.” At that moment, the lights came back on, and Niku asked

whether Dr. Leif would like another Galaxy Express.

“Thank you, but I have to get back to the lab.”

Disappointed, she said, “But aren’t you going to tell me what the

Fourth Dee is?”

“No, not right now. Let’s continue this conversation at our website:

<www.analog.com/library/analogdialogue/leif1.html>. You must

excuse me. By the way,” he said, as he started from the table, “I

would be honored if you should wish to join my team.”

And Niku felt sure the white emitters lickered .

Barrie Gilbert , the irst-appointed ADI

Fellow, has “spent a lifetime in pursuit

of analog excellence.” Barrie was born

in Bour nemout h, England, in 1937.

Before joining A DI, he worked with

irst-generation transistors at SRDE in

1954. At Mullard, Ltd. in the late ’50s,

he pioneered transistorized sampling

oscilloscopes, and in 1964 became a

leading ’scope designer at Tektronix.

He spent two years as a group leader at Plessey Research Labs

before joining Analog Devices in 1972, where he is now director

of the Northwest Labs in Beaverton, Oregon. Barrie is a Life

Fellow of the IEEE and has received numerous service awards.

He has about 70 issued patents, has authored some 50 papers,

is a reviewer for several professional journals, and a co-author

or co-editor of ive books. In 1997, he was awarded an honorary

doctorate of engineering from Oregon State University.

Analog Dialogue volume 39 Number 4

i Coupler ® Digital Isolators Protect

rS-232, rS-485, and CAN Buses

in Industrial, Instrumentation,

and Computer Applications

rapid changes in ground potential, often as large as hundreds, or

thousands, of volts. When this occurs, the logic-level switching

signal expected by the remote system would be superimposed on a

high voltage with respect to its local ground. Without isolation, this

voltage could corrupt the signal or damage the system. Referring

all devices connected to the bus to a single ground will protect the

system against this destructive energy, and isolating the devices

will prevent ground loops and electrical surges.

To completely isolate the system, all signal lines and power supplies

must be isolated. An isolated dc-to-dc converter can provide power

supply isolation, while the i Coupler digital isolator provides the

signal isolation.

By Scott Wayne [ scott.wayne@analog.com ]

INtrODUCtION

In applications such as industrial process control, power supply

regulation, and point-to-point communications between

computers, serial communication buses transmit data over

various types of physical networks, such as RS-232, RS-485, and

the Controller Area Network (CAN). Each of the interconnected

systems usually has its own power supply, and the systems are

often separated by long distances, so galvanic isolation is typically

required to break up ground loops, protect the system from

high-voltage transients, and reduce signal distortion, as well as

for physical safety.

i Coupler technology

i Coupler isolators are magnetic couplers based on chip-scale

transformers (Figure 2), as compared with the LEDs and

photodiodes used in optocouplers. The planar transformers use

CMOS metal layers, plus a gold layer that is placed on top of the

passivation. A high breakdown polyimide layer underneath the

gold layer insulates the top transformer coil from the bottom.

High-speed CMOS circuits connected to the top and bottom

coils provide the interface between each transformer and its

external signals. Wafer-scale processing provides a low-cost

method for integrating multiple isolation channels, as well as other

semiconductor functions, in a single package. i Coupler technology

eliminates the uncertain current transfer ratios, nonlinear transfer

functions, and drift (with time and temperature) associated with

optocouplers; reduces power consumption by as much as 90%; and

eliminates the need for external drivers or discrete devices.

Isolation

Transformers, coupling capacitors, optocouplers—and now,

i Couplers—are typical means of providing galvanic isolation,

which blocks current from lowing between two points, while

allowing data to pass unimpeded (Figure 1). Isolation is used

to protect against high voltages or currents caused by line

surges or ground loops, which can occur in any system that

has multiple ground paths. System grounds that are separated

by long cables will not be at the same potential, so ground

current will low between the two systems. Without isolation,

this current could introduce noise, degrade measurements, or

even destroy system components.

TOP COIL

BOTTOM COIL

POLYIMIDE LAYERS

POINT A

ISOLATOR

POINT B

INFORMATION

FLOW

NO CURRENT

FLOW

Figure 2. i Coupler cross section.

PROTECT HUMANS/EQUIPMENT

ELIMINATE GROUNDING PROBLEMS

IMPROVE SYSTEM PERFORMANCE

Circuitry on the primary side of the transformer encodes the input

logic transitions into 1-ns pulses, which are then coupled through

the transformer; circuitry on the secondary side detects them and

recreates the input signal, as shown in Figure 3. A refresh circuit

on the input side ensures that the output state matches the input

state even if no input transitions are present. This is important in

power-up situations and for input waveforms with low data rates

or constant dc inputs.

ISOLATION

BARRIER

Figure 1. Galvanic isolation allows information low

but prevents current low.

Currents that are inductively coupled into the long cables found

in industrial environments by motors switching on and off ,

electrostatic discharge (ESD), or nearby lightning strikes can cause

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Figure 3. The digital input is recreated at the output of the i Coupler.

Analog Dialogue volume 39 Number 4

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