volume30-number3(1).pdf

Editor’s Notes

spikes the equipment is responding to—or producing—are causing

problems an analog designer has to solve.

The big ray of hope here is digital signal processing . DSP to the

rescue of analog! Some very hairy analog problems are indeed

being solved with DSP. Your computer’s modem, cellular phone,

and ac motor controller are doing things via the analog variables

in ordinary phone lines, rf, and motors that many of us never

believed possible 30 years ago. It’s still analog, but it’s aided by

digital (programmers’) intelligence and the space-, cost-, and power

savings of the analog hardware that we call DSPs. A

Dan.Sheingold@analog.com

ANALOG DIALECTIC

A little more than 30 years ago,

the undersigned held a

responsible technical marketing

position with George A.

Philbrick Researches, Inc., and

was editor of a journal called

“The Lightning Empiricist.” For

the edification of a sizeable

portion of our readers who

weren’t around at the time,

GAP/R, a company with annual sales of $6M, virtually owned the

operational amplifier market—such as it was. The only significant

competitors were then a lot smaller—Tom Brown’s Burr-Brown

Research Corporation, and Al Pearlman and the late Roger Noble’s

Nexus Research Laboratory, Inc.

Aye, what’s in a name? Do you find it interesting that “Research”

was part of all three names? The founders of a new company, Ray

Stata and Matt Lorber, did. They believed that the existing

companies were getting prices* that would interest only researchers

(who had much fatter budgets in those days), and that a significant

industrial volume market for operational amplifier modules with

a wide variety of performance was waiting to be tapped (ICs with

decent performance were still pretty far over the horizon, so you

didn’t need venture capital to buy an expensive fab; you could

still build modules in your own garage, or in this case, a loft—or

was it a basement?—in Cambridge, MA).

So they gave their company the highly descriptive and serviceable

name, Analog Devices, Inc., and offered high-performance op amps

at reasonable prices, along with strong application support. They

were right, of course! The company’s sales took off almost

immediately. And ADI was aided by an incredible stroke of luck—

both Philbrick and Nexus were acquired by the giant Teledyne,

Inc., and merged. As often happens in such circumstances, the

effect was like to putting two resistors in parallel; their joint

effectiveness actually decreased. Not only that, but ADI was able

to acquire the services of a number of talented, and perhaps

disgruntled, former employees of Teledyne Philbrick Nexus, a move

that proved synergistic. The rest is history! Should the next sentence

read: “And with its primacy in analog devices, ADI lived happily

ever after?”

Ah, but what’s in a name? Don’t you find it interesting that one of

the fastest-growing product lines at Analog Devices is the SHARC

general-purpose digital signal processor—a DIGITAL

COMPUTER, for gosh sakes!? Yes, the analog stuff is doing very

well indeed, too, but isn’t that a bit of a stretch? If we some day

end up getting more revenue from “digital” products than “analog”

products, (in no way a sure thing, say our “analog” guys!), will

“Analog” in our name be a misnomer?

No! As we’re endlessly fond of saying, hardware design problems

are analog problems. If the name of the game is signal processing ,

any signal that’s a voltage or a current—or even a time interval—

has resolution limitations that are analog in nature (noise, jitter,

bandwidth, etc.), even if the voltage or current level you’re trying

to recognize is representing a “1” or a “0”, and the time is a

sampling interval. And don’t forget interference: those parasitic

*For example, the P2 solid-state parametric electrometer was priced at $227

(in mid-1960s US dollars!)

THE AUTHORS

Dave Robertson (page 3) is a

design engineer in the Analog

Devices High-Speed Converter

group in Wilmington, MA. He

joined Analog on graduating from

Dartmouth College, with BA and

BE degrees. He has worked on

high-speed D/A and A/D con-

verters, including the AD568,

AD668, AD773, AD872, and

AD871, and is now developing and adapting high-speed converters

for communications systems. In his free time Dave plays rugby

and entertains his two children.

Doug Mercer (page 7) is an

Analog Devices Fellow doing

circuit design on CMOS DAC

architectures in the High Speed

Converter Group in Wilmington,

MA. He joined ADI in 1977 after

graduating from Rensselaer

Polytechnic Institute with a BSEE.

In his free time, he tries to improve

his golf game.

Joe DiPilato (page 7) is a Product

Marketing Manager in the High

Speed Converter group at Analog

Devices in Wilmington, MA. He

has a BSEE from Worcester

Polytechnic Institute and an MBA

from Anna Maria College. He is

focused on defining and marketing

high-speed A/D- and D/A-con-

verter-based products for use in

the communications marketplace. When he’s not working, Joe

enjoys water sports, tennis, golf, the beach, and spending quality

time with his wife, Lisa.

[ more authors on page 22 ]

Cover: The cover illustration was designed and executed by

Shelley Miles , of Design Encounters , Hingham MA.

One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106

Published by Analog Devices, Inc. and available at no charge to engineers and

scientists who use or think about I.C. or discrete analog, conversion, data handling

and DSP circuits and systems. Correspondence is welcome and should be addressed

to Editor, Analog Dialogue , at the above address. Analog Devices, Inc., has

representatives, sales offices, and distributors throughout the world. Our web site is

http://www.analog.com/ . For information regarding our products and their

applications, you are invited to use the enclosed reply card, write to the above address,

or phone 617-937-1428, 1-800-262-5643 (U.S.A. only) or fax 617-821-4273.

ISSN 0161–3626

©Analog Devices, Inc. 1996

Analog Dialogue 30-3 (1996)

Selecting Mixed-Signal

Components for

Digital Communication

Systems—An

Introduction

by Dave Robertson

Communications is about moving information from point A to

point B, but the computer revolution is fundamentally changing

the nature of communication. Information is increasingly created,

manipulated, stored, and transmitted in digital form—even signals

that are fundamentally analog. Audio recording/playback, wired

telephony, wireless telephony, audio and video broadcast—all of

these nominally analog communications media have adopted, or

are adopting, digital standards. Entities responsible for providing

communications networks, both wired and wireless, are faced with

the staggering challenge of keeping up with the exponentially growing

demand for digital communications traffic. More and more,

communications is about moving bits from point A to point B.

Digital communications embraces an enormous variety of

applications, with radically different constraints. The transmission

medium can be a twisted pair of copper wire, coaxial cable, fiber-

optic cable, or wireless—via any number of different frequency

bands. The transmission rate can range from a few bits per second

for an industrial control signal communicating across a factory

floor to 32 kbits/second for compressed voice, 2 Mb/s for MPEG

compressed video, 155 Mbps for a SONET data trunk, and

beyond. Some transmission schemes are constrained by formal

standards, others are free-lance or developmental. The richness of

design and architectural alternatives produced by such variety

boggles the mind. The digital communications topic is so vast as to

defy a comprehensive treatment in anything less than a shelf of books.

A communications jargon and a bewildering array of acronyms

have developed, making it sometimes difficult for the

communications system engineer and the circuit hardware designer

to communicate with one another. Components have often been

selected based on voltage-oriented specifications in the time

domain for systems whose specifications are expressed in frequency

and power. Our purpose here, and in future articles, will be to

take a fairly informal overview of some of the fundamentals, with

an emphasis on tracing the sometimes complex relationship

between component performance and system performance.

The “communications perspective” and analytic tool set have also

contributed substantially in solving problems not commonly

thought of as “communications” problems. For example, the

approach has provided great insight into some of the speed/

bandwidth limits inherent in disk-drive data-recovery problems,

where the channel from A to B includes the writing and reading of

data in a magnetic medium—and in moving data across a high

speed bus on a processing board.

Shannon’s law—the fundamental constraint : In general, the

objective of a digital communications system is:

• to move as much data as possible per second

• across the designated channel

• with as narrow a bandwidth as possible

• using the cheapest, lowest-power, smallest-space (etc.)

equipment available.

System designers are concerned with each of these dimensions to

different degrees. Claude Shannon, in 1948, established the

theoretical limit on how rapidly data can be communicated:

This means that the maximum information that can be transmitted

through a given channel in a given time increases linearly with the

channel’s bandwidth, and noise reduces the amount of information

that can be effectively transmitted in a given bandwidth, but with

a logarithmic sensitivity (a thousandfold increase in noise may

result in a tenfold reduction in maximum channel capacity).

Essentially, the “bucket” of information has two dimensions:

bandwidth and signal-to-noise ratio (SNR). For a given capacity

requirement, one could use a wide-bandwidth channel with

relatively poor SNR, or a narrowband channel with relatively good

SNR (Figure 1). In situations where bandwidth is plentiful, it is

common to use cheap, bandwidth-hungry communications

schemes because they tend to be insensitive to noise and

implementation imperfections. However, as demand for data

communication capacity increases (e.g., more cellular phones)

bandwidth is becoming increasingly scarce. The trend in most

systems is towards greater spectral efficiency, or bits capacity per

unit of bandwidth used. By Shannon’s law, this suggests moving

to systems with better SNR and greater demands on the transmit

and receive hardware and software.

Let’s examine the dimensions of bandwidth (time/frequency

domain) and SNR (voltage/power domain) a little more closely by

considering some examples.

BEST SPECTRAL EFFICIENCY

LEAST SNR REQUIREMENTS

(CHEAPEST?)

BANDWIDTH – Hz

Figure 1. Shannon’s capacity limit: equal theoretical capacity.

IN THIS ISSUE

Volume 30, Number 3, 1996, 24 Pages

Editor’s Notes, Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Selecting mixed-signal components for digital communication systems—Introduction . 3

TxDAC™ CMOS D/A converters are optimized for the communication

Transmit path . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Buffered multiplexers for video applications . . . . . . . . . . . . . . . . . . . . . . . . 10

Single-chip direct digital synthesis (DDS) vs. the analog phase-locked loop . . . 12

For efficient signal processing in embedded systems, take a DSP, not a RISC . . 14

New-Product Briefs:

Operational Amplifiers and variable-compression Audio Preamplifier . . 16

DSPs and Digital and Analog Audio chips . . . . . . . . . . . . . . . . . . . . . 17

ADC, 2-channel Data-Acquisition system, and Direct Digital

Synthesis chip . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Transceivers and switched-cap Regulated Inverter . . . . . . . . . . . . . . . . 19

Ask The Applications Engineer—22: Current-feedback amplifiers—I . . . . . 20

Worth Reading, More authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

Potpourri . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Analog Dialogue 30-3 (1996)

PCM: A simple (but common) case: Consider the simple case

of transmitting the bit stream illustrated in Figure 2a, from a

transmitter at location A to a receiver at location B (one may

assume, that the transmission is via a pair of wires, though it could

be any medium.) We will also assume that the transmitter and

receiver have agreed upon both the voltage levels to be transmitted

and the timing of the transmitted signals. The transmitter sends

“high” and “low” voltages at the agreed-upon times, corresponding

to 1s and 0s in its bit stream. The receiver applies a decision element

(comparator) at the agreed-upon time to discriminate between a

transmitted “high” and “low”, thereby recovering the transmitted

bit stream. This scheme is called pulse code modulation (or PCM).

Application of the decision element is often referred to as “slicing”

the input signal stream, since a determination of what bit is being

sent is based on the value of the received signal at one instant in

(slice of) time. To transmit more information down this wire, the

transmitter increases the rate at which it updates its output signal,

with the receiver increasing its “slicing” rate correspondingly.

threshold could then be set at the “average” value of the incoming

bit stream, which should be some value between the transmitted

“1” and transmitted “0” (half-way between, if the density of ones

and zeros are equal.) For timing, a phase-locked loop could be

used—with a center frequency somewhere near the agreed-upon

transmit frequency; it would “lock on” to the transmitted signal,

thereby giving us an exact frequency to slice at. This process is

usually called clock recovery ; the format requirements on the

transmit signal are related to the performance characteristics of

the phase-locked-loop. Figure 3 illustrates the elements of this

simplified pulse receiver.

Bandwidth Limitations: The real world is not quite so simple.

One of the first important physical limitations to consider is that

the transmission channel has finite bandwidth. Sharp-edged square

wave pulses sent from the transmitter will be “rounded off” by a

low bandwidth channel. The severity of this effect is a function of

0100111 00

OPTIMUM

THRESHOLD

OPTIMUM SLICING TIME

MISPLACED

THRESHOLD

0000000 00

BAD SLICING

FREQUENCY

Figure 2. Simplified bit voltage transmission (PCM).

This simple case, familiar to anyone who has had an introductory

course in digital circuit design, reveals several of the important

elements in establishing a digital communications system. First,

the transmitter and receiver must agree upon the “levels” that are

to be transmitted: in this case, what voltage constitutes a

transmitted “1”, and what voltage level constitutes a transmitted

“0”. This allows the receiver to select the right threshold for its

decision element; incorrect setting of this threshold means that

the transmitted data will not be recovered (Figure 2b). Second,

the transmitter and receiver must agree on the transmission

frequency; if the receiver “slices” at a different rate than the bits

are being transmitted, the correct bit sequence will not be recovered

(2c). In fact, as we’ll see in a moment, there must be agreement

on both frequency and phase of the transmitted signal.

How difficult are these needs to implement? In a simplified world,

one could assume that the transmitted signal is fairly “busy”,

without long strings of consecutive ones or zeros. The decision

PHASE

LOCKED

LOOP

RECOVERED CLOCK

RECEIVED SIGNAL

CLK

RECOVERED

DATA

COMPARATOR

DATA THRESHOLD

AVERAGER

KEY ELEMENTS – DECISION ELEMENT, REFERENCE RECOVERY,

TIMING RECOVERY

Figure 3. Idealized PCM.

Figure 4. Scope waveforms vs. time (L) and eye diagrams (R).

Analog Dialogue 30-3 (1996)

the channel bandwidth. (Figure 4). In the extreme case, the

transmitted signal never gets to a logical “1” or “0”, and the

transmitted information is essentially lost. Another way of viewing

this problem is to consider the impulse response of the channel.

An infinite bandwidth channel passes an impulse undistorted

(perhaps with just a pure time delay). As the bandwidth starts to

decrease, the impulse response “spreads out”. If we consider the

bit signal to be a stream of impulses, inter-symbol interference

(ISI) starts to appear; the impulses start to interfere with one-

another as the response from one pulse extends into the next pulse.

The voltage seen at the Receive end of the wire is no longer a

simple function of the bit sent by the transmitter at time t 1 , but is

also dependent on the previous bit (sent at time t 0 ), and the

following bit (sent at time t 2 ).

Figure 4 illustrates what might be seen with an oscilloscope

connected to the Receive end of the line in the simple noisy

communications system described above for the case where the

bandwidth restriction is a first-order lag (single R-C). Two kinds

of response are shown, a portion of the actual received pulse train

and a plot triggered on each cycle so that the responses are all

overlaid. This latter, known as an “eye” diagram, combines

information about both bandwidth and noise; if the “eye” is open

sufficiently for all traces, 1s can be easily distinguished from 0s. In

the adequate bandwidth case of Figure 4a, one can see

unambiguous 1s, 0s, and sharp transitions from 1 to 0. As the

bandwidth is progressively reduced, (4b, 4c, 4d, 4e), the 1s and 0s

start to collapse towards one another, increasing both timing- and

voltage uncertainty. In reduced-bandwidth and/or excessive-noise

cases, the bits bleed into one another, making it difficult to

distinguish 1s from 0s; the “eye” is said to be closed (4e).

As one would expect, it is much easier to design a circuit to recover

the bits from a signal like 4a than from 4d or 4e. Any misplacement

of the decision element, either in threshold level or timing, will be

disastrous in the bandlimited cases (d, e), while the wideband case

would be fairly tolerant of such errors. As a rule of thumb, to send

a pulse stream at rate F S , a bandwidth of at least F S /2 will be needed

to maintain an open eye, and typically wider bandwidths will be

used. This excess bandwidth is defined by the ratio of actual

bandwidth to F S /2. The bandwidth available is typically limited by

the communication medium being used (whether 2000 ft. of

twisted-pair wire, 10 mi of coaxial cable etc.), but it is also necessary

to ensure that the signal processing circuitry in the transmitter

and receiver do not limit the bandwidth.

Signal processing circuitry can often be used to help mitigate the

effects of the intersymbol interference introduced by the

bandlimited channel. Figure 5 shows a simplified block diagram

of a bandlimited channel followed by an equalizer, followed by

the bit “slicer”. The goal of the equalizer is to implement a transfer

function that is effectively the inverse of the transmission channel

over a portion of the band to extend the bandwidth. For example,

if the transmission channel is acting as a low pass filter, the equalizer

might implement a high-pass characteristic, such that a signal

passing through the two elements will come out of the equalizer

undistorted over a wider bandwidth.

Though straightforward in principle, this can be very difficult to

implement in practice. To begin with, the transfer function of the

transmission channel is not generally known with any great

precision, nor is it constant from one situation to the next. (You

1 The field of disk-drive read -channel design is a hotbed of equalizer

development in the ongoing struggle to improve access specs.

and your neighbor down the street have different length phone

wires running back to the phone company central office, and will

therefore have slightly different bandwidths.) This means that these

equalizers usually must be tunable or adaptive in some way.

Furthermore, considering Figure 5 further, we see that a passive

equalizer may flatten out the frequency response, but will also

attenuate the signal. The signal can be re-amplified, but with a

probable deterioration in signal-to-noise ratio. The ramifications

of that approach will be considered in the next section. While they

are not an easy cure-all, equalizers are an important part of many

communications systems, particularly those seeking the maximum

possible bit rate over a bandwidth-constrained channel. There are

extremely sophisticated equalization schemes in use today,

including decision feedback equalizers which, as their name

suggests, use feedback from the output of the decision element to

the equalization block in an attempt to eliminate trailing-edge

intersymbol interference. 1

RECEIVER

BANDLIMITED

CHANNEL

DECISION

ELEMENT

TRANSMITTER

EQUALIZER

CHANNEL

RESPONSE

EQUALIZER

RESPONSE

COMPOSITE RESPONSE

Figure 5. Channel equalization.

Multi-level symbols—sending more than one bit at a time:

Since the bandwidth limit sets an upper bound on the number of

pulses per second that can be effectively transmitted down the

line, one could decide to get more data down the channel by

transmitting two bits at a time. Instead of transmitting a “0” or

“1” in a binary system, one might transmit and receive 4 distinct

states, corresponding to a “0” (00), “1” (01), “2” (10), or “3”

(11). The transmitter could be a simple 2-bit DAC, and the receiver

could be a 2-bit ADC. (Figure 6). In this kind of modulation,

called pulse-amplitude modulation (PAM), additional information

has been encoded in the amplitude of the bit stream.

Communication is no longer one bit at a time; multiple-bit words,

or symbols , are being sent with each transmission event. It is then

necessary to distinguish between the system’s bit rate, or number

of bits transmitted per second, and its symbol rate, or baud rate,

which is the number of symbols transmitted per second. These two

rates are simply related:

bit rate = symbol rate (baud) × bits / symbol

The bandwidth limitations and intersymbol interference discussed

in the last section put a limit on the realizable symbol rate, since

they limit how closely spaced the “transmission events” can be in

time. However, by sending multiple bits per symbol, one can

increase the effective bit rate, employing a higher-order modulation

scheme. The transmitter and receiver become significantly more

complicated. The simple switch at the transmitter has now been

replaced with a DAC, and the single comparator in the receiver is

VARIABLE

GAIN

AMPLIFIER

n-BIT

SYMBOL

n-BIT

DAC

n-BIT

ADC

CHANNEL

ATTENUATION

ADDITIVE NOISE

GAIN

CONTROL

CLOCK

Figure 6. Simplified PAM transmitter/receiver.

Analog Dialogue 30-3 (1996)

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