volume32-number1(1).pdf

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

INTEGRATED ANALOG FRONT-ENDS PROCESS SIGNALS FROM CCD CHIPS (page 5)

X-FET™ Voltage References: Low Noise, Low Power, Better than Bandgap (page 3)

Oversampling A/D Converters Provide 16-Bit Resolution at 1 MHz (page 13)

Complete contents on page 3

Volume 32, Number 1, 1998

Editor’s Notes

NEW FELLOWS

We are pleased to note the intro-

duction of 3 new Fellows at our

1998 General Technical Conference:

Roy Gosser, Bill Hunt, and Chris

Mangelsdorf. Fellow , at Analog

Devices, represents the highest level

of achievement that a technical

contributor can achieve, on a par

with Vice President. The criteria

for promotion to Fellow are very

demanding. Fellows will have earned universal respect and

recognition from the technical community for unusual talent and

identifiable innovation at the state of the art; their creative technical

contributions in product or process technology will have led to

commercial success with a major impact on the company’s

net revenues.

Attributes include roles as mentor, consultant, entrepreneur,

organizational bridge, teacher, and ambassador. Fellows must also

be effective leaders and members of teams and in perceiving

customer needs. This trio’s technical abilities, accomplishments,

and personal qualities well-qualify them to join Derek Bowers

(1991), Paul Brokaw (1980), Lew Counts (1984), Barrie Gilbert

(1980) Jody Lapham (1988), Fred Mapplebeck (1989), Jack

Memishian (1980), Doug Mercer (1995), Mohammad Nasser

(1993), Wyn Palmer (1991), Carl Roberts (1992), Paul Ruggerio

(1994), Brad Scharf (1993), Mike Timko (1982), Mike Tuthill

(1988), Jim Wilson (1993), and Scott Wurcer (1996) as Fellows.

BILL HUNT

Since 1983 Bill has been Design

Engineering Manager at ADI’s site

in Limerick, Ireland. During this

time Bill has continually con-

tributed designs and leadership to

many core developments, in D/A

and A/D converters of all types,

including sigma-delta. He has been

chief proponent and architect of

devices in the servo section of hard-

disk drives (HDD). He led the design of baseband audio converters

for digital wireless telephony and products for basestations and wired

telephony. He also developed a line of DDS products.

He has shown a great ability to understand customer system

problems and to develop solutions in terms of new directions for

semiconductor technology. He has been active in developing

computer-aided design techniques, providing inputs to process-

technology developments and measurement techniques.

Bill graduated with a BSEE in 1967 and worked his way through

the development engineering ranks of Telectron Ltd, a

telecommunications equipment manufacturing company, before

joining Analog Devices in 1979 as a Design Engineer. During this

period, he gained insight into the emerging infrastructure of the

telecom industry and their inherent dependence on early adoption

of semiconductor technology as a competitive advantage.

CHRIS MANGELSDORF

Dr. Christopher W. Mangelsdorf

designed the AD770 8-bit, 200-

MSPS A/D converter, and went on

to lead a team that designed the

industry’s first CMOS 10-bit, 15-

MSPS ADC and the first high-

resolution integrated CCD signal

processing chip for digital cameras.

These have served as core designs

for many other CMOS high-

performance products. For the last 2 years, Chris has managed

the product design center that serves our customers in Japan.

Chris represents ADI on the Bipolar Circuits and ISSCC

conference committees; he has chaired panels and presented papers

at these and other conferences. He has published >10 technical

papers and has 13 patents (5 shared). He serves as a link to college

campuses and as a mentor to young team members.

He received a BS in Physics from Davidson College (NC) in 1977,

and went on to earn a Master’s degree, then a Ph.D., in Electrical

Engineering at MIT, where he held the Analog Devices Fellowship.

He has been associated with Analog Devices since summer

employment in 1980. He enjoys board sports, i.e., windsurfing,

surfing, and snowboarding.

ROY GOSSER

Royal A. Gosser is an innovator

whose design track record is

highlighted by products labeled

“First”, “Fastest”, and “Greatest

SFDR”. A perennial contributor of

new ideas, Roy has designed

products that include various A/D

converters, op amps, and track and

holds. Recent well-known products

include the AD9042A/D converter

(co-designed with Frank Murden), the AD8011 op amp, and the

AD8320 cable line driver. He holds 4 patents.

Equally as important as creative circuit design, a key to IC device

performance is the manufacturing process. Roy has supplied ideas

and other inputs to assist our process engineers in designing the

Analog Devices XFCB (eXtra-Fast Complementary Bipolar)

process, one that makes possible the manufacture of some of the

world’s highest-performance analog ICs in silicon.

Roy joined Analog Devices Computer Labs Division, in

Greensboro, NC, in 1982 as an IC design engineer, meeting the

challenge to build better interstage amplifiers for A/D converter

cards. After a 4-year interlude as Manager of Product Test

Engineering, he returned to the design of integrated circuits—

and hasn’t looked back! Before joining ADI, he had worked in

R/D at Litronix (now Siemens), as a design engineer at Hewlett

Packard (Palo Alto), and then at Harris Semiconductor. His

training included 4 years with Naval Air as an electronic technician,

followed by a BSEE from San Jose (CA) University and an MSEE

from National Technological University (NTU).

[ More Editor’s Notes on page 21 ]

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 781-937-1428, 1-800-262-5643 (U.S.A. only) or fax 781-821-4273.

ISSN 0161–3626

©Analog Devices, Inc. 1998

Analog Dialogue 32-1 (1998)

XFET™ References

Low noise, lower voltage than Zeners,

Micropower, better than bandgaps

by Roya Nasraty

In order for an analog signal to represent (or be represented by) a

digital number, a reference, usually voltage, is necessary to translate

the scale. Thus, an A/D converter produces a digital number

proportional to the ratio of an analog signal to a reference voltage;

and a D/A converter produces an output that is a fraction of the

full-scale voltage or current, established by a reference. If the

reference signal develops an error of +1%, it will cause a

proportional system error: the analog output of a DAC will increase

by 1%, and the digital output of an ADC will decrease by 1%.

In systems where absolute measurements are required, system

accuracy is highly dependent on the accuracy of the reference. In

high-resolution data-acquisition systems, especially those that must

operate over a wide temperature range, high-stability references

are a must. The accuracy of any converter is limited by the

temperature sensitivity and long term drift of its voltage reference.

If the voltage reference is allowed to contribute an error equivalent

to only 1/2 of a least-significant bit (1˚ LSB˚ =˚ 2 - n of full scale), it

may be surprising to see just how good the reference must be,

even for small temperature excursions. And when temperature

changes are large, the reference design is a major problem.

For instance, an autocalibrated true 16-bit A/D converter has an

LSB of 15.2 parts per million (ppm) of full scale. For the ADC to

have an absolute accuracy of 16 bits, the voltage-reference error

over the entire operating temperature range must be less than or

equal to 1/2˚ LSB, or 7.6˚ ppm. If the reference drift is 1˚ ppm/°C,

then (neglecting all other error sources) the total temperature swing

must not exceed 7.6

long term instability, the buried breakdown diode is less noisy and

more stable than surface Zeners. However, it requires a power

supply of at least 6˚ V and must draw several hundred microamperes

to keep the noise to a practical level.

Bandgaps: Another popular design technique for voltage

references uses the bandgap principle: the V be of any silicon

transistor has a negative tempco of about 2˚ mV/

C, which can be

extrapolated to approximately 1.2˚ V at absolute zero (the bandgap

voltage of silicon). The difference in base-emitter voltage between

matched transistors operating at differing current densities will be

proportional to absolute temperature (PTAT). This voltage, added

to a V be with its negative temperature coefficient, will achieve the

constant bandgap voltage. This temperature-invariant voltage can

be used as a “low-voltage Zener diode” in a shunt connection

(AD1580). More often, it is amplified and buffered to produce a

standard voltage value, such as 2.5 or 5˚ V. The bandgap voltage

reference has attained a high degree of refinement since its

introduction and is widely used; yet it lacks the precision demanded

by many of today’s electronic systems. Practical bandgap references

are not noted for good noise performance, exhibit considerable

temperature hysteresis, and have long-term stability dependent

on the absolute value of at least one on-chip resistor.

A new principle—the XFET™: With the proliferation of

systems using 5-V supplies and the growing need for operation at

and below 3 volts, designers of ICs and systems need high-

performance voltage references that can operate from supply rails

well below the >6˚ V needed for buried-Zener diodes. Such a device

must combine low-power operation with low noise and low drift.

Also desirable are linear temperature coefficient, good long-term

stability and low thermal hysteresis. To meet these needs, a new

reference architecture has been created to provide this much-

desired voltage reference. The technique, dubbed XFET™ (eXtra

implanted FET), yields a low-noise reference that requires low

supply current and provides improved temperature coefficient

linearity with low thermal hysteresis.

The core of the XFET reference consists of two junction field-

effect transistors, one of which has an extra channel implant to

raise its pinch-off voltage. With both JFETs running at the same

drain current, the difference in pinch-off voltage is amplified and

used to form a highly stable voltage reference. The intrinsic

reference voltage is about 500˚ mV, with a negative temperature

coefficient of about 120˚ ppm/K. This slope is essentially locked in

C to maintain true 16-bit accuracy. Anther

sources of error, often overlooked, is reference noise ; keeping it

low (typically less than 1/4˚ LSB) is critical for high accuracy.

Nonlinearity of the reference’s temperature coefficient and large

thermal hysteresis are other sources of error that can significantly

affect overall system accuracy.

TYPES OF REFERENCES

Zener* diodes: Widely used for many years is the temperature-

compensated Zener diode, produced by the reverse breakdown of

the base-emitter junction at the surface of the device. Zeners have

constant voltage drop, especially when used in a circuit that can

provide a constant current derived from a higher supply voltage.

Zeners are available in a wide range of voltage options: from about

6˚ V to 200˚ V, tolerances of 1.0% to 20%, and power dissipation

from a fraction of a watt to 40 or 50˚ W. However they have many

shortcomings. They often require additional circuitry to obtain

low output impedance, the voltage tolerance of low-cost devices is

generally poor; they are noisy and very sensitive to changes in current

and temperature, and they are susceptible to change with time.

The buried , or subsurface Zener is the preferred reference source

for accurate IC devices. In a subsurface Zener reference, the reverse

breakdown area is covered by a protective diffusion to keep it well

below the impurities, mechanical stresses and crystal imperfections

found at the surface. Since these effects contribute to noise and

IN THIS ISSUE

Volume 32, Number 1, 1998, 24 Pages

Editor’s Notes (New Fellows: Roy Gosser, Bill Hunt, Chris Mangelsdorf) . . 2

XFET™ References: Low noise, lower voltages than Zeners, better than

bandgaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1.5-W loudspeaker amplifier delivers sound performance . . . . . . . . . . . . 5

Integrated solutions for CCD signal processing . . . . . . . . . . . . . . . . . . . 6

DSP101, Part 4: Programming considerations for real-time I/O . . . . . . . 9

Oversampling ADCs for 16-bit resolution and inputs >1-MHz . . . . . . . 13

New-Product Briefs:

Analog-to-Digital Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Digital-to-Analog Converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Amplifiers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Power-Management ICs & RS-232/RS-485 Transceivers . . . . . . . 18

Switches, Video Encoders, Fast 8/10-Bit ADCs . . . . . . . . . . . . . . . 19

Magnetic Sensor, Digital Isolators, Log Amp, etc. . . . . . . . . . . . . 20

Worth Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

More Editor’s Notes: Frank Goodenough— In Memoriam,

New on the Net, Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22

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

*Note: Reference diodes can use two types of breakdown phenomena, Zener

and avalanche. Most reference diodes employ the higher-voltage avalanche

mode, but all have come to be called “Zener” diodes.

Analog Dialogue 32-1 (1998)

to the dielectric constant of silicon and is closely compensated for

by adding a correction term generated in the same manner as the

proportional-to-absolute temperature (PTAT) term used to

compensate bandgap references. However, the intrinsic

temperature coefficient of the XFET is some thirty times lower

than that of a bandgap. As a result, much less correction is needed.

This tends to result in much less noise, since most of the noise of

a bandgap reference comes from the temperature-compensation

circuitry. The temperature correction term is provided by a current,

I PTAT , which is positive and proportional to absolute temperature

(Figure 1).

not consistent from part to part, so a simple ROM/software look-

up table cannot be used for temperature coefficient correction.

Temperature coefficient linearity is a very important specification

for DVM applications. Another major advantage of the XFET is

its excellent long term stability. Its drift is less than one-fifth that

of a bandgap reference and comparable to that of Zener references

(see Table).

Despite the low quiescent current, the ADR29x family are capable

of delivering 5˚ mA to the load from a low-dropout PNP output

stage; and there is no requirement for an output decoupling

capacitor. Thermal hysteresis with the XFET design is much better

than with bandgaps. Production devices exhibit approximately

200˚ mV of recoverable and non-cumulative shift when subjected

to a 100-kelvin thermal shock vs. a 500 to 1000-mV shift in

comparable bandgaps. The overall performance advantage offered

by ADI’s proprietary XFET architecture in portable systems

requiring precision, stability, and low power is unmatched by

existing bandgap or Zener references.

Application—current source : The ADR29x Series are useful

for many low-power, low-voltage precision reference applications,

including negative references and “beefed-up” precision regulators

using external low-quiescent rail-to-rail amplifiers with Kelvin

feedback connections. The low and insensitive quiescent current

(about 12˚ ± ˚ 2˚ mA over temperature) permits the ADR29x family

members to serve as precision current sources, operating from

low supply voltage.

Figure 2 shows a basic connection for a floating current source

with a grounded load. The precision regulated output voltage

causes a current of ( V OUT / R SET ), to flow through R SET , which is the

sum of a fixed and an adjustable external resistance. This current,

V IN

I 1

V OUT

V P

I PTAT

GND

* EXTRA CHANNEL IMPLANT

R 1

R 2

R 3

V OUT 5

3 D

V P 1

I PTAT 3

Figure 1. Simplified schematic diagram of ADR29x reference.

The ADR29x series* are the first of a growing family of references

based on the XFET architecture. They operate from supply rails

from 2.7 to 15˚ V and draw just 12˚

A. Output voltage options

include 2.048˚ V (ADR290), 2.5˚ V (ADR291), 4.096˚ V (ADR292),

and 5 V (ADR293).

Fruits of the new technology: The XFET circuit topology has

significant advantages over most bandgap and Zener references.

When operating at the same current, peak-to-peak noise voltage

from a XFET reference at frequencies between 0.1 and 10˚ Hz is

typically 3 times less than that for a bandgap (see comparison

between the REF192 and ADR291). Alternatively, a bandgap

reference needs to run at typically 20 times the supply current of

an XFET reference in order to provide equivalent peak-to-peak

noise performance (ADR291 vs. AD680). The XFET reference

has a very flat or linear temperature coefficient over the extended

industrial operating temperature range. The best bandgap and

Zener voltage references typically have non-linear temperature

coefficients at the temperature extremes. These nonlinearities are

5˚ mA, adds to the quiescent current to form the load current

through R L . Thus, predictable currents from 12˚

A to 5˚ mA can

be programmed to flow through the load.

V IN

ADR29x

V OUT

R 1

GND

R SET

I SY

ADJUST

I Q

P 1

I OUT

V OUT

R SET

I OUT = I Q +

R L

*For data, consult our Web site, www.analog.com (Product Center), AnalogFax

line 800-446-6212 (with Faxcode 2110 ), or use the reply card. Circle 1

Figure 2. Precision current source.

Table 1. Comparison of Zener, Bandgap, and XFET References

Parameter

ADR291

AD586

AD680

REF192

Reference Topology

XFET

Buried Zener

Bandgap

Supply Voltage (V)

+3.0

+15.0

+5.0

+3.3

Voltage Output (V)

2.5

Initial Accuracy (mV)* max

± 2

± 5

± 2

Temperature Coefficient (ppm/°C)* max

8 (–25 to +85)

2 (0 to +70)

20 (–40 to +85)

5 (–40 to +85)

Noise Voltage 0.1–10 Hz (mV p-p)

Quiescent Current (mA) max, 25°C

3000

250

Line Regulation (ppm/V)*, max

100

Load Regulation (ppm/mA)* max

100

Operating Temperature Range (°C)

–40 to +125

–40 to +85

*Top Grade

Analog Dialogue 32-1 (1998)

1.5-W Loudspeaker

Amplifier Delivers

Sound Performance

by Troy Murphy

The SSM2211* speaker amplifier, from the Analog Devices audio

amplifier group, is an operational power amplifier designed to

deliver up to 1.5˚ W of power into a 4-W speaker when powered by

a +5-V single supply. Its current drive, sound quality, and heat

dissipation are substantially improved over earlier integrated

speaker amplifiers. Its SO-8 package uses a patented Thermal

Coastline ® technique for significantly improved heat dissipation

in a small space. This allows the device to deliver power at elevated

ambient temperatures.

The pushpull-output SSM2211 consists of an input amplifier

(Figure 1), that can be configured for gain like a standard op amp,

and a unity-gain inverting amplifier—with appropriate biasing—

producing a differential output voltage across a floating “bridge-

tied” load (BTL) with maximum swing approaching twice the

supply voltage (hence four times the single-ended power output

into a resistive load). Both amplifiers have high-current output

stages (to within 400˚ mV of the rails at full power). A reference

voltage is available to bias the two amplifiers for single-supply use,

and the device can be put into a low-current shutdown mode,

drawing typically less than˚ 10˚ nA; this makes it very suitable for

battery-powered applications, such as portable PC audio and

mobile radios.

At maximum output power, the total harmonic distortion (THD)

is only 0.1%, a significant improvement over IC speaker amplifiers

currently on the market.

Design objectives There are two major challenges in designing a

power amplifier to be housed in a small-outline (SOIC) package.

One is to deliver the maximum power efficiently from a single

supply voltage. The other is to dissipate the heat the device

generates at high output power levels without excessive temperature

rise.

To drive a load connected from the single-ended output of an

amplifier to ground, the maximum sine-wave power available is

simply V P 2 /(2 R ), where V P is the peak voltage. In the ideal case

(rail-to-rail), V P would be half the supply voltage, and max output

would be V s 2 /(8 R ). With the amplifier biased halfway in a single-

supply application, a capacitor must be used to couple a speaker

to a single-ended output to block direct current from the speaker.

Because the typical resistance of a speaker can be 8˚ W or less, the

capacitance must be at least several hundred microfarads to

minimize attenuation at low frequencies. The capacitor adds cost

to the system design and takes up precious board space. The

efficiency of this arrangement is low.

By connecting a speaker across both outputs in a pushpull, or

bridge-tied load (BTL) configuration, the need for a coupling

capacitor is eliminated, because both output terminals are biased

to the same dc voltage. The BTL configuration also doubles the

V DD

COPPER

LEAD-FRAME

20k V

SSM2211

20k

V IN

V O1

50k

LOUD

SPEAKER

OUTPUT

50k

COPPER PADDLE

V O2

50k

BIAS

CONTROL

0.1

SHUTDOWN

Figure 1. SSM2211

Simplified Schematic.

Figure 2. Thermal Coastline.

voltage swing across the output. Because the output power is

proportional to the square of the voltage, this allows four times as

much power to be delivered to the speaker, a loudness increase of

12˚ dB. In addition, efficiency can be greater.

The maximum power dissipation of the SSM2211 is a function of

the supply voltage and the resistance of the speaker it is driving. It

can be found by the formula:

DISS

, max

where V DD is the supply voltage and R L is the speaker resistance.

With a +5-V supply and an 8-W speaker, the maximum power

dissipation of the device is 633˚ mW. This can result in a significant

heat increase in a standard SO-8 package. To improve the heat

dissipation from the package, the SSM2211 uses a modified

package for lowered thermal resistance. This proprietary package,

developed by Analog Devices, uses an internal modification called

a Thermal Coastline ® to improve the thermal resistance in a SOIC

package by more than 30%.

The modification, done inside the package, is invisible to the user.

In a standard package, the die sits on a rectangular paddle with

the bonding pads coming out to the die. In a package with a

Thermal Coastline, the area of the paddle is increased; the bonding

pads are extended and curve around the paddle, as shown in

Figure˚ 2. This provides a path with increased thermal conductivity

for heat to flow from the die into the package case, thereby lowering

the thermal resistance from the die to the ambient surroundings.

For a standard SOIC package, typical junction-to-ambient-

temperature thermal resistance (q J A ) is 158°C/W. In a Thermal

Coastline SOIC package,

C/W. Thus, a die in a Thermal

Coastline package will not get as hot as a die in a standard package

with the same power dissipation.

As a result of this packaging, the SSM2211 can deliver 1˚ W into

an 8-W load at temperatures up to +85°C. This is a significant

improvement over IC power amplifiers in conventional small-

outline packages, which can only deliver this magnitude of output

power at temperatures less than +44°C.

Analog Devices Thermal Coastline technology is not limited to

small outline packages; it can be applied to practically any package

type. Besides high-power audio, these new thermally efficient

packages have useful applications in power management and

temperature sensing devices. You can expect to see more small

packages of this sort with greater power output on increasing

numbers of new products from Analog Devices.

q J A is 98

*For data, consult our Web site, www.analog.com (Product Center), AnalogFax

line 800-446-6212 (with Faxcode 1979 ), or use the reply card. Circle 2

Analog Dialogue 32-1 (1998)

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