volume30-number4(1).pdf

Editor’s Notes

THIRTY YEARS OF ANALOG DIALOGUE

In Volume 15-1 (1981), in cele-

bration of 15 years in print, we

listed the first 15 years of Analog

Dialogue cover features. At that

time, we wrote: “. . . will mark the

15th year of publication of this

journal, and the 13th year of our

stewardship. While adding a copy

of the [most recent] issue to our

bulging binder, we nostalgically

turned the pages of issues long forgotten. The amount of

technological progress reported in them surprised even us. Before

the list expands beyond the capacity of this column, we thought

you might be interested in seeing a roll of just the cover stories

alone, though much significant progress never attained the cover.”

In this spirit, in celebration of our 30th year in print (and 28th of

stewardship), here are the second 15 years of cover themes:

THE AUTHORS

Howard Samuels (page 3) is

a Product Development Engineer

in ADI’s Micromachined Products

Division in Wilmington, MA. He

joined Analog after graduating

from Carnegie Mellon University

in 1982 with a BS in EE and

Applied Mathematics. He holds

several patents and has published

articles in trade magazines.

Howard has designed signal conditioners and isolators, including

the AD210 3-port isolator. When not at work, he enjoys caring for

and taking pictures of his new baby.

Matt Smith (page 6) is a Senior

Applications Engineer at our

Limerick, Ireland, facility. He is

responsible for Interface and

Supervisory products. He holds a

B.Eng from the University of

Limerick. His leisure interests

include playing squash, motor

maintenance and more recently,

woodworking.

Bill Slattery (page 9) has

marketing responsibility for

products of the Digital Video

Group at ADI’s Limerick facility,

including video DACs, RAM-

DACs, and video encoders &

decoders. Earlier, he worked as a

Senior Engineer in the Applications

Group in Limerick. Bill has a BSc

(Eng) degree from the University of

Dublin (Trinity College) and an MBA from the University of

Limerick. A licensed private pilot, he enjoys flying his airplane.

Jürgen Kühnel (page 13) is a

Senior Marketing Engineer for

Power Management products,

located in Munich. Since joining

ADI in 1984, he has had various

roles in Sales and Marketing in

Central Europe. His Dipl. Ing. is

from the Technische Fachhochschule

in Berlin, and he worked for several

years as a system designer in

medical and chemical instrumentation. In his spare time he likes

riding on two wheels (powered or not) and designing electronic

systems for his model railroad.

1981 15-2 D/A converters for graphic displays

1982 16-1 High-performance hybrid-circuit isolation amp (AD293)

16-2 Dual monolithic multiplying DACs (AD7528)

16-3 Monolithic instrumentation amplifier (AD524)

and thermocouple preamp (AD594)

1983 17-1 CMOS ICs for digital signal processing

17-2 Quad CMOS DAC with buffered voltage outputs

1984 18-1 Amplifier noise basics revisited

18-2 Monolithic V-out

P-compatible 12-bit DAC (AD667)

18-3 An intelligent vision system for industrial image analysi s

1985 19-1 Multifunction analog IC computes y(z/x) m , etc. (AD538)

1986 20-1 Low-cost, high-performance, compact iso amps (AD202/4)

20-2 Fast, flexible CMOS DSP µ P (ADSP-2100)

1987 24-1 Monolithic process-control transmitters (AD693)

21-2 Complete 8-bit 400-ksps analog I/O (AD7569)

1988 22-1 Chipset for 50-Mbit/s digital data recovery (AD890/891)

22-2 200-MSPS 8-bit IC ADC with 250-MHz BW (AD770)

1989 23-1 Isolated sensor-to-serial with a screwdriver (6B Series)

23-2 Single-chip DSP microcomputer (ADSP-2101)

23-3 DC-120-MHz IC log amp—accurate compression (AD640)

23-4 Pin electronics for high-speed ATE (AD1315/1521/22)

1990 24-1 Monolithic 75-MSPS 10-bit flash converter (AD9060)

24-2 Mixed-signal processor: DSP/ADC/DAC (ADSP-21msp50)

24-3 RAM-DAC upgrade enhances VGA graphics (AD7148)

1991 25-1 Pro-Logic decoder: Dolby“Surround Sound” (SSM2125)

25-2 ADSP-21020 floating-point high-speed DSP

1992 26-1 Mixed-signal chips drive digital radio (AD7001/7002)

26-2 Wideband “linear in dB”VCA (AD600/602)

1993 27-1 Fast precise 155-Mbps fiberoptic timing recovery (AD802)

27-2 Single-chip micromachined accelerometer (ADXL50)

1994 28-1 Dual-setpoint single-chip temperature controller (TMP01)

28-2 Complete, low-distortion 500-MHz IC mixer (AD831)

28-3 SHARC Floating-point DSP: tops in memory, performance

1995 29-1 Integrated stereo codecs for multimedia (AD1843/1845)

29-2 Meeting the challenges of high speed

29-3 Considerations in low-power, single-supply system design

1996 30-1 Read-channel processor uses PRML with MR heads

30-2 DSP-based chip set for ac motor control (ADMC201)

30-3 CMOS DACs optimized for transmit path (AD976x family)

30-4 Dual-axis accelerometer (ADXL250) A

Dan.Sheingold@analog.com

[ 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 30-4 (1996)

Single- and Dual-Axis

Micromachined

Accelerometers

ADXL150 & ADXL250: New complete

low-noise 50- g accelerometers

by Howard Samuels

The ADXL150 and ADXL250 represent the newest generation

of surface-micromachined monolithic accelerometers* from

Analog Devices. Like the landmark ADXL50 ( Analog Dialogue

27-2, 1993), the new devices include both the signal conditioning

circuitry and the sensor, fabricated together on a single monolithic

chip—providing acceleration measurement at very low cost with

high reliability. As with the ADXL50, the sensor structure is a

differential capacitor, but it is modified to take advantage of the

experience gained from producing millions of ADXL50s, further

advancing the state of the art of micromachined sensor design.

The sensor: The silhouettes in Figure 1 compare the sensors used

in the ADXL50 and the ADXL150. Both sensors have numerous

fingers along each side of the movable center member; they

constitute the center plates of a paralleled set of differential

capacitors. Pairs of fixed fingers attached to the substrate interleave

with the beam fingers to form the outer capacitor plates. The beam

is supported by tethers, which serve as mechanical springs. The

voltage on the moving plates is read via the electrically conductive

tether anchors that support the beam.

The polysilicon support springs (tethers) are highly reliable. Many

devices have been tested by deflecting the beam with the equivalent

of > 250

10 10 cycles, with zero

failures, as part of the product qualification process.

The ADXL50’s tethers extend straight out from the beam in an

‘H’ configuration. On the ADXL150, however, the tethers are

folded, reducing the size of the sensor and halving the number of

anchors (Figure 2). Since each anchor adds parasitic capacitance,

the smaller number of anchors reduces capacitive load, increasing

the sensor’s acceleration sensitivity. In addition, the tether geometry

minimizes sensitivity to mechanical die-stress; this allows the

ADXL150 to be packaged in standard cerdip and surface-mount

cerpak packages, which require higher sealing temperatures (and

associated thermal stress) than metal cans. The folded tether was

first used in the ADXL05 low- g accelerometer; its higher sensitivity

makes die stress more of a concern.

In addition to the sense fingers projecting from both sides of the

beam, the ADXL150 has 12 force fingers (visible near both ends

the force of gravity, for > 7

TETHER

BEAM

AXIS OF

ACCELERATION

AXIS OF

ACCELERATION

SENSE

FINGERS

SENSE

FINGERS

FORCE

FINGERS

FORCE

FINGERS

ANCHOR

TETHER

FOLDED TETHER

Figure 2. Partial aerial SEM view of one end of the

ADXL150’s sensor.

IN THIS ISSUE

Volume 30, Number 4, 1996, 24 Pages

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

Single- and dual-axis micromachined accelerometers (ADXL150, ADXL250) . 3

EMC, CE Mark, IEC801 . . .What’s it all about? . . . . . . . . . . . . . . . . . . 6

Integrated digital video encoders—studio quality video at consumer video prices 9

Selecting mixed-signal components for digital communication systems—II . . 11

Voltage regulators for power management . . . . . . . . . . . . . . . . . . . . . . . . 13

New-Product Briefs:

Three new op amp families . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

A/D and D/A converters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

DSPs and Mixed-signal processors . . . . . . . . . . . . . . . . . . . . . . . . 18

Mixed bag: Circuit protectors, Temperature to current,

Switched-cap regulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Ask The Applications Engineer—23: Current-feedback amplifiers—II . . . 20

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

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

Figure 1. Silhouette plots of ADXL50 (upper) and ADXL150

(lower). Axis of motion is vertical.

*For technical data, visit our Web site, http://www.analog.com. Data is also avail-

able in North America around the clock by Analogfax™, 1-800-446-6212;

request 2060 ; or use the reply card. Circle 1

Analog Dialogue 30-4 (1996)

of the beam), used for self-test actuation. The plates of a parallel-

plate capacitor attract each other with an electrostatic force of:

becomes unbalanced. The beam waveform is a square wave with

amplitude proportional to the amount of displacement, and hence,

acceleration magnitude. The phase of the beam voltage relative to

the excitation determines the acceleration polarity.

The beam output is connected directly to a noninverting amplifier,

which provides buffering for the high impedance beam node, as

well as gain for the 100-kHz output signal.

The output is demodulated in a synchronous demodulator that

samples the amplifier output after it has settled in each half of the

excitation cycle. By detecting the difference between the amplifier’s

output levels for the two states, the offset voltage of the amplifier

is eliminated, much like that of a chopper stabilized amplifier. Since

the demodulator is phase synchronized with the excitation, the

output signal polarity correctly indicates the direction of the applied

acceleration.

The ADXL150 has a 2-pole gain-of-3 Bessel low-pass filter on

board [the ADXL250—see below—includes a 2-pole filter for each

channel]. These filters can be used to prevent aliasing of high-

frequency components in the demodulator output with A/D

converter clock frequencies in associated data-acquisition circuitry.

A second input to the filter is connected to a resistive divider with

a gain of 1/6, brought out to a package pin. It provides a convenient

offset adjustment point for the accelerometer, with a net gain of

+0.5 for the applied voltage.

Because extensive use is made of CMOS logic, and the open-loop

architecture allows simpler signal conditioning circuitry, the device

draws only 1.8 mA of supply current at 5 V (including the 2-pole

output filter), a >80% reduction from the ADXL50.

The increased excitation levels used, along with carefully executed

chopper modulation/demodulation techniques, yield a noise

density of just 1 m g /

AV 2

2 d 2

where ε is the permittivity of the material between the plates, A is

the area of the plates, V is the voltage across the capacitor, and d is

the distance between the plates.

In normal operation, the fixed fingers on either side of the force

fingers are at the same voltage potential as the beam and its fingers.

With no voltage between the force fingers on the beam and the

fixed fingers on the substrate, there is no electrostatic force.

However, when a digital self-test input pin is activated, the fixed

fingers on one side of the force section are driven to a nonzero dc

voltage, applying a force to the sense fingers, deflecting the beam.

The forcing voltage is laser-trimmed to produce a net electrostatic

force on the beam equivalent to a 10- g acceleration. This voltage

will depend on the specific electrical and mechanical characteristics

of each individual device.

The self-test circuitry operates independently of the normal

accelerometer signal path. When self-test is activated, the deflection

it produces is measured by the device in the same way as a

deflection produced by accelerating the entire device. Since the

full-scale deflection of the sensor is only about 1.5% of the gap

between the capacitor fingers, the self-test response is nearly

constant, adding to the deflection caused by any existing

acceleration. Like an externally applied acceleration, the deflection

produced by the self-test circuitry makes full use of the

measurement circuitry of the normally functioning accelerometer

to generate an output, so it is a highly reliable indicator of the

device’s ability to function correctly.

Circuit Architecture: As Figure 3 shows, the fixed fingers are

driven with antiphase square waves. Unlike the ADXL50, which

uses a dc bias between the excitations and the beam as a means of

providing a force-balance feedback path, the ADXL150 employs

an open-loop architecture. With zero average dc voltage on the

beam, the excitation square waves can swing to the power supply

rails, with the beam biased at one half the supply voltage. The

larger amplitude of the 100-kHz excitation in the ADXL150 results

in reduced sensitivity to electronic device noise and is a contributing

factor to its improved noise performance.

If the beam is perfectly centered, both sides of the differential

capacitor have equal capacitance, and the ac voltage on the beam

is zero. However, if the beam is off center due to an applied

acceleration (or self-test deflection), the differential capacitor

= ε

Hz , less than 1/6 that of the ADXL50. The

improved dynamic range enables the ADXL150 to be used in

applications such as machine health, vibration monitoring, shock

sensing, and instrumentation.

The ADXL150 has a sensitivity of 38 mV/ g , measured at the output

pin. The full scale range is

√

50 g , for a total signal swing of 3.8 V,

with a single 5-V supply. This significant output voltage range allows

the designer to take full advantage of the input range of a single-

supply A/D converter, such as might be found in a microprocessor

system.

The output voltage is given by the relationship:

2 + α⋅

0.038 V

V O =

V S

TEST

INPUT

V S /2

ADXL150

R OS

–

V ST

R I

R F

V S

SENSOR

AMP

DEMOD

V O

V S

– V ST

V O '

TWO-POLE

FILTER

V S /2

SENSE

FINGERS

–

DEMOD

G = 3

FORCE

FINGERS

V O

V S /2

ADXL150 WITH ADDITIONAL GAIN AV = R F /R I

OFFSET ADJUSTMENT RANGE OF ± ,

AND ADDITIONAL SINGLE POLE LPF, –3dB = .

V S

R F

R OS

V S /2

2 π R F C

CLOCK/

TIMING

V S /2

ZERO-g

ADJUST

Figure 4. ADXL150 with an external op amp for additional

gain and filtering.

Figure 3. ADXL150 electrical block diagram.

Analog Dialogue 30-4 (1996)

9.8 m/s 2 ), and

V S is the power supply and reference voltage, nominally 5 V. If V S

is also used as the reference for a ratiometric A/D converter, the

system will reject variations in V S . With zero applied acceleration,

the output of the ADXL150 is V S /2, which is half scale of the A/D

converter. Even if V s is not exactly 5 V, the digital output code of

the A/D converter still reads half scale. For any applied acceleration,

the output of the A/D converter will be essentially independent of

changes in V S .

Without external manipulation of the filter’s offset, the device

provides a convenient reference point at one half the power supply

voltage. An external operational amplifier can be used (Figure 4),

for additional gain with respect to this voltage to increase the

sensitivity of the accelerometer. An additional external capacitor

can be used in this circuit to add a third pole after the internal

two-pole filter. The offset can be adjusted by current injected into

the summing node of the external amplifier.

The ADXL250 adds a new dimension: The ADXL250, a single

monolithic chip (Figure 5), measures both the x and y coordinates

of acceleration in a given plane (e.g., forward-back and side-to-

side). Because the sensitive axis of the ADXL150’s sensor is in the

plane of the chip, twin sensors can be fabricated on the same die,

with one rotated 90 degrees from the other. The ADXL250 is the

world’s first commercially available two-axis monolithic

accelerometer.

Both channels share the clock generator, demodulator timing, self

test logic, and bias voltage. Each sensor receives the clock signals

via its own CMOS inverter drivers, and the signals generated by

the sensors are treated completely independently.

The single self-test pin activates both sensors simultaneously,

simplifying the interface to a microprocessor. As in the ADXL150,

the test signal deflects each sensor by an amount equivalent to a

10- g acceleration. Each channel has its own offset adjustment pin

and its own output voltage pin. Both channels have the same

sensitivity.

The total power-supply current of the two-channel ADXL250, is

typically 3.5 mA (5 mA maximum, including the output filters—

just half the typical supply current of the earlier ADXL50). Both

devices have A and J versions, specified for temperature ranges –

40 to +85°C and 0 to +70°C. Prices (100s) start at $12.45

(ADXL150JQC) and $19.95 (ADXL250JQC).

How do I use them? The ADXL150 is a complete sensor on a

chip. Just connect a single 5-V power supply (with clean output,

bypassed to ground by a decent-quality ceramic capacitor) and

connect the output to its readout destination.

is the applied acceleration expressed in g s (1 g

≈

If the self-test pin is left open-circuited, an internal pulldown

resistor ensures normal operation. With nothing connected to the

offset adjust pin, the output voltage is unmodified.

To adjust the output zero- g voltage level, use the offset adjust pin.

The offset can be adjusted by applying an analog dc voltage,

including the supply voltage or ground. Computer control can be

achieved in various ways, e.g., by a serial or parallel D/A converter,

or by a modulated duty cycle with an R-C averager. A choice of

three offset adjustment values can be achieved with a three-state

digital output bit and a series resistor.

The ADXL150 and ADXL250 were developed by multidisciplinary

product teams in ADI’s Micromachined Products Division,

Wilmington, MA.

MOUNTING AND MECHANICAL CONSIDERATIONS

When an accelerometer is mounted on a PC board, the IC

becomes part of a larger mechanical system. Accelerations of

50 g cause the sensor to deflect within the IC package; in

addition, the PC board and its mounting structure will deflect

and deform. The motion of the board generates a false

acceleration signal, which the accelerometer can sense. If the

resonant frequency of the supporting structure is within the

signal band or not much higher than the filter rolloff, the

vibrations of the PC board and its mounting system will show

up in the sensor output.

The best way to minimize these effects is to make the mounting

scheme as stiff as possible, thereby transmitting the system

acceleration more faithfully to the sensor and increasing the

resonant frequency. Since a PC board is much stiffer in its plane

than perpendicular to its surface, the accelerometer’s sensitive

axis (both axes, if dual) should be in the plane of the board.

Because the ADXL150 and ADXL250 have their sensitive axes

in the plane of the chip, and the surface of the chip is parallel

to the base of the package, the accelerometers receive the benefit

of the PC board’s stiffness when simply soldered to the board.

If the sensitive axis were perpendicular to the plane of the chip

(as is the case for some bulk-micromachined sensors), soldering

the package to the board would render the measurement most

susceptible to PC-board flexibility. A right-angle mounting

system could be used to orient the sensitive axis parallel to the

PC board, but the mounting system itself can deform, producing

false acceleration readings. The mounting system, and any PC

board stiffeners, add cost to the acceleration measurement. Also,

the additional mass of the mounting system lowers its resonant

frequency, causing larger false acceleration signals.

X DEMOD

V OUTX

X ZERO-g ADJUST

Y DEMOD

V OUTY

Y ZERO-g ADJUST

Figure 5. ADXL250 block diagram (L) and partial chip photo showing sensors at right angles in plane of chip (R).

Analog Dialogue 30-4 (1996)

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