AD22279-A-R2 - ANALOG DEVICES

Single-Axis, High-g,

iMEMS® Accelerometers

ADXL78

Rev. B

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FEATURES

Complete acceleration measurement system on a single

monolithic IC

Available in ±35 g, ±50 g, or ±70 g output full-scale ranges

Full differential sensor and circuitry for high resistance

to EMI/RFI

Environmentally robust packaging

Complete mechanical and electrical self-test on

digital command

Output ratiometric to supply

Sensitive axes in the plane of the chip

High linearity (0.2% of full scale)

Frequency response down to dc

Low noise

Low power consumption (1.3 mA)

Tight sensitivity tolerance and 0 g offset capability

Largest available prefilter clipping headroom

400 Hz, 2-pole Bessel filter

Single-supply operation

Compatible with Sn/Pb and Pb-free solder processes

Qualified for automotive applications

APPLICATIONS

Vibration monitoring and control

Vehicle collision sensing

Shock detection

GENERAL DESCRIPTION

The ADXL78 is a low power, complete single-axis accelerometer

with signal conditioned voltage outputs that are on a single

monolithic IC. This product measures acceleration with a full-

scale range of ±35 g, ±50 g, or ±70 g (minimum). It can also

measure both dynamic acceleration (vibration) and static

acceleration (gravity).

The ADXL78 is the fourth-generation surface micromachined

iMEMS® accelerometer from ADI with enhanced performance

and lower cost. Designed for use in front and side impact airbag

applications, this product also provides a complete cost-

effective solution useful for a wide variety of other applications.

The ADXL78 is temperature stable and accurate over the

automotive temperature range, with a self-test feature that fully

exercises all the mechanical and electrical elements of the

sensor with a digital signal applied to a single pin.

The ADXL78 is available in a 5 mm × 5 mm × 2 mm,

8-terminal ceramic LCC package.

FUNCTIONAL BLOCK DIAGRAM

05368-001

ADXL78

DD2

DIFFERENTIAL

SENSOR

EXC DEMOD

AMP X

OUT

400Hz

BESSEL

FILTER

TIMING

GENERATOR

SELF-TEST

Figure 1.

ADXL78

Rev. B | Page 2 of 12

TABLE OF CONTENTS

Features .............................................................................................. 1

Applications ....................................................................................... 1

General Description ......................................................................... 1

Functional Block Diagram .............................................................. 1

Specifications ..................................................................................... 3 

Absolute Maximum Ratings ............................................................ 4

ESD Caution .................................................................................. 4

Pin Configuration and Function Descriptions ............................. 5

Theory of Operation ........................................................................ 7

Applications ........................................................................................8

Power Supply Decoupling ............................................................8

Self-Test ..........................................................................................8

Clock Frequency Supply Response .............................................8

Signal Distortion ...........................................................................8

Outline Dimensions ..........................................................................9

ADXL78 Ordering Guide .............................................................9

Automotive Products ....................................................................9

REVISION HISTORY

7/10—Rev. A to Rev. B

Updated Format ................................................................. Universal

Change to Features Section ............................................................. 1

Updated Outline Dimensions ......................................................... 9

Changes to Ordering Guide ............................................................ 9

Added Automotive Products Section ............................................ 9

5/05—Rev. 0 to Rev. A

ADXL78

Rev. B | Page 3 of 12

SPECIFICATIONS1

At TA = −40°C to +105°C, 5.0 V dc ± 5%, acceleration = 0 g, unless otherwise noted.

Table 1.

Model No. AD22279 Model No. AD22280 Model No. AD22281

Parameter Conditions Min Typ Max Min Typ Max Min Typ Max Unit

SENSOR

Output Full-Scale Range IOUT ≤ ±100 μA 37 55 70 g

Nonlinearity 0.2 2 0.2 2 0.2 2 %

Package Alignment Error 1 1 1 Degree

Cross-Axis Sensitivity −5 +5 −5 +5 −5 +5 %

Resonant Frequency 24 24 24 kHz

Sensitivity, Ratiometric

(Over Temperature)

VDD = 5 V, 100 Hz 52.25 55 57.75 36.1 38 39.9 25.65 27 28.35 mV/g

OFFSET

Zero-g Output Voltage

(Over Temperature)2

VOUT − VDD/2,

VDD = 5 V

−200 +200 −150 +150 −150 +150 mV

NOISE

Noise Density 10 Hz − 400 Hz,

5 V

1.1 3 1.4 3 1.8 3.5 mg/√Hz

Clock Noise 5 5 5 mV p-p

FREQUENCY RESPONSE 2-pole Bessel

−3 dB Frequency 360 400 440 360 400 440 360 400 440 Hz

−3 dB Frequency Drift 25°C to

TMIN or TMAX

2 2 2 Hz

SELF-TEST

Output Change

(Cube vs. VDD)3

VDD = 5 V 440 550 660 304 380 456 216 270 324 mV

Logic Input High VDD = 5 V 3.5 3.5 3.5 V

Logic Input Low VDD = 5 V 1 1 1 V

Input Resistance Pull-down

resistor to GND

30 50 30 50 30 50 kΩ

OUTPUT AMPLIFIER

Output Voltage Swing IOUT = ±400 μA 0.25 VDD − 0.25 0.25 VDD − 0.25 0.25 VDD − 0.25 V

Capacitive Load Drive 1000 1000 1000 pF

PREFILTER HEADROOM 280 400 560 g

CFSR @ 400 kHz 5 4 3 V/V

POWER SUPPLY (VDD) 4.75 5.25 4.75 5.25 4.75 5.25 V

Functional Range 3.5 6 3.5 6 3.5 6 V

Quiescent Supply Current VDD = 5 V 1.3 2 1.3 2 1.3 2 mA

TEMPERATURE RANGE −40 +105 −40 +105 −40 +105 °C

1 All minimum and maximum specifications are guaranteed. Typical specifications are not guaranteed.

2 Zero g output is ratiometric.

3 Self-test output at VDD = (Self-Test Output at 5 V) × (VDD/5 V)3.

ADXL78

Rev. B | Page 4 of 12

ABSOLUTE MAXIMUM RATINGS

Table 2.

Parameter Rating

Acceleration (Any Axis, Unpowered) 4000 g

Acceleration (Any Axis, Powered) 4000 g

VS −0.3 V to +7.0 V

All Other Pins (COM − 0.3 V) to

(VS + 0.3 V)

Output Short-Circuit Duration

(Any Pin to Common)

Indefinite

Operating Temperature Range −65°C to +150°C

Storage Temperature −65°C to +150°C

Stresses above those listed under Absolute Maximum Ratings

may cause permanent damage to the device. This is a stress

rating only; functional operation of the device at these or any

other conditions above those indicated in the operational

section of this specification is not implied. Exposure to absolute

maximum rating conditions for extended periods may affect

device reliability.

ESD CAUTION

ADXL78

Rev. B | Page 5 of 12

PIN CONFIGURATION AND FUNCTION DESCRIPTIONS

05368-002

NC = NO CONNECT

OUT

TOP VIEW

(Not to Scale)

ADXL78

DD2

Figure 2. Pin Configuration

Table 3. Pin Function Descriptions

Pin No. Mnemonic Description

1 NC Do Not Connect

2 NC Do Not Connect

3 COM Common

4 ST Self-Test

5 NC Do Not Connect

6 XOUT X Channel Output

7 VDD 3.5 V to 6 V

8 VDD2 3.5 V to 6 V

ADXL78

Rev. B | Page 6 of 12

05368-003

25°C TO PEAK

PREHEAT

CRITICAL ZONE

TO T

TEMPERATURE

TIME

RAMP-DOWN

RAMP-UP

SMIN

SMAX

Figure 3. Recommended Soldering Profile

Table 4. Recommended Soldering Profile

Profile Feature Sn63/Pb37 Pb-Free

AVERAGE RAMP RATE (TL TO TP) 3°C/s max 3°C/s max

PREHEAT

Minimum Temperature (TSMIN) 100°C 150°C

Maximum Temperature (TSMAX) 150°C 200°C

TIME (TSMIN TO TSMAX), tS 60 s − 120 s 60 s − 150 s

TSMAX TO TL

Ramp-Up Rate 3°C/s 3°C/s

TIME MAINTAINED ABOVE LIQUIDOUS (TL)

Liquidous Temperature (TL) 183°C 217°C

Time (tL) 60 s − 150 s 60 s − 150 s

PEAK TEMPERATURE (TP) 240°C + 0°C/−5°C 260°C + 0°C/−5°C

TIME WITHIN 5°C OF ACTUAL PEAK TEMPERATURE (tP) 10 s − 30 s 20 s − 40 s

RAMP-DOWN RATE 6°C/s max 6°C/s max

TIME 25°C TO PEAK TEMPERATURE 6 min max 8 min max

05368-004

EARTH'S SURFACE

XXXXX

XXXX

22280

PIN 8

XOUT = 2.462V

XXXXX

XXXX

22280

XOUT = 2.500V

XXXXX

XXXX

22280

XXXXX

XXXX

22280

OUT = 2.500V

XOUT = 2.538V

XOUT = 2.500V

Figure 4. Output Response vs. Orientation

ADXL78

Rev. B | Page 7 of 12

THEORY OF OPERATION

The ADXL78 provides a fully differential sensor structure and

circuit path, resulting in the industry’s highest resistance to

EMI/RFI effects. This latest generation uses electrical feedback

with zero-force feedback for improved accuracy and stability.

The sensor resonant frequency is significantly higher than the

signal bandwidth set by the on-chip filter, avoiding the signal

analysis problems caused by resonant peaks near the signal

bandwidth.

05368-005

UNIT

SENSING

CELL

MOVABLE

FRAME

FIXED

PLATES

UNIT

FORCING

CELL

ANCHOR

MOVING

PLATE

CAPACITORS

ACCELERATION

ANCHOR

Figure 5 is a simplified view of one of the differential sensor

elements. Each sensor includes several differential capacitor

unit cells. Each cell is composed of fixed plates attached

to the substrate and movable plates attached to the frame.

Displacement of the frame changes the differential capacitance,

which is measured by the on-chip circuitry. Figure 5. Simplified View of Sensor Under Acceleration

Complementary 400 kHz square waves drive the fixed plates.

Electrical feedback adjusts the amplitudes of the square waves

such that the ac signal on the moving plates is 0. The feedback

signal is linearly proportional to the applied acceleration. This

unique feedback technique ensures that there is no net

electrostatic force applied to the sensor. The differential

feedback control signal is also applied to the input of the filter,

where it is filtered and converted to a single-ended signal.

ADXL78

Rev. B | Page 8 of 12

APPLICATIONS

POWER SUPPLY DECOUPLING

For most applications, a single 0.1 μF capacitor, CDC, adequately

decouples the accelerometer from noise on the power supply.

However, in some cases, particularly where noise is present at

the 400 kHz internal clock frequency (or any harmonic

thereof), noise on the supply can cause interference on the

ADXL78’s output. If additional decoupling is needed, a 50 Ω (or

smaller) resistor or ferrite bead can be inserted in the supply

line. Additionally, a larger bulk bypass capacitor (in the 1 μF to

4.7 μF range) can be added in parallel to CDC.

SELF-TEST

The fixed fingers in the forcing cells are normally kept at the

same potential as that of the movable frame. When the self-test

digital input is activated, the voltage on the fixed fingers on one

side of the moving plate in the forcing cells is changed. This

creates an attractive electrostatic force, which causes the frame

to move toward those fixed fingers. The entire signal channel is

active; therefore, the sensor displacement causes a change in

VOUT. The ADXL78 self-test function is a comprehensive

method of verifying the operation of the accelerometer.

Because electrostatic force is independent of the polarity of the

voltage across capacitor plates, a positive voltage is applied in

half of the forcing cells, and its complement in the other half of

the forcing cells. Activating self-test causes a step function force

to be applied to the sensor, while the capacitive coupling term is

canceled. The ADXL78 has improved self-test functionality,

including excellent transient response and high speed switching

capabilities. Arbitrary force waveforms can be applied to the

sensor by modulating the self-test input, such as test signals to

measure the system frequency response or even crash signals to

verify algorithms within the limits of the self-test swing.

The ST pin should never be exposed to voltages greater than

VS + 0.3 V. If this cannot be guaranteed due to the system

design (for instance, if there are multiple supply voltages), then

a low VF clamping diode between ST and VS is recommended.

CLOCK FREQUENCY SUPPLY RESPONSE

In any clocked system, power supply noise near the clock

frequency may have consequences at other frequencies. An

internal clock typically controls the sensor excitation and the

signal demodulator for micromachined accelerometers.

If the power supply contains high frequency spikes, they may be

demodulated and interpreted as an acceleration signal. A signal

appears as the difference between the noise frequency and the

demodulator frequency. If the power supply spikes are 100 Hz

away from the demodulator clock, there is an output term at

100 Hz. If the power supply clock is at exactly the same frequency

as the accelerometer clock, the term appears as an offset.

If the difference frequency is outside of the signal bandwidth,

the filter attenuates it. However, both the power supply clock

and the accelerometer clock may vary with time or temperature,

which can cause the interference signal to appear in the output

filter bandwidth.

The ADXL78 addresses this issue in two ways. First, the high

clock frequency eases the task of choosing a power supply clock

frequency such that the difference between it and the accelero-

meter clock remains well outside of the filter bandwidth.

Second, the ADXL78 is the only micromachined accelerometer

to have a fully differential signal path, including differential

sensors. The differential sensors eliminate most of the power

supply noise before it reaches the demodulator. Good high

frequency supply bypassing, such as a ceramic capacitor close to

the supply pins, also minimizes the amount of interference.

The clock frequency supply response (CFSR) is the ratio of the

response at VOUT to the noise on the power supply near the

accelerometer clock frequency. A CFSR of 3 means that the

signal at VOUT is 3× the amplitude of an excitation signal at VDD

near the accelerometer internal clock frequency. This is

analogous to the power supply response, except that the

stimulus and the response are at different frequencies. The

ADXL78’s CFSR is 10× better than a typical single-ended

accelerometer system.

SIGNAL DISTORTION

Signals from crashes and other events may contain high

amplitude, high frequency components. These components

contain very little useful information and are reduced by the

2-pole Bessel filter at the output of the accelerometer. However,

if the signal saturates at any point, the accelerometer output

does not look like a filtered version of the acceleration signal.

The signal may saturate anywhere before the filter. For example,

if the resonant frequency of the sensor is low, the displacement

per unit acceleration is high. The sensor may reach the

mechanical limit of travel if the applied acceleration is high

enough. This can be remedied by locating the accelerometer

where it does not see high values of acceleration, and by using a

higher resonant frequency sensor such as the ADXL78.

Also, the electronics may saturate in an overload condition

between the sensor output and the filter input. Ensuring that

the internal circuit nodes operate linearly to at least several

times the full-scale acceleration value can minimize electrical

saturation. The ADXL78’s circuits are linear to approximately

8× full scale.

ADXL78

Rev. B | Page 9 of 12

OUTLINE DIMENSIONS

BOTTOM VIEW

(PLATING OPTION 1,

SEE DETAIL A

FOROPTION2)

DETAIL A

(OPTION 2)

TOP VIEW

0.075 REF

R0.008

(4 PLCS)

0.203

0.197 SQ

0.193 0.020

0.015

0.010

(R 4 PLCS)

0.180

0.177 SQ

0.174

0.087

0.078

0.069

0.008

0.006

0.004 0.077

0.070

0.063

0.054

0.050

0.046

0.030

0.025

0.020 0.028

0.020 DIA

0.012

0.019 SQ

0.106

0.100

0.094

R0.008

(8 PLCS)

05-21-2010-D

Figure 6. 8-Terminal Ceramic Leadless Chip Carrier [LCC]

(E-8-1)

Dimensions shown in inches

ADXL78 ORDERING GUIDE

Model1, 2, 3

Parts

per

Reel

Measurement

Range

Specified

Voltage (V)

Temperature

Range Package Description

Package

Option

AD22279-A-R2 250 ±35 g 5 −40°C to +105°C 8-Terminal Ceramic Leadless Chip Carrier E-8-1

AD22279-A 3,000 ±35 g 5 −40°C to +105°C 8-Terminal Ceramic Leadless Chip Carrier E-8-1

ADW22279ZC 3,000 ±35 g 5 −40°C to +105°C 8-Terminal Ceramic Leadless Chip Carrier E-8-1

ADW22279ZC-RL7 250 ±35 g 5 −40°C to +105°C 8-Terminal Ceramic Leadless Chip Carrier E-8-1

AD22280-R2 250 ±50 g 5 −40°C to +105°C 8-Terminal Ceramic Leadless Chip Carrier E-8-1

AD22280 3,000 ±50 g 5 −40°C to +105°C 8-Terminal Ceramic Leadless Chip Carrier E-8-1

ADW22280ZC 3,000 ±50 g 5 −40°C to +105°C 8-Terminal Ceramic Leadless Chip Carrier E-8-1

ADW22280ZC-RL7 250 ±50 g 5 −40°C to +105°C 8-Terminal Ceramic Leadless Chip Carrier E-8-1

AD22281-R2 250 ±70 g 5 −40°C to +105°C 8-Terminal Ceramic Leadless Chip Carrier E-8-1

AD22281 3,000 ±70 g 5 −40°C to +105°C 8-Terminal Ceramic Leadless Chip Carrier E-8-1

ADW22281ZC 3,000 ±70 g 5 −40°C to +105°C 8-Terminal Ceramic Leadless Chip Carrier E-8-1

ADW22281ZC-RL7 250 ±70 g 5 −40°C to +105°C 8-Terminal Ceramic Leadless Chip Carrier E-8-1

1 All models are on tape and reel and are RoHS compliant parts.

2 Z = RoHS Compliant Part.

3 W = Qualified for Automotive Applications.

AUTOMOTIVE PRODUCTS

The ADW22279, ADW22280, and ADW22281 models are available with controlled manufacturing to support the quality and reliability

requirements of automotive applications. Note that these automotive models may have specifications that differ from the commercial

models; therefore, designers should review the Specifications section of this data sheet carefully. Only the automotive grade products

shown are available for use in automotive applications. Contact your local Analog Devices account representative for specific Automotive

Reliability reports for these models.

ADXL78

Rev. B | Page 10 of 12

NOTES

ADXL78

Rev. B | Page 11 of 12

NOTES

ADXL78

Rev. B | Page 12 of 12

NOTES

registered trademarks are the property of their respective owners.

D05368-0-7/10(B)