AD AD22279-A-R2

Single-Axis, High-g,
iMEMS® Accelerometers
ADXL78
FEATURES
GENERAL DESCRIPTION
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
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 fullscale 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 costeffective 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.
APPLICATIONS
Vibration monitoring and control
Vehicle collision sensing
Shock detection
FUNCTIONAL BLOCK DIAGRAM
VS
ADXL78
TIMING
GENERATOR
VDD
VDD2
DIFFERENTIAL
SENSOR
SELF-TEST
DEMOD
AMP
400Hz
BESSEL
FILTER
XOUT
05368-001
EXC
Figure 1.
Rev. A
Information furnished by Analog Devices is believed to be accurate and reliable.
However, no responsibility is assumed by Analog Devices for its use, nor for any
infringements of patents or other rights of third parties that may result from its use.
Specifications subject to change without notice. No license is granted by implication
or otherwise under any patent or patent rights of Analog Devices. Trademarks and
registered trademarks are the property of their respective owners.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A.
Tel: 781.329.4700
www.analog.com
Fax: 781.461.3113
©2005 Analog Devices, Inc. All rights reserved.
ADXL78
TABLE OF CONTENTS
Specifications..................................................................................... 3
Power Supply Decoupling ............................................................8
Absolute Maximum Ratings............................................................ 4
Self-Test ..........................................................................................8
ESD Caution.................................................................................. 4
Clock Frequency Supply Response .............................................8
Pin Configuration and Function Descriptions............................. 5
Signal Distortion ...........................................................................8
Theory of Operation ........................................................................ 7
Outline Dimensions ..........................................................................9
Applications....................................................................................... 8
Ordering Guide .............................................................................9
REVISION HISTORY
5/05—Rev. 0 to Rev. A
Rev. A | Page 2 of 12
ADXL78
SPECIFICATIONS 1
At TA = −40°C to +105°C, 5.0 V dc ± 5%, acceleration = 0 g, unless otherwise noted.
Table 1.
Parameter
SENSOR
Output Full-Scale Range
Nonlinearity
Package Alignment Error
Cross-Axis Sensitivity
Resonant Frequency
Sensitivity, Ratiometric
(Over Temperature)
OFFSET
Zero-g Output Voltage
(Over Temperature) 2
NOISE
Noise Density
Clock Noise
FREQUENCY RESPONSE
−3 dB Frequency
−3 dB Frequency Drift
SELF-TEST
Output Change
(Cube vs. VDD) 3
Logic Input High
Logic Input Low
Input Resistance
OUTPUT AMPLIFIER
Output Voltage Swing
Capacitive Load Drive
PREFILTER HEADROOM
CFSR @ 400 kHz
POWER SUPPLY (VDD)
Functional Range
Quiescent Supply Current
TEMPERATURE RANGE
Conditions
Model No. AD22279
Min
Typ Max
Model No. AD22280
Min
Typ Max
Model No. AD22281
Min
Typ Max
IOUT ≤ ±100 μA
37
55
70
0.2
1
−5
VDD = 5 V, 100 Hz
52.25
VOUT − VDD/2,
VDD = 5 V
−200
+5
24
55
1.1
10 Hz − 400 Hz,
5V
2
0.2
1
−5
57.75
36.1
+200
−150
3
+5
24
38
1.4
5
2
0.2
1
−5
39.9
25.65
+150
−150
3
1.8
5
28.35
g
%
Degree
%
kHz
mV/g
+150
mV
3.5
mg/√Hz
2
+5
24
27
Unit
5
mV p-p
2-pole Bessel
360
400
2
440
360
400
2
440
360
400
2
440
Hz
Hz
VDD = 5 V
440
550
660
304
380
456
216
270
324
mV
VDD = 5 V
VDD = 5 V
Pull-down
resistor to GND
3.5
1
V
V
kΩ
25°C to
TMIN or TMAX
IOUT = ±400 μA
3.5
3.5
1
30
50
0.25
1000
1
30
VDD − 0.25
0.25
1000
280
5
4.75
3.5
VDD = 5 V
1.3
−40
50
30
VDD − 0.25
0.25
1000
400
4
5.25
6
2
+105
4.75
3.5
1.3
−40
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.
Rev. A | Page 3 of 12
50
VDD − 0.25
560
3
5.25
6
2
+105
4.75
3.5
1.3
−40
5.25
6
2
+105
V
pF
g
V/V
V
V
mA
°C
ADXL78
ABSOLUTE MAXIMUM RATINGS
Table 2.
Parameter
Acceleration (Any Axis, Unpowered)
Acceleration (Any Axis, Powered)
VS
All Other Pins
Output Short-Circuit Duration
(Any Pin to Common)
Operating Temperature Range
Storage Temperature
Rating
4000 g
4000 g
−0.3 V to +7.0 V
(COM − 0.3 V) to
(VS + 0.3 V)
Indefinite
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.
−65°C to +150°C
−65°C to +150°C
ESD CAUTION
ESD (electrostatic discharge) sensitive device. Electrostatic charges as high as 4000 V readily accumulate
on the human body and test equipment and can discharge without detection. Although this product features
proprietary ESD protection circuitry, permanent damage may occur on devices subjected to high energy
electrostatic discharges. Therefore, proper ESD precautions are recommended to avoid performance
degradation or loss of functionality.
Rev. A | Page 4 of 12
ADXL78
PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
VDD2
8
NC 1
7
VDD
ADXL78
NC 2
6 XOUT
TOP VIEW
(Not to Scale)
COM 3
5
NC
05368-002
4
ST
NC = NO CONNECT
Figure 2. Pin Configuration
Table 3. Pin Function Descriptions
Pin No.
1
2
3
4
5
6
7
8
Mnemonic
NC
NC
COM
ST
NC
XOUT
VDD
VDD2
Description
Do Not Connect
Do Not Connect
Common
Self-Test
Do Not Connect
X Channel Output
3.5 V to 6 V
3.5 V to 6 V
Rev. A | Page 5 of 12
ADXL78
CRITICAL ZONE
TL TO TP
tP
TP
TEMPERATURE
RAMP-UP
TL
tL
TSMAX
TSMIN
tS
RAMP-DOWN
05368-003
PREHEAT
t25°C TO PEAK
TIME
Figure 3. Recommended Soldering Profile
Table 4. Recommended Soldering Profile
Profile Feature
AVERAGE RAMP RATE (TL TO TP)
PREHEAT
Minimum Temperature (TSMIN)
Maximum Temperature (TSMAX)
TIME (TSMIN TO TSMAX), tS
TSMAX TO TL
Ramp-Up Rate
TIME MAINTAINED ABOVE LIQUIDOUS (TL)
Liquidous Temperature (TL)
Time (tL)
PEAK TEMPERATURE (TP)
TIME WITHIN 5°C OF ACTUAL PEAK TEMPERATURE (tP)
RAMP-DOWN RATE
TIME 25°C TO PEAK TEMPERATURE
Sn63/Pb37
3°C/s max
Pb-Free
3°C/s max
100°C
150°C
60 s − 120 s
150°C
200°C
60 s − 150 s
3°C/s
3°C/s
183°C
60 s − 150 s
240°C + 0°C/−5°C
10 s − 30 s
6°C/s max
6 min max
217°C
60 s − 150 s
260°C + 0°C/−5°C
20 s − 40 s
6°C/s max
8 min max
PIN 8
XXXXX
XXXX
XOUT = 2.462V
22280
XXXXX
XXXX
22280
XOUT = 2.500V
22280
XXXXX
XXXX
XOUT = 2.538V
XOUT = 2.500V
EARTH'S SURFACE
Figure 4. Output Response vs. Orientation
Rev. A | Page 6 of 12
05368-004
XOUT = 2.500V
XXXXX
XXXX
22280
ADXL78
THEORY OF OPERATION
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.
Rev. A | Page 7 of 12
MOVABLE
FRAME
PLATE
CAPACITORS
UNIT
SENSING
CELL
FIXED
PLATES
UNIT
FORCING
CELL
MOVING
PLATE
ANCHOR
Figure 5. Simplified View of Sensor Under Acceleration
05368-005
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.
ANCHOR
ACCELERATION
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.
ADXL78
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 accelerometer 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.
Rev. A | Page 8 of 12
ADXL78
OUTLINE DIMENSIONS
5.00
SQ
1.27
1.78
1.27
4.50
SQ
7
0.50 DIAMETER
1
1.90
2.50
TOP VIEW
1.27
R 0.38
0.20
5
3
0.64 2.50
0.38 DIAMETER
R 0.20
BOTTOM VIEW
Figure 6. 8-Terminal Ceramic Leadless Chip Carrier [LCC]
(E-8)
Dimensions shown in millimeters
ADXL78 ORDERING GUIDE
Model 1
AD22279-A-R2
AD22279-A
AD22280-R2
AD22280
AD22281-R2
AD22281
1
Parts
per Reel
250
3000
250
3000
250
3000
Measurement
Range
±35 g
±35 g
±50 g
±50 g
±70 g
±70 g
Specified
Voltage (V)
5
5
5
5
5
5
Temperature
Range
−40°C to +105°C
−40°C to +105°C
−40°C to +105°C
−40°C to +105°C
−40°C to +105°C
−40°C to +105°C
All models are on tape and reel and are Pb-free parts.
Rev. A | Page 9 of 12
Package Description
8-Terminal Ceramic Leadless Chip Carrier
8-Terminal Ceramic Leadless Chip Carrier
8-Terminal Ceramic Leadless Chip Carrier
8-Terminal Ceramic Leadless Chip Carrier
8-Terminal Ceramic Leadless Chip Carrier
8-Terminal Ceramic Leadless Chip Carrier
Package
Option
E-8
E-8
E-8
E-8
E-8
E-8
ADXL78
NOTES
Rev. A | Page 10 of 12
ADXL78
NOTES
Rev. A | Page 11 of 12
ADXL78
NOTES
©2005 Analog Devices, Inc. All rights reserved. Trademarks and
registered trademarks are the property of their respective owners.
D05368–0–5/05(A)
Rev. A | Page 12 of 12