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Low Cost, Ultracompact 2 g Dual-Axis Accelerometer ADXL311
FEATURES
Low cost High resolution Dual-axis accelerometer on a single IC chip 5 mm x 5 mm x 2 mm CLCC package Low power < 400 A (typ) X-axis and Y-axis aligned to within 0.1 (typ) BW adjustment with a single capacitor Single-supply operation High shock survival
GENERAL DESCRIPTION
The ADXL311 is a low cost, low power, complete dual-axis accelerometer with signal conditioned voltage outputs, all on a single monolithic IC. The ADXL311 is built using the same proven iMEMS(R) process used in over 100 million Analog Devices accelerometers shipped to date, with demonstrated 1 FIT reliability (1 failure per 1 billion device operating hours).
APPLICATIONS
Tilt and motion sensing in cost-sensitive applications Smart handheld devices Computer security Input devices Pedometers and activity monitors Game controllers Toys and entertainment products
The ADXL311 will measure acceleration with a full-scale range of 2 g. The ADXL311 can measure both dynamic acceleration (e.g., vibration) and static acceleration (e.g., gravity). The outputs are analog voltages proportional to acceleration. The typical noise floor is 300 g/Hz allowing signals below 2 mg (0.1 of inclination) to be resolved in tilt sensing applications using narrow bandwidths (10 Hz). The user selects the bandwidth of the accelerometer using capacitors CX and CY at the XFILT and YFILT pins. Bandwidths of 1 Hz to 2 kHz may be selected to suit the application. The ADXL311 is available in a 5 mm x 5 mm x 2 mm 8-terminal hermetic CLCC package
3.0V VDD
CX XOUT SELF TEST
X SENSOR DEMOD OSCILLATOR
RFILT 32k
CDC
ADXL311JE
DEMOD
BIAS 200k
Y SENSOR
32k RFILT
COM
YOUT CY
Figure 1. Functional Block Diagram
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 companies.
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: 781.329.4700 www.analog.com Fax: 781.326.8703 (c) 2003 Analog Devices, Inc. All rights reserved.
ADXL311
TABLE OF CONTENTS
Specifications..................................................................................... 3 Absolute Maximum Ratings............................................................ 4 Typical Performance Characteristics ............................................. 5 Theory of Operation ........................................................................ 7 Applications................................................................................... 7 Design Trade-Offs for Selecting Filter Characteristics: The Noise/BW Trade-Off.................................................................... 7 Using the ADXL311 as a Dual-Axis Tilt Sensor....................... 8 Pin Configuration and Functional Descriptions...........................9 Outline Dimensions ....................................................................... 10 Ordering Guide .......................................................................... 10
REVISION HISTORY
7/03--Data sheet changed from Rev. 0 to Rev. A. Change to OUTLINE DIMENSIONS.......................................... 10 Revision 0: Initial Version
Rev. A | Page 2 of 12
ADXL311
SPECIFICATIONS
Table 1. TA = 25oC, VDD = 3 V, RBIAS = 125 k, Acceleration = 0 g, unless otherwise noted.)
Parameter SENSOR INPUT Measurement Range Nonlinearity Aligment Error1 Aligment Error Cross Axis Sensitivity2 SENSITIVITY Sensitivity at XFILT, YFILT Sensitivity Change due to Temperature3 ZERO g BIAS LEVEL 0 g Voltage XFILT, YFILT 0 g Offset vs. Temperature NOISE PERFORMANCE Noise Density FREQUENCY RESPONSE 3 dB Bandwidth Sensor Resonant Frequency FILTER RFILT Tolerance Minimum Capacitance SELF TEST XFILT, YFILT POWER SUPPLY Operating Voltage Range Quiescent Supply Current Turn-On Time TEMPERATURE RANGE Operating Range Conditions Each Axis Best Fit Straight Line X Sensor to Y Sensor Each Axis VDD = 3 V Delta from 25C Each Axis VDD = 3 V Delta from 25C @25C At Pins XFILT, YFILT Min Typ 2 0.2 1 0.01 2 140 167 -0.025 1.5 2.0 300 6 10 15 1000 45 2.7 0.4 160 x CFILT + 0.3 0 5.25 1.0 195 Max Units g % of FS Degrees Degrees % mV/g %/C V mg/C g/Hz RMS kHz kHz % pF mV V mA ms C
1.2
1.8
32 k Nominal At Pins XFILT, YFILT Self Test 0 to 1
70
1 2
Alignment error is specified as the angle between the true and indicated axis of sensitivity (Figure 1). Cross axis sensitivity is the algebraic sum of the alignment and the inherent sensitivity errors. 3 Defined as the output change from ambient to maximum temperature or ambient to minimum temperature.
Rev. A | Page 3 of 12
ADXL311
ABSOLUTE MAXIMUM RATINGS
Table 2.
Parameter Acceleration (Any Axis, Unpowered) Acceleration (Any Axis, Powered, VDD = 3 V) VDD Output Short-Circuit Duration, (Any Pin to Commom) Operating Temperature Range Storage Temperature Rating 3,500 g, 0.5 ms 3,500 g, 0.5 ms -0.3 V to +0.6 V Indefinite -55C to +125C -65C to +150C
Table 3. Package Characteristics
Package Type 8-Lead CLCC JA 120C/W JC TBDC/W Device Weight <1.0 gram
Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only and 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.
Rev. A | Page 4 of 12
ADXL311
TYPICAL PERFORMANCE CHARACTERISTICS
10 9 8
PERCENT OF PARTS
14 12 10 8 6 4 2 0 0.153
6 5 4 3 2 1 0 1.33 1.37 1.41 1.45 1.49 V 1.53 1.57 1.61
PERCENT OF PARTS
7
0.157
0.161
0.165 V/g
0.169
0.173
0.177
0.181
Figure 2. X-Axis Zero g BIAS Output Distribution
9 8 7
PERCENT OF PARTS SENSITIVITY - %
110 108 106 104 102 100 98 96 94 92 90
Figure 5. Y-Axis Sensitivity Distribution at YOUT
6
5
4 3 2 1 0 1.33
1.37
1.41
1.45
1.49 V
1.53
1.57
1.61
0
10
20
30
40
50
60
70
80
TEMPERATURE - C
Figure 3. Y-Axis Zero g BIAS Output Distribution
14 12 10 8 6 4 2 0 0.156
30
Figure 6. Normalized Sensitivity vs. Temperature
25
PERCENT OF PARTS
PERCENT OF PARTS
20
15
10
5
0.16
0.164
0.168 V/g
0.172
0.176
0.18
0 150
200
250
300
350
400
450
500
550
NOISE DENSITY - g/Hz
Figure 4. X-Axis Output Sensitivity Distribution at XOUT
Figure 7. Noise Density Distribution
Rev. A | Page 5 of 12
ADXL311
0.45 0.4 0.35
3
VDD
CURRENT - mA
0.3 0.25 0.2
2
XOUT
CFILT = 0.01 F
V
1 0
0.15 0.1 0.05 0
0
10
20
30
40
50
60
70
80
0
0.4
0.8 TIME - ms
1.2
1.4
TEMPERATURE - C
Figure 8. Typical Supply Current vs. Temperature
Figure 9. Typical Turn-On Time
Rev. A | Page 6 of 12
ADXL311
THEORY OF OPERATION
The ADXL311 is a complete, dual-axis acceleration measurement system on a single monolithic IC. It contains a polysilicon surface-micromachined sensor and signal conditioning circuitry to implement an open-loop acceleration measurement architecture. The output signals are analog voltage proportional to acceleration. The ADXL311 is capable of measuring both positive and negative accelerations to at least 2 g. The accelerometer can measure static acceleration forces, such as gravity, allowing it to be used as a tilt sensor. The sensor is a surface-micromachined polysilicon structure built on top of the silicon wafer. Polysilicon springs suspend the structure over the surface of the wafer and provide a resistance against acceleration forces. Deflection of the structure is measured using a differential capacitor that consists of independent fixed plates and central plates attached to the moving mass. The fixed plates are driven by 180 out of phase square waves. Acceleration will deflect the beam and unbalance the differential capacitor, resulting in an output square wave whose amplitude is proportional to acceleration. Phase sensitive demodulation techniques are then used to rectify the signal and determine the direction of the acceleration. The output of the demodulator is amplified and brought offchip through a 32 k resistor. At this point, the user can set the signal bandwidth of the device by adding a capacitor. This filtering improves measurement resolution and helps prevent aliasing.
F-3dB = 5 F / C( X,Y )
The tolerance of the internal resistor (RFILT) can vary typically as much as 15% of its nominal value of 32 k; thus, the bandwidth will vary accordingly. A minimum capacitance of 1000 pF for CX and CY is required in all cases.
Table 4. Filter Capacitor Selection, CX and CY
Bandwidth 10 Hz 50 Hz 100 Hz 200 Hz 500 Hz 5 kHz Capacitor (F) 0.47 0.10 0.05 0.027 0.01 0.001
SELF TEST
The ST pin controls the self-test feature. When this pin is set to VDD, an electrostatic force is exerted on the beam of the accelerometer. The resulting movement of the beam allows the user to test if the accelerometer is functional. The typical change in output will be 270 mg (corresponding to 45 mV). This pin may be left open circuit or connected to common in normal use.
RBIAS SELECTION
A bias resistor (RBIAS) must always be used. If no resistor is present, the ADXL311 may appear to work but will suffer degraded noise performance. The value of the resistor used is not critical. Any value from 50 k to 2 M can be used. Using a 2 M resistor rather than a 50 k will save roughly 25 A of supply current.
Applications
POWER SUPPLY DECOUPLING
For most applications, a single 0.1 F capacitor, CDC, will adequately decouple the accelerometer from noise on the power supply. However, in some cases, particularly where noise is present at the 100 kHz internal clock frequency (or any harmonic thereof), noise on the supply may cause interference on the ADXL311 output. If additional decoupling is needed, a 100 (or smaller) resistor or ferrite beads may be inserted in the supply line of the ADXL311. Additionally, a larger bulk bypass capacitor (in the 1 F to 4.7 F range) may be added in parallel to CDC.
Design Trade-Offs for Selecting Filter Characteristics: The Noise/BW Trade-Off
The accelerometer bandwidth selected will ultimately determine the measurement resolution (smallest detectable acceleration). Filtering can be used to lower the noise floor, which improves the resolution of the accelerometer. Resolution is dependent on the analog filter bandwidth at XOUT and YOUT. The output of the ADXL311 has a typical bandwidth of 5 kHz. The user must filter the signal at this point to limit aliasing errors. The analog bandwidth must be no more than half the A/D sampling frequency to minimize aliasing. The analog bandwidth may be further decreased to reduce noise and improve resolution. The ADXL311 noise has the characteristics of white Gaussian noise that contributes equally at all frequencies and is described in terms of g/Hz, i.e., the noise is proportional to the square
SETTING THE BANDWIDTH USING CX AND CY
The ADXL311 has provisions for bandlimiting the XOUT and YOUT pins. Capacitors must be added at these pins to implement low-pass filtering for antialiasing and noise reduction. The equation for the 3 dB bandwidth is
F-3dB = 1/ 2(32 k )x C( X,Y )
or, more simply
(
)
Rev. A | Page 7 of 12
ADXL311
root of the bandwidth of the accelerometer. It is recommended that the user limit bandwidth to the lowest frequency needed by the application, to maximize the resolution and dynamic range of the accelerometer. With the single pole roll-off characteristic, the typical noise of the ADXL202E is determined by The zero g bias output is also ratiometric, so the zero g output is nominally equal to VDD/2 at all supply voltages. The output noise is not ratiometric but absolute in volts; therefore, the noise density decreases as the supply voltage increases. This is because the scale factor (mV/g) increases while the noise voltage remains constant. The self-test response is roughly proportional to the square of the supply voltage. At VDD = 5 V, the self-test response will be approximately equivalent to 800 mg (typical). The supply current increases as the supply voltage increases. Typical current consumption at VDD = 5 V is 600 A.
RMS NOISE = 300 g / Hz x
At 100 Hz the noise will be
(
)(
BW x 1.6
)
RMS NOISE = 300g / Hz x 100 x 1.6 = 3.8 mg
Often the peak value of the noise is desired. Peak-to-peak noise can only be estimated by statistical methods. Table 5 is useful for estimating the probabilities of exceeding various peak values, given the rms value.
(
)(
)
Using the ADXL311 as a Dual-Axis Tilt Sensor
One of the most popular applications of the ADXL311 is tilt measurement. An accelerometer uses the force of gravity as an input vector to determine the orientation of an object in space. An accelerometer is most sensitive to tilt when its sensitive axis is perpendicular to the force of gravity, i.e., parallel to the earth's surface. At this orientation, its sensitivity to changes in tilt is highest. When the accelerometer is oriented on axis to gravity, i.e., near its +1 g or -1 g reading, the change in output acceleration per degree of tilt is negligible. When the accelerometer is perpendicular to gravity, its output will change nearly 17.5 mg per degree of tilt, but at 45 degrees, it is changing only at 12.2 mg per degree and resolution declines.
Table 5. Estimation of Peak-to-Peak Noise
Peak-to-Peak Value 2 x RMS 4 x RMS 6 x RMS 8 x RMS % of Time That Noise Will Exceed Nominal Peak-to-Peak Value 32 4.6 0.27 0.006
The peak-to-peak noise value will give the best estimate of the uncertainty in a single measurement. Table 6 gives the typical noise output of the ADXL311 for various CX and CY values.
Table 6. Filter Capacitor Selection (CX, CY)
Bandwidth (Hz) 10 50 100 500 CX, CY (F) 0.47 0.1 0.047 0.01 RMS Noise (mg) 1.2 2.7 3.8 8.5 Peak-to-Peak Noise Estimate (mg) 7.2 16.2 22.8 51
DUAL-AXIS TILT SENSOR: CONVERTING ACCELERATION TO TILT
When the accelerometer is oriented so both its X-axis and Y-axis are parallel to the earth's surface, it can be used as a two axis tilt sensor with a roll axis and a pitch axis. Once the output signal from the accelerometer has been converted to an acceleration that varies between -1 g and +1 g, the output tilt in degrees is calculated as follows:
USING THE ADXL311 WITH OPERATING VOLTAGES OTHER THAN 3 V
The ADXL311 is tested and specified at VDD = 3 V; however, it can be powered with VDD as low as 2.7 V or as high as 5.25 V. Some performance parameters will change as the supply voltage is varied. The ADXL311 output is ratiometric, so the output sensitivity (or scale factor) will vary proportionally to supply voltage. At VDD = 5 V the output sensitivity is typically 312 mV/g.
PITCH = ASIN(AX /1 g ) ROLL = ASIN(AY /1 g )
Be sure to account for overranges. It is possible for the accelerometers to output a signal greater than 1 g due to vibration, shock, or other accelerations.
Rev. A | Page 8 of 12
ADXL311
PIN CONFIGURATION AND FUNCTIONAL DESCRIPTIONS
VDD
8
XOUT YOUT NC
7 6 5 4
1
ST BIAS COM
ADXL311
BOTTOM VIEW
2 3
NC
Figure 10. 8-Lead CLCC
Table 7. Pin Function Descriptions--8-Lead CLCC
Pin No. 1 2 3 4 5 6 7 8 Mnemonic ST BIAS COM NC NC YOUT XOUT VDD Description Self Test Bias Resistor (200 k) Common Do Not Connect Do Not Connect Y Channel Output X Channel Output 2.7 V to 5.25 V
Rev. A | Page 9 of 12
ADXL311
OUTLINE DIMENSIONS
5.00 SQ
1.78 1.27
1.27
7
0.50 DIAMETER
1
1.90
4.50 SQ
2.50
TOP VIEW
1.27 R 0.20 0.20
5
3
0.64 2.50
R 0.20
0.38 DIAMETER
BOTTOM VIEW
Figure 11. 8-Terminal Ceramic Leadless Chip Carrier [CLCC] (E-8) Dimensions shown in millimeters
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.
Ordering Guide
ADXL311Products ADXL311JE ADXL311JE-REEL ADXL311EB Evaluation Board Number of Axes 2 2 Specified Voltage 3V 3V Temperature Range 0C to 70C 0C to 70C
Rev. A | Page 10 of 12
ADXL311
NOTES
Rev. A | Page 11 of 12
ADXL311
NOTES
(c) 2003 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective companies. C03582-0-7/03(A)
Rev. A | Page 12 of 12

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