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Small and Thin 18 g Accelerometer ADXL321
FEATURES
Small and thin 4 mm x 4 mm x 1.45 mm LFCSP package 3 mg resolution at 50 Hz Wide supply voltage range: 2.4 V to 6 V Low power: 350 A at VS = 2.4 V (typ) Good zero g bias stability Good sensitivity accuracy X-axis and Y-axis aligned to within 0.1 (typ) BW adjustment with a single capacitor Single-supply operation 10,000 g shock survival Compatible with Sn/Pb and Pb-free solder processes
GENERAL DESCRIPTION
The ADXL321 is a small and thin, low power, complete dualaxis accelerometer with signal conditioned voltage outputs, which is all on a single monolithic IC. The product measures acceleration with a full-scale range of 18 g (typical). It can also measure both dynamic acceleration (vibration) and static acceleration (gravity). The ADXL321's typical noise floor is 320 g/Hz, allowing signals below 3 mg to be resolved in tilt-sensing applications using narrow bandwidths (<50 Hz). The user selects the bandwidth of the accelerometer using capacitors CX and CY at the XOUT and YOUT pins. Bandwidths of 0.5 Hz to 2.5 kHz may be selected to suit the application. The ADXL321 is available in a very thin 4 mm x 4 mm x 1.45 mm, 16-lead, plastic LFCSP.
APPLICATIONS
Vibration monitoring and compensation Abuse event detection Sports equipment
FUNCTIONAL BLOCK DIAGRAM
+3V
VS
ADXL321
CDC
AC AMP SENSOR
DEMOD
OUTPUT AMP
OUTPUT AMP
RFILT 32k
RFILT 32k
COM
ST
Figure 1.
Rev. 0
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: 781.326.8703 (c) 2004 Analog Devices, Inc. All rights reserved.
05291-001
YOUT CY
XOUT CX
ADXL321 TABLE OF CONTENTS
Specifications..................................................................................... 3 Absolute Maximum Ratings............................................................ 4 ESD Caution.................................................................................. 4 Pin Configuration and Function Descriptions............................. 5 Typical Performance Characteristics (VS = 3.0 V) ....................... 7 Theory of Operation ...................................................................... 11 Performance ................................................................................ 11 Applications..................................................................................... 12 Power Supply Decoupling ......................................................... 12 Setting the Bandwidth Using CX and CY ................................. 12 Self-Test ....................................................................................... 12 Design Trade-Offs for Selecting Filter Characteristics: The Noise/BW Trade-Off.................................................................. 12 Use with Operating Voltages Other than 3 V ............................. 13 Use as a Dual-Axis Tilt Sensor ................................................. 13 Outline Dimensions ....................................................................... 14 Ordering Guide .......................................................................... 14
REVISION HISTORY
12/04--Revision 0: Initial Version
Rev. 0 | Page 2 of 16
ADXL321 SPECIFICATIONS1
TA = 25C, VS = 3 V, CX = CY = 0.1 F, Acceleration = 0 g, unless otherwise noted. Table 1.
Parameter SENSOR INPUT Measurement Range Nonlinearity Package Alignment Error Alignment Error Cross Axis Sensitivity SENSITIVITY (RATIOMETRIC)2 Sensitivity at XOUT, YOUT Sensitivity Change due to Temperature3 ZERO g BIAS LEVEL (RATIOMETRIC) 0 g Voltage at XOUT, YOUT 0 g Offset vs. Temperature NOISE PERFORMANCE Noise Density FREQUENCY RESPONSE4 CX, CY Range5 RFILT Tolerance Sensor Resonant Frequency SELF-TEST6 Logic Input Low Logic Input High ST Input Resistance to Ground Output Change at XOUT, YOUT OUTPUT AMPLIFIER Output Swing Low Output Swing High POWER SUPPLY Operating Voltage Range Quiescent Supply Current Turn-On Time7 TEMPERATURE Operating Temperature Range Conditions Each axis % of full scale X sensor to Y sensor Each axis VS = 3 V VS = 3 V Each axis VS = 3 V Min Typ 18 0.2 1 0.1 2 51 57 0.01 1.5 2 320 0.002 32 15% 5.5 0.6 2.4 50 18 0.3 2.6 2.4 0.49 20 -20 +70 6 10 63 Max Unit g % Degrees Degrees % mV/g %/C V mg/C g/Hz rms F k kHz V V k mV V V V mA ms C
1.4
1.6
@ 25C
Self-test 0 to 1 No load No load
1 2
All minimum and maximum specifications are guaranteed. Typical specifications are not guaranteed. Sensitivity is essentially ratiometric to VS. 3 Defined as the change from ambient-to-maximum temperature or ambient-to-minimum temperature. 4 Actual frequency response controlled by user-supplied external capacitor (CX, CY). 5 Bandwidth = 1/(2 x x 32 k x C). For CX, CY = 0.002 F, bandwidth = 2500 Hz. For CX, CY = 10 F, bandwidth = 0.5 Hz. Minimum/maximum values are not tested. 6 Self-test response changes cubically with VS. 7 Larger values of CX, CY increase turn-on time. Turn-on time is approximately 160 x CX or CY + 4 ms, where CX, CY are in F.
Rev. 0 | Page 3 of 16
ADXL321 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 10,000 g 10,000 g -0.3 V to +7.0 V (COM - 0.3 V) to (VS + 0.3 V) Indefinite -55C to +125C -65C to +150C
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
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. 0 | Page 4 of 16
ADXL321 PIN CONFIGURATION AND FUNCTION DESCRIPTIONS
NC
16
VS
15
VS
14
NC
13 12
NC 1
ST 2
COM 3
NC 4
5 6 7 8
XOUT
NC
YOUT
NC
ADXL321
TOP VIEW (Not to Scale)
11
10
9
COM COM COM
NC = NO CONNECT
NC
Figure 2. Pin Configuration
Table 3. Pin Function Descriptions
Pin No. 1, 4, 8, 9, 11, 13, 16 2 3, 5 to 7 10 12 14, 15 Mnemonic NC ST COM YOUT XOUT VS Description Do Not Connect Self-Test Common Y Channel Output X Channel Output 2.4 V to 6 V
Rev. 0 | Page 5 of 16
05291-002
ADXL321
CRITICAL ZONE TL TO TP
TP RAMP-UP
tP
TEMPERATURE
TL
TSMAX TSMIN
tL
tS
PREHEAT
RAMP-DOWN
05291-003
t25C 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) Minimum 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 5C of Actual Peak Temperature (tP) Ramp-Down Rate Time 25C to Peak Temperature Sn63/Pb37 3C/s max 100C 150C 60 s - 120 s 3C/s 183C 60 s - 150 s 240C + 0C/-5C 10 s - 30 s 6C/s max 6 min max Pb-Free 3C/s max 150C 200C 60 s - 150 s 3C/s 217C 60 s - 150 s 260C + 0C/-5C 20 s - 40 s 6C/s max 8 min max
Rev. 0 | Page 6 of 16
ADXL321 TYPICAL PERFORMANCE CHARACTERISTICS (VS = 3.0 V)
25
25
20
20
% OF POPULATION
15
% OF POPULATION
05291-004
15
10
10
5
5
05291-007
0 1.40 1.42 1.44 1.46 1.48 1.50 1.52 1.54 1.56 1.58 1.60 VOLTS
0 1.40 1.42 1.44 1.46 1.48 1.50 1.52 1.54 1.56 1.58 1.60 VOLTS
Figure 4. X-Axis Zero g Bias at 25C
25 25
Figure 7. Y-Axis Zero g Bias at 25C
20
20
% OF POPULATION
15
% OF POPULATION
05291-005
15
10
10
5
5
05291-008
0 -10 -8 -6 -4 -2 0 mg/C 2 4 6 8 10
0 -10 -8 -6 -4 -2 0 mg/C 2 4 6 8 10
Figure 5. X-Axis Zero g Bias Temperature Coefficient
50 45 40 50 45 40
Figure 8. Y-Axis Zero g Bias Temperature Coefficient
% OF POPULATION
30 25 20 15 10
05291-006
% OF POPULATION
35
35 30 25 20 15 10 5 0 52 53 54 55 56 mV/g 57 58 59 60
05291-009
5 0 52 53 54 55 56 mV/g 57 58 59 60
Figure 6. X-Axis Sensitivity at 25C
Figure 9. Y-Axis Sensitivity at 25C
Rev. 0 | Page 7 of 16
ADXL321
1.600 1.575
OUTPUT (SCALE = 56mV/g)
0.060 0.059 0.058
SENSITIVITY (mV/g)
05291-010
1.550 1.525 1.500 1.475 1.450 1.425 1.400 -30
0.057 0.056 0.055 0.054
05291-013
0.053 0.052 -30
-20
-10
0
10
20
30
40
50
60
70
80
-10
10
30
50
70
TEMPERATURE (C)
TEMPERATURE (C)
Figure 10. Zero g Bias vs. Temperature--Parts Soldered to PCB
30
Figure 13. Sensitivity vs. Temperature--Parts Soldered to PCB
30
25
25
% OF POPULATION
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% OF POPULATION
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20
15
15
10
10
5
5
05291-014
0 200 220 240 260 280 300 320 340 NOISE DENSITY (g/ Hz)
0 200 220 240 260 280 300 320 340 NOISE DENSITY (g/ Hz)
Figure 11. X-Axis Noise Density at 25C
25 30
Figure 14. Y-Axis Noise Density at 25C
20
25
% OF POPULATION
% OF POPULATION
05291-012
20
15
15
10
10
5
5
05291-015
0 -5 -4 -3 -2 -1 0 1 2 3 4 5 PERCENT SENSITIVITY (%)
0 -5 -4 -3 -2 -1 0 1 2 3 4 5 PERCENT SENSITIVITY (%)
Figure 12. Z vs. X Cross-Axis Sensitivity
Figure 15. Z vs. Y Cross-Axis Sensitivity
Rev. 0 | Page 8 of 16
ADXL321
16 14 12
% OF POPULATION
10 8 6 4
05291-016
2 0 10 11 12 13 14 15 16 mV 17 18 19 20 21
Figure 16. X-Axis Self-Test Response at 25C
40 35 30
Figure 19. Turn-On Time--CX, CY = 0.1 F, Time Scale = 2 ms/DIV
% OF POPULATION
25 20 15 10
05291-017
5 0 420 430 440 450 460 470 480 490 500 510 520 530 CURRENT (A)
Figure 17. Supply Current at 25C
16 14 12
% OF POPULATION
10 8 6 4
05291-018
2 0 10 11 12 13 14 15 16 mV 17 18 19 20 21
Figure 18. Y-Axis Self-Test Response at 25C
Rev. 0 | Page 9 of 16
05291-019
ADXL321
XL 321J #1234 5678P
XOUT = 1.443V YOUT = 1.500V
XL 321J #1234 5678P
XL 321J #1234 5678P
XOUT = 1.500V YOUT = 1.557V
XOUT = 1.500V YOUT = 1.443V
EARTH'S SURFACE
Figure 20. Output Response vs. Orientation (Top View)
Rev. 0 | Page 10 of 16
05291-020
XL 321J #1234 5678P
XOUT = 1.557V YOUT = 1.50V XOUT = 1.500V YOUT = 1.500V
ADXL321 THEORY OF OPERATION
The ADXL321 is a complete acceleration measurement system on a single monolithic IC. The ADXL321 has a measurement range of 18 g. It contains a polysilicon surface-micromachined sensor and signal conditioning circuitry to implement an openloop acceleration measurement architecture. The output signals are analog voltages that are proportional to acceleration. The accelerometer measures static acceleration forces, such as gravity, which allows it to be used as a tilt sensor. The sensor is a polysilicon surface-micromachined structure built on top of a 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 plates attached to the moving mass. The fixed plates are driven by 180 out-of-phase square waves. Acceleration deflects the beam and unbalances 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 demodulator's output is amplified and brought off-chip through a 32 k resistor. The user then sets the signal bandwidth of the device by adding a capacitor. This filtering improves measurement resolution and helps prevent aliasing.
PERFORMANCE
Rather than using additional temperature compensation circuitry, innovative design techniques have been used to ensure high performance is built-in. As a result, there is neither quantization error nor nonmonotonic behavior, and temperature hysteresis is very low (typically less than 10 mg over the -20C to +70C temperature range). Figure 10 shows the zero g output performance of eight parts (X- and Y-axis) over a -20C to +70C temperature range. Figure 13 demonstrates the typical sensitivity shift over temperature for supply voltages of 3 V. This is typically better than 1% over the -20C to +70C temperature range.
Rev. 0 | Page 11 of 16
ADXL321 APPLICATIONS
POWER SUPPLY DECOUPLING
For most applications, a single 0.1 F capacitor, CDC, adequately decouples the accelerometer from noise on the power supply. However, in some cases, particularly where noise is present at the 140 kHz internal clock frequency (or any harmonic thereof), noise on the supply may cause interference on the ADXL321 output. If additional decoupling is needed, a 100 (or smaller) resistor or ferrite bead may be inserted in the supply line. 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 ultimately determines 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 ADXL321 has a typical bandwidth of 2.5 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 ADXL321 noise has the characteristics of white Gaussian noise, which contributes equally at all frequencies and is described in terms of g/Hz (the noise is proportional to the square root of the accelerometer's bandwidth). The user should limit bandwidth to the lowest frequency needed by the application in order to maximize the resolution and dynamic range of the accelerometer. With the single-pole, roll-off characteristic, the typical noise of the ADXL321 is determined by
SETTING THE BANDWIDTH USING CX AND CY
The ADXL321 has provisions for band-limiting 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-3 dB = 1/(2(32 k) x C(X, Y)) or more simply, F-3 dB = 5 F/C(X, Y) The tolerance of the internal resistor (RFILT) typically varies as much as 15% of its nominal value (32 k), and the bandwidth varies accordingly. A minimum capacitance of 2000 pF for CX and CY is required in all cases. Table 5. Filter Capacitor Selection, CX and CY
Bandwidth (Hz) 1 10 50 100 200 500 Capacitor (F) 4.7 0.47 0.10 0.05 0.027 0.01
rmsNoise = (320 g/ Hz ) x ( BW x1.6 )
At 100 Hz bandwidth the noise will be
rmsNoise = (320 g/ Hz ) x ( 100 x1.6 ) = 4 mg
Often, the peak value of the noise is desired. Peak-to-peak noise can only be estimated by statistical methods. A factor of 6 is generally used to convert rms to peak-to-peak. Table 6 is useful for estimating the probabilities of exceeding various peak values, given the rms value. Table 6. 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 Exceeds Nominal Peak-to-Peak Value 32 4.6 0.27 0.006
SELF-TEST
The ST pin controls the self-test feature. When this pin is set to VS, an electrostatic force is exerted on the accelerometer beam. The resulting movement of the beam allows the user to test if the accelerometer is functional. The typical change in output is 315 mg (corresponding to 18 mV). This pin may be left opencircuit or connected to common (COM) in normal use. 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.
Rev. 0 | Page 12 of 16
ADXL321
Peak-to-peak noise values give the best estimate of the uncertainty in a single measurement. Table 7 gives the typical noise output of the ADXL321 for various CX and CY values. Table 7. 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) 1.3 2.9 4 9.1 Peak-to-Peak Noise Estimate (mg) 7.8 17.4 24 54.6
USE AS A DUAL-AXIS TILT SENSOR
An accelerometer is most sensitive to tilt when its sensitive axis is perpendicular to the force of gravity (that is, when it is 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 (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 changes nearly 17.5 mg per degree of tilt. At 45, its output changes at only 12.2 mg per degree of tilt, and resolution declines.
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 2-axis tilt sensor with both a roll axis and 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 PITCH = arcsine(AX/1 g) ROLL = arcsine(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.
USE WITH OPERATING VOLTAGES OTHER THAN 3 V
The ADXL321 is tested and specified at VS = 3 V; however, it can be powered with VS as low as 2.4 V or as high as 6 V. Note that some performance parameters change as the supply voltage is varied. The ADXL321 output is ratiometric, so the sensitivity (or scale factor) varies proportionally to supply voltage. At VS = 5 V, the sensitivity is typically 100 mV/g. At VS = 2.4 V, the sensitivity is typically 45 mV/g. The zero g bias output is also ratiometric, so the zero g output is nominally equal to VS/2 at all supply voltages. The output noise is not ratiometric but is 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. At VS = 5 V, the noise density is typically 190 g/Hz, while at VS = 2.4 V, the noise density is typically 400 g/Hz, Self-test response in g is roughly proportional to the square of the supply voltage. However, when ratiometricity of sensitivity is factored in with supply voltage, the self-test response in volts is roughly proportional to the cube of the supply voltage. For example, at VS = 5 V, the self-test response for the ADXL321 is approximately 80 mV. At VS = 2.4 V, the self-test response is approximately 8 mV. The supply current decreases as the supply voltage decreases. Typical current consumption at VS = 5 V is 750 A, and typical current consumption at VS = 2.4 V is 350 A.
Rev. 0 | Page 13 of 16
ADXL321 OUTLINE DIMENSIONS
0.20 MIN 0.20 MIN
13 16 1
PIN 1 INDICATOR 2.43 1.75 SQ 1.08
PIN 1 INDICATOR
TOP VIEW
4.15 4.00 SQ 3.85 0.65 BSC
12
BO TTOM VIEW
9 8 5 4
0.55 0.50 0.45 1.50 1.45 1.40 SEATING PLANE 0.05 MAX 0.02 NOM 0.35 0.30 0.25 COPLANARITY 0.05
1.95 BSC
Figure 21. 16-Lead Lead Frame Chip Scale Package [MQ_LFCSP] 4 mm x 4 mm Body, Thick Quad (CP-16-5) Dimensions shown in millimeters (Drawing Not to Scale)
ORDERING GUIDE
Model ADXL321JCP1 ADXL321JCP-REEL1 ADXL321EB Measurement Range 18 g 18 g Specified Voltage (V) 3 3 Temperature Range -20C to +70C -20C to +70C Package Description 16-Lead LFCSP 16-Lead LFCSP Evaluation Board Package Option CP-16-5 CP-16-5
1
Lead finish--Matte tin.
Rev. 0 | Page 14 of 16
ADXL321 NOTES
Rev. 0 | Page 15 of 16
ADXL321 NOTES
(c) 2004 Analog Devices, Inc. All rights reserved. Trademarks and registered trademarks are the property of their respective owners. D05291-0-12/04(0)
Rev. 0 | Page 16 of 16

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