2-Axis a
FEATURES Acceleration Sensor on a Single IC Chip Measures Static Acceleration as Well as Dynamic Acceleration Duty Cycle Output with User Adjustable Period Low Power <0.6 mA Faster Response than Electrolytic, Mercury or Thermal Tilt Sensors Bandwidth Adjustment with a Single Capacitor Per Axis 5 m g Resolution at 60 Hz Bandwidth +3 V to +5.25 V Single Supply Operation 1000 g Shock Survival APPLICATIONS 2-Axis Tilt Sensing Computer Peripherals Inertial Navigation Seismic Monitoring Vehicle Security Systems Battery Powered Motion Sensing
Low Cost 2 g/ 10 g Dual Axis iMEMS Accelerometers with Digital Output ADXL202/ADXL210
GENERAL DESCRIPTION
The ADXL202/ADXL210 are low cost, low power, complete 2-axis accelerometers with a measurement range of either 2 g/ 10 g. The ADXL202/ADXL210 can measure both dynamic acceleration (e.g., vibration) and static acceleration (e.g., gravity). The outputs are digital signals whose duty cycles (ratio of pulsewidth to period) are proportional to the acceleration in each of the 2 sensitive axes. These outputs may be measured directly with a microprocessor counter, requiring no A/D converter or glue logic. The output period is adjustable from 0.5 ms to 10 ms via a single resistor (RSET ). If a voltage output is desired, a voltage output proportional to acceleration is available from the XFILT and YFILT pins, or may be reconstructed by filtering the duty cycle outputs. The bandwidth of the ADXL202/ADXL210 may be set from 0.01 Hz to 5 kHz via capacitors CX and C Y. The typical noise floor is 500 g/Hz allowing signals below 5 mg to be resolved for bandwidths below 60 Hz. The ADXL202/ADXL210 is available in a hermetic 14-lead Surface Mount CERPAK, specified over the 0C to +70C commercial or 40C to +85C industrial temperature range.
FUNCTIONAL BLOCK DIAGRAM
+3.0V TO +5.25V CX VDD VDD XFILT SELF TEST
X SENSOR CDC DEMOD
RFILT 32k
ADXL202/ ADXL210
X OUT DUTY CYCLE MODULATOR (DCM) C O U N T E R
OSCILLATOR RFILT 32k DEMOD Y SENSOR
P
Y OUT
COM
YFILT CY
T2 RSET
T2
T1 A(g) = (T1/T2 0.5)/12.5% 0g = 50% DUTY CYCLE T2 = RSET/125M
iMEMS is a registered trademark of Analog Devices, Inc.
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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 which may result from its use. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices.
AIN2 =
One Technology Way, P.O. Box 9106, Norwood, MA 02062-9106, U.S.A. Tel: [phone] World Wide Web Site: http://www.analog.com Fax: [phone] Analog Devices, Inc., 1999
, T = +25 for ADXL202/ADXL210SPECIFICATIONS (T ==T125tok T , AccelerationC = 0 JgGrade only, V = +5 V, R , unless otherwise noted)
A MIN MAX A DD SET
Parameter SENSOR INPUT Measurement Range 1 Nonlinearity Alignment Error 2 Alignment Error Transverse Sensitivity 3 SENSITIVITY Duty Cycle per g Sensitivity, Analog Output Temperature Drift 4 ZERO g BIAS LEVEL 0 g Duty Cycle Initial Offset 0 g Duty Cycle vs. Supply 0 g Offset vs. Temperature 4 NOISE PERFORMANCE Noise Density5 FREQUENCY RESPONSE 3 dB Bandwidth 3 dB Bandwidth Sensor Resonant Frequency FILTER RFILT Tolerance Minimum Capacitance SELF TEST Duty Cycle Change DUTY CYCLE OUTPUT STAGE FSET FSET Tolerance Output High Voltage Output Low Voltage T2 Drift vs. Temperature Rise/Fall Time POWER SUPPLY Operating Voltage Range Specified Performance Quiescent Supply Current Turn-On Time6 TEMPERATURE RANGE Operating Range Specified Performance
Conditions Each Axis Best Fit Straight Line X Sensor to Y Sensor Each Axis T1/T2 @ +25C At Pins XFILT, YFILT from +25C Each Axis T1/T2 from +25C @ +25C Duty Cycle Output At Pins XFILT, YFILT
ADXL202/JQC/AQC Min Typ Max 1.5 2 0.2 1 0.01 2 12.5 312 0.5 50 2 1.0 2.0 500 500 5 10 15 1000 10 125 M/RSET 0.7 VS 200 mV 35 200 3.0 4.75 5.25 5.25 1.0 15
ADXL210/JQC/AQC Min Typ Max 8 10 0.2 1 0.01 2 4.0 100 0.5 50 2 1.0 2.0 500 500 5 14 15 1000 10 125 M/RSET 0.7 VS 200 mV 35 200 2.7 4.75 0.6 160 CFILT + 0.3 0 40 5.25 5.25 1.0 4.8
Units g % of FS Degrees Degrees % %/g mV/g % Rdg % g %/V mg/C g/Hz Hz kHz kHz % pF %
10
3.2
25
75 4.0
42
58 4.0
1000
1000
32 k Nominal At XFILT, YFILT Self-Test 0 to 1
RSET = 125 k I = 25 A I = 25 A
1.3 200
1.3 200
kHz mV mV ppm/C ns V V mA ms C C
To 99% JQC AQC 0 40
0.6 160 CFILT + 0.3
+70 +85
+70 +85
NOTES 1 For all combinations of offset and sensitivity variation. 2 Alignment error is specified as the angle between the true and indicated axis of sensitivity. 3 Transverse sensitivity is the algebraic sum of the alignment and the inherent sensitivity errors. 4 Specification refers to the maximum change in parameter from its initial at +25 C to its worst case value at T MIN to T MAX . 5 Noise density (g/Hz) is the average noise at any frequency in the bandwidth of the part. 6 CFILT in F. Addition of filter capacitor will increase turn on time. Please see the Application section on power cycling. All min and max specifications are guaranteed. Typical specifications are not tested or guaranteed. Specifications subject to change without notice.
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ADXL202/ADXL210
ABSOLUTE MAXIMUM RATINGS* PIN CONFIGURATION
Acceleration (Any Axis, Unpowered for 0.5 ms) . . . . . . 1000 g Acceleration (Any Axis, Powered for 0.5 ms) . . . . . . . . . 500 g +VS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 0.3 V to +7.0 V Output Short Circuit Duration (Any Pin to Common) . . . . . . . . . . . . . . . . . . . . . . Indefinite Operating Temperature . . . . . . . . . . . . . . . . . 55C to +125C Storage Temperature . . . . . . . . . . . . . . . . . . . 65C to +150C
*Stresses above those listed under Absolute Maximum Ratings may cause permanent damage to the device. This is a stress rating only; the functional operation of the device at these or any other conditions above those indicated in the operational sections of this specification is not implied. Exposure to absolute maximum rating conditions for extended periods may affect device reliability.
NC 1 VTP 2 ST 3
14 VDD
12 XFILT TOP VIEW COM 4 (Not to Scale) 11 YFILT T2 5 NC 6 COM 7 AY AX 10 XOUT 9 YOUT 8 NC
ADXL202/ ADXL210
13 VDD
NC = NO CONNECT
Drops onto hard surfaces can cause shocks of greater than 1000 g and exceed the absolute maximum rating of the device. Care should be exercised in handling to avoid damage.
PIN FUNCTION DESCRIPTIONS
Figure 1 shows the response of the ADXL202 to the Earths gravitational field. The output values shown are nominal. They are presented to show the user what type of response to expect from each of the output pins due to changes in orientation with respect to the Earth. The ADXL210 reacts similarly with output changes appropriate to its scale.
TYPICAL OUTPUT AT PIN: 9 = 50% DUTY CYCLE 10 = 62.5% DUTY CYCLE 11 = 2.5V 12 = 2.188V
Pin 1 2 3 4 5 6 7 8 9 10 11 12 13 14
Name NC VTP ST COM T2 NC COM NC YOUT XOUT YFILT XFILT VDD VDD
Description No Connect Test Point, Do Not Connect Self Test Common Connect RSET to Set T2 Period No Connect Common No Connect Y Axis Duty Cycle Output X Axis Duty Cycle Output Connect Capacitor for Y Filter Connect Capacitor for X Filter +3 V to +5.25 V, Connect to 14 +3 V to +5.25 V, Connect to 13
TYPICAL OUTPUT AT PIN: 9 = 37.5% DUTY CYCLE 10 = 50% DUTY CYCLE 11 = 2.812V 12 = 2.5V
TYPICAL OUTPUT AT PIN: 9 = 62.5% DUTY CYCLE 10 = 50% DUTY CYCLE 11 = 2.188V 12 = 2.5V
1g TYPICAL OUTPUT AT PIN: 9 = 50% DUTY CYCLE 10 = 37.5% DUTY CYCLE 11 = 2.5V 12 = 2.812V EARTH'S SURFACE
PACKAGE CHARACTERISTICS
Package 14-Lead CERPAK
JA
JC
Device Weight 5 Grams
110C/W
30C/W
Figure 1. ADXL202/ADXL210 Nominal Response Due to Gravity
ORDERING GUIDE
Model ADXL202JQC ADXL202AQC ADXL210JQC ADXL210AQC g Range 2 2 10 10 Temperature Range 0C to +70C 40C to +85C 0C to +70C 40C to +85C Package Description 14-Lead CERPAK 14-Lead CERPAK 14-Lead CERPAK 14-Lead CERPAK Package Option QC-14 QC-14 QC-14 QC-14
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 the ADXL202/ADXL210 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.
WARNING!
ESD SENSITIVE DEVICE
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ADXL202/ADXL210 TYPICAL CHARACTERISTICS
1.06 PERIOD NORMALIZED TO 1 AT 25 C 1.04 CHANGE IN SENSITIVITY 30 15 0 15 30 45 TEMPERATURE C 60 75 90
(@ +25 C RSET = 125 k , VDD = +5 V, unless otherwise noted)
4% 3% 2% 1% 0% 1% 2% 3%
1.02
1.00
0.98
0.96
0.94 45
4% 40
25 TEMPERATURE C
85
Figure 2. Normalized DCM Period (T2) vs. Temperature
Figure 5. Typical X Axis Sensitivity Drift Due to Temperature
0.8 0.6 ZERO g OFFSET SHIFT IN g 0.4 0.2 0 0.2 0.4 0.6 0.8 40 30 20 10
0 2 CFILT = 0.01 F 3
VOLTS
1
0
10 20 30 40 50 TEMPERATURE C
60
70
80
90
0
0.4
0.8 FREQUENCY ms
1.2
1.4
Figure 3. Typical Zero g Offset vs. Temperature
Figure 6. Typical Turn-On Time
0.7 0.6 SUPPLY CURRENT mA
20 18 VS = 5 VDC PERCENTAGE OF SAMPLES 100 16 14 12 10 8 6 4 2 20 0 20 40 60 80 0 0.87g 0.64g 0.41g 0.17g 0.06g 0.29g g/DUTY CYCLE OUTPUT 0.52g 0.75g
0.5 VS = 3.5 VDC 0.4 0.3
0.2 0.1 0 40
TEMPERATURE C
Figure 4. Typical Supply Current vs. Temperature
Figure 7. Typical Zero g Distribution at +25C
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ADXL202/ADXL210
9 8 12 PERCENTAGE OF SAMPLES 7 6 5 4 3 2 1 0 11.3 11.5 11.7 11.9 12.2 12.4 12.6 12.8 13.1 13.3 13.5 13.7 DUTY CYCLE OUTPUT % per g TOTAL RMS NOISE mg 10 8 6 CFILT = 0.047 F BW = 100Hz CFILT = 0.1 F BW = 50Hz CFILT = 0.47 F BW = 10Hz 14 CFILT = 0.01 F BW = 500Hz T2 = 1ms
4 2 0
1
4 16 NUMBER OF AVERAGE SAMPLES
64
Figure 8. Typical Sensitivity per g at +25 C
Figure 10. Typical Noise at Digital Outputs
14 12
20 18 16
TOTAL RMS NOISE mg
10 % OF PARTS 8 6
14 12 10 8 6 4
4 2 0 0.01 F 500Hz
2 0.0125 0.0125 0.625 1.375 1.125 0.875 0.625 0.375 0.375 0.875 1.125 1.375 0 0.047 F 100Hz 0.1 F 50Hz 0.47 F 10Hz
CX, CY BANDWIDTH
DEGREES OF MISALIGNMENT
Figure 9. Typical Noise at XFILT Output
Figure 11. Rotational Die Alignment
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ADXL202/ADXL210
DEFINITIONS
APPLICATIONS
POWER SUPPLY DECOUPLING
T1 Length of the on portion of the cycle. T2 Length of the total cycle. Duty Cycle Ratio of the on time (T1) of the cycle to the total cycle (T2). Defined as T1/T2 for the ADXL202/ ADXL210. Pulsewidth Time period of the on pulse. Defined as T1 for the ADXL202/ADXL210.
THEORY OF OPERATION
The ADXL202/ADXL210 are complete dual axis acceleration measurement systems on a single monolithic IC. They contain a polysilicon surface-micromachined sensor and signal conditioning circuitry to implement an open loop acceleration measurement architecture. For each axis, an output circuit converts the analog signal to a duty cycle modulated (DCM) digital signal that can be decoded with a counter/timer port on a microprocessor. The ADXL202/ADXL210 are capable of measuring both positive and negative accelerations to a maximum level of 2 g or 10 g. The accelerometer measures 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. An 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 drives a duty cycle modulator (DCM) stage through a 32 k resistor. At this point a pin is available on each channel to allow the user to set the signal bandwidth of the device by adding a capacitor. This filtering improves measurement resolution and helps prevent aliasing. After being low-pass filtered, the analog signal is converted to a duty cycle modulated signal by the DCM stage. A single resistor sets the period for a complete cycle (T2), which can be set between 0.5 ms and 10 ms (see Figure 12). A 0 g acceleration produces a nominally 50% duty cycle. The acceleration signal can be determined by measuring the length of the T1 and T2 pulses with a counter/timer or with a polling loop using a low cost microcontroller. An analog output voltage can be obtained either by buffering the signal from the XFILT and YFILT pin, or by passing the duty cycle signal through an RC filter to reconstruct the dc value. The ADXL202/ADXL210 will operate with supply voltages as low as 3.0 V or as high as 5.25 V.
T2 T1 A(g) = (T1/T2 0.5)/12.5% 0g = 50% DUTY CYCLE T2(s) = RSET( )/125M
For most applications a single 0.1 F capacitor, CDC , will adequately decouple the accelerometer from signal and noise on the power supply. However, in some cases, especially where digital devices such as microcontrollers share the same power supply, digital noise on the supply may cause interference on the ADXL202/ ADXL210 output. This is often observed as a slowly undulating fluctuation of voltage at XFILT and YFILT. If additional decoupling is needed, a 100 (or smaller) resistor or ferrite beads, may be inserted in the ADXL202/ADXL210s supply line.
DESIGN PROCEDURE FOR THE ADXL202/ADXL210
The design procedure for using the ADXL202/ADXL210 with a duty cycle output involves selecting a duty cycle period and a filter capacitor. A proper design will take into account the application requirements for bandwidth, signal resolution and acquisition time, as discussed in the following sections.
VDD
The ADXL202/ADXL210 have two power supply (VDD) Pins: 13 and 14. These two pins should be connected directly together.
COM
The ADXL202/ADXL210 have two commons, Pins 4 and 7. These two pins should be connected directly together and Pin 7 grounded.
VTP
This pin is to be left open; make no connections of any kind to this pin. A 0.1 F capacitor is recommended from VDD to COM for power supply decoupling.
ST Decoupling Capacitor C DC
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 10% at the duty cycle outputs (corresponding to 800 mg). This pin may be left open circuit or connected to common in normal use.
Duty Cycle Decoding
The ADXL202/ADXL210s digital output is a duty cycle modulator. Acceleration is proportional to the ratio T1/T2. The nominal output of the ADXL202 is: 0 g = 50% Duty Cycle Scale factor is 12.5% Duty Cycle Change per g The nominal output of the ADXL210 is: 0 g = 50% Duty Cycle Scale factor is 4% Duty Cycle Change per g These nominal values are affected by the initial tolerance of the device including zero g offset error and sensitivity error. T2 does not have to be measured for every measurement cycle. It need only be updated to account for changes due to temperature, (a relatively slow process). Since the T2 time period is shared by both X and Y channels, it is necessary only to measure it on one channel of the ADXL202/ADXL210. Decoding algorithms for various microcontrollers have been developed. Consult the appropriate Application 6 Note. REV. B
Figure 12. Typical Output Duty Cycle
ADXL202/ADXL210
+3.0V TO +5.25V CX VDD VDD XFILT SELF TEST
X SENSOR CDC DEMOD
RFILT 32k
ADXL202/ ADXL210
X OUT DUTY CYCLE MODULATOR (DCM) Y OUT C O U N T E R T1 P A(g) = (T1/T2 0.5)/12.5% 0g = 50% DUTY CYCLE T2 = RSET/125M T2
OSCILLATOR RFILT 32k DEMOD Y SENSOR YFILT CY
COM
T2 RSET
Figure 13. Block Diagram
Setting the Bandwidth Using C X and CY Table II. Resistor Values to Set T2
The ADXL202/ADXL210 have provisions for bandlimiting the XFILT and YFILT 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 =
T2 1 ms 2 ms 5 ms 10 ms
RSET 125 k 250 k 625 k 1.25 M
(2 (32 k) C(x, y))
1
or, more simply, F 3 dB
5 F = C(X ,Y )
The tolerance of the internal resistor (RFILT), can vary as much as 25% of its nominal value of 32 k; so the bandwidth will vary accordingly. A minimum capacitance of 1000 pF for C(X, Y) is required in all cases.
Table I. Filter Capacitor Selection, CX and CY
Note that the RSET should always be included, even if only an analog output is desired. Use an RSET value between 500 k and 2 M when taking the output from XFILT or YFILT. The RSET resistor should be place close to the T2 Pin to minimize parasitic capacitance at this node.
Selecting the Right Accelerometer
Bandwidth 10 Hz 50 Hz 100 Hz 200 Hz 500 Hz 5 kHz
Setting the DCM Period with R SET
Capacitor Value 0.47 F 0.10 F 0.05 F 0.027 F 0.01 F 0.001 F
For most tilt sensing applications the ADXL202 is the most appropriate accelerometer. Its higher sensitivity (12.5%/g allows the user to use a lower speed counter for PWM decoding while maintaining high resolution. The ADXL210 should be used in applications where accelerations of greater than 2 g are expected.
MICROCOMPUTER INTERFACES
The ADXL202/ADXL210 were specifically designed to work with low cost microcontrollers. Specific code sets, reference designs, and application notes are available from the factory. This section will outline a general design procedure and discuss the various trade-offs that need to be considered. The designer should have some idea of the required performance of the system in terms of: Resolution: the smallest signal change that needs to be detected. Bandwidth: the highest frequency that needs to be detected. Acquisition Time: the time that will be available to acquire the signal on each axis. These requirements will help to determine the accelerometer bandwidth, the speed of the microcontroller clock and the length of the T2 period. When selecting a microcontroller it is helpful to have a counter timer port available. The microcontroller should have provisions for software calibration. While the ADXL202/ADXL210 are highly accurate accelerometers, they have a wide tolerance for 7
The period of the DCM output is set for both channels by a single resistor from RSET to ground. The equation for the period is: T2 = RSET () 125 M
A 125 k resistor will set the duty cycle repetition rate to approximately 1 kHz, or 1 ms. The device is designed to operate at duty cycle periods between 0.5 ms and 10 ms.
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ADXL202/ADXL210
initial offset. The easiest way to null this offset is with a calibration factor saved on the microcontroller or by a user calibration for zero g. In the case where the offset is calibrated during manufacture, there are several options, including external EEPROM and microcontrollers with one-time programmable features.
DESIGN TRADE-OFFS FOR SELECTING FILTER CHARACTERISTICS: THE NOISE/BW TRADE-OFF
Table IV gives typical noise output of the ADXL202/ADXL210 for various CX and C Y values.
Table IV. Filter Capacitor Selection, C X and CY
Bandwidth 10 Hz 50 Hz 100 Hz 200 Hz 500 Hz
CX , CY 0.47 F 0.10 F 0.05 F 0.027 F 0.01 F
rms Noise 1.9 mg 4.3 mg 6.1 mg 8.7 mg 13.7 mg
Peak-to-Peak Noise Estimate 95% Probability (rms 4) 7.6 mg 17.2 mg 24.4 mg 35.8 mg 54.8 mg
The accelerometer bandwidth selected will determine the measurement resolution (smallest detectable acceleration). Filtering can be used to lower the noise floor and improve the resolution of the accelerometer. Resolution is dependent on both the analog filter bandwidth at XFILT and YFILT and on the speed of the microcontroller counter. The analog output of the ADXL202/ADXL210 has a typical bandwidth of 5 kHz, much higher than the duty cycle stage is capable of converting. The user must filter the signal at this point to limit aliasing errors. To minimize DCM errors the analog bandwidth should be less than 1/10 the DCM frequency. Analog bandwidth may be increased to up to 1/2 the DCM frequency in many applications. This will result in greater dynamic error generated at the DCM. The analog bandwidth may be further decreased to reduce noise and improve resolution. The ADXL202/ADXL210 noise has the characteristics of white Gaussian noise that contributes equally at all frequencies and is described in terms of g per root Hz; i.e., the noise is proportional to the square 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 ADXL202/ADXL210 is determined by the following equation: Noise rms = 500 g / Hz BW 1.5 At 100 Hz the noise will be: Noise rms = 500 g / Hz 100 (1.5) = 6.12 mg Often the peak value of the noise is desired. Peak-to-peak noise can only be estimated by statistical methods. Table III is useful for estimating the probabilities of exceeding various peak values, given the rms value.
Table III. Estimation of Peak-to-Peak Noise
CHOOSING T2 AND COUNTER FREQUENCY: DESIGN TRADE-OFFS
The noise level is one determinant of accelerometer resolution. The second relates to the measurement resolution of the counter when decoding the duty cycle output. The ADXL202/ADXL210s duty cycle converter has a resolution of approximately 14 bits; better resolution than the accelerometer itself. The actual resolution of the acceleration signal is, however, limited by the time resolution of the counting devices used to decode the duty cycle. The faster the counter clock, the higher the resolution of the duty cycle and the shorter the T2 period can be for a given resolution. The following table shows some of the trade-offs. It is important to note that this is the resolution due to the microprocessorss counter. It is probable that the accelerometers noise floor may set the lower limit on the resolution, as discussed in the previous section.
Table V. Trade-Offs Between Microcontroller Counter Rate, T2 Period and Resolution of Duty Cycle Modulator
ADXL202/ ADXL210 RSET Sample T2 (ms) (k ) Rate 1.0 1.0 1.0 5.0 5.0 5.0 10.0 10.0 10.0 124 124 124 625 625 625 1250 1250 1250 1000 1000 1000 200 200 200 100 100 100 CounterClock Counts Rate per T2 Counts Resolution (MHz) Cycle per g (mg) 2.0 1.0 0.5 2.0 1.0 0.5 2.0 1.0 0.5 2000 1000 500 10000 5000 2500 20000 10000 5000 250 125 62.5 1250 625 312.5 2500 1250 625 4.0 8.0 16.0 0.8 1.6 3.2 0.4 0.8 1.6
( )
( )
Nominal Peak-to-Peak Value 2.0 rms 4.0 rms 6.0 rms 8.0 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.
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ADXL202/ADXL210
STRATEGIES FOR USING THE DUTY CYCLE OUTPUT WITH MICROCONTROLLERS A DUAL AXIS TILT SENSOR: CONVERTING ACCELERATION TO TILT
Application notes outlining various strategies for using the duty cycle output with low cost microcontrollers are available from the factory.
USING THE ADXL202/ADXL210 AS A DUAL AXIS TILT SENSOR
When the accelerometer is oriented so both its X and Y axes are parallel to the earths surface it can be used as a two axis tilt sensor with a roll 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: 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.
MEASURING 360 OF TILT
One of the most popular applications of the ADXL202/ADXL210 is tilt measurement. An accelerometer uses the force of gravity as an input vector to determine 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 earths 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. The following table illustrates the changes in the X and Y axes as the device is tilted 90 through gravity.
+90 Y 0 X 90 1g
It is possible to measure a full 360 of orientation through gravity by using two accelerometers oriented perpendicular to one another (see Figure 15). When one sensor is reading a maximum change in output per degree, the other is at its ...