Low Cost ⴞ2 g/ⴞ10 g Dual Axis iMEMS® Accelerometers with Digital Output ADXL202/ADXL210 a GENERAL DESCRIPTION FEATURES 2-Axis 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 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. APPLICATIONS 2-Axis Tilt Sensing Computer Peripherals Inertial Navigation Seismic Monitoring Vehicle Security Systems Battery Powered Motion Sensing 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 µg/√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 0°C to +70°C commercial or –40°C to +85°C industrial temperature range. FUNCTIONAL BLOCK DIAGRAM +3.0V TO +5.25V CX VDD X SENSOR SELF TEST XFILT VDD ADXL202/ ADXL210 RFILT 32kV X OUT DEMOD CDC DUTY CYCLE MODULATOR (DCM) OSCILLATOR RFILT 32kV Y OUT DEMOD C O U N T E R mP Y SENSOR COM YFILT T2 RSET CY T2 T1 iMEMS is a registered trademark of Analog Devices, Inc. A(g) = (T1/T2 – 0.5)/12.5% 0g = 50% DUTY CYCLE T2 = RSET/125MV REV. B 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: 781/329-4700 World Wide Web Site: http://www.analog.com Fax: 781/326-8703 © Analog Devices, Inc., 1999 , T = +25ⴗC for J Grade only, V = +5 V, ADXL202/ADXL210–SPECIFICATIONS (TR ==T125tok⍀,T Acceleration = 0 g , unless otherwise noted) A MIN MAX A DD SET ADXL202/JQC/AQC Min Typ Max ADXL210/JQC/AQC Min Typ Max ± 1.5 ±2 0.2 ±1 ± 0.01 ±2 ±8 ± 10 0.2 ±1 ± 0.01 ±2 10 12.5 312 ± 0.5 15 3.2 4.0 100 ± 0.5 4.8 %/g mV/g % Rdg 25 75 42 50 ±2 1.0 2.0 58 ∆ from +25°C 50 ±2 1.0 2.0 % g %/V mg/°C @ +25°C 500 1000 500 1000 Duty Cycle Output At Pins XFILT, YFILT 500 5 10 Parameter SENSOR INPUT Measurement Range 1 Nonlinearity Alignment Error 2 Alignment Error Transverse Sensitivity 3 Best Fit Straight Line SENSITIVITY Duty Cycle per g Sensitivity, Analog Output Temperature Drift 4 Each Axis T1/T2 @ +25°C At Pins XFILT, YFILT ∆ from +25°C ZERO g BIAS LEVEL 0 g Duty Cycle Initial Offset 0 g Duty Cycle vs. Supply 0 g Offset vs. Temperature 4 Each Axis T1/T2 NOISE PERFORMANCE Noise Density5 FREQUENCY RESPONSE 3 dB Bandwidth 3 dB Bandwidth Sensor Resonant Frequency Conditions Each Axis X Sensor to Y Sensor FILTER RFILT Tolerance Minimum Capacitance 32 kΩ Nominal At XFILT, YFILT SELF TEST Duty Cycle Change Self-Test “0” to “1” DUTY CYCLE OUTPUT STAGE FSET FSET Tolerance Output High Voltage Output Low Voltage T2 Drift vs. Temperature Rise/Fall Time RSET = 125 kΩ I = 25 µA I = 25 µA 4.0 ± 15 1000 125 MΩ/RSET 0.7 VS – 200 mV 1.3 To 99% TEMPERATURE RANGE Operating Range Specified Performance JQC AQC 3.0 4.75 ± 15 % pF 10 % 0.6 160 CFILT + 0.3 1.3 200 35 200 5.25 5.25 1.0 2.7 4.75 +70 +85 0 –40 µg/√Hz Hz kHz kHz 200 0 –40 4.0 500 5 14 125 MΩ/RSET 0.7 VS – 200 mV 35 200 POWER SUPPLY Operating Voltage Range Specified Performance Quiescent Supply Current Turn-On Time6 g % of FS Degrees Degrees % 1000 10 Units 0.6 160 CFILT + 0.3 kHz mV mV ppm/°C ns 5.25 5.25 1.0 V V mA ms +70 +85 °C °C 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 (µg/√Hz) is the average noise at any frequency in the bandwidth of the part. 6 CFILT in µF. 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. –2– REV. B ADXL202/ADXL210 PIN CONFIGURATION ABSOLUTE MAXIMUM RATINGS* 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 . . . . . . . . . . . . . . . . . –55°C to +125°C Storage Temperature . . . . . . . . . . . . . . . . . . . –65°C to +150°C NC 1 VTP 2 ST 3 PIN FUNCTION DESCRIPTIONS Pin Name Description 1 2 3 4 5 6 7 8 9 10 11 12 13 14 NC VTP ST COM T2 NC COM NC YOUT XOUT YFILT XFILT VDD VDD 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 13 VDD 12 XFILT TOP VIEW COM 4 (Not to Scale) 11 YFILT AX 10 XOUT T2 5 *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. 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. 14 VDD ADXL202/ ADXL210 9 YOUT NC 6 COM 7 AY 8 NC NC = NO CONNECT Figure 1 shows the response of the ADXL202 to the Earth’s 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 TYPICAL OUTPUT AT PIN: 9 = 62.5% DUTY CYCLE 10 = 50% DUTY CYCLE 11 = 2.188V 12 = 2.5V TYPICAL OUTPUT AT PIN: 9 = 37.5% DUTY CYCLE 10 = 50% DUTY CYCLE 11 = 2.812V 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 JA JC Device Weight 14-Lead CERPAK 110°C/W 30°C/W 5 Grams Figure 1. ADXL202/ADXL210 Nominal Response Due to Gravity ORDERING GUIDE Model g Range Temperature Range Package Description Package Option ADXL202JQC ADXL202AQC ADXL210JQC ADXL210AQC ±2 ±2 ± 10 ± 10 0°C to +70°C –40°C to +85°C 0°C to +70°C –40°C to +85°C 14-Lead CERPAK 14-Lead CERPAK 14-Lead CERPAK 14-Lead CERPAK 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. REV. B –3– WARNING! ESD SENSITIVE DEVICE ADXL202/ADXL210 TYPICAL CHARACTERISTICS (@ +25ⴗC RSET = 125 k⍀, VDD = +5 V, unless otherwise noted) 4% 3% 1.04 CHANGE IN SENSITIVITY PERIOD NORMALIZED TO 1 AT 25 8C 1.06 1.02 1.00 0.98 2% 1% 0% –1% –2% 0.96 –3% 0.94 –45 –30 –15 0 15 30 45 TEMPERATURE – 8C 60 75 –4% –40 90 Figure 2. Normalized DCM Period (T2) vs. Temperature 25 TEMPERATURE – 8C 85 Figure 5. Typical X Axis Sensitivity Drift Due to Temperature 0.8 3 0.4 CFILT = 0.01mF 2 0.2 VOLTS ZERO g OFFSET SHIFT IN g 0.6 0 –0.2 1 –0.4 –0.6 0 –0.8 –40 –30 –20 –10 0 10 20 30 40 50 TEMPERATURE – 8C 60 70 80 0 90 0.4 0.8 1.2 1.4 FREQUENCY – ms Figure 6. Typical Turn-On Time Figure 3. Typical Zero g Offset vs. Temperature 20 0.7 18 VS = 5 VDC 16 PERCENTAGE OF SAMPLES SUPPLY CURRENT – mA 0.6 0.5 VS = 3.5 VDC 0.4 0.3 0.2 0.1 0 –40 14 12 10 8 6 4 2 –20 0 20 40 60 80 0 –0.87g 100 TEMPERATURE – 8C –0.64g –0.41g –0.17g 0.06g 0.29g g/DUTY CYCLE OUTPUT 0.52g 0.75g Figure 7. Typical Zero g Distribution at +25°C Figure 4. Typical Supply Current vs. Temperature –4– REV. B ADXL202/ADXL210 9 14 12 7 TOTAL RMS NOISE – mg PERCENTAGE OF SAMPLES T2 = 1ms CFILT = 0.01mF BW = 500Hz 8 6 5 4 3 2 CFILT = 0.1mF BW = 50Hz 8 CFILT = 0.47mF BW = 10Hz 6 4 1 2 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 0 Figure 8. Typical Sensitivity per g at +25 °C CFILT = 0.047mF BW = 100Hz 10 1 4 16 NUMBER OF AVERAGE SAMPLES 64 Figure 10. Typical Noise at Digital Outputs 20 14 18 12 14 % OF PARTS TOTAL RMS NOISE – mg 16 10 8 6 12 10 8 6 4 4 2 Figure 11. Rotational Die Alignment –5– 1.375 1.125 0.875 0.625 0.375 0.0125 –0.0125 DEGREES OF MISALIGNMENT Figure 9. Typical Noise at XFILT Output REV. B –0.375 0.47mF 10Hz –0.625 CX, CY BANDWIDTH 0.1mF 50Hz –0.875 0 0.047mF 100Hz –1.125 0 0.01mF 500Hz –1.375 2 ADXL202/ADXL210 DEFINITIONS APPLICATIONS 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. POWER SUPPLY DECOUPLING For most applications a single 0.1 µF 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 Ω (or smaller) resistor or ferrite beads, may be inserted in the ADXL202/ADXL210’s supply line. 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. 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 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 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. Decoupling Capacitor C DC A 0.1 µF capacitor is recommended from VDD to COM for power supply decoupling. ST 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. The output of the demodulator drives a duty cycle modulator (DCM) stage through a 32 kΩ 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. Duty Cycle Decoding The ADXL202/ADXL210’s 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 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 nominal output of the ADXL210 is: 0 g = 50% Duty Cycle Scale factor is 4% Duty Cycle Change per g The ADXL202/ADXL210 will operate with supply voltages as low as 3.0 V or as high as 5.25 V. These nominal values are affected by the initial tolerance of the device including zero g offset error and sensitivity error. T2 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 Note. T1 A(g) = (T1/T2 – 0.5)/12.5% 0g = 50% DUTY CYCLE T2(s) = RSET(V)/125MV Figure 12. Typical Output Duty Cycle –6– REV. B ADXL202/ADXL210 +3.0V TO +5.25V CX X SENSOR SELF TEST XFILT VDD VDD RFILT 32kV ADXL202/ ADXL210 X OUT T2 DEMOD CDC DUTY CYCLE MODULATOR (DCM) OSCILLATOR RFILT 32kV Y OUT DEMOD C O U N T E R T1 mP A(g) = (T1/T2 – 0.5)/12.5% 0g = 50% DUTY CYCLE T2 = RSET/125MV Y SENSOR YFILT COM T2 CY 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 = or, more simply, F –3 dB 1 (2 π (32 kΩ) × C(x, y)) 5 µF = C(X ,Y ) The tolerance of the internal resistor (RFILT), can vary as much as ± 25% of its nominal value of 32 kΩ; 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 Bandwidth Capacitor Value 10 Hz 50 Hz 100 Hz 200 Hz 500 Hz 5 kHz 0.47 µF 0.10 µF 0.05 µF 0.027 µF 0.01 µF 0.001 µF Setting the DCM Period with R SET 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 (Ω) 125 MΩ A 125 kΩ 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. REV. B T2 RSET 1 ms 2 ms 5 ms 10 ms 125 kΩ 250 kΩ 625 kΩ 1.25 MΩ Note that the RSET should always be included, even if only an analog output is desired. Use an RSET value between 500 kΩ and 2 MΩ 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 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– 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. 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 DESIGN TRADE-OFFS FOR SELECTING FILTER CHARACTERISTICS: THE NOISE/BW TRADE-OFF Bandwidth CX , CY rms Noise Peak-to-Peak Noise Estimate 95% Probability (rms ⴛ 4) 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. 10 Hz 50 Hz 100 Hz 200 Hz 500 Hz 0.47 µF 0.10 µF 0.05 µF 0.027 µF 0.01 µF 1.9 mg 4.3 mg 6.1 mg 8.7 mg 13.7 mg 7.6 mg 17.2 mg 24.4 mg 35.8 mg 54.8 mg 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. 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/ADXL210’s 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 microprocessors’s counter. It is probable that the accelerometer’s noise floor may set the lower limit on the resolution, as discussed in the previous section. 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 µg 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: Table V. Trade-Offs Between Microcontroller Counter Rate, T2 Period and Resolution of Duty Cycle Modulator ( ) Noise rms = 500 µg / Hz × BW × 1.5 At 100 Hz the noise will be: ( ) Noise rms = 500 µg / 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 Nominal Peak-to-Peak Value % of Time that Noise Will Exceed Nominal Peak-to-Peak Value 2.0 × rms 4.0 × rms 6.0 × rms 8.0 × rms 32% 4.6% 0.27% 0.006% ADXL202/ ADXL210 RSET Sample T2 (ms) (k⍀) Rate CounterClock Counts Rate per T2 Counts Resolution (MHz) Cycle per g (mg) 1.0 1.0 1.0 5.0 5.0 5.0 10.0 10.0 10.0 2.0 1.0 0.5 2.0 1.0 0.5 2.0 1.0 0.5 124 124 124 625 625 625 1250 1250 1250 1000 1000 1000 200 200 200 100 100 100 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 The peak-to-peak noise value will give the best estimate of the uncertainty in a single measurement. –8– REV. B 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. When the accelerometer is oriented so both its X and Y axes are parallel to the earth’s 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: USING THE ADXL202/ADXL210 AS A DUAL AXIS TILT SENSOR 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 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. The following table illustrates the changes in the X and Y axes as the device is tilted ± 90° through gravity. 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 360ⴗ OF TILT 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 minimum. +908 Y 3608 OF TILT Y 08 1g X 1g X –908 X OUTPUT Y OUTPUT ( g) X AXIS ⌬ PER ⌬ PER ORIENTATION DEGREE OF DEGREE OF TO HORIZON (ⴗ) X OUTPUT ( g) TILT (mg ) Y OUTPUT (g) TILT (mg ) – – – – – – – –90 –75 –60 –45 –30 –15 0 15 30 45 60 75 90 –1.000 –0.966 –0.866 –0.707 –0.500 –0.259 0.000 0.259 0.500 0.707 0.866 0.966 1.000 –0.2 4.4 8.6 12.2 15.0 16.8 17.5 16.9 15.2 12.4 8.9 4.7 0.2 0.000 0.259 0.500 0.707 0.866 0.966 1.000 0.966 0.866 0.707 0.500 0.259 0.000 Figure 15. Using a Two-Axis Accelerometer to Measure 360 ° of Tilt 17.5 16.9 15.2 12.4 8.9 4.7 0.2 –4.4 –8.6 –12.2 –15.0 –16.8 –17.5 Figure 14. How the X and Y Axes Respond to Changes in Tilt REV. B –9– ADXL202/ADXL210 USING THE ANALOG OUTPUT Power Cycling When Using the Digital Output The ADXL202/ADXL210 was specifically designed for use with its digital outputs, but has provisions to provide analog outputs as well. An alternative is to run the microcontroller at a higher clock rate and put it into shutdown between readings, allowing the use of the digital output. In this approach the ADXL202/ ADXL210 should be set at its fastest sample rate (T2 = 0.5 ms), with a 500 Hz filter at XFILT and YFILT. The concept is to acquire a reading as quickly as possible and then shut down the ADXL202/ADXL210 and the microcontroller until the next sample is needed. Duty Cycle Filtering An analog output can be reconstructed by filtering the duty cycle output. This technique requires only passive components. The duty cycle period (T2) should be set to 1 ms. An RC filter with a 3 dB point at least a factor of 10 less than the duty cycle frequency is connected to the duty cycle output. The filter resistor should be no less than 100 kΩ to prevent loading of the output stage. The analog output signal will be ratiometric to the supply voltage. The advantage of this method is an output scale factor of approximately double the analog output. Its disadvantage is that the frequency response will be lower than when using the XFILT, YFILT output. XFILT, YFILT Output The second method is to use the analog output present at the XFILT and YFILT pin. Unfortunately, these pins have a 32 kΩ output impedance and are not designed to drive a load directly. An op amp follower may be required to buffer this pin. The advantage of this method is that the full 5 kHz bandwidth of the accelerometer is available to the user. A capacitor still must be added at this point for filtering. The duty cycle converter should be kept running by using RSET <10 MΩ. Note that the accelerometer offset and sensitivity are ratiometric to the supply voltage. The offset and sensitivity are nominally: 0 g Offset = VDD/2 ADXL202 Sensitivity = (60 mV × VS)/g ADXL210 Sensitivity = (20 mV × VS)/g In either of the above approaches, the ADXL202/ADXL210 can be turned on and off directly using a digital port pin on the microcontroller to power the accelerometer without additional components. The port should be used to switch the common pin of the accelerometer so the port pin is “pulling down.” CALIBRATING THE ADXL202/ADXL210 The initial value of the offset and scale factor for the ADXL202/ ADXL210 will require calibration for applications such as tilt measurement. The ADXL202/ADXL210 architecture has been designed so that these calibrations take place in the software of the microcontroller used to decode the duty cycle signal. Calibration factors can be stored in EEPROM or determined at turn-on and saved in dynamic memory. For low g applications, the force of gravity is the most stable, accurate and convenient acceleration reference available. A reading of the 0 g point can be determined by orientating the device parallel to the earth’s surface and then reading the output. A more accurate calibration method is to make a measurements at +1 g and –1 g. The sensitivity can be determined by the two measurements. 2.5 V at +5 V 300 mV/g at +5 V, VDD 100 mV/g at +5 V, VDD To calibrate, the accelerometer’s measurement axis is pointed directly at the earth. The 1 g reading is saved and the sensor is turned 180° to measure –1 g. Using the two readings, the sensitivity is: USING THE ADXL202/ADXL210 IN VERY LOW POWER APPLICATIONS An application note outlining low power strategies for the ADXL202/ADXL210 is available. Some key points are presented here. It is possible to reduce the ADXL202/ADXL210’s average current from 0.6 mA to less than 20 µA by using the following techniques: Let A = Accelerometer output with axis oriented to +1 g Let B = Accelerometer output with axis oriented to –1 g then: Sensitivity = [A – B]/2 g For example, if the +1 g reading (A) is 55% duty cycle and the –1 g reading (B) is 32% duty cycle, then: 1. Power Cycle the accelerometer. 2. Run the accelerometer at a Lower Voltage, (Down to 3 V). Sensitivity = [55% – 32%]/2 g = 11.5%/g Power Cycling with an External A/D Depending on the value of the XFILT capacitor, the ADXL202/ ADXL210 is capable of turning on and giving a good reading in 1.6 ms. Most microcontroller based A/Ds can acquire a reading in another 25 µs. Thus it is possible to turn on the ADXL202/ ADXL210 and take a reading in <2 ms. If we assume that a 20 Hz sample rate is sufficient, the total current required to take 20 samples is 2 ms × 20 samples/s × 0.6 mA = 24 µA average current. Running the part at 3 V will reduce the supply current from 0.6 mA to 0.4 mA, bringing the average current down to 16 µA. These equations apply whether the output is analog, or duty cycle. Application notes outlining algorithms for calculating acceleration from duty cycle and automated calibration routines are available from the factory. The A/D should read the analog output of the ADXL202/ ADXL210 at the XFILT and Y FILT pins. A buffer amplifier is recommended, and may be required in any case to amplify the analog output to give enough resolution with an 8-bit to 10-bit converter. –10– REV. B ADXL202/ADXL210 OUTLINE DIMENSIONS Dimensions shown in inches and (mm). C3037b–2–4/99 14-Lead CERPAK (QC-14) 0.390 (9.906) MAX 14 8 0.291 (7.391) 0.285 (7.239) 0.419 (10.643) 0.394 (10.008) 1 7 PIN 1 0.300 (7.62) 0.195 (4.953) 0.115 (2.921) 0.020 (0.508) 0.004 (0.102) 0.215 (5.461) 0.119 (3.023) 0.050 (1.27) BSC 0.020 (0.508) 0.013 (0.330) 0.0125 (0.318) 0.009 (0.229) 8° 0° 0.050 (1.270) 0.016 (0.406) PRINTED IN U.S.A. SEATING PLANE 0.345 (8.763) 0.290 (7.366) REV. B –11–