a FEATURES 2-Axis Acceleration Sensor on a Single IC Chip 5 mm 5 mm 2 mm Ultrasmall Chip Scale Package 2 mg Resolution at 60 Hz Low Power < 0.6 mA Direct Interface to Low-Cost Microcontrollers via Duty Cycle Output BW Adjustment with a Single Capacitor 3 V to 5.25 V Single-Supply Operation 1000 g Shock Survival APPLICATIONS 2-Axis Tilt Sensing with Faster Response than Electrolytic, Mercury, or Thermal Sensors Computer Peripherals Information Appliances Alarms and Motion Detectors Disk Drives Vehicle Security GENERAL DESCRIPTION The ADXL210E is a low-cost, low-power, complete 2-axis accelerometer with a digital output, all on a single monolithic IC. It is an improved version of the ADXL210AQC/JQC. The ADXL210E will measure accelerations with a full-scale range of ± 10 g. The ADXL210E can measure both dynamic acceleration (e.g., vibration) and static acceleration (e.g., gravity). The outputs are analog voltage or digital signals whose duty cycles (ratio of pulsewidth to period) are proportional to acceleration. The duty cycle outputs can be directly measured by a microprocessor counter without an A/D converter or glue logic. The duty cycle period is adjustable from 0.5 ms to 10 ms via a single resistor (RSET). Low-Cost 10 g Dual-Axis Accelerometer with Duty Cycle ADXL210E FUNCTIONAL BLOCK DIAGRAM 3V TO 5.25V CX VDD XFILT RFILT 32k X SENSOR XOUT DEMOD CDC OSCILLATOR ANALOG TO DUTY CYCLE (ADC) ADXL210E DEMOD Y SENSOR COM SELF-TEST RFILT 32k YOUT YFILT C O U N P T E R T2 CY RSET T2 T1 A(g) = (T1/T2 – 0.5)/4% 0g = 50% DUTY CYCLE T2 = RSET/125M The typical noise floor is 200 g√Hz, allowing signals below 2 mg (at 60 Hz bandwidth) to be resolved. The bandwidth of the accelerometer is set with capacitors CX and CY at the XFILT and YFILT pins. An analog output can be reconstructed by filtering the duty cycle output. The ADXL210E is available in a 5 mm ⫻ 5 mm ⫻ 2 mm 8-lead hermetic LCC package. 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. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. 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 © Analog Devices, Inc., 2002 (TA = TMIN to TMAX, TA = 25C for J Grade only, VDD = 5 V, RSET = 125 k, Acceleration = 0 g, ADXL210E–SPECIFICATIONS unless otherwise noted.) Parameter Conditions SENSOR INPUT Measurement Range1 Nonlinearity Alignment Error2, 3 Alignment Error Cross-Axis Sensitivity2, 4 Each Axis ±8 ± 10 0.2 ±1 0.01 ±2 3.3 3.2 85 45 4.0 3.8 100 55 ± 0.5 44 40 2.3 1.35 Delta from 25⬚C 50 50 2.5 1.5 1.0 2.0 @ 25⬚C At Pins XFILT, YFILT X Sensor to Y Sensor Each Axis T1/T2, VDD = 5 V T1/T2, VDD = 3 V VDD = 5 V VDD = 3 V Delta from 25⬚C ZERO g BIAS LEVEL 0 g Duty Cycle2 0 g Duty Cycle2 0 g Voltage XFILT, YFILT2 0 g Voltage XFILT, YFILT2 0 g Duty Cycle vs. Supply2 0 g Offset vs. Temperature2, 5 Each Axis T1/T2, VDD = 5 V T1/T2, VDD = 3 V VDD = 5 V VDD = 3 V FREQUENCY RESPONSE 3 dB Bandwidth Sensor Resonant Frequency FILTER RFILT Tolerance Minimum Capacitance 32 kΩ Nominal At Pins XFILT, YFILT SELF-TEST Duty Cycle Change Self-Test “0” to “1” DUTY CYCLE OUTPUT STAGE RSET = 125 kΩ FSET Output High Voltage I = 25 µA Output Low Voltage I = 25 µA T2 Drift vs. Temperature Rise/Fall Time POWER SUPPLY Operating Voltage Range Quiescent Supply Current Turn-On Time ADXL210JE Typ Max Best Fit Straight Line SENSITIVITY Duty Cycle per g2 Duty Cycle per g2 Sensitivity XFILT, YFILT2 Sensitivity XFILT, YFILT2 Temperature Drift2, 5 NOISE PERFORMANCE Noise Density2 Min ADXL210AE Typ Max ± 10 0.2 ±1 0.01 ±2 4.9 4.4 125 65 3.2 3.0 80 40 4.0 3.8 100 55 ± 0.5 5 4.6 130 70 %/g %/g mV/g mV/g % 56 60 2.7 1.65 4.0 42 38 2.3 1.3 50 50 2.5 1.5 1.0 2.0 58 62 2.7 1.7 4.0 % % V V %/V mg/⬚C 200 200 1000 µg√Hz rms 6 10 6 10 kHz kHz ± 15 % pF 3 % ± 15 g % of FS Degrees Degrees % 1000 3 0.7 VS – 200 mV 1.3 0.7 VS – 200 mV 1.3 200 200 50 200 CFILT in µF 50 200 5.25 0.6 1.0 160 ⫻ CFILT + 0.3 0 Unit ±8 1000 3 TEMPERATURE RANGE Specified Performance AE Operating Range Min 70 3.0 5.25 0.6 1.0 160 ⫻ CFILT + 0.3 –40 –40 +85 +85 kHz V mV ppm/⬚C ns V mA ms ⬚C ⬚C NOTES 1 Guaranteed by measurement of initial offset and sensitivity. 2 See Typical Performance Characteristics. 3 Alignment error is specified as the angle between the true and indicated axis of sensitivity (see TPC 15). 4 Cross-axis sensitivity is the algebraic sum of the alignment and the inherent sensitivity errors. 5 Defined as the output change from ambient to maximum temperature or ambient to minimum temperature. Specifications subject to change without notice. –2– REV. 0 ADXL210E 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 +6.0 V Output Short Circuit Duration, (Any Pin to Common) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Indefinite Operating Temperature . . . . . . . . . . . . . . . . –55⬚C to +125⬚C Storage Temperature . . . . . . . . . . . . . . . . . . –65⬚C to +150⬚C VDD 8 PACKAGE CHARACTERISTICS Package Weight JA JC Device 8-Lead LCC 120°C/W TBD°C/W <1.0 grams 7 1 ST YFILT 6 2 T2 XOUT 5 3 COM 4 YOUT BOTTOM VIEW *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 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. XFILT PIN FUNCTION DESCRIPTIONS Pin No. Mnemonic Description 1 2 3 4 5 6 7 8 ST T2 COM YOUT XOUT YFILT XFILT VDD Self-Test Connect RSET to Set T2 Period Common Y-Channel Duty Cycle Output X-Channel Duty Cycle Output Y-Channel Filter Pin X-Channel Filter Pin 3 V to 5.25 V ORDERING GUIDE Model No. of Axes Specified Voltage Temperature Range Package Description Package Option ADXL210JE ADXL210AE* 2 2 3 V to 5 V 3 V to 5 V 0 to 70⬚C –40⬚C to +85⬚C 8-Lead LCC 8-Lead LCC E-8 E-8 *Available Soon 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 ADXL210E 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 –3– WARNING! ESD SENSITIVE DEVICE ADXL210E–Typical Performance Characteristics* VDD = 5 V 35 35 30 30 25 25 PERCENT OF PARTS PERCENT OF PARTS VDD = 3 V 20 15 10 1.44 1.46 1.48 1.50 1.52 VOLTS 1.54 1.56 1.58 1.60 35 35 30 30 25 25 20 15 10 5 0 20 15 10 5 0 1.40 1.42 1.44 1.46 1.48 1.50 1.52 1.54 1.56 1.58 1.60 VOLTS TPC 2. Y-Axis Zero g Bias Distribution at YFILT, VDD = 3 V 2.38 2.40 2.43 2.45 2.48 2.50 2.53 2.55 2.58 2.60 2.63 VOLTS TPC 5. Y-Axis Zero g Bias Distribution at YFILT, VDD = 5 V 70 30 60 25 50 PERCENT OF PARTS 35 20 15 10 5 0 2.38 2.40 2.43 2.45 2.48 2.50 2.53 2.55 2.58 2.60 2.63 VOLTS TPC 4. X-Axis Zero g Bias Distribution at XFILT, VDD = 5 V PERCENT OF PARTS PERCENT OF PARTS 10 0 1.42 TPC 1. X-Axis Zero g Bias Distribution at XFILT, VDD = 3 V PERCENT OF PARTS 15 5 5 0 20 40 30 20 10 52.5 53.3 54.2 55.0 55.8 56.7 mV/g 57.5 58.3 59.2 0 60.0 TPC 3. X-Axis Sensitivity Distribution at XFILT, VDD = 3 V 97.5 100.0 103.0 105.0 mV/g 108.0 110.0 113.0 TPC 6. X-Axis Sensitivity Distribution at XFILT, VDD = 5 V *Data taken from 14,500 parts over 3 lots minimum. –4– REV. 0 ADXL210E VDD = 3 V 35 70 30 60 25 50 PERCENT OF PARTS PERCENT OF PARTS VDD = 3 V 20 15 10 54.2 53.3 55.0 55.8 56.7 mV/g 57.5 58.3 59.2 60.0 70 60 60 50 50 PERCENT OF PARTS PERCENT OF PARTS 70 40 30 20 0 97.5 100.0 103.0 105.0 mV/g 108.0 110.0 113.0 40 30 20 10 3.5 3.6 3.7 3.8 3.9 4.0 PERCENT DUTY CYCLE PER g 0 4.1 TPC 8. X-Axis Sensitivity Distribution at XOUT, VDD = 3 V 3.9 4.0 4.1 4.3 4.2 PERCENT DUTY CYCLE PER g 4.4 TPC 11. X-Axis Sensitivity Distribution at XOUT, VDD = 5 V 70 60 60 50 50 PERCENT OF PARTS 70 40 30 20 10 0 95.0 TPC 10. Y-Axis Sensitivity Distribution at YFILT, VDD = 5 V 10 PERCENT OF PARTS 20 0 52.5 TPC 7. Y-Axis Sensitivity Distribution at YFILT, VDD = 3 V 40 30 20 10 3.4 3.5 3.7 3.8 3.9 3.6 PERCENT DUTY CYCLE PER g 4.0 0 4.1 TPC 9. Y-Axis Sensitivity Distribution at YOUT, VDD = 3 V REV. 0 30 10 5 0 40 3.8 3.9 4.0 4.3 4.2 4.1 PERCENT DUTY CYCLE PER g 4.4 TPC 12. Y-Axis Sensitivity Distribution at YOUT, VDD = 5 V –5– ADXL210E 35 25 30 PERCENT OF PARTS PERCENT OF PARTS 20 15 10 25 20 15 10 5 5 0 0 250 230 270 290 310 330 350 370 NOISE DENSITY – g Hz rms 390 410 TPC 13. Noise Density Distribution, VDD = 3 V 190 210 230 250 270 NOISE DENSITY – g Hz rms 290 310 TPC 16. Noise Density Distribution, VDD = 5 V 40 0.7 0.6 35 VS = 5 VDC 30 0.5 PERCENT OF PARTS SUPPLY CURRENT – mA 170 150 VS = 3.5 VDC 0.4 0.3 0.2 25 20 15 10 0.1 5 0 –40 0 –20 20 40 60 80 0 100 –2 –3 –1 TEMPERATURE – C TPC 14. Typical Supply Current vs. Temperature 0 PERCENT 1 2 3 TPC 17. Cross-Axis Sensitivity Distribution 20 18 VDD 3 14 CFILT = 0.01F XOUT 2 VOLTS 12 10 8 1 6 4 0 0 1.375 1.125 0.875 0.625 0.375 0.125 –0.125 –0.375 –0.625 –0.875 0 –1.125 2 –1.375 PERCENT OF PARTS 16 0.4 0.8 1.2 1.4 TIME – ms DEGREES OF MISALIGNMENT TPC 15. Rotational Die Alignment TPC 18. Typical Turn-On Time –6– REV. 0 20 18 18 16 16 14 14 8 mg/C 60 60 50 50 PERCENT OF PARTS PERCENT OF PARTS 3.06 2.15 2.60 1.70 TPC 22. Y-Axis Zero g Drift Due to Temperature Distribution, –40 °C to +85 °C 40 30 20 40 30 20 10 10 0 0 –0.0292 –0.0245 –0.0198 –0.0152 –0.0105 –0.0058 –0.0012 –0.0156 –0.0123 –0.0090 –0.0056 –0.0023 0.0010 0.0043 0.0077 PERCENT/C PERCENT/C TPC 20. X-Axis Sensitivity Drift at XFILT Due to Temperature Distribution, –40 °C to +85 °C TPC 23. Y-Axis Sensitivity Drift at XFILT Due to Temperature Distribution, –40 °C to +85 °C 2.57 2.60 2.55 2.58 2.53 2.56 2.51 2.54 VOLTS VOLTS 0.79 mg/C TPC 19. X-Axis Zero g Drift Due to Temperature Distribution, –40 °C to +85 °C 2.49 2.52 2.47 2.50 2.45 2.48 2.43 2.46 2.41 –40 –30 –20 –10 0 10 20 30 40 50 TEMPERATURE – C 60 70 80 2.44 –40 –30 –20 –10 90 TPC 21. Typical X-Axis Zero g Output vs. Temperature for 16 Parts REV. 0 1.24 0 0.34 0 –0.12 2 –1.02 4 2 –0.57 4 –1.93 6 –1.47 6 10 –2.83 8 –2.38 10 12 –3.29 12 –3.74 PERCENT OF PARTS 20 –5.34 –4.83 –4.32 –3.81 –3.30 –2.79 –2.28 –1.77 –1.26 –0.76 –0.25 0.26 0.77 1.28 1.79 2.30 2.81 3.32 3.83 4.34 PERCENT OF PARTS ADXL210E 0 10 20 30 40 50 TEMPERATURE – C 60 70 80 90 TPC 24. Typical Y-Axis Zero g Output vs. Temperature for 16 Parts –7– ADXL210E PERIOD NORMALIZED TO 1 @ 25C 1.06 1.04 1.02 1.00 0.98 0.96 0.94 –45 –30 –15 0 15 30 45 TEMPERATURE – C 60 90 75 TPC 25. Normalized DCM Period (T2) vs. Temperature 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. DEFINITIONS 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 ADXL210E/ ADXL210. Pulsewidth Time period of the “on” pulse. Defined as T1 for the ADXL210E/ADXL210. 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 ADXL210E will operate with supply voltages as low as 3.0 V or as high as 5.25 V. THEORY OF OPERATION T2 The ADXL210E 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. 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 ADXL210E is capable of measuring both positive and negative accelerations to ± 10 g. The accelerometer can measure static acceleration forces such as gravity, allowing it to be used as a tilt sensor. T1 A(g) = (T1/T2 – 0.5)/4% 0g = 50% DUTY CYCLE T2(s) = RSET()/125M Figure 1. Typical Output Duty Cycle APPLICATIONS 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 ADXL210E output. This may be 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 supply line of the ADXL210E. 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. FERRITE BEAD VDD 100 VDD CDC 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. XOUT ADXL210E COM YOUT ST XFILT XFILT T2 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 TPC 12). A 0 g acceleration produces a RSET YFILT YFILT Figure 2. –8– REV. 0 ADXL210E DESIGN PROCEDURE FOR THE ADXL210E Setting the Bandwidth Using C X and CY The design procedure for using the ADXL210E 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. The ADXL210E has 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 = Decoupling Capacitor C DC A 0.1 µF capacitor is recommended from VDD to COM for power supply decoupling. F –3 dB = 5 µF C(X ,Y ) The tolerance of the internal resistor (RFILT), can vary typically as much as ± 15% 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. Duty Cycle Decoding Table I. Filter Capacitor Selection, CX and CY The ADXL210E’s digital output is a duty cycle modulator. Acceleration is proportional to the ratio T1/T2. The nominal output of the ADXL210E 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 ADXL210E. Decoding algorithms for various microcontrollers have been developed. Consult the appropriate Application Note. ) or, more simply, 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 3% at the duty cycle outputs (corresponding to 800 mg). This pin may be left open circuit or connected to common in normal use. ( 1 2 π (32 kΩ) × C(x, y) 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 3V TO 5.25V CX VDD XFILT RFILT 32k X SENSOR XOUT DEMOD CDC OSCILLATOR DEMOD Y SENSOR COM ANALOG TO DUTY CYCLE (ADC) ADXL210E 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. SELF-TEST RFILT 32k YOUT YFILT C O U N P T E R Table II. Resistor Values to Set T2 T2 CY RSET T2 T1 A(g) = (T1/T2 – 0.5)/4% 0g = 50% DUTY CYCLE T2 = RSET/125M Figure 3. Block Diagram 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 placed close to the T2 Pin to minimize parasitic capacitance at this node. Selecting the Right Accelerometer For most tilt sensing applications the ADXL202E 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 ADXL210E should be used in applications where accelerations of greater than ±2 g are expected. REV. 0 –9– ADXL210E MICROCOMPUTER INTERFACES The ADXL210E is 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. With the single pole roll-off characteristic, the typical noise of the ADXL210E is determined by the following equation: ( ) ( Noise (rms) = 200 µg / Hz × BW × 1.6 ) At 100 Hz the noise will be: ( The designer should have some idea of the required performance of the system in terms of: ) Noise (rms) = 200 µg / Hz × 100 × (1.6) = 2.53 mg 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. 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. These requirements will help to determine the accelerometer bandwidth, the speed of the microcontroller clock and the length of the T2 period. Table III. Estimation of Peak-to-Peak Noise When selecting a microcontroller it is helpful to have a counter timer port available. The microcontroller should have provisions for software calibration. While the ADXL210E is a highly accurate accelerometer, it has a wide tolerance for 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. 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% The peak-to-peak noise value will give the best estimate of the uncertainty in a single measurement. Table IV gives typical noise output of the ADXL210E for various CX and CY values. DESIGN TRADE-OFFS FOR SELECTING FILTER CHARACTERISTICS: THE NOISE/BW TRADE-OFF 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. Table IV. Filter Capacitor Selection, CX and CY Bandwidth CX, CY rms Noise Peak-to-Peak Noise Estimate 95% Probability (rms 4) The analog output of the ADXL210E has a typical bandwidth of 5 kHz, while the duty cycle modulators’ bandwidth is 500 Hz. The user must filter the signal at this point to limit aliasing errors. To minimize DCM errors the analog bandwidth should be less than one-tenth the DCM frequency. Analog bandwidth may be increased to up to half the DCM frequency in many applications. This will result in greater dynamic error generated at the DCM. 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 0.8 mg 1.8 mg 2.5 mg 3.6 mg 5.7 mg 3.2 mg 7.2 mg 10.1 mg 14.3 mg 22.6 mg The analog bandwidth may be further decreased to reduce noise and improve resolution. The ADXL210E 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. 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. CHOOSING T2 AND COUNTER FREQUENCY: DESIGN TRADE-OFFS The ADXL210E’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’ counter. It is probable that the accelerometer’s noise floor may set the lower limit on the resolution, as discussed in the previous section. –10– REV. 0 ADXL210E Table V. Trade-Offs Between Microcontroller Counter Rate, T2 Period, and Resolution of Duty Cycle Modulator CounterADXL210E Clock Counts RSET Sample Rate per T2 Counts Resolution T2 (ms) (k) Rate (MHz) Cycle per g (mg) 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 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 80 40 20 400 200 100 800 400 200 12.50 25.00 50.00 2.50 5.00 10.00 1.25 2.50 5.00 USING THE ANALOG OUTPUT The ADXL210E was specifically designed for use with its digital outputs, but has provisions to provide analog outputs as well. 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 X FILT 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 ADXL210E Sensitivity = (20 mV ⫻ VS)/g Power Cycling with an External A/D Depending on the value of the XFILT capacitor, the ADXL210E 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 ADXL210E 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 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. The A/D should read the analog output of the ADXL210E at the XFILT and YFILT 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. Power Cycling When Using the Digital Output 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 ADXL210E 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 ADXL210E and the microcontroller until the next sample is needed. In either of the above approaches, the ADXL210E can be turned on and off directly using a digital port pin on the microcontroller to power the accelerometer without additional components. CALIBRATING THE ADXL210E The initial value of the offset and scale factor for the ADXL210E will require calibration for applications such as tilt measurement. The ADXL210E 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 measurements at +1 g and –1 g. The sensitivity can be determined by the two measurements. 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: 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 USING THE ADXL210E IN VERY LOW POWER APPLICATIONS An application note outlining low power strategies for the ADXL210E is available. Some key points are presented here. It is possible to reduce the ADXL210E’s average current from 0.6 mA to less than 20 µA by using the following techniques: 1. Power cycle the accelerometer. 2. Run the accelerometer at a lower voltage (down to 3 V). REV. 0 For example, if the +1 g reading (A) is 55% duty cycle and the –1 g reading (B) is 47% duty cycle, then: Sensitivity = [55% – 47%]/2 g = 4%/g 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. –11– ADXL210E OUTLINE DIMENSIONS Dimensions shown in inches and (mm). 0.197 (5.00) SQ 0.177 (4.50) SQ 0.070 (1.78) 0.050 (1.27) 0.015 7 1 5 3 0.050 (1.27) C02778–0–2/02(0) 8-Terminal Ceramic Leadless Chip Carrier (E-8) (0.38) 0.075 (1.91) 0.099 (2.50) 0.025 (0.64) 0.099 (2.50) TOP VIEW 0.050 (1.27) R0.008 0.015 (0.38) (0.20) 0.008 BOTTOM VIEW (0.20) CONTROLLING DIMENSIONS ARE IN MILLIMETERS PRINTED IN U.S.A. R0.028 (0.70) –12– REV. 0