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ADXL362BCCZ-R2 Arkusz danych(PDF) 13 Page - Analog Devices |
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ADXL362BCCZ-R2 Arkusz danych(HTML) 13 Page - Analog Devices |
13 / 44 page Data Sheet ADXL362 Rev. B | Page 13 of 44 THEORY OF OPERATION The ADXL362 is a complete 3-axis acceleration measurement system that operates at extremely low power consumption levels. It measures both dynamic acceleration, resulting from motion or shock, and static acceleration, such as tilt. Acceleration is reported digitally and the device communicates via the SPI protocol. Built-in digital logic enables autonomous operation and implements functionality that enhances system level power savings. MECHANICAL DEVICE OPERATION The moving component of the sensor is a polysilicon surface- micromachined structure that is 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 differential capacitors that consist of independent fixed plates and plates attached to the moving mass. Acceleration deflects the structure and unbalances the differential capacitor, resulting in a sensor output whose amplitude is proportional to acceleration. Phase sensitive demodulation determines the magnitude and polarity of the acceleration. OPERATING MODES The ADXL362 has two operating modes: measurement mode for continuous, wide bandwidth sensing; and wake-up mode for limited bandwidth activity detection. In addition, measurement can be suspended altogether by placing the device in standby. Measurement Mode Measurement mode is the normal operating mode of the ADXL362. In this mode, acceleration data is read continuously and the accelerometer consumes less than 3 µA (typical) across its entire range of output data rates of up to 400 Hz using a 2.0 V supply. All features described in this datasheet are available when operating the ADXL362 in this mode. The ability to continuously output data from the minimum 12.5 Hz to the maximum 400 Hz data rate while still delivering less than 3 µA (typical) of current consumption is what defines the ADXL362 as an ultralow power accelerometer. Other accel- erometers derive low current by using a specific low power mode that power cycles acceleration sensing. The result is a small effective bandwidth in the low power modes and undersampling of input data; therefore, unwanted aliasing can occur. Under- sampling and aliasing do not occur with the ADXL362 because it continuously samples the full bandwidth of its sensor at all data rates. Wake-Up Mode Wake-up mode is ideal for simple detection of the presence or absence of motion at extremely low power consumption (270 nA at a 2.0 V supply voltage). Wake-up mode is useful particularly for implementation of a motion activated on/off switch, allowing the rest of the system to be powered down until activity is detected. Wake-up mode reduces current consumption to a very low level by measuring acceleration only about six times per second to determine whether motion is present. If motion is detected, the accelerometer can respond autonomously in the following ways: • Switch into full bandwidth measurement mode • Signal an interrupt to a microcontroller • Wake up downstream circuitry, depending on the configuration In wake-up mode, all accelerometer features are available with the exception of the activity timer. All registers can be accessed, and real-time data can be read and/or stored in the FIFO. Standby Placing the ADXL362 in standby suspends measurement and reduces current consumption to 10 nA (typical). Pending interrupts and data are preserved and no new interrupts are generated. The ADXL362 powers up in standby with all sensor functions turned off. SELECTABLE MEASUREMENT RANGES The ADXL362 has selectable measurement ranges of ±2 g, ±4 g, and ±8 g. Acceleration samples are always converted by a 12-bit ADC; therefore, sensitivity scales with g range. Ranges and corresponding sensitivity values are listed in Table 1. When acceleration exceeds the measurement extremes, data is clipped at the full-scale value (0x0FFF), and no damage is caused to the accelerometer. Table 2 lists the absolute maximum ratings for acceleration, indicating the acceleration level that can cause permanent damage to the device. SELECTABLE OUTPUT DATA RATES The ADXL362 can report acceleration data at various data rates ranging from 12.5 Hz to 400 Hz. The internal low-pass filter pole is automatically set to ¼ or ½ the selected ODR (based on the HALF_BW setting) to ensure the Nyquist sampling criterion is met and no aliasing occurs. |
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