Data Sheet
ADXL36±
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.
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:
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.
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•
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Switch into full bandwidth measurement mode
Signal an interrupt to a microcontroller
Wake up downstream circuitry, depending on the
configuration
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.
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.
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.
The ADXL362 powers up in standby with all sensor functions
turned off.
SELECTABLE MEASUREMENT RANGES
Measurement Mode
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.
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.
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.
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.
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.
Rev. B | Page 13 of 44
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