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ADXL362 Datasheet(Arkusz danych) 8 Page - Analog Devices

Numer części ADXL362
Szczegółowy opis  Micropower Three-Axis ±2g/±4g/±8g
Pobierz  40 Pages
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
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ADXL362
Preliminary Technical Data
Rev. PrB | Page 8 of 40
FUNCTIONAL DESCRIPTION
The ADXL362 is a complete three-axis acceleration
measurement system that operates at extremely low power
consumption levels and enables system-level power savings. 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.
MECHANICAL DEVICE OPERATION
The moving component of the sensor is a polysilicon surface-
micromachined structure, also referred to as a beam, 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 beam
and unbalances the differential capacitor, resulting in a sensor
output whose amplitude is proportional to acceleration. Phase-
sensitive demodulation is used to determine 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. 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 up to 400 Hz, with a 2.0V
supply. All features described in this datasheet are available
when operating in this mode.
The ability to continuously output data from the minimum
12.5Hz to the maximum 400Hz data rate without aliasing any
input signal while also still delivering less than 3 µA (typical) of
current consumption is what makes the ADXL362 truly a
breakthrough ultra-low power accelerometer. Other
accelerometers derive low current by using a specific low power
mode that power cycles acceleration sensing. This leads to a
small effective bandwidth in the low-power modes and
undersampling of input data and thus unwanted aliasing can
occur. Undersampling and aliasing will not happen with the
ADXL362 since 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 (300 nA
at a 2.0V supply voltage). This 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 several ways: it can
switch into full-bandwidth measurement mode, signal an interrupt
to a microcontroller, and/or wake up downstream circuitry,
depending on configuration.
Standby
Placing the ADXL362 in Standby suspends measurement and
reduces current consumption to merely 10nA (typical). Pending
interrupts and data are preserved. No new interrupts are
generated.
The ADXL362 powers up in Standby with all sensor functions
off.
POWER/NOISE TRADEOFF
The ADXL362 offers a few options for decreasing noise at the
expense of only a small increase in current consumption.
The noise performance of the ADXL362 in normal operation,
typically 7 LSB RMS at 100 Hz bandwidth, will be adequate for
most applications, depending upon bandwidth and the desired
resolution. For cases where lower noise is needed, the ADXL362
provides two lower-noise operating modes that trade reduced
noise for somewhat higher supply current.
Table 6 shows the supply current values and noise densities
obtained for normal operation and the two lower-noise modes,
at a typical 2.0V supply.
Operating the ADXL362 at a higher supply voltage also
decreases noise. Table 7 shows the supply current values and
noise densities obtained for normal operation and the two
lower-noise modes the highest recommended supply, 3.3V.




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