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ADXL362 Datasheet(Arkusz danych) 10 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 10 of 40
POWER SAVINGS FEATURES
The ADXL362 was designed for the most power-conscious
applications, and as such includes several features, described in
this section, for enabling power savings at the system level as
well as at the device level.
ULTRA-LOW POWER CONSUMPTION IN ALL
MODES
At the device level, the most obvious power saving feature of the
ADXL362 is its ultra-low current consumption in all
configurations: the ADXL362 consumes between 1.1 µA (typ)
and 5 µA (typ) across all data rates up to 400 Hz and all supply
voltages up to 3.6V (see Figure 4). An even lower-power, 350
nA (typ) motion-triggered Wake-Up Mode is provided for
simple motion detection applications that require even lower
power consumption than 1 µA.
At these current levels, the accelerometer consumes less power
in full operation than many other system components’ standby
currents, and is therefore optimal for applications that require
continuous acceleration monitoring and very long battery life.
As the accelerometer is always on, it can act as a motion-
activated switch and signal to the rest of the system when to
turn on, thereby managing power at the system level.
No less important than its low operating current, the
ADXL362’s 10 nA (typ) Standby current contributes to much
longer battery life in applications that spend most of their time
in a “sleep” state and wake up via an external trigger.
MOTION DETECTION
The ADXL362 features built-in logic that detects Activity
(presence of acceleration above a threshold) and Inactivity (lack
of acceleration above a threshold).
Detection of an activity or inactivity event is indicated in the
Status register and can also be configured to generate an
interrupt. In addition, the activity status of the device – that is,
whether it is “moving” or “stationary” – is indicated by the
Awake bit, described on page 11.
Activity and Inactivity detection can be used when the
accelerometer is in either Measurement Mode or Wake-Up
Modes.
Activity Detection
An Activity event is detected when acceleration stays above a
specified threshold for a specified time period.
Referenced and Absolute Configurations
Activity detection can be configured as Referenced or Absolute.
When using Absolute Activity detection, acceleration samples
are compared to a user-set threshold to determine whether
motion is present. For example, if a threshold of 0.5g is set and
the acceleration on the z-axis is 1g for longer than the user-
defined Activity time, the Activity status is asserted.
In many applications, it is advantageous for activity detection to
be based not on an absolute threshold but on a deviation from a
reference point or orientation. This is particularly useful as it
removes the effect on Activity detection of the static 1g imposed
by gravity. When an accelerometer is stationary, its output can
reach 1g, even when it is not moving. In Absolute Activity, if the
threshold were set to less than 1g, activity would immediately be
detected in this case.
In the Referenced configuration, Activity is detected when
acceleration samples are at least a user-set amount above an
internally-defined reference, for the user-defined amount of
time. The reference is calculated when Activity detection is
engaged, so activity is only detected when the acceleration has
deviated sufficiently from the initial orientation.
The Referenced configuration results in a very sensitive Activity
detection that detects even the most subtle motion events.
Fewer False Positives
Ideally, the intent of Activity detection is to wake up a system
only when motion is intentional, ignoring noise or small,
unintentional movements. In addition to being sensitive to
subtle motion events, the ADXL362 Activity detection
algorithm is also designed to be robust in filtering out
undesired triggers.
The ADXL362 activity detection functionality includes a timer
that can be used to filter out unwanted motion and ensure that
only sustained motion is recognized as Activity. The duration of
this timer, as well as the acceleration threshold, are user-
adjustable from 1 sample (i.e., no timer) to up to 20 seconds of
motion.
Note that the Activity timer is operational in Measurement
Mode only. In Wake-Up Mode, one-sample Activity detection is
used.
Inactivity Detection
An Inactivity event is detected when acceleration remains below
a specified threshold for a specified time. Inactivity detection
can also be configured as Referenced or Absolute.
When using Absolute Inactivity detection, acceleration samples
are compared to a user-set threshold for the user-set time to
determine the absence of motion. The Absolute configuration
should be used for implementing free-fall detection.
When using Referenced Inactivity detection, Inactivity is
detected if acceleration samples are within less than a user-
specified amount of an internally-defined reference, for a user-
defined amount of time.
Referenced Inactivity, like Referenced Activity, is particularly
useful for eliminating the effects of the 1g due to gravity. With




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