ADXL366 datasheet(21/71 Pages) AD

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Data Sheet

ADXL366

POWER SAVINGS FEATURES

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Rev. 0 | 21 of 71

Designed for the most power conscious applications, the ADXL366

includes several features 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

ADXL366 is its ultra-low current consumption in all configurations.

The ADXL366 consumes between 0.96µA (typical) and 1.3µA (typi-

cal) across all data rates up to 400Hz and all supply voltages up to

3.6V (see Figure 42). At an even lower power of 191nA (typical),

a motion triggered wake-up mode is provided for simple motion

detection applications that require a power consumption lower than

0.2µA.

At these current levels, the accelerometer consumes less power

in full operation than the standby currents of many other system

components, and is, therefore, optimal for applications that require

continuous acceleration monitoring and long battery life. Because

the accelerometer is always on, it can act as a motion activation

switch. The accelerometer signals to the rest of the system when to

turn on, thereby managing power at the system level.

As important as its low operating current, the 47nA (typical) standby

current of the ADXL366 contributes to a much longer battery life

in applications that spend most of their time in a sleep state and

wake-up via an external trigger.

PEDOMETER

The ADXL366 has a built-in step counting feature that, when

enabled, only adds 120nA (typical) of current consumption. The

step counter feature estimates the number of steps taken based on

the peak detection analysis of the vector of acceleration produced

by a step, over a predefined time window. To reduce false positives,

the step counter only considers steps as valid if eight or more steps

are detected, ensuring that the step count only increments when

the user is actively walking or running, as shown in the Figure 45.

Figure 45. Acceleration Profile of Person Walking 10 Steps in One Direction

and Then Turning Around and Walking 10 Steps More; The Step Count Is

Shown as Black Lines

Figure 46 shows, graphically, how the acceleration data is interpret-

ed and analyzed by the step counting feature. In Figure 46, within

the first second, the pedometer captured that a single step is

first taken, and the person then rests in place. The possible step

count increases to one after the first detected minimum; however,

because the second maximum does not meet the algorithm condi-

tion to be considered a possible step, the possible step count is

reset to zero. The person starts walking again at approximately

1.4sec. At approximately 6.5sec, the possible step count reaches

eight, and thus all eight possible steps and consecutive possible

steps are certified as valid steps and added to the pedometer step

count output registers.

Figure 46. Step Counter Acceleration Signal Processing

The step counter can be enabled by setting to 1 the PEDOM-

ETER_EN bit of PEDOMETER_CTL (Register 0x49), and the

current steps counted can be accessed by reading the PED-

OMETER_STEP_CNT_H register (Register 0x47) and PEDOME-

TER_STEP_CNT_L register (Register 0x48).

The two user-configurable parameters include the following:

►

The pedometer initial threshold is the initial offset that is fed to

the step counter when enabled. Its default value is 4000LSB,

and it can be modified by the user by writing the desired value

to the PEDOMETER_TRHES_H register (Register 0x4A) and

PEDOMETER_TRHES_L register (Register 0x4B). The scale

factor for these registers is 4000LSB/g. Internally, the threshold

is dynamically updated during operation every time the difference

between a pair of maximum and a minimum peaks is greater

than the pedometer sensitivity setting.

►

Pedometer sensitivity. defines a zone near the dynamic threshold

amplitude that helps to discard signals that are most likely due to

undesired motion. For a pair of maximum and minimum peaks to

be considered as a possible step, the following conditions must

be satisfied:

►

Maximum Peak > (Dynamic Threshold + Pedometer Sensitivi-

ty/2)

►

Minimum Peak < (Dynamic Threshold – Pedometer Sensitivi-

ty/2)

ADXL366 Datasheet(PDF) 21 Page - Analog Devices

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