CN-0274 Datasheet, PDF (2/6 Pages) Analog Devices – Ultralow Power, 3-Axis, Motion Activated Switch

CN-0274
Circuit Note
Basic Operation of the ADXL362
The ADXL362 is a three-axis, ultralow power acceleration
measurement system capable of measuring dynamic acceleration
(resulting from motion or shock) as well as static acceleration
(that is, gravity).
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. Each capacitor consists of independent fixed plates
and plates attached to the moving mass. Any 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.
Modes of Operation
The three basic modes of operation for the ADXL362 are
standby, measurement, and wake-up.
• Placing the ADXL362 in standby mode suspends measurement
and reduces current consumption to 10 nA. Any pending data
or interrupts are preserved; however, no new information
is processed. The ADXL362 powers up in standby mode
with all sensor functions turned off.
• Measurement mode is the normal operating mode of the
ADXL362. In this mode, acceleration data is continuously
read, and the accelerometer consumes less than 3 µA across
its entire range of output data rates of up to 400 Hz using a
2.0 V supply. All described features are available while
operating 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 of current
consumption is what defines the ADXL362 as an ultralow
power accelerometer. 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 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 6 times a second to determine whether motion is present.
In wake-up mode, all accelerometer features are available
with the exception of the activity timer. All registers are
accessible, and real-time data is available from the part.
The CN0274 evaluation software uses the wake-up mode of the
ADXL362. That is, the ADXL362 is asleep until it detects motion at
which point it enters measurement mode.
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, is 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 1. ADXL362 Noise vs. Supply Current
Mode
Noise
Current Consumption
(µg/√Hz Typical) (µA Typical)
Normal Operation 380
2.7
Low Noise
280
4.5
Ultralow Noise
175
15
Table 1 shows the supply current values and noise densities
obtained for normal operation and the two lower noise modes,
at a typical 3.3 V supply.
The CN0274 evaluation software uses the normal operation
noise mode of the ADXL362.
Motion Detection
The ADXL362 has built-in logic that detects activity (acceleration
above a certain threshold) and inactivity (lack of acceleration
above a certain 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.
Activity and inactivity detection can be used when the
accelerometer is in either measurement mode or wake-up mode.
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