CN-0274 [ADI]
Ultralow Power, 3-Axis, Motion Activated Switch; 超低功耗， 3轴，人体感应开关
| 型号： | CN-0274 |
| 厂家： | 
ADI |
| 描述： | Ultralow Power, 3-Axis, Motion Activated Switch |
| 文件： | 总6页 (文件大小：261K) |
| 中文： | 中文翻译 | 下载： | 下载PDF数据表文档文件 |
Circuit Note
CN-0274
Devices Connected/Referenced
Circuits from the Lab™ reference circuits are engineered and
tested for quick and easy system integration to help solve today’s ADXL362
analog, mixed-signal, and RF design challenges. For more
Nanopower, 3-Axis, 2 g, 4 g, and 8 g
Digital Output MEMS Accelerometer
Logic Controlled, High-Side Power Switch
with Reverse Current Blocking
information and/or support, visit www.analog.com/CN0274.
ADP195
Ultralow Power, 3-Axis, Motion Activated Switch
The ADXL362 is an ultralow power, 3-axis accelerometer
EVALUATION AND DESIGN SUPPORT
that consumes less than 100 nA in wake-up mode. Unlike
accelerometers that use power duty cycling to achieve low power
consumption, the ADXL362 does not alias input signals by under
sampling; it samples continuously at all data rates. There is also
an on-chip, 12-bit temperature sensor accurate to 0.5°.
Circuit Evaluation Boards
CN-0274 Circuit Evaluation Board (EVAL-CN0274-SDPZ)
System Demonstration Platform (EVAL-SDP-CS1Z)
Design and Integration Files
Schematics, Layout Files, Bill of Materials
The ADXL362 provides 12-bit output resolution and has three
operating ranges, 2 g, 4 g, and 8 g. It is specified over a
minimum temperature range of −40°C to +85°C. For applications
where a noise level less than 480 µg/√Hz is desired, either of its
two lower noise modes (down to 120 µg/√Hz) can be selected at
a minimal increase in supply current.
CIRCUIT FUNCTION AND BENEFITS
The combination of parts shown in Figure 1 provides an ultralow
power, 3-axis, motion activated power switch solution capable of
controlling up to 1.1 A of load current. The circuit is ideal for
applications where extended battery life is critical. When the switch is
off, the battery current is less than 300 nA, and when the switch is on,
it draws less than 3 µA. The circuit provides an industry leading,
low power motion sensing solution suitable for wireless sensors,
metering devices, home healthcare, and other portable applications.
The ADP195 is a high-side load switch designed for operation
between 1.1 V and 3.6 V and is protected against reverse current
flow from output to input. The device contains a low on-resistance,
P-channel MOSFET that supports over 1.1 A of continuous load
current and minimizes power losses.
The 3-axis accelerometer controls the high-side switch by
monitoring the acceleration in three axes and closes or opens
the switch depending on the presence or absence of motion.
3.3V
3.3V
C
C
IO
S
V
V
DD I/O
S
ADXL362
MOSI
INTERRUPT
CONTROL
MISO
SCLK
CS
INT1
INT2
SPI
INTERFACE
GND
AWAKE
REVERSE
POLARITY
PROTECTION
ADP195
VOUT
LOAD
VIN
VS
GND
LEVEL SHIFT
AND SLEW
RATE CONTROL
EN
Figure 1. Ultralow Power Standalone Motion Switch (Simplified Schematic: Decoupling and All Connections Not Shown)
Rev. A
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©2012 Analog Devices, Inc. All rights reserved.
CN-0274
Circuit Note
Basic Operation of the ADXL362
Power/Noise Tradeoff
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 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.
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.
Table 1. ADXL362 Noise vs. Supply Current
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.
Noise
Current Consumption
Mode
(µg/√Hz Typical) (µA Typical)
Normal Operation 380
2.7
4.5
15
Low Noise
Ultralow Noise
280
175
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.
Modes of Operation
The three basic modes of operation for the ADXL362 are
standby, measurement, and wake-up.
The CN0274 evaluation software uses the normal operation
noise mode of the ADXL362.
•
•
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.
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.
•
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.
Rev. A | Page 2 of 6
Circuit Note
CN-0274
Activity Detection
Linking Activity and Inactivity Detection
An activity event is detected when acceleration stays above a
specified threshold for a user-specified time period. The two
activity detection events are absolute and referenced.
The activity and inactivity detection functions can be used
concurrently, and processed manually by a host processor, or
they can be configured to interact in several ways:
•
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 0.5 g is set,
and the acceleration on any axis is 1 g 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 because it removes the effect on activity detection of
the static 1 g imposed by gravity. When an accelerometer is
stationary, its output can reach 1 g, even when it is not moving.
In absolute activity, if the threshold is set to less than 1 g,
activity is immediately detected in this case.
In the referenced activity detection, 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, and the first sample obtained is used as a reference
point. Activity is only detected when the acceleration has
deviated sufficiently from this initial orientation. The
referenced configuration results in a very sensitive activity
detection that detects even the most subtle motion events.
•
In default mode, activity and inactivity detection are both
enabled, and all interrupts must be serviced by a host
processor; that is, a processor must read each interrupt
before it is cleared and can be used again.
•
In linked mode, activity and inactivity detection are linked
to each other such that only one of the functions is enabled
at any given time. Once activity is detected, the device is
assumed moving or awake and stops looking for activity:
inactivity is expected as the next event so only inactivity
detection operates. When inactivity is detected, the device
is assumed stationary or asleep. Activity is now expected as
the next event so that only activity detection operates. In
this mode, a host processor must service each interrupt
before the next is enabled.
In loop mode, motion detection operates as previously
described in linked mode; however, interrupts do not need
to be serviced by a host processor. This configuration
simplifies the implementation of commonly used motion
detection and enhances power savings by reducing the
amount of power used in bus communication.
•
•
•
When enabling autosleep mode in linked mode or loop
mode, it causes the device to autonomously enter wake-up
mode when inactivity is detected, and reenter measurement
mode when activity is detected.
The CN0274 evaluation software uses the referenced mode of
operation when searching for activity.
The CN0274 evaluation software uses the autosleep and loop
modes to demonstrate the functionality of the ADXL362.
Inactivity Detection
An inactivity event is detected when acceleration remains below
a specified threshold for a specified time. The two inactivity
detection events are absolute and referenced.
The AWAKE Bit
The AWAKE bit is a status bit that indicates whether the ADXL362
is awake or asleep. The device is awake when it has seen an activity
condition, and the device is asleep when it has seen an inactivity
condition.
•
In absolute inactivity detection, acceleration samples are
compared to a user set threshold for the user set time to
determine the absence of motion.
•
In referenced inactivity detection, acceleration samples are
compared to a user specified reference for a user defined
amount of time. When the part first enters the awake state,
the first sample is used as a reference point, and the threshold
is applied around it. If the acceleration stays inside the
threshold, the part enters the asleep state. If an acceleration
value moves outside the threshold, this point is then used
as a new reference, and the thresholds are reapplied to this
new point.
The awake signal can be mapped to the INT1 or INT2 pin and
can thus be used as a status output to connect or disconnect
power to downstream circuitry based on the awake status of
the accelerometer. Used in conjunction with loop mode, this
configuration implements a trivial, autonomous motion-
activated switch.
If the turn-on time of the downstream circuitry can be tolerated,
this motion switch configuration can save significant system-
level power by eliminating the standby current consumption of
the rest of the application. This standby current can often exceed
the full operating current of the ADXL362.
The CN0274 evaluation software uses the referenced mode of
operation when searching for inactivity.
Rev. A | Page 3 of 6
CN-0274
Circuit Note
Interrupts
Several of the built-in functions of the ADXL362 can trigger
interrupts to alert the host processor of certain status conditions.
Interrupts may be mapped to either (or both) of two designated
output pins, INT1 and INT2, by setting the appropriate bits in
the INTMAP1 and INTMAP2 registers. All functions can be
used simultaneously. If multiple interrupts are mapped to one
pin, the OR combination of the interrupts determines the status
of the pin.
If no functions are mapped to an interrupt pin, that pin is
automatically configured to a high impedance (high-Z) state.
The pins are placed in this state upon a reset as well.
When a certain status condition is detected, the pin that condition
is mapped to is activated. The configuration of the pin is active
high by default, so that when it is activated, the pin goes high.
However, this configuration can be switched to active low by
setting the INT_LOW pin in the appropriate INTMAP register.
Figure 2. Screen Shot of Evaluation Software Output
For better visibility, the X-axis and Z-axis plots are disabled
using the radio buttons above the chart.
The INT pins may be connected to the interrupt input of a host
processor and interrupts responded to with an interrupt routine.
Because multiple functions can be mapped to the same pin, the
STATUS register can be used to determine which condition
caused the interrupt to trigger.
The output of the ADP195, or the interrupt pin itself, was
measured using a digital multimeter. When the ADXL362 is awake,
the interrupt goes high and drives the EN pin of the ADP195
high, which in turn drives the gate of the MOSFET low, causing the
switch to close, connecting any downstream circuitry to the power
supply. Conversely, when the ADXL362 is asleep, the interrupt
drives the EN pin of the ADP195 low, which in turn drives the
gate of the MOSFET high, causing the switch to open.
The CN0274 evaluation software configures the ADXL362 such
that when activity is detected, the INT1 pin is high, and when
inactivity is detected, the INT1 pin is low.
Test Results
All testing was performed using the EVAL-CN0274-SDPZ and
the EVAL-SDP-CS1Z. Functionality of the part is demonstrated
by setting the activity threshold at 0.5 g, the inactivity threshold
at 0.75 g, and the number of inactivity samples at 20. When
looking for activity, only one acceleration sample on any axis is
required to cross the threshold.
PCB Layout Considerations
In any circuit where accuracy is crucial, it is important to
consider the power supply and ground return layout on the
board. The PCB should isolate the digital and analog sections as
much as possible. The PCB for this system was constructed in a
4-layer stack up with large area ground plane layers and power
plane polygons. See the MT-031 Tutorial for more discussion on
layout and grounding, and the MT-101 Tutorial for information
on decoupling techniques.
Starting with the circuit oriented so that the battery pack is flat
against the table, the printed circuit board (PCB) can be slowly
rotated 90° in any direction causing the acceleration to cross the
threshold as it approaches perpendicular to the initial orientation.
Decouple the power supply to the ADXL362 with 1 µF and
0.1 µF capacitors to properly suppress noise and reduce ripple.
Place the capacitors as close to the device as possible. Ceramic
capacitors are advised for all high frequency decoupling.
Figure 2 shows a screen shot of the CN0274 evaluation software
showing the ADXL362 first asleep, looking for activity. Then,
when Sample 11 crosses the threshold, the ADXL362 enters the
awake state and begins looking for inactivity. The thresholds
adjust to show the device is now looking for inactivity.
Power supply lines should have as large a trace width as possible
to provide low impedance paths and reduce glitch effects on the
supply line. Shield clocks and other fast switching digital signals
from other parts of the board by digital ground. A photo of the
PCB is shown in Figure 3.
A complete design support package for this circuit note can be
found at www.analog.com/CN0274-DesignSupport.
Rev. A | Page 4 of 6
Circuit Note
CN-0274
Figure 3. Photo of EVAL-CN0274-SDPZ PCB
Because the ADXL362 requires a relatively small amount of
COMMON VARIATIONS
power in both the asleep and awake states, it is possible to
power the EVAL-CN0274-SDPZ from the digital data lines
coming out of the EVAL-SDP-CS1Z.
The maximum continuous operating current of the ADP195 is
fixed at 1.1 A. For applications requiring more downstream power,
a higher current rated switch can be used in place of the ADP195.
Equipment Needed
By sacrificing approximately 15 µA of quiescent current, the
ADP197 is capable of providing 3 A of current to downstream
circuitry. For applications requiring less downstream power, the
ADP190 can be used. It has a continuous current of 500 mA
and is available in a smaller WLCSP package than the ADP195.
The following equipment is needed:
• A PC with a USB port and Windows® XP or Windows Vista®
(32-bit), or Windows® 7 (32-bit)
• The EVAL-CN0274-SDPZ evaluation board
• The EVAL-SDP-CS1Z evaluation board
• The CN0274 Evaluation Software
• A power supply: 3.0 V or 2 AAA batteries.
Getting Started
A second variant of the provided solution is to create a free fall
detection system. This function can be implemented using the
inactivity interrupt. When an object is in true free-fall, acceleration
on all axes is 0 g. Thus, free-fall detection is achieved by looking
for acceleration on all axes to fall below a certain threshold (close to
0 g) for a certain amount of time.
Load the evaluation software by placing the CN0274 evaluation
software CD into the PC. Using My Computer, locate the drive
that contains the evaluation software CD and open the Readme
file. Follow the instructions contained in the Readme file for
installing and using the evaluation software.
The ADXL362 functions as a free-fall detector by setting the
inactivity threshold (300 mg to 600 mg) and inactivity time
(150 ms to 350 ms). The register setting for these values varies
based on the g-range setting of the device.
CIRCUIT EVALUATION AND TEST
This circuit uses the EVAL-SDP-CS1Z System Demonstration
Platform (SDP) evaluation board and the EVAL-CN0274-SDPZ
circuit board. The two boards have 120-pin mating connectors,
allowing for the quick setup and evaluation of the performance
of the circuit.
The EVAL-CN0274-SDPZ contains the circuit to be evaluated,
as described in this note, and the EVAL-SDP-CS1Z is used with
the CN0274 evaluation software to capture the data from the
EVAL-CN0274-SDPZ.
Rev. A | Page 5 of 6
CN-0274
Circuit Note
Functional Block Diagram
Test
See Figure 4 for the test setup block diagram, and the EVA L -
CN0274-SDPZ-SCH-RevA.pdf file for the circuit schematics.
This file is contained in the CN0274 Design Support Package.
Apply power to the J3 screw terminal or place batteries in the J2
connector on the bottom of the PCB batteries (move Jumper J6
to the left-hand position for battery operation). Launch the
CN0274 evaluation software and connect the USB cable from
the PC to the mini-USB connector on the EVAL-SDP-CS1Z.
3V
PC
POWER
SUPPLY
Once USB communications are established, the EVA L-SDP-CS1Z
can now be used to send, receive, and capture serial data from
the EVAL-CN0274-SDPZ.
USB
J3
Information regarding the EVAL-SDP-CS1Z can be found in
the SDP User Guide.
EVAL-SDP-CS1Z
EVAL-CN0274-SDPZ
Information and details regarding test setup and calibration, and
how to use the evaluation software for data capture can be found in
the software Readme file found at: www.analog.com/CN0274-
UserGuide.
120-PIN
CONNECTOR
J1
J2
OPTIONAL
AAA BATTERIES
LEARN MORE
Figure 4. Test Setup Block Diagram
CN-0274 Design Support Package:
http://www.analog.com/CN0274-DesignSupport
Setup
MT-031 Tutorial, Grounding Data Converters and Solving the
Mystery of “AGND” and “DGND”, Analog Devices.
Connect the 120-pin connector on the EVAL-CN0274-SDPZ to
the connector on the EVAL-SDP-CS1Z. Use nylon hardware to
firmly secure the two boards, using the holes provided at the
ends of the 120-pin connectors.
MT-101 Tutorial, Decoupling Techniques, Analog Devices.
AN-688 Application Note, Phase and Frequency Response of
iMEMS Accelerometers and Gyros, Analog Devices
With power to the supply off, connect a 3.0 V power supply to the
J3 connector. Alternatively, Connector J2 can be used on the
bottom of the PCB to power the entire circuit off two AAA
batteries. Connect the USB cable supplied with the EVAL-SDP-
CS1Z to the USB port on the PC. Note: Do not connect the USB
cable to the mini-USB connector on the SDP board at this time.
Data Sheets and Evaluation Boards
CN0274 Circuit Evaluation Board (EVAL-CN0274-SDPZ)
System Demonstration Platform (EVAL-SDP-CS1Z)
ADXL362 Data Sheet
ADP195 Data Sheet
ADP195 Evaluation Board
REVISION HISTORY
11/12—Rev. 0 to Rev. A
Changes to Circuit Note Title and Circuit Function and Benefits
Section.................................................................................................1
Changes to Common Variations .....................................................5
9/12—Rev. 0: Initial Version
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CN10773-0-11/12(A)
Rev. A | Page 6 of 6
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