The circuit shown in Figure 1 uses a three-axis ADXL362 digital accelerometer and an ADP195 high-end power switch to build an ultra-low-power, motion-sensitive switch.

This combination of devices provides an industry-leading low-power solution for independent motion switches that control load power.

The ADXL362  is an ultra-low power three-axis accelerometer that consumes less than 100 nA in wake mode. Unlike accelerometers, which use power duty cycles to achieve low power consumption, the ADXL362 does not alias the input signal through undersampling; it samples continuously at the full data rate. There is also an on-chip, 12-bit temperature sensor with an accuracy of ±0.5°.

The output resolution of the ADXL362 is 12 bits and supports three operating ranges of ±2 g, ±4 g and ±8 g. The resolution within the ±2 g range is 1 mg/LSB. Applications requiring noise levels below 480μg/√Hz can select one of two low-noise modes (as low as 120μg/√Hz) with minimal increase in supply current.

The ADP195 is a high-end load switch that operates from a 1.1 V to 3.6 V supply and prevents reverse current flow from the output to the input. The device incorporates a low on-resistance P-channel MOSFET, which supports continuous load currents of more than 1.1 A and minimizes power losses.

Basic working principle of ADXL362

The ADXL362 is a three-axis, ultra-low power acceleration measurement system capable of measuring both dynamic acceleration (caused by motion or impact) and static acceleration (i.e., gravity).

The sensor's moving elements are micromachined polysilicon surface structures (also called beams) placed on top of a silicon wafer. Polysilicon springs are suspended from the structure on the wafer surface to provide resistance to acceleration forces.

Structural deflection is measured by differential capacitance. Each capacitor is composed of an independent fixed plate and a movable mass connecting plate. Any acceleration deflects the beam, imbalances the differential capacitance, and causes the amplitude of the sensor output to be proportional to the acceleration. Phase-sensitive demodulation is used to determine the magnitude and polarity of acceleration.

Operating mode

The three basic operating modes of the ADXL362 are standby, measurement and wake-up.

• Placing the ADXL362 in standby mode suspends measurements and reduces power consumption to 10nA. Any pending data or interrupts are retained, but new information is not 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, the device continuously reads acceleration data. When powered from a 2.0 V supply, the accelerometer consumes less than 3 μA over the entire range of output data rates up to 400 Hz. When working in this mode, all the functions described are available. As an ultra-low-power accelerometer, the ADXL362 is capable of continuously outputting data at data rates from 12.5 Hz (minimum) to 400 Hz (maximum) while still consuming less than 3 μA. Capable of continuously sampling the full bandwidth of its sensor at all data rates, the ADXL362 is immune to undersampling and aliasing.


• Wake-up mode is ideal for simply detecting the presence of motion with very low power consumption (270 nA at a supply voltage of 2.0 V). Wake-up mode is especially useful when implementing a motion-activated switch that allows the rest of the system to remain off until motion is detected. In wake mode, only 6 acceleration measurements are taken per second to determine whether motion is present, which reduces power consumption to very low levels. In wake mode, all features of the accelerometer are available except the activity timer. All registers can be accessed and real-time data can be obtained from the device.

The CN0274 evaluation software uses the ADXL362 wake-up mode. That is, the ADXL362 remains dormant until motion is detected, and enters measurement mode once motion is detected.

Power/Noise Tradeoff

The ADXL362 offers several options for noise reduction, but will result in a slight increase in power consumption when used.

At a bandwidth of 100 Hz, the noise performance of the ADXL362 in normal operation is typically 7 LSB rms, which is suitable for most applications, depending on the bandwidth and required resolution. For situations where lower noise is required, the ADXL362 offers two low-noise operating modes that reduce noise at the expense of slightly increased power consumption.

Table 1 shows the power consumption values ​​and noise density in normal operating mode and two low-noise modes, where the supply voltage is typically 3.3 V.

The CN0274 evaluation software uses the ADXL362's normal operating noise mode.

motion detection

The ADXL362's built-in logic detects motion (acceleration exceeds a certain threshold) and inactivity (acceleration does not exceed a certain threshold).

Detection of motion or inactivity events is indicated by the status register, which can also be configured to generate an interrupt. In addition, the motion status of the device (that is, whether the device is moving or stationary) is indicated by the AWAKE bit.

Motion and inactivity detection is available when the accelerometer is in measurement mode or wake mode.

motion detection

A motion event is detected when acceleration remains above a specified threshold for a user-specified period of time. There are two types of motion detection events: absolute motion detection and reference motion detection.

• When using absolute motion detection , acceleration samples are compared to a user-set threshold to determine whether motion is present. For example, if the threshold is set to 0.5 g, the acceleration in any axis is 1 g and the duration exceeds the user-defined motion time, the motion state is set. In many applications, motion detection based on deviations from a reference point or orientation is preferable to motion detection based on absolute thresholds. This is particularly useful as it eliminates the static 1 g effect on motion detection caused by gravity. When the accelerometer is stationary, although it is not moving, its output can still reach 1 g. When using absolute motion detection, if the threshold is set to less than 1 g, motion can be detected immediately.


• In reference motion detection mode , motion is detected when acceleration samples are above an internally defined reference value by at least a user-set amount for a user-defined period of time. When motion detection is enabled a reference value is calculated and the first sample acquired is used as a reference point. Movement is only detected if the acceleration deviates sufficiently from this initial orientation. The reference configuration makes motion detection extremely sensitive and can detect even the subtlest motion events.

The CN0274 evaluation software uses the reference operating mode when searching for motion.

Stationary detection

A stationary event is detected when the acceleration remains below a specified threshold for a specified period of time. There are two types of non-motion detection events: absolute stillness detection and reference stillness detection.

• When using absolute stationary detection , acceleration samples are compared to a user-set threshold over a user-set period of time to determine if motion is absent.


• When using a reference stationary check , the acceleration samples are compared to a user-specified reference for a user-defined period of time. When the device first enters wake-up state, the first sample is used as a reference point and a threshold is applied around that point. If the acceleration remains within the threshold, the device will enter sleep state. If the acceleration value falls outside the threshold range, that point is used as a new reference and the threshold is reapplied for that point.

The CN0274 evaluation software uses reference operating mode when searching for quiescence.

Link motion and stillness detection

Motion and inactivity detection can be used simultaneously and then handled manually by the host processor, or can be configured to interact in a variety of ways:

• In default mode , both motion and inactivity detection features are enabled, and all interrupts must be handled by the host processor; that is, each interrupt must be read by the processor before it can be cleared and used again.


• In linked mode , the motion and inactivity detection functions are linked to each other so that only one function is enabled at any given time. Once motion is detected, the device is considered to be in motion or awake, and then no more motion is searched for: the next event is expected to be stationary, so only stationary detection works. If quiescence is detected, the device is considered to be in quiescence or sleep state. At this point the next event is expected to be motion, so only motion detection works. In this mode, the host processor must handle each interrupt and then enable the next operation.


• In loop mode , motion detection works in the same way as linked mode described above; however, interrupts do not need to be handled by the host processor. This configuration simplifies the implementation of commonly used motion detection and enhances power savings by reducing bus communication power consumption.


• If auto-sleep mode is enabled in link mode or loop mode , the device autonomously enters wake-up mode upon detection of a stationary event and re-enters measurement mode upon detection of a motion event.

The CN0274 evaluation software uses automatic sleep and loop modes to demonstrate the functionality of the ADXL362.

AWAKE Bit

The AWAKE bit is a status bit that indicates whether the ADXL362 is awake or sleeping. Detection of a motion condition indicates that the device is awake, and detection of a stationary condition indicates that the device is in sleep state.

The wake-up signal can be mapped to the INT1 or INT2 pin and therefore can be used as a status output to connect or disconnect power from downstream circuitry depending on the accelerometer's wake-up status. When used with loop mode, this configuration enables a tiny autonomous motion-activated switch.

If the on-time of the downstream circuitry is within acceptable limits, this motion switch configuration can significantly reduce system-level power consumption by eliminating standby power from the rest of the application. This standby power consumption typically exceeds the entire power consumption range of the ADXL362.

interrupt

The ADXL362 has built-in features that trigger interrupts to alert the host processor of certain status conditions.

Interrupts can be mapped to one (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 a pin, the OR combination of interrupts determines the state of the pin

If no function is mapped to an interrupt pin, the pin is automatically configured in a high-impedance state (high-impedance state). The pin also enters this state after reset

When a specific status condition is detected, the pin to which the condition is mapped is activated. By default, the pin is configured to be active high, so when activated the pin goes high. However, the configuration can be switched to active low by setting the INT_LOW pin in the appropriate INTMAP register

The INT pin can be connected to the interrupt input of the host processor and respond to interrupts with an interrupt routine. Since multiple functions can be mapped to the same pin, the STATUS register can be used to determine the specific conditions that caused the interrupt to trigger.

The CN0274 evaluation software configures the ADXL362 as follows: after motion is detected, the INT1 pin is high; after motion is detected, the INT1 pin is low.

Test Results

All tests were performed using EVAL-CN0274-SDPZ and EVAL-SDP-CS1Z . To demonstrate the functionality of the device, the motion threshold was set to 0.5 g, the rest threshold was set to 0.75 g, and the number of rest samples was set to 20. When searching for motion, only one acceleration sample in any axis crosses the threshold.

Initially, with the circuit positioned so that the battery pack is flush with the table, the printed circuit board (PCB) is slowly rotated 90° in any direction, causing acceleration to cross the threshold as it approaches a position perpendicular to the initial orientation.

Figure 2 shows a screenshot of the CN0274 evaluation software, where the ADXL362 is initially sleeping and searching for motion. Then, when sample 11 crosses the threshold, the ADXL362 wakes up and begins searching for inactivity. The threshold is adjusted to indicate that the device is searching for quiescence.

For better presentation, the X-axis and Z-axis curves have been disabled using the radio buttons on the graph.

The output of the ADP195 (or the interrupt pin itself) is measured with 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, which turns on any downstream circuitry and Power connection. Conversely, when the ADXL362 is in sleep mode, an interrupt drives the ADP195's EN pin low, which in turn drives the MOSFET's gate high, causing the switch to open.

PCB layout considerations

In any circuit where precision is important, power and ground return layout on the circuit board must be carefully considered. The PCB should isolate the digital and analog parts as much as possible. The PCB of this system is stacked with 4-layer boards, with large area polygons for the ground layer and power layer. See the MT-031 guide for a detailed discussion of layout and grounding, and the MT-101 guide for information on decoupling techniques .

The ADXL362 power supply should be decoupled with 1 μF and 0.1 μF capacitors for proper noise suppression and ripple reduction. These capacitors should be placed as close to the device as possible. For all high frequency decoupling, ceramic capacitors are recommended.

Power traces should be as wide as possible to provide a low impedance path and reduce the effects of glitches on the power lines. Clocks and other fast-switching digital signals are digitally shielded so that they do not affect other components on the circuit board. A photo of the PCB is shown in Figure 3.

For the complete design support package for this circuit note, please visit www.analog.com/CN0274-DesignSupport .