CN-0274

Circuit Note

Rev. A | Page 2 of 6

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

(µg/√Hz Typical)

Current Consumption

(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.