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ADXL313WACPZ

Digital Accelerometer / 32-Lead Lead Frame Chip

Analog Devices 

ADXL313

APPLICATIONS INFORMATION

POWER SUPPLY DECOUPLING

A 1 μF tantalum capacitor (CS) at VS and a 0.1 μF ceramic capacitor

(CI/O) at VDD I/O placed close to the ADXL313 supply pins is

recommended to adequately decouple the accelerometer from

noise on the power supply. If additional decoupling is necessary,

a resistor or ferrite bead, no larger than 100 Ω, in series with VS

may be helpful. Additionally, increasing the bypass capacitance

on VS to a 10 μF tantalum capacitor in parallel with a 0.1 μF

ceramic capacitor may also improve noise.

Take care to ensure that the connection from the ADXL313

ground to the power supply ground has low impedance because

noise transmitted through ground has an effect similar to noise

transmitted through VS. It is recommended that VS and VDD I/O

be separate supplies to minimize digital clocking noise on the

VS supply. If this is not possible, additional filtering of the

supplies as previously mentioned may be necessary.

VS

VDD I/O

CS

CI/O

INTERRUPT

CONTROL

VS

VDD I/O

ADXL313

SDA/SDI/SDIO

INT1 SDO/ALT ADDRESS

INT2

SCL/SCLK

GND

CS

3-WIRE OR

4-WIRE SPI

OR I2C

INTERFACE

Figure 23. Application Diagram

MECHANICAL CONSIDERATIONS FOR MOUNTING

Mount the ADXL313 on the PCB in a location close to a hard

mounting point of the PCB to the case. Mounting the ADXL313

at an unsupported PCB location, as shown in Figure 24, may

result in large, apparent measurement errors due to undamped

PCB vibration. Placing the accelerometer near a hard mounting

point ensures that any PCB vibration at the accelerometer is above

the accelerometer’s mechanical sensor resonant frequency and,

therefore, effectively invisible to the accelerometer. Multiple

mounting points close to the sensor and/or a thicker PCB also

help to reduce the effect of system resonance on the performance

of the sensor.

ACCELEROMETERS

PCB

MOUNTING POINTS

Figure 24. Incorrectly Placed Accelerometers

Data Sheet

THRESHOLD

The lower output data rates are achieved by decimating a

common sampling frequency inside the device. The activity

detection function is performed using undecimated data.

Because the bandwidth of the output data varies with the data

rate and is lower than the bandwidth of the undecimated data,

the high frequency and high g data that are used to determine

activity may not be present if the output of the accelerometer is

examined. This may result in functions triggering when accelera-

tion data does not appear to meet the conditions set by the user

for the corresponding function.

LINK MODE

The function of the link bit in the POWER_CTL register

(Address 0x2D) is to reduce the number of activity interrupts

that the processor must service by setting the device to look

for activity only after inactivity. For proper operation of this

feature, the processor must still respond to the activity and

inactivity interrupts by reading the INT_SOURCE register

(Address 0x30) and, therefore, clearing the interrupts. If an activity

interrupt is not cleared, the part cannot go into autosleep mode.

SLEEP MODE vs. LOW POWER MODE

In applications where a low data rate and low power consumption

are desired (at the expense of noise performance), it is recom-

mended that low power mode be used. The use of low power

mode preserves the functionality of the DATA_READY

interrupt and the FIFO for postprocessing of the acceleration

data. Sleep mode, while offering a low data rate and low power

consumption, is not intended for data acquisition.

However, when sleep mode is used in conjunction with the

autosleep mode and the link mode, the part can automatically

switch to a low power, low sampling rate mode when inactivity

is detected. To prevent the generation of redundant inactivity

interrupts, the inactivity interrupt is automatically disabled and

activity is enabled. When the ADXL313 is in sleep mode, the host

processor can also be placed into sleep mode or low power

mode to save significant system power. When activity is

detected, the accelerometer automatically switches back to the

original data rate of the application and provides an activity

interrupt that can be used to wake up the host processor.

Similar to when inactivity occurs, detection of activity events is

disabled and inactivity is enabled.

Rev. 0 | Page 22 of 28

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