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Data Sheet

ADXL314

APPLICATIONS INFORMATION

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Rev. A | 23 of 27

POWER SUPPLY REQUIREMENTS

The ADXL314 operates using supply voltage rails ranging from 2.0

V to 3.9 V. The operating voltage range (VS) specified in Table

1 ranges from 2.0 V to 3.6 V to account for inaccuracies and

transients of up to ±10% on the supply voltage.

When powering up the ADXL314, the VS and VDD I/O rise time must

be linear and within 250 µs to reach 2.0 V. When powering down,

VS and VDD I/O must be fully discharged to ground level (0 V) for

at least 200 ms before the device is powered back up. To enable

supply discharge, it is recommended to power the device from a

microcontroller general-purpose input and output (GPIO), connect a

shutdown discharge switch to the supply, or use a voltage regulator

with a shutdown discharge feature.

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 ADXL314 supply pins

are 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 performance.

Make sure that the connection from the ADXL314 ground to the

power supply ground has low impedance because noise transmit-

ted 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 it

is not possible to use separate supplies, additional filtering of the

supplies, as previously mentioned, may be necessary.

Figure 29. Application Diagram

MECHANICAL CONSIDERATIONS FOR

MOUNTING

Mount the ADXL314 on the PCB in a location close to a hard

mounting point of the PCB to the case. Mounting the ADXL314 at

an unsupported PCB location, as shown in Figure 30, may result

in large, apparent measurement errors due to undampened PCB

vibration. Locating the accelerometer near a hard mounting point

ensures that any PCB vibration at the accelerometer is above

the mechanical sensor resonant frequency of the accelerometer

and is, therefore, effectively invisible to the accelerometer. Multiple

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

to reduce the effects of system resonance on the performance of

the sensor.

Figure 30. Incorrectly Placed Accelerometers

THRESHOLD

The lower output data rates are achieved by decimating a common

sampling frequency inside the device. The activity detection func-

tion 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 determine activity may not be present if the output of the

accelerometer is examined, which can result in functions triggering

when acceleration data does not appear to meet the conditions set

by the user for the corresponding function.

LINK MODE

The function of the link bit 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 device 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 power consumption, is not

intended for data acquisition.

However, when sleep mode is used in conjunction with the auto-

sleep mode and the link mode, the device 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 ADXL314 is in sleep mode, the host processor can also

be placed into sleep mode or low power mode to save significant

system power. Once activity is detected, the accelerometer auto-

matically 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.

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