ADXL346ACCZ-R2 datasheet(31/40 Pages) AD

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Data Sheet

ADXL346

THRESHOLD

The lower output data rates are achieved by decimating a

common sampling frequency inside the device. The activity,

free-fall, and single-tap/double-tap detection functions without

improved tap enabled are performed using undecimated data.

As 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 is used to determine activity,

free-fall, and single-tap/double-tap events may not be present if

the output of the accelerometer is examined. This may 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 part cannot go into autosleep mode.

The asleep bit in the ACT_TAP_STATUS register (Address 0x2B)

indicates whether the part is asleep.

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 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 ADXL346 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 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.

OFFSET CALIBRATION

Accelerometers are mechanical structures containing elements

that are free to move. These moving parts can be very sensitive

to mechanical stresses, much more so than solid-state electronics.

The 0 g bias or offset is an important accelerometer metric because

it defines the baseline for measuring acceleration. Additional

stresses can be applied during assembly of a system containing

an accelerometer. These stresses can come from, but are not

limited to, component soldering, board stress during mounting,

and application of any compounds on or over the component. If

calibration is deemed necessary, it is recommended that calibration

be performed after system assembly to compensate for these effects.

A simple method of calibration is to measure the offset while

assuming that the sensitivity of the ADXL346 is as specified in

Table 1. The offset can then be automatically accounted for by

using the built-in offset registers (Register 0x1E, Register 0x1F, and

DATAY, and DATAZ registers (Address 0x32 to Address 0x37)

already compensating for any offset.

In a no-turn or single-point calibration scheme, the part is oriented

such that one axis, typically the z-axis, is in the 1 g field of gravity

and the remaining axes, typically the x- and y-axes, are in a 0 g

field. The output is then measured by taking the average of a

series of samples. The number of samples averaged is a choice of

the system designer, but a recommended starting point is 0.1 sec

worth of data for data rates of 100 Hz or greater. This corresponds

to 10 samples at the 100 Hz data rate. For data rates of less than

100 Hz, it is recommended that at least 10 samples be averaged

together. These values are stored as X0g, Y0g, and Z+1g for the 0 g

measurements on the x- and y-axes and the 1 g measurement

on the z-axis, respectively.

The values measured for X0g and Y0g correspond to the offset of

the x- and y-axes, and compensation is done by subtracting those

values from the output of the accelerometer to obtain the actual

acceleration:

XACTUAL = XMEAS − X0g

YACTUAL = YMEAS − Y0g

Because the z-axis measurement is done in a 1 g field, a no-turn or

single-point calibration scheme assumes an ideal sensitivity, SZ,

for the z-axis. This is subtracted from Z+1g to attain the z-axis

offset, which is then subtracted from future measured values to

obtain the actual value:

Z0g = Z1g − SZ

ZACTUAL = ZMEAS − Z0g

The ADXL346 can automatically compensate the output for offset

by using the offset registers (Register 0x1E, Register 0x1F, and

value that is automatically added to all measured acceleration

values, and the result is then placed into the DATAX, DATAY,

and DATAZ registers. Because the value placed in an offset register

is additive, a negative value is placed into the register to eliminate a

positive offset and vice versa for a negative offset. The register

has a scale factor of 15.6 mg/LSB and is independent of the

selected g range.

As an example, assume that the ADXL346 is placed into full-

resolution mode with a sensitivity of typically 256 LSB/g. The

part is oriented such that the z-axis is in the field of gravity and

the outputs of the x-, y-, and z-axes are measured as +10 LSB,

−13 LSB, and +9 LSB, respectively. Using the previous equations,

X0g is +10 LSB, Y0g is −13 LSB, and Z0g is +9 LSB. Each LSB of

Rev. C | Page 31 of 40

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