ADXL3ꢀ1
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.
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.
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
Register 0x20). This results in the data acquired from the DATAX,
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.
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.
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:
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.
X
ACTUAL = XMEAS − X0g
ACTUAL = YMEAS − Y0g
Y
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:
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.
Z0g = Z1g − SZ
Z
ACTUAL = ZMEAS − Z0g
The ADXL346 can automatically compensate the output for offset
by using the offset registers (Register 0x1E, Register 0x1F, and
Register 0x20). These registers contain an 8-bit, twos complement
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.
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,
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
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