ADXL375 Ver la hoja de datos (PDF) - Analog Devices
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ADXL375 Datasheet PDF : 32 Pages
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ADXL375
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. Low power mode preserves
the functionality of the DATA_READY interrupt and the FIFO
buffer for postprocessing of the acceleration data. To enable low
power mode, set the LOW_POWER bit (Bit D4) in the BW_RATE
register (Address 0x2C).
Sleep mode also provides a low data rate and low power consump-
tion, but it is not intended for data acquisition. However, when
sleep mode is used in conjunction with the autosleep and link
modes, 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 the activity interrupt is
enabled. To enable autosleep mode, set the AUTO_SLEEP bit
(Bit D4) and the link bit (Bit D5) in the POWER_CTL register
(Address 0x2D).
When the ADXL375 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 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 detection of 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 it 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 ADXL375 is as specified in
Table 1. The offset can then be automatically accounted for by
using the built-in offset registers. The result of this calibration is
that the data acquired from the data registers already compensates
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.
Data Sheet
The number of samples averaged is selected by the system
designer, but a recommended starting point is 0.1 sec worth of
data for data rates of 100 Hz or greater—that is, 10 samples at
the 100 Hz data rate. For data rates less than 100 Hz, it is recom-
mended that at least 10 samples be averaged. 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 x- and
y-axis offsets, and compensation is performed by subtracting
these values from the output of the accelerometer to obtain the
actual acceleration, as follows:
XACTUAL = XMEAS − X0g
YACTUAL = YMEAS − Y0g
Because the z-axis measurement is performed in a +1 g field,
a no-turn or single-point calibration scheme assumes an ideal
sensitivity, SZ, for the z-axis. This value is subtracted from Z+1g
to obtain the z-axis offset, which is then subtracted from future
measured values to obtain the actual value, as follows:
Z0g = Z+1g − SZ
ZACTUAL = ZMEAS − Z0g
The ADXL375 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; the result is then placed into the data registers. Because
the value placed in an offset register is additive, a negative value
in the register eliminates a positive offset, and a positive value in
the register eliminates a negative offset. The register has a scale
factor of 1.56 g/LSB.
As with all registers in the ADXL375, the offset registers do not
retain the values written into them when power is removed from
the part. Power cycling the ADXL375 returns the offset registers
to their default value of 0x00.
Because the no-turn or single-point calibration method assumes
an ideal sensitivity in the z-axis, any error in the sensitivity results
in offset error.
DATA FORMATTING AT OUTPUT DATA RATES OF
3200 HZ AND 1600 HZ
When using the 3200 Hz or 1600 Hz output data rate, the
LSB of the output data-word is always 0. When the data is right
justified, the LSB corresponds to Bit D0 of the DATAx0 register;
when the data is left justified, the LSB corresponds to Bit D3 of
the DATAx0 register.
Rev. 0 | Page 28 of 32
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