ADXL375 datasheet(28/32 Pages) AD

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ADXL375

Data Sheet

Rev. 0 | Page 28 of 32

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

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.

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

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.

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