ADXL346ACCZ-R2 datasheet(32/40 Pages) AD

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ADXL346

Data Sheet

output in full-resolution is 3.9 mg or one-quarter of an LSB of

the offset register.

Because the offset register is additive, the 0 g values are negated

and rounded to the nearest LSB of the offset register:

XOFFSET = −Round(10/4) = −3 LSB

YOFFSET = −Round(−13/4) = 3 LSB

ZOFFSET = −Round(9/4) = −2 LSB

These values are programmed into the OFSX, OFSY, and OFXZ

registers, respectively, as 0xFD, 0x03, and 0xFE. As with all

registers in the ADXL346, the offset registers do not retain the

value written into them when power is removed from the part.

Power-cycling the ADXL346 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. For instance, if the actual sensitivity was 250 LSB/g

in the previous example, the offset would be 15 LSB, not 9 LSB.

To help minimize this error, an additional measurement point

can be used with the z-axis in a 0 g field, and the 0 g measurement

can be used in the ZACTUAL equation.

USING SELF-TEST

The self-test change is defined as the difference between the

acceleration output of an axis with self-test enabled and the

acceleration output of the same axis with self-test disabled (see

Endnote 8 of Table 1). This definition assumes that the sensor

does not move between these two measurements, because if the

sensor moves, a non–self-test related shift corrupts the test.

Proper configuration of the ADXL346 is also necessary for an

accurate self-test measurement. The part should be set with a data

rate of 100 Hz through 800 Hz, or 3200 Hz. This is done by

ensuring that a value of 0x0A through 0x0D, or 0x0F is written

into the rate bits (Bit D3 through Bit D0) in the BW_RATE

normal power operation by ensuring that the LOW_POWER

bit (Bit D4) in the BW_RATE register is cleared

(LOW_POWER bit = 0) for accurate self-test measurements. It

is recommended that the part be set to full-resolution, 16 g

mode to ensure that there is sufficient dynamic range for the

entire self-test shift. This is done by setting the FULL_RES bit

(Bit D3) and writing a value of 0x03 to the range bits (Bit D1

and Bit D0) of the DATA_FORMAT register (Address 0x31).

This results in a high dynamic range for measurement and a

3.9 mg/LSB scale factor.

After the part is configured for accurate self-test measurement,

several samples of acceleration data for the x-, y-, and z-axes

should be retrieved from the sensor and averaged together. 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. The

averaged values should be stored and labeled appropriately as the

self-test disabled data, that is, XST_OFF, YST_OFF, and ZST_OFF.

Next, self-test should be enabled by setting Bit D7 of the

DATA_FORMAT register (Address 0x31). The output needs some

time (about four samples) to settle once self-test is enabled. After

allowing the output to settle, several samples of acceleration data

for the x-, y-, and z-axes should be taken again and averaged. It

is recommended that the same number of samples be taken for

this average as was previously taken. These averaged values should

again be stored and labeled appropriately as the value with self-

test enabled, that is, XST_ON, YST_ON, and ZST_ON. Self-test can then

be disabled by clearing Bit D7 of the DATA_FORMAT register

(Address 0x31).

With the stored values for self-test enabled and disabled, the

self-test change is as follows:

XST = XST_ON − XST_OFF

YST = YST_ON − YST_OFF

ZST = ZST_ON − ZST_OFF

Because the measured output for each axis is expressed in LSBs,

XST, YST, and ZST are also expressed in LSBs. These values can be

converted to g’s of acceleration by multiplying each value by the

3.9 mg/LSB scale factor, if configured for full-resolution mode.

Additionally, Table 15 through Table 18 correspond to the self-test

range converted to LSBs and can be compared with the measured

self-test change when operating at a VS of 2.6 V. For other voltages,

the minimum and maximum self-test output values should be

adjusted based on (multiplied by) the scale factors shown in

Table 14. If the part was placed into ±2 g, 10-bit or full-resolution

mode, the values listed in Table 15 should be used. Although

the fixed 10-bit mode or a range other than 16 g can be used, a

different set of values, as indicated in Table 16 through Table 18,

would need to be used. Using a range below 8 g may result in

insufficient dynamic range and should be considered when

selecting the range of operation for measuring self-test.

If the self-test change is within the valid range, the test is considered

successful. Generally, a part is considered to pass if the minimum

magnitude of change is achieved. However, a part that changes

by more than the maximum magnitude is not necessarily a failure.

Another effective method for using the self-test to verify accel-

erometer functionality is to toggle the self-test at a certain rate

and then perform an FFT on the output. The FFT should have a

corresponding tone at the frequency the self-test was toggled.

This methodology removes the dependency of the test on supply

voltage and on self-test magnitude, which can vary within a rather

wide range.

ORIENTATION SENSING

The orientation function of the ADXL346 reports both 2D

and 3D orientation concurrently through the orient register

(Address 0x3C). The V2 and V3 bits (Bit D6 and Bit D3 in the

orient register) report the validity of the 2D and 3D orientation

Rev. C | Page 32 of 40

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