EVAL-ADXL346Z Analog Devices Inc, EVAL-ADXL346Z Datasheet - Page 32

• Manufacturer Part Number: EVAL-ADXL346Z
• Description: Inertial Sensor Evaluation System
• Manufacturer: Analog Devices Inc

Specifications of EVAL-ADXL346Z

• Silicon Manufacturer: Analog Devices
• Silicon Core Number: ADXL346
• Kit Application Type: Sensing - Motion / Vibration / Shock
• Application Sub Type: Accelerometer
• Silicon Family Name: IMEMS
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant

ADXL346
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:
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 Z
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 7 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 greater than or equal to 100 Hz. This is done by ensuring that
a value greater than or equal to 0x0A is written into the rate bits
(Bit D3 through Bit D0) in the BW_RATE register (Address 0x2C).
The part also must be placed into 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
X
Y
Z
OFFSET
OFFSET
OFFSET
= −Round(−13/4) = 3 LSB
= −Round(9/4) = −2 LSB
= −Round(10/4) = −3 LSB
ACTUAL
equation.
Rev. A | Page 32 of 40
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, X
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, X
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:
Because the measured output for each axis is expressed in LSBs,
X
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 V
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.
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
codes. If V2 or V3 are set, their respective code is a valid
orientation. If V2 or V3 are cleared, the orientation of the
accelerometer is unknown, such as when the orientation is
within the dead zone between valid regions.
ST
, Y
X
Y
Z
ST
ST
ST
ST
, and Z
= Z
= X
= Y
ST_ON
ST_ON
ST_ON
ST
are also expressed in LSBs. These values can be
− Z
− Y
− X
ST_OFF
ST_OFF
ST_OFF
ST_ON
, Y
ST_ON
ST_OFF
, and Z
, Y
S
of 2.6 V. For other voltages,
ST_OFF
ST_ON
, and Z
. Self-test can then
ST_OFF
.