EVAL-ADXL344Z Analog Devices, EVAL-ADXL344Z Datasheet - Page 30

• Manufacturer Part Number: EVAL-ADXL344Z
• Description: Daughter Cards & OEM Boards EB
• Manufacturer: Analog Devices
• Series: ADXL344r

Specifications of EVAL-ADXL344Z

• Rohs: yes
• Product: Breakout Boards
• Description/function: 3 axis accelerometer evaluation board
• Interface Type: I2C, SPI
• Maximum Operating Temperature: + 85 C
• Minimum Operating Temperature: - 40 C
• Operating Supply Voltage: 1.7 V to 2.75 V
• Factory Pack Quantity: 1
• For Use With: ADXL344

ADXL344
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 ADXL344, the offset registers do not retain the
value written into them when power is removed from the part.
Power-cycling the
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 8 of Table 1). This definition assumes that the sensor
does not move between these two measurements. If the sensor
moves, the additional shift, which is unrelated to self-test,
corrupts the test.
Proper configuration of the
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 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(9/4) = −2 LSB
= −Round(10/4) = −3 LSB
= −Round(−13/4) = 3 LSB
ADXL344
ACTUAL
equation.
ADXL344
returns the offset registers to their
is also necessary for
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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.
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.
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
Data Sheet
ST_OFF
.