ADXL313 Datasheet, PDF (23/28 Pages) Analog Devices – 3-Axis, ±0.5 g/±1 g/±2 g/±4 g Digital Accelerometer

Data Sheet
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 4 of Table 1). This definition assumes that the sensor
does not move between these two measurements because, if the
sensor moves, a nonself test related shift corrupts the test.
Proper configuration of the ADXL313 is also necessary for an
accurate self test measurement. Set the part 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 must also be placed into normal power operation by
ensuring that the LOW_POWER bit in the BW_RATE register
is cleared (LOW_POWER bit = 0) for accurate self test measure-
ments. It is recommended that the part be set to full resolution,
±4 g mode to ensure that there is sufficient dynamic range for
the entire self test shift. This is done by setting Bit D3 of the
DATA_FORMAT register (Address 0x31) and writing a value of
0x03 to the range bits (Bit D1 and Bit D0) of the DATA_FORMAT
register. This results in a high dynamic range for measurement
and 1024 LSB/g sensitivity.
After the part is configured for accurate self test measurement,
several samples of x-, y-, and z-axis acceleration data should be
retrieved from the sensor and averaged together. The number of
samples averaged is a choice of the system designer, but a recom-
mended starting point is 0.1 sec worth of data, which corresponds
to 10 samples at 100 Hz data rate. Store and label the averaged
values appropriately as the self test disabled data, that is, XST_OFF,
YST_OFF, and ZST_OFF.
ADXL313
Next, enable self test by setting Bit D7 of the DATA_FORMAT
register (Address 0x31). The output needs some time (about
four samples) to settle after enabling self test. After allowing the
output to settle, take several samples of the x-, y-, and z-axis
acceleration data, and average them. It is recommended that the
same number of samples be taken for this average as was previously
taken. Store and label these averaged values 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 acceleration (g) by multiplying each value by the
sensitivity, 1024 LSB/g, when configured for full resolution
mode. When operating in 10-bit mode, the self test delta in
LSBs varies according to the selected g range, even though the
self test force, in g, remains unchanged. Using a range below
±4 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.
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