Datasheet

ADXL345 Data Sheet
Rev. E | Page 22 of 40
SELF-TEST
The ADXL345 incorporates a self-test feature that effectively
tests its mechanical and electronic systems simultaneously.
When the self-test function is enabled (via the SELF_TEST bit
in the DATA_FORMAT register, Address 0x31), an electrostatic
force is exerted on the mechanical sensor. This electrostatic force
moves the mechanical sensing element in the same manner as
acceleration, and it is additive to the acceleration experienced
by the device. This added electrostatic force results in an output
change in the x-, y-, and z-axes. Because the electrostatic force
is proportional to V
S
2
, the output change varies with V
S
. This
effect is shown in Figure 43. The scale factors shown in Table 14
can be used to adjust the expected self-test output limits for
different supply voltages, V
S
. The self-test feature of the ADXL345
also exhibits a bimodal behavior. However, the limits shown in
Table 1 and Table 15 to Table 18 are valid for both potential self-
test values due to bimodality. Use of the self-test feature at data
rates less than 100 Hz or at 1600 Hz may yield values outside
these limits. Therefore, the part must be in normal power operation
(LOW_POWER bit = 0 in BW_RATE register, Address 0x2C)
and be placed into a data rate of 100 Hz through 800 Hz or 3200 Hz
for the self-test function to operate correctly.
–6
–4
–2
0
2
4
6
2.0 2.5 3.3 3.6
V
S
(V)
SELF-TEST SHIFT LIMIT (g)
X HIGH
X LOW
Y HIGH
Y LOW
Z HIGH
Z LOW
07925-242
Figure 43. Self-Test Output Change Limits vs. Supply Voltage
Table 14. Self-Test Output Scale Factors for Different Supply
Voltages, V
S
Supply Voltage, V
S
(V) X-Axis, Y-Axis Z-Axis
2.00 0.64 0.8
2.50 1.00 1.00
3.30 1.77 1.47
3.60 2.11 1.69
Table 15. Self-Test Output in LSB for ±2 g, 10-Bit or Full
Resolution (T
A
= 25°C, V
S
= 2.5 V, V
DD I/O
= 1.8 V)
Axis Min Max Unit
X 50 540 LSB
Y −540 −50 LSB
Z 75 875 LSB
Table 16. Self-Test Output in LSB for ±4 g, 10-Bit Resolution
(T
A
= 25°C, V
S
= 2.5 V, V
DD I/O
= 1.8 V)
Axis Min Max Unit
X 25 270 LSB
Y −270 −25 LSB
Z 38 438 LSB
Table 17. Self-Test Output in LSB for ±8 g, 10-Bit Resolution
(T
A
= 25°C, V
S
= 2.5 V, V
DD I/O
= 1.8 V)
Axis Min Max Unit
X 12 135 LSB
Y −135 −12 LSB
Z 19 219 LSB
Table 18. Self-Test Output in LSB for ±16 g, 10-Bit Resolution
(T
A
= 25°C, V
S
= 2.5 V, V
DD I/O
= 1.8 V)
Axis Min Max Unit
X 6 67 LSB
Y −67 −6 LSB
Z 10 110 LSB