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ADXL311_15 Datasheet(Arkusz danych) 9 Page - Analog Devices

Numer części ADXL311_15
Szczegółowy opis  Ultracompact 2g Dual-Axis Accelerometer
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ADXL311
Rev. B | Page 9 of 12
The output of the ADXL311 has a typical bandwidth of 3 kHz.
The user must filter the signal at this point to limit aliasing
errors. The analog bandwidth must be no more than half the
A/D sampling frequency to minimize aliasing. The analog
bandwidth can be further decreased to reduce noise and
improve resolution.
The ADXL311 noise has the characteristics of white Gaussian
noise that contribute equally at all frequencies and are described
in terms of µg/√Hz, i.e., the noise is proportional to the square
root of the bandwidth of the accelerometer. It is recommended
that the user limits the bandwidth to the lowest frequency
needed by the application to maximize the resolution and
dynamic range of the accelerometer.
With the single-pole roll-off characteristic, the typical noise of
the ADXL311 is determined by
(
) (
)6
.
1
Hz
/
300
×
×
µ
=
BW
g
Noise
RMS
At 100 Hz the noise will be
(
) (
)
g
g
Noise
RMS
m
8
.
3
6
.
1
100
Hz
/
300
=
×
×
µ
=
Often the peak value of the noise is desired. Peak-to-peak noise
can only be estimated by statistical methods. Table 6 shows the
probabilities of exceeding various peak values, given the rms value.
Table 6. Estimation of Peak-to-Peak Noise
Peak-to-Peak
Value
% of Time That Noise Exceeds Nominal
Peak-to-Peak Value
2 × rms
32
4 × rms
4.6
6 × rms
0.27
8 × rms
0.006
The peak-to-peak noise value gives the best estimate of the
uncertainty in a single measurement. Table 7 gives the typical
noise output of the ADXL311 for various CX and CY values.
Table 7. Filter Capacitor Selection, CX and CY
Bandwidth
(Hz)
CX, CY
(µF)
RMS Noise
(mg)
Peak-to-Peak Noise
Estimate (mg)
10
0.47
1.2
7.2
50
0.1
2.7
16.2
100
0.047
3.8
22.8
500
0.01
8.5
51
USING THE ADXL311 WITH OPERATING
VOLTAGES OTHER THAN 3 V
The ADXL311 is tested and specified at VDD = 3 V; however, it can
be powered with VDD as low as 2.4 V, or as high as 5.25 V. Some
performance parameters change as the supply voltage varies.
The ADXL311 output is ratiometric, so the output sensitivity
(or scale factor) varies proportionally to the supply voltage. At
VDD = 5 V, the output sensitivity is typically 312 mV/g.
The 0 g bias output is also ratiometric, so the 0 g output is
nominally equal to VDD/2 at all supply voltages.
The output noise is not ratiometric, but absolute in volts;
therefore, the noise density decreases as the supply voltage
increases. This is because the scale factor (mV/g) increases
while the noise voltage remains constant.
The self-test response is roughly proportional to the square of
the supply voltage. At VDD = 5 V, the self-test response is
approximately equivalent to 750 mg (typical).
The supply current increases as the supply voltage increases.
Typical current consumption at VDD = 5 V is 750 µA.
USING THE ADXL311 AS A DUAL-AXIS
TILT SENSOR
One of the most popular applications of the ADXL311 is tilt
measurement. An accelerometer uses the force of gravity as an
input vector to determine the orientation of an object in space.
An accelerometer is most sensitive to tilt when its sensitive axis
is perpendicular to the force of gravity, i.e., parallel to the earth’s
surface. When the accelerometer is oriented parallel to the gravity
vector, i.e., near its +1 g or –1 g reading, the change in output
acceleration per degree of tilt is negligible. When the acceler-
ometer is perpendicular to gravity, its output changes nearly
17.5 mg per degree of tilt, but at 45° degrees, it changes only
12.2 mg per degree, and resolution declines.
Dual-Axis Tilt Sensor: Converting
Acceleration to Tilt
When the accelerometer is oriented so both its X-axis and Y-axis
are parallel to the earth’s surface, it can be used as a two-axis tilt
sensor with a roll axis and a pitch axis. Once the output signal
from the accelerometer has been converted to an acceleration
that varies between –1 g and +1 g, the output tilt in degrees is
calculated as follows:
(
)g
A
SIN
Pitch
X
1
/
A
=
(
)g
A
SIN
Roll
Y
1
/
A
=
Be sure to account for overranges. It is possible for the
accelerometers to output a signal greater than ±1 g due to
vibration, shock, or other accelerations.




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