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ADXL311JE Datasheet(Arkusz danych) 8 Page - Analog Devices

Numer części ADXL311JE
Szczegółowy opis  Low Cost, Ultracompact ±2 g Dual-Axis Accelerometer
Pobierz  12 Pages
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
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ADXL311
root of the bandwidth of the accelerometer. It is recommended
that the user limit 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 ADXL202E 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 5 is useful
for estimating the probabilities of exceeding various peak val-
ues, given the rms value.
Table 5. Estimation of Peak-to-Peak Noise
Peak-to-Peak
Value
% of Time That Noise Will Exceed 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 will give the best estimate of the
uncertainty in a single measurement. Table 6 gives the typical
noise output of the ADXL311 for various CX and CY values.
Table 6. Filter Capacitor Selection (CX, CY)
Bandwidth
(Hz)
CX, CY
(µF)
RMS Noise
(m
g)
Peak-to-Peak Noise
Estimate (m
g)
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 2.7 V or as high as 5.25 V.
Some performance parameters will change as the supply
voltage is varied.
The ADXL311 output is ratiometric, so the output sensitivity
(or scale factor) will vary proportionally to supply voltage. At
VDD = 5 V the output sensitivity is typically 312 mV/
g.
The zero
g bias output is also ratiometric, so the zero g output is
nominally equal to VDD/2 at all supply voltages.
The output noise is not ratiometric but absolute in volts; there-
fore, 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 will be
approximately equivalent to 800 m
g (typical).
The supply current increases as the supply voltage increases.
Typical current consumption at VDD = 5 V is 600 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. At this orientation, its sensitivity to changes in tilt is
highest. When the accelerometer is oriented on axis to gravity,
i.e., near its +1
g or –1 g reading, the change in output accelera-
tion per degree of tilt is negligible. When the accelerometer is
perpendicular to gravity, its output will change nearly 17.5 m
g
per degree of tilt, but at 45° degrees, it is changing only at
12.2 m
g 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 accel-
eration that varies between –1
g and +1 g, the output tilt in de-
grees is calculated as follows:
(
)g
A
A
PITCH
X 1
/
SIN
=
(
)g
A
A
ROLL
Y 1
/
SIN
=
Be sure to account for overranges. It is possible for the acceler-
ometers to output a signal greater than ±1
g due to vibration,
shock, or other accelerations.
Rev. A | Page 8 of 12




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