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ADXL204EB Datasheet(Arkusz danych) 11 Page - Analog Devices
AD [Analog Devices]
Rev. A | Page 11 of 12
Peak-to-peak noise values give the best estimate of the uncertainty
in a single measurement and is estimated by 6 × rms. Table 8
gives the typical noise output of the ADXL204 for various C
Table 8. Filter Capacitor Selection (C
USING THE ADXL204 WITH OPERATING VOLTAGES
OTHER THAN 3.3 V
The ADXL204 is tested and specified at V
= 3.3 V; however, it
can be powered with V
as low as 3 V or as high as 6 V. Some
performance parameters change as the supply voltage is varied.
The ADXL204 output is ratiometric, so the output sensitivity, or
scale factor, varies proportionally to supply voltage. At V
= 3 V,
the output sensitivity is typically 560 mV/g. At V
= 5 V, the
output sensitivity is typically 1000 mV/g.
The zero g bias output is also ratiometric, so the zero g output is
nominally equal to V
/2 at all supply voltages.
The output noise is not ratiometric but is 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. At V
= 3 V, the noise
density is typically 190 μg/√Hz. At V
= 5 V, the noise density is
typically 110 μg/√Hz.
Self-test response in g is roughly proportional to the square of
the supply voltage. However, when ratiometricity of sensitivity
is factored in with supply voltage, self-test response in volts is
roughly proportional to the cube of the supply voltage. This
means at V
= 3 V, the self-test response is approximately
equivalent to 150 mV, or equivalent to 270 mg (typical). At
= 5 V, the self-test response is approximately equivalent to
750 mV, or equivalent to 750 mg (typical).
The supply current decreases as the supply voltage decreases.
Typical current consumption at V
= 5 V is 750 μA.
USING THE ADXL204 AS A DUAL-AXIS TILT SENSOR
One of the most popular applications of the ADXL204 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, that is, 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, that is, near its +1 g or –1 g reading, the change in
output acceleration per degree of tilt is negligible. When the
accelerometer is perpendicular to gravity, its output changes
nearly 17.5 mg per degree of tilt. At 45°, its output changes
at 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 2-axis
tilt sensor with a roll axis and a pitch axis. Once the output
signal from the accelerometer is converted to an acceleration
that varies between –1 g and +1 g, the output
tilt in degrees is calculated as:
PITCH = ASIN(A
ROLL = ASIN(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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