
ADXL103/ADXL203
Peak-to-peak noise values give the best estimate of the
uncertainty in a single measurement. Table 6 gives the typical
noise output of the ADXL103/ADXL203 for various C
X
and C
Y
values.
Table 6. Filter Capacitor Selection (C
X
, C
Y
)
C
X
, C
Y
(μF)
(m
g
)
10
0.47
0.4
50
0.1
1.0
100
0.047
1.4
500
0.01
3.1
USING THE ADXL103/ADXL203 WITH OPERATING
VOLTAGES OTHER THAN 5 V
The ADXL103/ADXL203 is tested and specified at V
S
= 5 V;
however, it can be powered with V
S
as low as 3 V or as high as
6 V. Some performance parameters will change as the supply
voltage is varied.
Rev. 0 | Page 10 of 12
Bandwidth(Hz)
RMS Noise
Peak-to-Peak Noise
Estimate (m
g
)
2.6
6
8.4
18.7
The ADXL103/ADXL203 output is ratiometric, so the output
sensitivity (or scale factor) will vary proportionally to supply
voltage. At V
S
= 3 V the output sensitivity is typically 560 mV/
g
.
The zero
g
bias output is also ratiometric, so the zero
g
output is
nominally equal to V
S
/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
S
= 3 V, the noise
density is typically 190 μ
g
/√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. So at
V
S
= 3 V, the self-test response will be approximately equivalent
to 150 mV, or equivalent to 270 m
g
(typical).
The supply current decreases as the supply voltage decreases.
Typical current consumption at V
DD
= 3 V is 450 μA.
USING THE ADXL203 AS A DUAL-AXIS TILT
SENSOR
One of the most popular applications of the ADXL203 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
acceleration 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. At 45°, its output changes at
only 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 2-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:
PITCH
=
ASIN
(
A
X
/1
g
)
ROLL
=
ASIN
(
A
Y
/1
g
)
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.