Datasheet

Data Sheet ADXL103/ADXL203
Rev. D | Page 15 of 16
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 change as the supply
voltage is varied.
The ADXL103/ADXL203 output is ratiometric, so the output
sensitivity (or scale factor) varies proportionally to the 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 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. So at
V
S
= 3 V, the self test response is approximately equivalent to
150 mV or equivalent to 270 mg (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, 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 has been converted to an acceleration that varies
between –1 g and +1 g, the output tilt in degrees is calculated as
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.