MEMSIC MXR7311GL Rev.D
Page 4 of 4
2004.03.15
MXR7311GL PIN DESCRIPTIONS
VDD – This is the supply input for the circuits and the
sensor heater in the accelerometer. The DC voltage should
be between 2.7 and 5.25 volts. Refer to the section on PCB
layout and fabrication suggestions for guidance on external
parts and connections recommended.
COM– This is the ground pin for the accelerometer.
TP- This pin should be connected to the ground.
Xout – This pin is the output of the x-axis acceleration
sensor. The user should ensure the load impedance is
sufficiently high as to not source/sink >250
µA typical.
While the sensitivity of this axis has been programmed at
the factory to be the same as the sensitivity for the y-axis,
the accelerometer can be programmed for non-equal
sensitivities on the x- and y-axes. Contact the factory for
additional information.
Yout –
This pin is the output of the y-axis acceleration
sensor. The user should ensure the load impedance is
sufficiently high as to not source/sink >250
µA typical.
While the sensitivity of this axis has been programmed at
the factory to be the same as the sensitivity for the x-axis,
the accelerometer can be programmed for non-equal
sensitivities on the x- and y-axes. Contact the factory for
additional information.
DISCUSSION OF TILT APPLICATIONS AND
RESOLUTION
Tilt Applications: One of the most popular applications of
the MEMSIC accelerometer product line is in
tilt/inclination measurement. An accelerometer uses the
force of gravity as an input to determine the inclination
angle of an object.
A MEMSIC accelerometer is most sensitive to changes in
position, or tilt, when the accelerometer’s sensitive axis is
perpendicular to the force of gravity, or parallel to the
Earth’s surface. Similarly, when the accelerometer’s axis is
parallel to the force of gravity (perpendicular to the Earth’s
surface), it is least sensitive to changes in tilt.
Table 1 and Figure 2 help illustrate the output changes in
the X- and Y-axes as the unit is tilted from +90
° to 0°.
Notice that when one axis has a small change in output per
degree of tilt (in mg), the second axis has a large change in
output per degree of tilt. The complementary nature of
these two signals permits low cost accurate tilt sensing to
be achieved with the MEMSIC device (reference
application note AN-00MX-007).
Figure 2: Accelerometer Position Relative to Gravity
X-Axis
Y-Axis
X-Axis
Orientation
To Earth’s
Surface
(deg.)
X Output
(g)
Change
per deg.
of tilt
(mg)
Y Output
(g)
Change
per deg.
of tilt
(mg)
90
-1.000
0.15
0.000
17.45
85
-0.996
1.37
0.087
17.37
80
-0.985
2.88
0.174
17.16
70
-0.940
5.86
0.342
16.35
60
-0.866
8.59
0.500
15.04
45
-0.707
12.23
0.707
12.23
30
-0.500
15.04
0.866
8.59
20
-0.342
16.35
0.940
5.86
10
-0.174
17.16
0.985
2.88
5
-0.087
17.37
0.996
1.37
0
0.000
17.45
1.000
0.15
Table 1: Changes in Tilt for X- and Y-Axes
Resolution: The accelerometer resolution is limited by
noise. The output noise will vary with the measurement
bandwidth. With the reduction of the bandwidth, by
applying an external low pass filter, the output noise drops.
Reduction of bandwidth will improve the signal to noise
ratio and the resolution. The output noise scales directly
with the square root of the measurement bandwidth. The
maximum amplitude of the noise, its peak- to- peak value,
approximately defines the worst case resolution of the
measurement. With a simple RC low pass filter, the rms
noise is calculated as follows:
Noise (mg rms) = Noise(mg/ Hz ) *
)
6
.
1
*
)
(
(
Hz
Bandwidth
The peak-to-peak noise is approximately equal to 6.6 times
the rms value (for an average uncertainty of 0.1%).
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