
PIN DESCRIPTIONS
V
DD
– This is the supply input for the digital circuits and
the sensor heater in the accelerometer. The DC voltage
should be between 2.70 and 5.25 volts.
V
DA
– This is the power supply input for the analog
amplifiers in the accelerometer. The DC voltage should
be between 2.70 and 5.25 volts
Gnd
– This is the ground pin for the accelerometer.
A
OUTX
– 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
>100
μ
A.
A
OUTY
–
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 >100
μ
A.
T
OUT
– This pin is the buffered output of the
temperature sensor. The analog voltage at T
OUT
is an
indication of the die temperature. This voltage is
useful as a differential measurement of temperature
from ambient and not as an absolute measurement of
temperature.
Sck
–
This pin should be grounded.
V
ref
– A reference voltage is available from this pin. It is
set at 2.50V typical and has 100
μ
A of drive capability.
DISCUSSION OF TILT APPLICATIONS AND
MINIMUM 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.
Following table and figure 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 m
g
), 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).
MEMSIC MXA2500J/K Rev.A
Page 6 of
8
2/17/2006
Top View
X
Y
+90
0
0
0
gravity
M
Accelerometer Position Relative to Gravity
X-Axis
Orientation
To Earth’s
Surface
(deg.)
90
85
80
70
60
45
30
20
10
X-Axis
Y-Axis
X
Output
(
g
)
1.000
0.996
0.985
0.940
0.866
0.707
0.500
0.342
0.174
0.087
0.000
Change
per deg.
of tilt
(m
g
)
0.15
1.37
2.88
5.86
8.59
12.23
15.04
16.35
17.16
17.37
17.45
Y
Output
(
g
)
0.000
0.087
0.174
0.342
0.500
0.707
0.866
0.940
0.985
0.996
1.000
Change
per deg.
of tilt
(m
g
)
17.45
17.37
17.16
16.35
15.04
12.23
8.59
5.86
2.88
1.37
0.15
5
0
Changes in Tilt for X- and Y-Axes
Minimum 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.
The peak-to-peak noise is approximately equal to 6.6
times the rms value (with an average uncertainty
of .1%).
EXTERNAL FILTERS
AC Coupling
: For applications where only dynamic
accelerations (vibration) are to be measured, it is
recommended to ac couple the accelerometer output as
shown in following figure. The advantage of ac coupling
is that variations from part to part of zero
g
offset and
zero
g
offset versus temperature can be eliminated.
Following figure is a HPF (high pass filter) with a –3dB
breakpoint given by the equation:
RC
π
2
f
1
=
. In many
applications it may be desirable to have the HPF –3dB
point at a very low frequency in order to detect very low
frequency accelerations. Sometimes the