mxd2020e memsic, mxd2020e Datasheet - Page 5

• Manufacturer Part Number: mxd2020e
• Description: Ultra Low Noise, Low Offset Drift ?1 G Dual Axis Accelerometer With Digital Outputs
• Manufacturer: memsic

X-Axis
Orientation
To Earth’s
Surface
(deg.)
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 to 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).
Resolution: Accelerometers can be used in a wide variety
of low g applications such as tilt and orientation. The
device noise floor will vary with the measurement
bandwidth. With the reduction of the bandwidth the noise
floor drops. This will improve the signal to noise ratio of
the measurement and 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
MEMSIC MXD2020E/F Rev H
90
85
80
70
60
45
30
20
10
5
0
Figure 2: Accelerometer Position Relative to Gravity
Y
Table 1: Changes in Tilt for X- and Y-Axes
Top View
X Output
1.000
0.996
0.985
0.940
0.866
0.707
0.500
0.342
0.174
0.087
0.000
( g )
X-Axis
X
per deg.
Change
of tilt
(m g )
12.23
15.04
16.35
17.16
17.37
17.45
+90
0.15
1.37
2.88
5.86
8.59
0
Y Output
0
0
0.000
0.087
0.174
0.342
0.500
0.707
0.866
0.940
0.985
0.996
1.000
( g )
Y-Axis
gravity
per deg.
Change
17.45
17.37
17.16
16.35
15.04
12.23
of tilt
(m g )
8.59
5.86
2.88
1.37
0.15
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resolution of the measurement. With a simple RC low pass
filter, the rms noise is calculated as follows:
Noise (mg rms) = Noise(mg/ Hz ) *
The peak-to-peak noise is approximately equal to 6.6 times
the rms value (for an average uncertainty of 0.1%).
DIGITAL INTERFACE
The MXD2020E/F is easily interfaced with low cost
microcontroller. For the digital output accelerometer, one
digital input port is required to read one accelerometer
output. For the analog output accelerometer, many low cost
microcontroller are available today that feature integrated
a/d (analog to digital converters) with resolutions ranging
from 8 to 12 bits.
In many applications the microcontroller provides an
effective approach for the temperature compensation of the
sensitivity and the zero g offset. Specific code set, reference
designs, and applications notes are available from the
factory. The following parameters must be considered in a
digital interface:
Resolution : smallest detectable change in input acceleration
Bandwidth : detectable accelerations in a given period of
time
Acquisition Time : the duration of the measurement of the
acceleration signal
DUTY CYCLE DEFINITION
The MXD2020E/F has two PWM duty cycle outputs (x,y).
The acceleration is proportional to the ratio T1/T2. The
zero g output is set to 50% duty cycle and the sensitivity
scale factor is set to 20% duty cycle change per g . These
nominal values are affected by the initial tolerance of the
device including zero g offset error and sensitivity error.
This device is offered from the factory programmed to
either a 10ms period (100 Hz) or a 2.5ms period (400Hz).
T1
T2 (Period)
Duty Cycle
Pulse width
T2= 2.5ms or 10ms (factory programmable)
Figure 3: Typical output Duty Cycle
Length of the “on” portion of the cycle.
Length of the total cycle.
Ratio of the “0n” time (T1) of the cycle to
the total cycle (T2). Defined as T1/T2.
Time period of the “on” pulse. Defined as
T1.
A (g)= (T1/T2 - 0.5)/0.2
At 0g T1=T2
T1
T2
(
Bandwidth
2/26/2007
(
Hz
* )
. 1
) 6