MEMSIC MXD2002A/B Rev.E
Page 5 of 8
5/15/2004
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
Minimum 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 resolution of the measurement. The peak-to-
peak noise is approximately equal to 6.6 times as the rms
value (with an average uncertainty of .1%). The maximum
noise for 1.0Hz bandwidth will be 1 mg/ Hz . If the
bandwidth is increased to 10 Hz for example: 3.162 mg is
the maximum rms noise and 20.87mg is the maximum
peak -to-peak noise.
DIGITAL INTERFACE
The MXD2002A/B is easily interfaced with low cost
microcontrollers. For the digital output accelerometer, one
digital input port is required to read one accelerometer
output. For the analog output accelerometer, many low cost
microcontrollers 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 MXD2002A/B 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 2% 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
Length of the “on” portion of the cycle.
T2 (Period)
Length of the total cycle.
Duty Cycle
Ratio of the “0n” time (T1) of the cycle to
the total cycle (T2). Defined as T1/T2.
Pulse width
Time period of the “on” pulse. Defined as
T1.
T2
T1
A (g)= (T1/T2 - 0.5)/2%
0g = 50% Duty Cycle
T2= 2.5ms or 10ms (factory programmable)
Figure 4: Typical output Duty C ycle
CHOOSING T2 AND COUNTER FREQUENCY
DESIGN TRADE-OFFS
The noise level is one determinant of accelerometer
resolution. The second relates to the measurement
resolution of the counter when decoding the duty cycle
output. The actual resolution of the acceleration signal is
limited by the time resolution of the counting devices used
to decode the duty cycle. The faster the counter clock, the
higher the resolution of the duty cycle and the shorter the
T2 period can be for a given resolution. Table 2 shows
some of the trade-offs. It is important to note that this is the
resolution due to the microprocessors’ counter. It is
probable that the accelerometer’s noise floor may set the
lower limit on the resolution.
T2 (ms)
MEMSIC
Sample
Rate
Counter-
Clock
Rate
(MHz)
Counts
Per T2
Cycle
Counts
per g
Reso-
lution
(mg)
2.5
400
2.0
5000
100
10
2.5
400
1.0
2500
50
20
2.5
400
0.5
1250
25
40
10.0
100
2.0
20000
400
2.5
10.0
100
1.0
10000
200
5.0
10.0
100
0.5
5000
100
10
Table 2: Trade-Offs Between Microcontroller Counter Rate and
T2 Period.
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