MXD2020EL Datasheet, PDF (5/8 Pages) List of Unclassifed Manufacturers – Ultra Low Noise, Low offset Drift +1g, -1g Dual Axis Accelerometer with Digital Outputs

these two signals permits low cost accurate tilt sensing to
be achieved with the MEMSIC device (reference
application note AN-00MX-007).
X
+90⁰
0⁰
gravity
Y
Top View
Figure 2: Accelerometer Position Relative to Gravity
X-Axis
Y-Axis
X-Axis
Orientation
Change
Change
To Earth’s X Output per deg. Y Output per deg.
Surface
(g)
of tilt
(g)
of tilt
(deg.)
(mg)
(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: 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. With a simple RC low pass
filter, the rms noise is calculated as follows:
Noise (mg rms) = Noise(mg/ Hz ) * _{(Bandwidth(Hz) *1.6)}
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
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 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
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.
T2
T1
A (g)= (T1/T2 - 0.5)/0.2
At 0g T1=T2
T2= 2.5ms or 10ms (factory programmable)
Figure 4: Typical output Duty Cycle
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.
MEMSIC MXD2020E/FL Rev D
Page 5 of 8
11/14/2003