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ADXL202AQC View Datasheet(PDF) - Analog Devices

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ADXL202AQC Datasheet PDF : 11 Pages
1 2 3 4 5 6 7 8 9 10
ADXL202/ADXL210
+3.0V TO +5.25V
VDD
VDD
CX
XFILT
SELF TEST
ADXL202/
X SENSOR
RFILT ADXL210
32k
X OUT
CDC
DEMOD
OSCILLATOR
RFILT
DUTY
CYCLE
MODULATOR
(DCM)
C
O
U
N
P
T
E
32k
DEMOD
Y OUT R
T2
T1
A(g) = (T1/T2 – 0.5)/12.5%
0g = 50% DUTY CYCLE
T2 = RSET/125M
Y SENSOR
COM
YFILT
T2
CY
RSET
Figure 13. Block Diagram
E Setting the Bandwidth Using CX and CY
The ADXL202/ADXL210 have provisions for bandlimiting the
T XFILT and YFILT pins. Capacitors must be added at these pins to
implement low-pass filtering for antialiasing and noise reduc-
tion. The equation for the 3 dB bandwidth is:
( ) E F–3dB =
1
2 π (32 k) × C(x, y)
L or, more simply,
F–3 dB
= 5 µF
C(X ,Y )
The tolerance of the internal resistor (RFILT), can vary as much
as ± 25% of its nominal value of 32 k; so the bandwidth will
vary accordingly. A minimum capacitance of 1000 pF for C(X, Y)
O is required in all cases.
Table I. Filter Capacitor Selection, CX and CY
S Bandwidth
Capacitor
Value
10 Hz
50 Hz
B 100 Hz
200 Hz
500 Hz
O 5 kHz
0.47 µF
0.10 µF
0.05 µF
0.027 µF
0.01 µF
0.001 µF
Table II. Resistor Values to Set T2
T2
1 ms
2 ms
5 ms
10 ms
RSET
125 k
250 k
625 k
1.25 M
Note that the RSET should always be included, even if only an
analog output is desired. Use an RSET value between 500 k
and 2 Mwhen taking the output from XFILT or YFILT. The
RSET resistor should be place close to the T2 Pin to minimize
parasitic capacitance at this node.
Selecting the Right Accelerometer
For most tilt sensing applications the ADXL202 is the most
appropriate accelerometer. Its higher sensitivity (12.5%/g allows
the user to use a lower speed counter for PWM decoding while
maintaining high resolution. The ADXL210 should be used in
applications where accelerations of greater than ±2 g are expected.
MICROCOMPUTER INTERFACES
The ADXL202/ADXL210 were specifically designed to work
with low cost microcontrollers. Specific code sets, reference
designs, and application notes are available from the factory.
This section will outline a general design procedure and discuss
the various trade-offs that need to be considered.
The designer should have some idea of the required perfor-
Setting the DCM Period with RSET
The period of the DCM output is set for both channels by a
single resistor from RSET to ground. The equation for the period
is:
mance of the system in terms of:
Resolution: the smallest signal change that needs to be detected.
Bandwidth: the highest frequency that needs to be detected.
Acquisition Time: the time that will be available to acquire the
T 2 = RSET ()
125 M
signal on each axis.
These requirements will help to determine the accelerometer
A 125 kresistor will set the duty cycle repetition rate to ap-
proximately 1 kHz, or 1 ms. The device is designed to operate at
duty cycle periods between 0.5 ms and 10 ms.
bandwidth, the speed of the microcontroller clock and the
length of the T2 period.
When selecting a microcontroller it is helpful to have a counter
timer port available. The microcontroller should have provisions
for software calibration. While the ADXL202/ADXL210 are
highly accurate accelerometers, they have a wide tolerance for
REV. B
–7–
 

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