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ADXL210_15 Datasheet(Arkusz danych) 7 Page - Analog Devices

Numer części ADXL210_15
Szczegółowy opis  Accelerometers with Digital Output
Pobierz  11 Pages
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ADXL202/ADXL210
–7–
REV. B
Setting the Bandwidth Using CX and CY
The ADXL202/ADXL210 have provisions for bandlimiting the
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:
F–3 dB =
1
2
π (32 kΩ) × C(x, y)
()
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 kΩ; so the bandwidth will
vary accordingly. A minimum capacitance of 1000 pF for C(X, Y)
is required in all cases.
Table I. Filter Capacitor Selection, CX and CY
Capacitor
Bandwidth
Value
10 Hz
0.47
µF
50 Hz
0.10
µF
100 Hz
0.05
µF
200 Hz
0.027
µF
500 Hz
0.01
µF
5 kHz
0.001
µF
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:
T 2
=
RSET (
Ω)
125 M
A 125 k
Ω resistor 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.
Table II. Resistor Values to Set T2
T2
RSET
1 ms
125 k
2 ms
250 k
5 ms
625 k
10 ms
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 M
Ω when taking the output from X
FILT 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-
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
signal on each axis.
These requirements will help to determine the accelerometer
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
DEMOD
RFILT
32k
RFILT
32k
OSCILLATOR
X SENSOR
Y SENSOR
X OUT
Y OUT
SELF TEST
XFILT
VDD
VDD
CX
+3.0V TO +5.25V
CDC
COM
YFILT
T2
CY
RSET
C
O
U
N
T
E
R
P
T2
A(
g) = (T1/T2 – 0.5)/12.5%
0
g = 50% DUTY CYCLE
T2 = RSET/125M
T1
DEMOD
DUTY
CYCLE
MODULATOR
(DCM)
ADXL202/
ADXL210
Figure 13. Block Diagram




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