ADXL210 Analog Devices, Inc., ADXL210 Datasheet - Page 9

• Manufacturer Part Number: ADXL210
• Description: Low-cost
  10 G Dual-axis Accelerometer With Duty Cycle
• Manufacturer: Analog Devices, Inc.

DESIGN PROCEDURE FOR THE ADXL210E
The design procedure for using the ADXL210E with a duty cycle
output involves selecting a duty cycle period and a filter capacitor.
A proper design will take into account the application requirements
for bandwidth, signal resolution and acquisition time, as discussed
in the following sections.
Decoupling Capacitor C
A 0.1 µF capacitor is recommended from V
supply decoupling.
ST
The ST pin controls the self-test feature. When this pin is set to V
an electrostatic force is exerted on the beam of the accelerometer.
The resulting movement of the beam allows the user to test if the
accelerometer is functional. The typical change in output will be 3%
at the duty cycle outputs (corresponding to 800 mg). This pin
may be left open circuit or connected to common in normal use.
Duty Cycle Decoding
The ADXL210E’s digital output is a duty cycle modulator.
Acceleration is proportional to the ratio T1/T2. The nominal
output of the ADXL210E is:
These nominal values are affected by the initial tolerance of the
device including zero g offset error and sensitivity error.
T2 does not have to be measured for every measurement cycle.
It need only be updated to account for changes due to tempera-
ture (a relatively slow process). Since the T2 time period is shared
by both X and Y channels, it is necessary only to measure it on
one channel of the ADXL210E. Decoding algorithms for various
microcontrollers have been developed. Consult the appropriate
Application Note.
C
DC
OSCILLATOR
Scale factor is 4% Duty Cycle Change per g
3V TO 5.25V
X SENSOR
Y SENSOR
V
COM
DD
DEMOD
DEMOD
0 g = 50% Duty Cycle
ADXL210E
DC
R
32k
R
32k
FILT
FILT
C
X
X
Y
FILT
FILT
C
Y
ANALOG
T1
CYCLE
DUTY
(ADC)
DD
TO
SELF-TEST
T2
A(g) = (T1/T2 – 0.5)/4%
0g = 50% DUTY CYCLE
T2 = R
R
T2
to COM for power
SET
SET
X
Y
/125M
OUT
OUT
C
O
U
N
T
E
R
P
DD
,
Setting the Bandwidth Using C
The ADXL210E has provisions for bandlimiting the X
Y
low-pass filtering for antialiasing and noise reduction. The equa-
tion for the 3 dB bandwidth is:
or, more simply,
The tolerance of the internal resistor (R
much as ± 15% of its nominal value of 32 kΩ; so the bandwidth
will vary accordingly. A minimum capacitance of 1000 pF for
C
Setting the DCM Period with R
The period of the DCM output is set for both channels by a single
resistor from R
A 125 kΩ resistor will set the duty cycle repetition rate to approxi-
mately 1 kHz, or 1 ms. The device is designed to operate at duty
cycle periods between 0.5 ms and 10 ms.
Note that the R
analog output is desired. Use an R
and 2 MΩ when taking the output from X
resistor should be placed close to the T2 Pin to minimize parasitic
capacitance at this node.
Selecting the Right Accelerometer
For most tilt sensing applications the ADXL202E 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 ADXL210E should be used in
applications where accelerations of greater than ±2 g are expected.
FILT
(X, Y)
pins. Capacitors must be added at these pins to implement
is required in all cases.
Table I. Filter Capacitor Selection, C
Bandwidth
10 Hz
50 Hz
100 Hz
200 Hz
500 Hz
5 kHz
Table II. Resistor Values to Set T2
SET
T2
1 ms
2 ms
5 ms
10 ms
SET
F
–3 dB
to ground. The equation for the period is:
should always be included, even if only an
=
F
(
T 2 =
2 π (32 kΩ) × C(x, y)
–3 dB
125 MΩ
R
=
X
SET
SET
C
5 µF
and C
(X ,Y )
1
SET
(Ω)
FILT
Capacitor
Value
0.47 µF
0.10 µF
0.05 µF
0.027 µF
0.01 µF
0.001 µF
R
125 kΩ
250 kΩ
625 kΩ
1.25 MΩ
value between 500 kΩ
FILT
Y
SET
), can vary typically as
ADXL210E
or Y
)
X
FILT
and C
. The R
FILT
Y
and
SET