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ADXL212 Datasheet(Arkusz danych) 10 Page - Analog Devices

Numer części ADXL212
Szczegółowy opis  Precision ±2 g Dual Axis, PWM Output Accelerometer
Pobierz  12 Pages
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
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ADXL212
Rev. 0 | Page 10 of 12
APPLICATIONS INFORMATION
POWER SUPPLY DECOUPLING
For most applications, a single 0.1 μF capacitor, CDC, adequately
decouples the accelerometer from noise on the power supply.
However, in some cases, particularly where noise is present at
the 140 kHz internal clock frequency (or any harmonic thereof),
noise on the supply may cause interference on the output of the
ADXL212. If additional decoupling is needed, insert a 100 Ω (or
smaller) resistor or ferrite beads in the supply line of the ADXL212.
In addition or as an alternative to adding the resistor or ferrite
beads, a larger bulk bypass capacitor (in the range of 1 μF to
22 μF) can be added in parallel to CDC.
SETTING THE BANDWIDTH USING CX AND CY
The ADXL212 has provisions for band limiting the XOUT and
YOUT pins. Capacitors must be added at these pins to implement
low-pass filtering for antialiasing and noise reduction. The
equation for the 3 dB bandwidth is
f3 dB = 1/(2π(32 kΩ) × C(X, Y))
or more simply,
f3 dB = 5 μF/C(X, Y)
The tolerance of the internal resistor (RFILT) can vary typically as
much as ±25% of its nominal value (32 kΩ); the bandwidth varies
accordingly. A minimum capacitance of 2000 pF for CX and CY
is required in all cases.
Table 6. Filter Capacitor Selection, CX and CY
Bandwidth (Hz)
Capacitor (μF)
1
4.7
10
0.47
50
0.10
100
0.05
200
0.027
500
0.01
SELF TEST
The ST pin controls the self test feature. When this pin is set to
VS, an electrostatic force is exerted on the beam of the accelero-
meter. The resulting movement of the beam allows the user to
test if the accelerometer is functional. The typical change in
output is 750 mg (corresponding to a duty cycle of 10%) and is
additive to the accelerometer outputs. The ST pin can remain
open circuit, or it can be connected to ground in normal use.
Never expose the ST pin to voltages greater than VS + 0.3 V. If
the system design is such that this condition cannot be guaranteed
(that is, multiple supply voltages are present), a low VF clamping
diode between ST and VS is recommended.
DESIGN TRADE-OFFS FOR SELECTING FILTER
CHARACTERISTICS: NOISE vs. BANDWIDTH
The chosen accelerometer bandwidth ultimately determines the
measurement resolution (smallest detectable acceleration). Filtering
can be used to lower the noise floor, which improves the resolu-
tion of the accelerometer. Resolution is dependent on the analog
filter capacitors at XFILT and YFILT.
The ADXL212 has a typical PWM bandwidth of 500 Hz. The
user must filter the signal to a bandwidth lower than 500 Hz to
limit aliasing errors.
The ADXL212 noise has the characteristics of white Gaussian
noise, which contributes equally at all frequencies and is described
in terms of μg/√Hz (that is, the noise is proportional to the square
root of the accelerometer bandwidth). To maximize the resolu-
tion and dynamic range of the accelerometer, limit bandwidth
to the lowest frequency needed by the application.
With the single pole roll-off characteristic, the typical noise of
the ADXL212 is determined by
)
6
.
1
(
)
Hz
/
μ
500
(
×
×
=
BW
g
Noise
rms
At 100 Hz, the noise is
g
g
Noise
rms
m
3
.
6
)
6
.
1
100
(
)
Hz
/
μ
500
(
=
×
×
=
Often, the peak value of the noise is desired. Peak-to-peak noise
can only be estimated by statistical methods. Table 7 is useful
for estimating the probabilities of exceeding various peak values,
given the rms value.
Table 7. Estimation of Peak-to-Peak Noise
Peak-to-Peak Value
% of Time that Noise Exceeds
Nominal Peak-to-Peak Value
2 × rms
32
4 × rms
4.6
6 × rms
0.27
8 × rms
0.006
For example, at 100 Hz bandwidth, peak noise exceeds 25.2 mg
4.6% of the time.
Peak-to-peak noise values provide the best estimate of the
uncertainty in a single measurement. Table 8 lists the typical
noise output of the ADXL212 for various CX and CY values.
Table 8. Filter Capacitor Selection (CX, CY)
Bandwidth(Hz)
CX, CY
(μF)
RMS Noise
(mg)
Peak-to-Peak Noise
Estimate (mg)
10
0.47
0.64
3.8
50
0.1
1.4
8.6
100
0.047
2
12
500
0.01
4.5
27.2




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