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ADXL204EB Datasheet(Arkusz danych) 10 Page - Analog Devices

Numer części ADXL204EB
Szczegółowy opis  Precisionc -1.7 g Single-/Dual-Axis i MEMS Accelerometer
Pobierz  12 Pages
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
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ADXL204
Rev. A | Page 10 of 12
APPLICATIONS
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 can cause interference on the ADXL204
output. If additional decoupling is needed, a 100 Ω, or smaller,
resistor or ferrite bead can be inserted in the supply line of the
ADXL204. Additionally, a larger bulk bypass capacitor, in the
1 μF to 22 μF range, can be added in parallel to CDC.
SETTING THE BANDWIDTH USING CX AND CY
The ADXL204 has provisions for bandlimiting 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
F–3 dB = 1/(2π(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 typically as
much as ±25% of its nominal value (32 kΩ); thus, the band-
width 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 325 mg (corresponding to 200 mV). This pin can be
left open-circuit or connected to common in normal use.
The ST pin should never be exposed to voltage greater than
VS + 0.3 V. If the system design is such that this condition
cannot be guaranteed (that is, multiple supply voltages present),
a low VF clamping diode between ST and VS is recommended.
DESIGN TRADE-OFFS FOR SELECTING FILTER
CHARACTERISTICS: THE NOISE/BW TRADE-OFF
The accelerometer bandwidth selected ultimately determines
the measurement resolution (smallest detectable acceleration).
Filtering can be used to lower the noise floor, which improves
the resolution of the accelerometer. Resolution is dependent on
the analog filter bandwidth at XOUT and YOUT.
The output of the ADXL204 has a typical bandwidth of 2.5 kHz.
The user must filter the signal at this point to limit aliasing
errors. The analog bandwidth must be no more than half the
A/D sampling frequency to minimize aliasing. The analog
bandwidth can be further decreased to reduce noise and
improve resolution.
The ADXL204 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’s bandwidth). The user
should limit bandwidth to the lowest frequency needed by the
application to maximize the resolution and dynamic range of
the accelerometer.
With the single-pole, roll-off characteristic, the typical noise of
the ADXL204 is determined by
rmsNoise = (170 μg/√Hz) × (√BW×1.6)
At 100 Hz the noise is
rmsNoise = (170 μg/√Hz) × (√BW×1.6) = 2.15 mg
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 Noise Exceeds
Nominal Peak-to-Peak Value
2 × rms
32
4 × rms
4.6
6 × rms
0.27
8 × rms
0.006




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