ADXL206(Rev0) View Datasheet(PDF) - Analog Devices
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ADXL206 Datasheet PDF : 12 Pages
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ADXL206
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. In
some cases, however, 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 ADXL206
output. If additional decoupling is needed, a 100 Ω (or smaller)
resistor or ferrite bead can be inserted in the supply line of the
ADXL206. 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 ADXL206 has provisions for band-limiting the XOUT and
YOUT pins. A capacitor must be added to the pin 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) × Cx)
or more simply,
f−3 dB = 5 μF/Cx
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 acceler-
ometer. The resulting movement of the beam allows the user to
test whether the accelerometer is functional. The typical change
in output is 800 mg (corresponding to 250 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 are
present), it is recommended that a clamping diode with low
forward voltage be connected between ST and VS.
DESIGN TRADE-OFFS FOR SELECTING FILTER
CHARACTERISTICS: NOISE/BANDWIDTH TRADE-OFF
The accelerometer bandwidth selected ultimately determines
the measurement resolution (smallest detectable acceleration).
Filtering can be used to lower the noise floor, improving the
resolution of the accelerometer. Resolution is dependent on the
analog filter bandwidth at XOUT.
The output of the ADXL206 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
analog-to-digital sampling frequency to minimize aliasing. The
analog bandwidth can be further decreased to reduce noise and
improve resolution.
The ADXL206 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). 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 ADXL206 is determined by
( ) ( ) rms Noise = 110 μg/ Hz × BW × 1.6
At 100 Hz, the noise is
( ) ( ) rms Noise = 110 μg/ Hz × 100 × 1.6 = 1.4 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 probability 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
Peak-to-peak noise values give the best estimate of the uncer-
tainty in a single measurement; peak-to-peak noise is estimated
by 6 × rms. Table 8 gives the typical noise output of the ADXL206
for various CX and CY values.
Table 8. Typical Noise Output for Various Capacitor Values
CX, CY RMS Noise Peak-to-Peak Noise
Bandwidth (Hz) (μF) (mg)
Estimate (mg)
10
0.47 0.4
2.6
50
0.1
1.0
6
100
0.047 1.4
8.4
500
0.01 3.1
18.7
Rev. 0 | Page 10 of 12
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