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參數資料
| 型號： | ADXL204EB |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Precisionc +-1.7 g Single-/Dual-Axis i MEMS Accelerometer |
| 中文描述： | Precisionc -1.7克Single-/Dual-Axis i MEMS加速度 |
| 文件頁數： | 10/12頁 |
| 文件大小： | 486K |
| 代理商： | ADXL204EB |
ADXL204
APPLICATIONS
POWER SUPPLY DECOUPLING
For most applications, a single 0.1 μF capacitor, C
DC
, 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 C
DC
.
Rev. A | Page 10 of 12
SETTING THE BANDWIDTH USING C
X
AND C
Y
The ADXL204 has provisions for bandlimiting the X
OUT
and
Y
OUT
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 (R
FILT
) 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 C
X
and C
Y
is required in all cases.
Table 6. Filter Capacitor Selection, C
X
and C
Y
Bandwidth (Hz)
1
10
50
100
200
500
SELF TEST
The ST pin controls the self-test feature. When this pin is set to
V
S
, 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 m
g
(corresponding to 200 mV). This pin can be
left open-circuit or connected to common in normal use.
Capacitor (μF)
4.7
0.47
0.10
0.05
0.027
0.01
The ST pin should never be exposed to voltage greater than
V
S
+ 0.3 V. If the system design is such that this condition
cannot be guaranteed (that is, multiple supply voltages present),
a low V
F
clamping diode between ST and V
S
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 X
OUT
and Y
OUT
.
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 μ
g
/√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 μ
g
/√Hz) × (√BW×1.6)
At 100 Hz the noise is
rmsNoise
= (170 μ
g
/√Hz) × (√BW×1.6) = 2.15 m
g
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
2 × rms
4 × rms
6 × rms
8 × rms
% of Time Noise Exceeds
Nominal Peak-to-Peak Value
32
4.6
0.27
0.006
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