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參數(shù)資料
| 型號： | ADXL321 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Small and Thin 18 g Accelerometer |
| 中文描述： | 小而薄的18克加速度 |
| 文件頁數(shù)： | 12/16頁 |
| 文件大小： | 379K |
| 代理商： | ADXL321 |
ADXL321
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 may cause interference on the
ADXL321 output. If additional decoupling is needed, a 100
(or smaller) resistor or ferrite bead may be inserted in the
supply line. Additionally, a larger bulk bypass capacitor (in the
1 μF to 4.7 μF range) may be added in parallel to C
DC
.
Rev. 0 | Page 12 of 16
SETTING THE BANDWIDTH USING C
X
AND C
Y
The ADXL321 has provisions for band-limiting 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 k) ×
C
(
X
,
Y
)
)
or more simply,
F
–3 dB
= 5 μF/
C
(
X
,
Y
)
The tolerance of the internal resistor (R
FILT
) typically varies as
much as ±15% of its nominal value (32 k), and the bandwidth
varies accordingly. A minimum capacitance of 2000 pF for C
X
and C
Y
is required in all cases.
Table 5. 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 accelerometer beam.
The resulting movement of the beam allows the user to test if
the accelerometer is functional. The typical change in output is
315 m
g
(corresponding to 18 mV). This pin may be left open-
circuit or connected to common (COM) 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 voltages greater than
V
S
+ 0.3 V. If this cannot be guaranteed due to the system design
(for instance, if there are multiple supply voltages), then 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 ADXL321 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 may be further decreased to reduce noise and
improve resolution.
The ADXL321 noise has the characteristics of white Gaussian
noise, which contributes equally at all frequencies and is
described in terms of μ
g
/√Hz (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 in order to maximize the resolution and dynamic
range of the accelerometer.
With the single-pole, roll-off characteristic, the typical noise of
the ADXL321 is determined by
)
1.6
BW
(
)
Hz
μg/
(320
×
×
=
rmsNoise
At 100 Hz bandwidth the noise will be
mg
4
)
1.6
100
(
)
Hz
μg/
(320
=
×
×
=
rmsNoise
Often, the peak value of the noise is desired. Peak-to-peak noise
can only be estimated by statistical methods. A factor of 6 is
generally used to convert rms to peak-to-peak. Table 6 is useful
for estimating the probabilities of exceeding various peak
values, given the rms value.
Table 6. Estimation of Peak-to-Peak Noise
% 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 Value
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| ADXL321JCP | 制造商:Analog Devices 功能描述:Accelerometer |
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