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參數(shù)資料
| 型號： | ADXL250 |
| 廠商： | Analog Devices, Inc. |
| 英文描述： | Low Power，65 g to 650 g, Low Noise,Single/Dual Axis Accelerometers(低功耗、測量范圍為65g到650g的低噪音單/雙軸雙軸加速器) |
| 中文描述： | 低功耗，65克至650克，低噪聲，單/雙軸加速度（低功耗，測量范圍為65克到650克的低噪音單/雙軸雙軸加速器） |
| 文件頁數(shù)： | 10/15頁 |
| 文件大?。?/td> | 291K |
| 代理商： | ADXL250 |
–10–
ADXL150/ADXL250
REV. 0
DEVICE BANDWIDTH VS. MEASUREMENT
RESOLUTION
Although an accelerometer is usually specified according to its
full-scale
g
level, the limiting resolution of the device, i.e., its
minimum discernible input level, is extremely important when
measuring low
g
accelerations.
3dB BANDWIDTH – Hz
100mg
1mg
10mg
10
1k
100
N
660mg
66mg
6.6mg
N
Figure 19.ADXL150/ADXL250 Noise Level vs. 3 dB
Bandwidth (Using a “Brickwall” Filter)
The limiting resolution is predominantly set by the measure-
ment noise “floor,” which includes the ambient background
noise and the noise of the ADXL150/ADXL250 itself. The level
of the noise floor varies directly with the bandwidth of the mea-
surement. As the measurement bandwidth is reduced, the noise
floor drops, improving the signal-to-noise ratio of the measure-
ment and increasing its resolution.
The bandwidth of the accelerometer can be easily reduced by
adding low-pass or bandpass filtering. Figure 19 shows the
typical noise vs. bandwidth characteristic of the ADXL150/
ADXL250.
The output noise of the ADXL150/ADXL250 scales with the
square root of the measurement bandwidth. With a single pole
roll-off, the equivalent rms noise bandwidth is
π
divided by 2 or
approximately 1.6 times the 3 dB bandwidth. For example, the
typical rms noise of the ADXL150 using a 100 Hz one pole post
filter is:
Noise rms
)
=
1
mg/ Hz
×
100 1.6
=
12
.
25
mg
Because the ADXL150/ADXL250’s noise is, for all practical
purposes, Gaussian in amplitude distribution, the highest noise
amplitudes have the smallest (yet nonzero) probability. Peak-
to-peak noise is therefore difficult to measure and can only be
estimated due to its statistical nature. Table I is useful for esti-
mating the probabilities of exceeding various peak values, given
the rms value.
Table I.
Nominal Peak-to-
Peak Value
2.0
×
rms
4.0
×
rms
6.0
×
rms
6.6
×
rms
8.0
×
rms
% of Time that Noise Will Exceed
Nominal Peak-to-Peak Value
32%
4.6%
0.27%
0.1%
0.006%
RMS and peak-to-peak noise (for 0.1% uncertainty) for various
bandwidths are estimated in Figure 19. As shown by the figure,
device noise drops dramatically as the operating bandwidth is
reduced. For example, when operated in a 1 kHz bandwidth,
the ADXL150/ADXL250 typically have an rms noise level of
32 m
g
. When the device bandwidth is rolled off to 100 Hz, the
noise level is reduced to approximately 10 m
g
.
Alternatively, the signal-to-noise ratio may be improved consid-
erably by using a microprocessor to perform multiple measure-
ments and then to compute the average signal level.
Low-Pass Filtering
The bandwidth of the accelerometer can easily be reduced by using
post filtering. Figure 20 shows how the buffer amplifier can be
connected to provide 1-pole post filtering, zero
g
offset trimming,
and output scaling. The table provides practical component values
Cf
R2
1M
V
+V
S
RT
200k
V
0gTRIM
SCALE
FACTOR
TRIM
(OPTIONAL)
R3
100k
V
+V
S
0.1
m
F
R1a
75k
V
V
OUT
0.1
m
F
+V
S
2
SELF-TEST
25k
V
5k
V
ADXL150
GAIN
AMP
OFFSET
NULL
COM
C1
0.1
m
F
BUFFER
AMP
DEMODULATOR
SENSOR
+V
S
TP
(DO NOT CONNECT)
14
9
10
7
5
CLOCK
OP196
3
4
6
2
8
7
+V
S
2
R1b
50k
V
EXT
AMP
GAIN
R3
VALUE
F.S.
RANGE
DESIRED
OUTPUT
SCALE FACTOR
76mV/g
100mV/g
200mV/g
400mV/g
Cf (
m
F)
100Hz
Cf (
m
F)
30Hz
Cf (
m
F)
10Hz
0.0082
0.0056
0.0033
0.0015
0.027
0.022
0.010
0.0056
0.082
0.056
0.033
0.015
6
25g
6
20g
6
10g
6
5g
2.0
2.6
5.3
10.5
200k
V
261k
V
536k
V
1M
V
Figure 20.One-Pole Post Filter Circuit with SF and Zero g Offset Trims
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參數(shù)描述 |
|---|---|
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