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ADXL250 Datasheet(Arkusz danych) 10 Page - Analog Devices

Numer części ADXL250
Szczegółowy opis  -5 g to -50 g, Low Noise, Low Power, Single/Dual Axis iMEMS Accelerometers
Pobierz  15 Pages
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Producent  AD [Analog Devices]
Strona internetowa  http://www.analog.com
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 10 page
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–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
100m
g
1m
g
10m
g
10
1k
100
660m
g
66m
g
6.6m
g
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
()=1mg/ Hz × 100 1.6
()=12.25mg
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-
% of Time that Noise Will Exceed
Peak Value
Nominal Peak-to-Peak Value
2.0
× rms
32%
4.0
× rms
4.6%
6.0
× rms
0.27%
6.6
× rms
0.1%
8.0
× rms
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 mg. When the device bandwidth is rolled off to 100 Hz, the
noise level is reduced to approximately 10 mg.
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
+VS
RT
200k
0
g TRIM
SCALE
FACTOR
TRIM
(OPTIONAL)
R3
100k
+VS
0.1 F
R1a
75k
VOUT
0.1 F
+VS
2
SELF-TEST
25k
5k
ADXL150
GAIN
AMP
OFFSET
NULL
COM
C1
0.1 F
BUFFER
AMP
DEMODULATOR
SENSOR
+VS
TP
(DO NOT CONNECT)
14
9
10
7
5
CLOCK
OP196
3
4
6
2
8
7
+VS
2
R1b
50k
EXT
AMP
GAIN
R3
VALUE
F.S.
RANGE
DESIRED
OUTPUT
SCALE FACTOR
Cf ( F)
100Hz
Cf ( F)
30Hz
Cf ( 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
76mV/
g
25
g
2.0
200k
100mV/
g
20
g
2.6
261k
200mV/
g
10
g
5.3
536k
400mV/
g
5
g
10.5
1M
Figure 20. One-Pole Post Filter Circuit with SF and Zero g Offset Trims




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