Analog Devices AN-374 Application Notes
AN-374
1.5
g
150m
g
15m
g
1.5m
g
150µ
g
3dB BANDWIDTH – Hz
NOISE LEVEL – Peak to Peak
NOISE LEVEL – rms
10
g
1m
g
10m
g
100m
g
1
g
10 100 1k
ADXL05
ADXL50
a
ONE TECHNOLOGY WAY • P.O. BOX 9106
Using Accelerometers in Low g Applications
INTRODUCTION
Accelerometers can be used in a wide variety of low
applications such as tilt and orientation, vibration analysis, motion detection, etc. This application note explains
how to best apply the ADXL50 (50
accelerometers when measuring signals at the low end
of their respective full-scale ranges. Although each
accelerometer is specified according to its full scale
(clipping)
i.e., its minimum discernible input level, is extremely important when measuring low
The limiting resolution is predominantly set by the measurement noise “floor” which includes the ambient
background noise and the noise of the accelerometer itself. The level of the noise floor varies directly with the
bandwidth of the measurement. As the measurement
bandwidth is reduced, the noise floor drops, improving
the signal-to-noise ratio of the measurement and its
limiting resolution.
g
level, the limiting resolution of the device,
g
) and ADXL05 (5 g)
g
accelerations.
•
NORWOOD, MASSACHUSETTS 02062-9106
by Charles Kitchin
g
APPLICATION NOTE
617/329-4700
•
DEVICE BANDWIDTH VS. MEASUREMENT RESOLUTION
The output noise of the ADXL50 and ADXL05 scales
with the square root of the measurement bandwidth.
The maximum amplitude of the noise, its peak-to-peak
value, approximately defines the worst-case resolution
of a measurement. The peak-to-peak noise is approximately equal to 6.6 times its rms value (for an average
uncertainty of 0.1%).
The bandwidth of the accelerometer can be easily reduced by adding low-pass or bandpass filtering. Figure
1 shows the noise vs. bandwidth characteristics of the
ADXL50 and ADXL05 devices.
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
ADXL05 typically has a peak-to-peak noise level of
130 m
g
. With ±5 g applied accelerations, this 130 m
resolution limit is normally quite satisfactory; but for
g
Figure 1. Noise Level vs. 3 dB Bandwidth
smaller acceleration levels the noise is now a much
greater percentage of the signal. As shown by Figure 1,
when the device bandwidth is rolled off to 100 Hz, the
peak-to-peak noise level is reduced to approximately
40 m
g
, and at 10 Hz it is down to 10 mg.
0
g
offset trimming, and output scaling. Two tables are included with the figure which provide practical component
values for various full-scale
g
levels and approximate circuit bandwidths. For bandwidths other than those listed,
use the formula:
Alternatively, the signal-to-noise ratio may be improved
considerably by using a microprocessor to perform
multiple measurements and then compute the average
signal level. When using this technique, the signal level
will be increased directly with the number of measurements while the noise will only increase by their square
root. For example, with 100 measurements, the signal-tonoise ratio will be increased by a factor of 10 (20 dB).
Low-Pass Filtering
The bandwidth of either accelerometer can be reduced by
providing post filtering. Figure 2 shows how the buffer
amplifier can be connected to provide 1-pole post filtering,
C2
4
0.022µF
0.022µF
C1
COM
C1
2
3
5
+3.4V
REF
ADXL50 OR ADXL05
PRE-AMP
6
8
V
PR
Capacitor C4 (Farads) =
2 π×R3(Ω)×3dB BW (Hz)
1
or simply scale the value of capacitor C4 accordingly, i.e.,
for an application with a 50 Hz bandwidth, the value of C4
will need to be twice as large as its 100 Hz value. If further
noise reduction is needed while maintaining the maximum possible bandwidth, then a 2- or 3-pole post filter is
recommended. These provide a much steeper roll-off of
noise above the pole frequency. Figure 3 shows a circuit
that uses the buffer amplifier to provide 2-pole post filtering. Component values for the 2-pole filter were selected
to operate the buffer at unity gain.
0.1µF
+5V
V
OUT
R1a
R1b
1.8V
10
BUFFER
–
V
IN
1
AMP
9
R3
OPTIONAL SCALE
0
g
LEVEL
TRIM
FACTOR TRIM*
R2
50kΩ
*TO OMIT THE OPTIONAL SCALE FACTOR
TRIM , REPLACE R1a AND R1b WITH A
C4
FIXED VALUE 1% METAL FILM RESISTOR.
SEE VALUES SPECIFIED IN TABLES BELOW.
ADXL50 COMPONENT VALUES FOR VARIOUS
FULL-SCALE RANGES AND BANDWIDTHS
FULL
SCALE
±10
±20
±10
±20
mV
3dB
R1a
per
g
BW (Hz)
200
g
g
g
g
100
200
100
100
100
10
10
3dB BW =
R1b
kΩ
kΩ
21.5
5
23.7
5
21.5
5
23.7
5
1
2π R3 C4
R3
kΩ
249
137
249
137
R2
kΩ
100
100
100
100
C4
µF
0.0068
0.01
0.068
0.01
ADXL05 COMPONENT VALUES FOR VARIOUS
FULL-SCALE RANGES AND BANDWIDTHS
FULL
SCALE
±1
g
±2
g
±4
g
±5
g
mV
per
2000
1000
500
400
3dB
g
BW (Hz)
10
100
200
300
3dB BW =
R1a
kΩ
10
10
10
10
R1b
kΩ
24.9
35.7
35.7
45.3
1
2π R3 C4
301
200
100
100
R3
R2
kΩ
100
100
100
100
C4
µF
0.056
0.0082
0.0082
0.0056
kΩ
Figure 2. Using the Buffer Amplifier to Provide 1-Pole Post Filtering Plus Scale Factor and 0 g Level Trimming
–2–
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