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ADXL05 Datasheet(PDF) 14 Page - Analog Devices

Part No. ADXL05
Description  Single Chip Accelerometer with Signal Conditioning
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Maker  AD [Analog Devices]
Homepage  http://www.analog.com
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ADXL05 Datasheet(HTML) 14 Page - Analog Devices

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ADXL05
REV. B
–14–
OPTIONAL CAPACITOR
FOR 3-POLE FILTERING
OUTPUT
SCALE
FACTOR
TRIM
20k
R7
71.5k
OFFSET AND
SCALING
AMPLIFIER
2-POLE FILTER
BUFFER
AMP
VOUT
VREF
1.8V
8
9
10
PRE-AMP
VPR
R4b
VIN–
ADXL05
R6
40.2k
0
g
LEVEL
TRIM
R4a
C4
2
4
7
6
AD820
R5
42.2k
R1
82.5k
R3
82.5k
C3
0.01µF
+5V
R5
3
6
+3.4V
2-POLE FILTER
COMPONENT VALUES
C4µF
0.0033
0.01
0.033
0.1
3dB
BW (Hz)
300
100
30
10
C3µF
0.027
0.082
0.27
0.82
OFFSET AND SCALING
AMPLIFIER COMPONENT VALUES
FULL
SCALE
±1g
±2g
±4g
±5g
mV
per
g
2000
1000
500
400
R4a
k
10
10
10
10
GAIN
10.00
4.98
2.50
2.00
R5
k
301
200
100
100
R4b
k
24.9
35.7
35.7
45.3
Figure 26. Two-Pole Filtering Circuit with Gain and 0 g
Offset Adjustment
Bandpass Filtering
Figure 27 shows how the combination of ac coupling and low-
pass filtering together form a bandpass filter that provides an
even greater improvement in noise reduction.
BUFFER
AMP
VOUT
1.8V
8
10
PRE-AMP
VPR
R1
R3
VIN–
ADXL05
C5
C4
VPR
R2
9
COMPONENT
VALUES ARE
APPROXIMATE.
0.002µF
0.002µF
0.0068µF
0.0068µF
0.068µF
1000
200
1000
200
200
30
30
3
1
0.1
49.9
249
49.9
249
249
0.10µF
0.022µF
1.0µF
0.68µF
6.8µF
249
249
249
249
249
640k
640k
640k
640k
640k
300
300
100
100
10
SCALE
FACTOR
IN
mV/
g
DESIRED
LOW
FREQUENCY
LIMIT, FL
R1
VALUE
IN k
CLOSEST
C4
VALUE
DESIRED
HIGH
FREQUENCY
LIMIT, FH
R3
IN
k
CLOSEST
C5
VALUE
VALUE
OF R2
FOR +2.5V
0
g LEVEL
Figure 27 AC Coupling and Low-Pass Filtering Used
Together to Provide a Bandpass Function
Low-Pass Filtering
The bandwidth of the accelerometer can easily be reduced by
using post filtering. Figure 25 shows how the buffer amplifier
can be connected to provide 1-pole post filtering, 0 g offset trim-
ming, and output scaling. The table provides practical compo-
nent values for various full-scale g levels and approximate circuit
bandwidths. For bandwidths other than those listed, use the
formula:
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 reduc-
tion is needed while maintaining the maximum possible band-
width, then a 2- or 3-pole post filter is recommended. These
provide a much steeper roll-off of noise above the pole fre-
quency. Figure 26 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. Ca-
pacitors C3 and C4 were chosen to provide 3 dB bandwidths of
10 Hz, 30 Hz, 100 Hz, and 300 Hz.
In this configuration, the nominal buffer amplifier output will be
+1.8 V
± the 200 mV/g scale factor of the accelerometer. An
AD820 external op amp allows noninteractive adjustment of 0 g
offset and scale factor. The external op amp offsets and scales
the output to provide a +2.5 V
± 2 V output over a wide range
of full-scale g levels.
BUFFER
AMP
0.1µF
+5V
VOUT
0.022µF
C1
C1
+3.4V
REF
COM
1.8V
8
9
10
1
2
3
4
0.022µF
C2
PRE-AMP
VPR
R1b
R3
VIN–
5
ADXL05
6
R2
0
g
LEVEL
TRIM
50k
R1a
C4
OPTIONAL SCALE
FACTOR TRIM
*
*TO OMIT THE OPTIONAL SCALE FACTOR
TRIM, REPLACE R1a AND R1b WITH A FIXED
VALUE 1% METAL FILM RESISTOR.
SEE VALUES SPECIFIED IN TABLES BELOW.
NOTE: FOR NONINTERACTIVE TRIMS,
SET SCALE FACTOR FIRST, THEN OFFSET.
COMPONENT VALUES FOR VARIOUS
FULL-SCALE RANGES AND BANDWIDTHS
FULL
SCALE
±1g
±2g
±4g
±5g
mV
per
g
2000
1000
500
400
R1a
k
10
10
10
10
R2
k
100
100
100
100
C4
µF
0.056
0.0082
0.0082
0.0056
3dB
BW (Hz)
10
100
200
300
R3
k
301
200
100
100
R1b
k
24.9
35.7
35.7
45.3
3dB BW =
1
2
π R3 C4
Figure 25. Using the Buffer Amplifier to Provide 1-Pole
Post Filtering Plus Scale Factor and 0 g Level Trimming
C 4
=
1
(2
π R3)Desired 3dB Bandwidth in Hz
OBSOLETE


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