ADXL150EM-1 Analog Devices Inc, ADXL150EM-1 Datasheet - Page 11

• Manufacturer Part Number: ADXL150EM-1
• Description: EVAL MODULE ACCELER TRI-AX 50G
• Manufacturer: Analog Devices Inc

Specifications of ADXL150EM-1

• Rohs Status: RoHS non-compliant

REV. 0
for various full-scale g levels and approximate circuit band-
widths. For bandwidths other than those listed, use the
formula:
or simply scale the value of capacitor Cf accordingly; i.e., for an
application with a 50 Hz bandwidth, the value of Cf 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, a 2- or 3-pole post filter is recommended. These provide
a much steeper roll-off of noise above the pole frequency. Fig-
ure 21 shows a circuit that provides 2-pole post filtering. Com-
ponent values for the 2-pole filter were selected to operate the
first op amp at unity gain. Capacitors C3 and C4 were chosen
to provide 3 dB bandwidths of 10 Hz, 30 Hz, 100 Hz and
300 Hz.
The second op amp offsets and scales the output to provide a
+2.5 V
APPLICATION HINTS
ADXL250 Power Supply Pins
When wiring the ADXL250, be sure to connect BOTH power
supply terminals, Pins 14 and 13.
Ratiometric Operation
Ratiometric operation means that the circuit uses the power
supply as its voltage reference. If the supply voltage varies, the
accelerometer and the other circuit components (such as an
ADC, etc.) track each other and compensate for the change.
2 V output over a wide range of full-scale g levels.
Cf
TYPICAL FILTER VALUES
300Hz 0.027 F 0.0033 F
100Hz 0.082 F 0.01 F
30Hz
10Hz
SCALE FACTOR
2 R3 Desired 3dB Bandwidth in Hz
BW
76mV/ g
100mV/ g
200mV/ g
400mV/ g
DESIRED
OUTPUT
0.27 F
0.82 F
C3
+V
RANGE
0.033 F
0.1 F
±25 g
±20 g
±10 g
±5 g
F.S.
S
0.1 F
C4
C1
1
GAIN
AMP
EXT
2.0
2.6
5.3
10.5
14
SENSOR
SELF-TEST
VALUE
(DO NOT CONNECT)
200k
261k
536k
1M
R5
Figure 21. Two-Pole Post Filter Circuit
ADXL150
9
TP
5
CLOCK
COM
GAIN
AMP
7
DEMODULATOR
–11–
OFFSET
0.1 F
NULL
Figure 22 shows how both the zero g offset and output sensitiv-
ity of the ADXL150/ADXL250 vary with changes in supply
voltage. If they are to be used with nonratiometric devices, such
as an ADC with a built-in 5 V reference, then both components
should be referenced to the same source, in this case the ADC
reference. Alternatively, the circuit can be powered from an
external +5 volt reference.
Since any voltage variation is transferred to the accelerometer’s
output, it is important to reduce any power supply noise. Simply
following good engineering practice of bypassing the power supply
right at Pin 14 of the ADXL150/ADXL250 with a 0.1 F ca-
pacitor should be sufficient.
C2
25k
OUTPUT
+V
2
S
2.65
2.60
2.55
2.50
2.45
2.40
2.35
8
5k
+V
2
Figure 22. Typical Ratiometric Operation
S
AMPLIFIER
5.25 5.20 5.15 5.10 5.05 5.00 4.95 4.90 4.85 4.80 4.75
BUFFER
SCALING
AMP
7
10
R5
82.5k
OP296
R1
4
C3
1/2
POWER SUPPLY VOLTAGE
82.5k
42.2k
5
6
+V
R3
R2
2
S
ADXL150/ADXL250
100k
1M
R4
R6
C4
3
2
OP296
1/2
+V
8
+V
2-POLE
FILTER
S
S
0.1 F
200k
0 g TRIM
1
35.75
40.25
39.50
38.75
38.00
37.25
36.50