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ADXL250JQC Datasheet(Arkusz danych) 9 Page - Analog Devices

Numer części ADXL250JQC
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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 9 page
background image
ADXL150/ADXL250
–9–
REV. 0
R3
100k
R2 (SEE NOTES)
+VS OR GND
+VS
R1
VOUT
+VS
2
SELF-TEST
+VS
2
25k
5k
ADXL150
GAIN
AMP
OFFSET
NULL
COM
BUFFER
AMP
DEMODULATOR
SENSOR
+VS
TP
(DO NOT CONNECT)
14
9
10
7
5
CLOCK
OP196
3
4
6
2
8
7
NOTES:
0
g “QUICK” CALIBRATION METHOD USING RESISTOR R2 AND A +5V SUPPLY.
(a) WITH ACCELEROMETER ORIENTED AWAY FROM EARTH’S
GRAVITY (i.e., SIDEWAYS), MEASURE PIN 10 OF THE ADXL150.
(b) CALCULATE THE OFFSET VOLTAGE THAT NEEDS TO BE NULLED:
VOS =(+2.5V – VPIN10)(R3/R1).
2.5V (R3)
VOS
(
c) R2 = ––––––––
(d) FOR VPIN 10 > +2.5V, R2 CONNECTS TO GND.
(e) FOR VPIN 10 < +2.5V, R2 CONNECTS TO +VS.
EXT
AMP
GAIN
R1
VALUE
FS
RANGE
DESIRED
OUTPUT
SCALE FACTOR
76mV/
g
25
g
2.0
49.9k
100mV/
g
20
g
2.6
38.3k
200mV/
g
10
g
5.3
18.7k
400mV/
g
5
g
10.5
9.53k
C2
0.1 F
C1
0.1 F
C4
0.1 F
Figure 17. “Quick Zero g Calibration” Connection
Adjusting the Zero g Bias Level
When a true dc (gravity) response is needed, the output from
the accelerometer must be dc coupled to the external amplifier’s
input. For high gain applications, a zero g offset trim will also be
needed. The external offset trim permits the user to set the zero g
offset voltage to exactly +2.5 volts (allowing the maximum output
swing from the external amplifier without clipping with a +5
supply).
With a dc coupled connection, any difference between the zero g
output and +2.5 V will be amplified along with the signal. To
obtain the exact zero g output desired or to allow the maximum
output voltage swing from the external amplifier, the zero g offset
will need to be externally trimmed using the circuit of Figure 20.
The external amplifier’s maximum output swing should be
limited to
±2 volts, which provides a safety margin of ±0.25
volts before clipping. With a +2.5 volt zero g level, the maxi-
mum gain will equal:
2 Volts
38 mV/g Times the Max Applied Acceleration in g
The device scale factor and zero g offset levels can be calibrated
using the earth’s gravity, as explained in the section “calibrating
the ADXL150/ADXL250.”
Using the Zero g “Quick-Cal” Method
In Figure 18 (accelerometer alone, no external op amp), a trim
potentiometer connects directly to the accelerometer’s zero g
null pin. The “quick offset calibration” scheme shown in Figure
17 is preferred over using a potentiometer, which could change
its setting over time due to vibration. The “quick offset calibra-
tion” method requires measuring only the output voltage of
the ADXL150/ADXL250 while it is oriented normal to the
earth’s gravity. Then, by using the simple equations shown in
the figures, the correct resistance value for R2 can be calculated.
In Figure 17, an external op amp is used to amplify the signal. A
resistor, R2, is connected to the op amp’s summing junction.
The other side of R2 connects to either ground or +VS depend-
ing on which direction the offset needs to be shifted.
OFFSET
NULL
+VS
200k
RIN AT PIN 8
30k
10k
SELF-TEST
+VS
2
25k
5k
ADXL150
GAIN
AMP
COM
BUFFER
AMP
DEMODULATOR
SENSOR
+VS
TP
(DO NOT CONNECT)
VOUT
14
8
9
10
7
5
CLOCK
C1
0.1 F
C2
0.1 F
Figure 18. Offset Nulling the ADXL150/ADXL250 Using a Trim Potentiometer




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