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ADXL250JQC Datasheet(Arkusz danych) 8 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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 8 page
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–8–
ADXL150/ADXL250
REV. 0
+VS
2
OUTPUT SCALE FACTOR = 38mV/
g ––
R1
R3
SELF-TEST
+VS
2
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
C4
0.1 F
+VS
OP196
8
3
4
7
6
R1
R3
VOUT
2
C2
0.1 F
Figure 15. Using an External Op Amp to Increase Output Scale Factor
+VS
C4
0.1 F
R2
OUTPUT
EXTERNAL AMP GAIN = ––––
R2
1M
+VS
2
VOUT
1M
BUFFER
GAIN
FS RANGE
R2
1Hz
3Hz
10Hz
20Hz
2
25
g
1M
0.15 F
0.05 F
0.015 F 0.0075 F
4
12.5
g
332k
0.47 F
0.15 F
0.047 F 0.022 F
5
10
g
249k
0.68 F
0.22 F
0.022 F 0.01 F
C3 VALUE FOR 3dB CORNER FREQ
TYPICAL COMPONENT VALUES FOR AC COUPLED CIRCUIT
C2
0.1 F
+VS
2
SELF-TEST
+VS
2
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
8
3
4
7
6
2
C3
Figure 16. AC Coupled Connection Using an External Op Amp
Increasing the
iMEMS Accelerometer’s Output
Scale Factor
Figure 15 shows the basic connections for using an external
buffer amplifier to increase the output scale factor.
The output multiplied by the gain of the buffer, which is simply
the value of resistor R3 divided by R1. Choose a convenient
scale factor, keeping in mind that the buffer gain not only ampli-
fies the signal, but any noise or drift as well. Too much gain can
also cause the buffer to saturate and clip the output waveform.
Note that the “+” input of the external op amp uses the offset
null pin of the ADXL150/ADXL250 as a reference, biasing the
op amp at midsupply, saving two resistors and reducing power
consumption. The offset null pin connects to the VS/2 reference
point inside the accelerometer via 30 k
Ω, so it is important not
to load this pin with more than a few microamps.
It is important to use a single-supply or “rail-to-rail” op amp for
the external buffer as it needs to be able to swing close to the
supply and ground.
The circuit of Figure 15 is entirely adequate for many applica-
tions, but its accuracy is dependent on the pretrimmed accuracy
of the accelerometer and this will vary by product type and grade.
For the highest possible accuracy, an external trim is recom-
mended. As shown by Figure 20, this consists of a potentiom-
eter, R1a, in series with a fixed resistor, R1b. Another option is
to select resistor values after measuring the device’s scale factor
(see Figure 17).
AC Coupling
If a dc (gravity) response is not required—for example in vibra-
tion measurement applications—ac coupling can be used be-
tween the accelerometer’s output and the external op amp’s
input as shown in Figure 16. The use of ac coupling virtually
eliminates any zero g drift and allows the maximum external
amp gain without clipping.
Resistor R2 and capacitor C3 together form a high pass filter
whose corner frequency is 1/(2
π R2 C3). This filter will reduce
the signal from the accelerometer by 3 dB at the corner fre-
quency, and it will continue to reduce it at a rate of 6 dB/octave
(20 dB per decade) for signals below the corner frequency.
Capacitor C3 should be a nonpolarized, low leakage type.
If ac coupling is used, the self-test feature must be monitored at
the accelerometer’s output rather than at the external amplifier
output (since the self-test output is a dc voltage).




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