2-26
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ni.com
Chapter 2
Analog Input and Temperature Sensor Measurements
Note
Filtering increases the settling time of the instrumentation amplifier to the
time constant of the filter used. Adding RC filters to scanning channels greatly
reduces the practical scanning rate, since the instrumentation amplifier settling time
can be increased to 10
T
or longer, where
T
= (
R
)(
C
).
Figure 2-23.
SCB-68A Circuit Diagram for Single-Ended Analog Input Lowpass Filter
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Analog Input Lowpass Filtering Applications
The following applications benefit from lowpass filtering:
•
Noise filtering
—You can use a lowpass filter to highly attenuate the noise frequency on a
measured signal. For example, power lines commonly add a noise frequency of 60 Hz.
Adding a filter with
f
c
<60 Hz at the input of the measurement system causes the noise
frequency to fall into the stopband.
Referring to Equation 2-2, fix the resistor value at 10 k
Ω
to calculate the capacitor value
and choose a commercial capacitor value that satisfies the following relationship:
(2-3)
AI
i
AI
i
+
8
y
S
C
x
E
A
F
G
B
C
D
+
+
+
+ 5 V
AI GND
+ 5 V
AI GND
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i
AI
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+
8
C
1
2
π
10 000
,
(
)
60
( )
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