© National Instruments
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3-5
Figure 3-7.
Response of a Real Filter to a Square Wave Input Signal
One-Pole Lowpass RC Filter
Figure 3-8 shows the transfer function of a simple series circuit consisting of a resistor (
R
) and
capacitor (
C
) when the voltage across
R
is assumed to be the output voltage (
V
m
).
Figure 3-8.
Simple RC Lowpass Filter
The transfer function is a mathematical representation of a one-pole lowpass filter, with a time
constant of:
Use Equation 3-1 to design a lowpass filter for a simple resistor and capacitor circuit, where the
values of the resistor and capacitor alone determine
f
c
:
(3-1)
where
G
is the DC gain and
s
represents the frequency domain.
Time (t)
V
o
lts (V)
R
C
V
in
V
m
1
2
π
RC
---------------
T s
( )
G
1
2
π
RC
(
)
s
+
-------------------------------
=
