National Instruments SCB-68A - One-Pole Highpass RC Filter; Selecting Components for Highpass Filtering

© National Instruments | 2-29
NI SCB-68A User Manual
One-Pole Highpass RC Filter
Figure 2-27 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 2-27. Simple RC Highpass Circuit
The transfer function is a mathematical representation of a one-pole highpass filter, with a time
constant of:
Use Equation 2-4 to design a lowpass filter for a simple resistor and capacitor circuit, where the
values of the resistor and capacitor alone determine f
c
:
(2-4)
where G is the DC gain and s represents the frequency domain.
Selecting Components for Highpass Filtering
To determine the value of the components in the circuit, fix R (10 kΩ is reasonable) and isolate
C from Equation 2-4 as follows:
(2-5)
The cutoff frequency in Equation 2-5 is f
c
.
For best results, choose a resistor that has the following characteristics:
• Low wattage of approximately 0.125 W
• Precision of at least 5%
• Temperature stability
• Tolerance of 5%
• AXL package (suggested)
• Carbon or metal film (suggested)
R
C
V
in
V
out
1
2πRC
---------------
Ts()
G
12πRC()s+
-------------------------------=
C
1
2πRf
c
---------------=