4608X-102-102 Datasheet, PDF (44/62 Pages) Bourns Electronic Solutions – Resistor Networks Product Selection Guide

Application Notes – EMI/RFI Filters 601 Series
General Description
Continual advances in digital IC technology are creating
stringent demands on EMI/RFI levels in equipment.
EMI/RFI low pass filters are required in personal
computers, data terminals, test equipment and process con-
trollers for high frequency suppression into or out of elec-
tronic equipment.
Filter Selection and Design Considerations
The “roll-off” frequency fc, defined as the frequency at
which the filter passes one-half the power it receives at its
input terminal, can be specified from the low megahertz
range up to about 100MHz. This frequency, also known as
the “-3 dB” frequency, will be determined by the R and C
values chosen. Custom resistor and capacitor values are
available to optimize system performance.
The specification of these values will depend on con-
straints relating to noise frequencies, system performance
and driver loading. The following procedure is suggested to
choose appropriate values of R and C.
The first step is to determine the desired roll-off
frequency of the filter, which will lie between the signal
frequency and the dominant frequencies of the EMI/RFI
noise. By determining the pole of the filter (setting the
denominator of the transfer function equal to zero), the
roll-off frequency can be expressed in terms of R and C:
fc
=
RS + RL + 2R
2πC(R + RS)(R + RL)
Furthermore, the RC combination must be chosen so
that the additional RC time delay will not result in exceeding
the sampling window of the receiving IC, due to excessive
lengthening of signal rise and fall times.
Rise time from 10% to 90% of the waveform amplitude
can be calculated in terms of the circuit’s RC time constant
using the 1 –exp (–t/RC) relationship for a charging capacitor.
At 10%, t _L = 0.1 time constants, and at 90%, tH = 2.3 time
constants. “Time constant” equals RthC, where Rth is the
Thevenin-equivalent resistance as seen by the capacitor.
Therefore, equating the difference in the two times to
the maximum tolerable rise (or fall) time:
tmax = t _H - tL = 2.2RthC
tmax = 2.2
(R + RS)(R + RL)C
RS + RL + 2R
A final consideration is the insertion loss. As mentioned
previously, the voltage drop across the two resistors will
attenuate the voltage reaching the load. Normally, logic high
and low levels will still be within valid limits. The signal
attenuation can be minimized by choosing small R values rel-
ative to the load impedance. Typical values for R range from
10 to 50 ohms.
Bourns Low-pass Filters for EMI/RFI Suppression
NO. OF LINES
7
8
8
BOURNS P/N
4118R-601-RC/CC
4120R-601-RC/CC
4420P-601-RC/CC
PACKAGE
DIP
Wide Body SMD
Standard Resistance/Capacitance Values And Codes
RC
R
CC
C
250
25Ω
500
50pF
270
27Ω
101
100pF
470
47Ω
181
180pF
820
82Ω
201
200pF
101
100Ω
320
Specifications are subject to change without notice.