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APPLICATION BULLETIN
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FILTER DESIGN PROGRAM FOR
THE UAF42 UNIVERSAL ACTIVE FILTER
By Johnnie Molina and R. Mark Stitt (602) 746-7592
Although active filters are vital in modern electronics, their
design and verification can be tedious and time consuming.
To aid in the design of active filters, Burr-Brown provides a
series of FilterPro™ computer-aided design programs. Us-
ing the FILTER42 program and the UAF42 it is easy to
design and implement all kinds of active filters. The UAF42
is a monolithic IC which contains the op amps, matched
resistors, and precision capacitors needed for a state-variable
filter pole-pair. A fourth, uncommitted precision op amp is
also included on the die.
Filters implemented with the UAF42 are time-continuous,
free from the switching noise and aliasing problems of
switched-capacitor filters. Other advantages of the state-
variable topology include low sensitivity of filter parameters
to external component values and simultaneous low-pass,
high-pass, and band-pass outputs. Simple two-pole filters
can be made with a UAF42 and two external resistors—see
Figure 1.
The DOS-compatible program guides you through the de-
sign process and automatically calculates component values.
Low-pass, high-pass, band-pass, and band-reject (or notch)
filters can be designed.
Active filters are designed to approximate an ideal filter
response. For example, an ideal low-pass filter completely
R
F1
15.8kΩ
eliminates signals above the cutoff frequency (in the stop-
band), and perfectly passes signals below it (in the pass-
band). In real filters, various trade-offs are made in an
attempt to approximate the ideal. Some filter types are
optimized for gain flatness in the pass-band, some trade-off
gain variation or ripple in the pass-band for a steeper rate of
attenuation between the pass-band and stop-band (in the
transition-band), still others trade-off both flatness and rate
of roll-off in favor of pulse-response fidelity. FILTER42
supports the three most commonly used all-pole filter types:
Butterworth, Chebyshev, and Bessel. The less familiar In-
verse Chebyshev is also supported. If a two-pole band-pass
or notch filter is selected, the program defaults to a resonant-
circuit response.
Butterworth
(maximally flat magnitude). This filter has the
flattest possible pass-band magnitude response. Attenuation
is –3dB at the design cutoff frequency. Attenuation beyond
the cutoff frequency is a moderately steep –20dB/decade/
pole. The pulse response of the Butterworth filter has mod-
erate overshoot and ringing.
Chebyshev
(equal ripple magnitude). (Other transliterations
of the Russian
Heby]ov
are Tschebychev, Tschebyscheff
or Tchevysheff). This filter response has steeper initial rate
of attenuation beyond the cutoff frequency than Butterworth.
R
F2
15.8kΩ
13
8
R
1
50kΩ
7
14
R
2
50kΩ
C
1
1000pF
C
2
1000pF
A
1
R
3
50kΩ
R
4
50kΩ
A
2
A
3
1
V
O
V
IN
2
UAF42
11
NOTE: A UAF42 and two external resistors make a unity-gain, two-pole, 1.25dB ripple
Chebyshev low-pass filter. With the resistor values shown, cutoff frequency is 10kHz.
FIGURE 1. Two-Pole Low-Pass Filter Using UAF42.
©
1991 Burr-Brown Corporation
AB-035C
1
Printed in U.S.A. July, 1993
BURR-BROWN [ BURR-BROWN CORPORATION ]
