1
FIGURE 1. Two-Pole Low-Pass Filter Using UAF42.
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.
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
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.
A
1
R
2
50kΩ
A
2
A
3
R
4
50kΩ
UAF42
11
R
1
50kΩ
R
F1
15.8kΩ
R
F2
15.8kΩ
C
1
1000pF
C
2
1000pF
13 8 7 14
2
V
IN
R
3
50kΩ
V
O
1
FILTER DESIGN PROGRAM FOR
THE UAF42 UNIVERSAL ACTIVE FILTER
By Johnnie Molina and R. Mark Stitt (602) 746-7592
APPLICATION BULLETIN
®
Mailing Address: PO Box 11400 • Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd. • Tucson, AZ 85706
Tel: (602) 746-1111 • Twx: 910-952-111 • Telex: 066-6491 • FAX (602) 889-1510 • Immediate Product Info: (800) 548-6132
©
1991 Burr-Brown Corporation AB-035C Printed in U.S.A. July, 1993
