Datasheet

V (s)
O
V (s)
I
A
LP n
w
2
s +s /Q+
n n
2 2
w w
=
V (s)
HP
V (s)
I
A
HP
s
2
s +s /Q+
n n
2 2
w w
=
V (s)
BP
V (s)
I
A
BP n
( /Q)sw
s +s /Q+
n n
2 2
w w
=
V (s)
BR
V (s)
I
A
BR n
(s + )
2 2
w
s +s /Q+
n n
2 2
w w
=
UAF42
www.ti.com SBFS002B –JULY 1992–REVISED OCTOBER 2010
APPLICATION INFORMATION
The UAF42 is a monolithic implementation of the The basic building element of the most
proven state-variable analog filter topology. This commonly-used filter types is the second-order
device is pin-compatible with the popular UAF41 section. This section provides a complex-conjugate
analog filter, and it provides several improvements. pair of poles. The natural frequency, w
n
, and Q of the
pole pair determine the characteristic response of the
The slew rate of the UAF42 has been increased to
section. The low-pass transfer function is shown in
10V/ms, versus 1.6V/ms for the UAF41.
Equation 1:
Frequency • Q product of the UAF42 has been
improved, and the useful natural frequency extended
by a factor of four to 100kHz. FET input op amps on
(1)
the UAF42 provide very low input bias current. The
monolithic construction of the UAF42 provides lower The high-pass transfer function is given by
cost and improved reliability. Equation 2:
DESIGN PROGRAM
(2)
Application report SBFA002 (available for download
The band-pass transfer function is calculated using
at www.ti.com) and a computer-aided design program
Equation 3:
also available from Texas Instruments, make it easy
to design and implement many kinds of active filters.
The DOS-compatible program guides you through the
(3)
design process and automatically calculates
component values.
A band-reject response is obtained by summing the
low-pass and high-pass outputs, yielding the transfer
Low-pass, high-pass, band-pass and band-reject
function shown in Equation 4:
(notch) filters can be designed. The program supports
the three most commonly-used all-pole filter types:
Butterworth, Chebyshev and Bessel. The less-familiar
(4)
inverse Chebyshev is also supported, providing a
smooth passband response with ripple in the stop
The most common filter types are formed with one or
band.
more cascaded second-order sections. Each section
is designed for w
n
and Q according to the filter type
With each data entry, the program automatically
(Butterworth, Bessel, Chebyshev, etc.) and cutoff
calculates and displays filter performance. This
frequency. While tabulated data can be found in
feature allows a spreadsheet-like what-if design
virtually any filter design text, the design program
approach. For example, a user can quickly determine,
eliminates this tedious procedure.
by trial and error, how many poles are required for a
desired attenuation in the stopband. Gain/phase plots
Second-order sections may be noninverting
may be viewed for any response type.
(Figure 1) or inverting (Figure 2). Design equations
for these two basic configurations are shown for
reference. The design program solves these
equations, providing complete results, including
component values.
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Product Folder Link(s): UAF42