LTC1562 Datasheet, PDF (7/28 Pages) Linear Technology – Very Low Noise, Low Distortion Active RC Quad Universal Filter

LTC1562
APPLICATIONS INFORMATION
Functional Description
The LTC1562 contains four matched, 2nd order, 3-termi-
nal universal continuous-time filter blocks, each with a
virtual-ground input node (INV) and two rail-to-rail out-
puts (V1, V2). In the most basic applications, one such
block and three external resistors provide 2nd order
lowpass and bandpass responses simultaneously (Figure
3, with a resistor for Z_IN). The three external resistors set
standard 2nd order filter parameters f_O, Q and gain. A
combination of internal precision components and exter-
nal resistor R2 sets the center frequency f_O of each 2nd
order block. The LTC1562 is trimmed at manufacture so
that f_O will be 100kHz ±0.5% if the external resistor R2 is
exactly 10k.
However, lowpass/bandpass filtering is only one specific
application for the 2nd order building blocks in the LTC1562.
Highpass response results if the external impedance Z_IN in
Figure 3 becomes a capacitor C_IN (whose value sets only
gain, not critical frequencies) as described below.
Responses with zeroes are available through other con-
nections (see Notches and Elliptic Responses). Moreover,
the virtual-ground input gives each 2nd order section the
built-in capability for analog operations such as gain
(preamplification), summing and weighting of multiple
inputs, handling input voltages beyond the power supplies
or accepting current or charge signals directly. These
Operational Filter^TM frequency-selective building blocks
are nearly as versatile as operational amplifiers.
The user who is not copying exactly one of the Typical
Applications schematics shown later in this data sheet is
urged to read carefully the next few sections through at
least Signal Swings, for orientation about the LTC1562,
before attempting to design custom application circuits.
Also available free from LTC, and recommended for de-
signing custom filters, is the general-purpose analog filter
design software FilterCAD^TM for Windows^®. This software
includes tools for finding the necessary f₀, Q and gain
parameters to meet target filter specifications such as
frequency response.
Setting f_O and Q
Each of the four 2nd order sections in the LTC1562 can be
programmed for a standard filter function (lowpass,
bandpass or highpass) when configured as in Figure 3
with a resistor or capacitor for Z_IN. These transfer func-
tions all have the same denominator, a complex pole pair
with center frequency ω_O = 2πf_O and quality parameter Q.
(The numerators depend on the response type as de-
scribed below.) External resistors R2 and R_Q set f_O and Q
as follows:
( ) fO =
2πC
1
(R1)R2

=

10kΩ ^
R2


100kHz
Q=
R_{Q =}
(R1)R2
R_Q
(10kΩ)R2
=
R_Q ^ 100kHz^
R2  f_O 
R1 = 10k and C = 159pF are internal to the LTC1562 while
R2 and R_Q are external.
A typical design procedure proceeds from the desired f_O
and Q as follows, using finite-tolerance fixed resistors.
First find the ideal R2 value for the desired f_O:
( ) ( ) ^{R2 Ideal}
=


100kHz
f_O


2
10kΩ
Then select a practical R2 value from the available finite-
tolerance resistors. Use the actual R2 value to find the
desired R_Q, which also will be approximated with finite
tolerance:
R_Q = Q (10kΩ)R2
The f_O range is approximately 10kHz to 150kHz, limited
mainly by the magnitudes of the external resistors
required. As shown above, R2 varies with the inverse
square of f_O. This relationship desensitizes f_O to R2’s
Operational Filter and FilterCAD are trademarks of Linear Technology Corporation.
Windows is a registered trademark of Microsoft Corporation.
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