7
LTC1562
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APPLICATIONS INFORMATION
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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% (±0.6% typical for PDIP
package) 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 op amps.
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 f
0
, 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, band-
pass or highpass) when configured as in Figure 3 with a
resistor or capacitor for Z
IN
. These transfer functions 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 described
below.) External resistors R2 and R
Q
set f
O
and Q as
follows:
f
CRR
k
R
kHz
O
==
()
1
212
10
2
100
π ()
Ω
Q
R
RR
R
kR
R
R
kHz
f
QQQ
O
== =
() ( )1 2 10 2
2
100
Ω
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
:
R Ideal
kHz
f
k
O
2
100
10
2
()
=
(
Ω
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:
RQ kR
Q
= ()10 2Ω
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.