LTC1562 Linear Technology, LTC1562 Datasheet - Page 20

• Manufacturer Part Number: LTC1562
• Description: Very Low Noise/ Low Distortion Active RC Quad Universal Filter
• Manufacturer: Linear Technology

LTC1562
APPLICATIONS
Notches and Elliptic Responses
The basic (essentially all-pole) LTC1562 circuit tech-
niques described so far will serve many applications.
However, the sharpest-cutoff lowpass, highpass and
bandpass filters include notches (imaginary zero pairs) in
the stopbands. A notch, or band-reject, filter has zero gain
at a frequency f
themselves to reject a narrow band of frequencies. A
number of circuit methods will give notch responses from
an Operational Filter block. Each method exhibits an input-
output transfer function that is a standard 2nd order band-
reject response:
with parameters
values as described below. (
Operational Filter block by its R2 and R
described earlier in Setting f
the gain magnitude |H
DC (f = 0) and H
addition to the notch, the gain changes by a factor:
The common principle in the following circuit methods is
to add a signal to a filtered replica of itself having equal gain
and 180 phase difference at the desired notch frequency
20
H
High Frequency Gain
BR
( )
s
DC Gain
s
2
N
–
. Notches are also occasionally used by
H s
N
N
N
U
at high frequencies (f >> f
O
BR
= 2 f
2
/
V
Q s
(j2 f)| has the value H
IN
INFORMATION
U
N
2
N
0
0
and H
2
O
N
2
and Q). Characteristically,
= 2 f
C
R
IN1
2
O
IN1
Figure 11. Feedforward Notch Configuration for f
W
N
0
INV
and Q are set for the
set by component
1/4 LTC1562
2nd ORDER
V1
Q
R
resistors as
Q1
N
V2
(f
U
N
N
R21
), so in
2
/f
0
R
R
2
IN2
FF2
) at
I
O
GROUND
VIRTUAL
f
notch depth (the completeness of cancellation) will be
infinite to the extent that the two paths have matching
gains. Three practical circuit methods are presented here,
with different features and advantages.
Examples and design procedures for practical filters using
these techniques appear in a series of articles
this data sheet on the Linear Technology web site
(www.linear-tech.com). Also available free is the analog
filter design software, FilterCAD for Windows, recom-
mended for designing filters not shown in the Typical
Applications schematics in this data sheet.
Elementary Feedforward Notches
A “textbook” method to get a 180 phase difference at
frequency f
order section (described earlier under Basic Bandpass),
which gives 180 phase shift at the section’s center
frequency f
bandpass section of Figure 6a, at its center frequency f
has a phase shift of 180 and a gain magnitude of H
R
into virtual ground have the same gains at the 180
frequency (then I
resistor values:
1
LTC1562 Quad Operational Filter.” Attached to this data sheet, available on the LTC web site
(www.linear-tech.com).
Nello Sevastopoulos, et al. “How to Design High Order Filters with Stopband Notches Using the
N
Q
. The two signals then cancel out at frequency f
/R
R
R
IN
FF
IN
2
2
. A notch results in Figure 11 if the paths summed
–
+
O
N
R
R
(Figure 11, with C
IN
for a notch is to dedicate a bandpass 2nd
Q
1
R
1
GAIN
N
O
= 0). This requires a constraint on the
f
O
1562 F11
V
OUT
IN1
= 0), so that f
1
attached to
N
= f
N
O
. The
. The
B
O
=
,