LTC1562 Linear Technology, LTC1562 Datasheet - Page 22

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

APPLICATIONS
LTC1562
robust against tolerances in the C
proaches f
attractive in narrow transition-band filters, because of the
relative cost of high accuracy capacitors. Further applica-
tion details appear in Part 1 of the series of articles.
Feedforward Notches for f
Just as feedforward around an inverting bandpass section
yields a notch at the section’s f
feedforward around an inverting lowpass section causes
a notch at zero frequency (which is to say, a highpass
response). Moreover, and this is what makes it useful,
introducing a capacitor for phase lead moves the notch
frequency up from DC, exactly as C
the notch frequency up from the center frequency f
Figure 14, the inverting lowpass output (V2) of the Opera-
tional Filter is summed, at a virtual ground, with a fed-
forward input signal. Capacitor C
notch frequency, f
notch with a highpass shape (Figure 15). The H
response parameters are now:
The constraint required for exact cancellation of the two
paths (i.e., for infinite notch depth) becomes:
22
H
N
N
R
R
O
0
GAIN
FF
(for f
2
1
–
N
N
R
/f
R
U
, up from zero, giving a low frequency
Q
0
1
1
1.4, as a rule of thumb) which is
INFORMATION
C
V
U
C
IN
IN
N
1
< f
0
0
(Figure 11 with C
R
R
IN
21
IN1
C
R
IN1
IN1
IN1
IN1
1
W
Figure 14. Feedforward Notch Configuration for f
shifts the resulting
in Figure 11 moves
value when f
INV
1/4 LTC1562
2nd ORDER
V1
R
U
Q1
IN1
V2
R21
N
BR
1
= 0),
0
. In
ap-
(s)
R
R
IN2
FF2
R1 and C are the internal precision components (in the
LTC1562, 10k and 159pF respectively) as described above
in Setting f
The configuration of Figure 14 is most useful as a lower
stopband notch in a bandpass filter, because the resistors
R
bandpass section made from a second Operational Filter,
as in Figure 6a. The configuration is robust against toler-
ances in the C
1.4, as a rule of thumb) which is attractive in narrow
transition-band filters, because of the relative cost of high
accuracy capacitors. Further application details appear in
Part 2 of the series of articles.
I
IN2
O
VIRTUAL
GROUND
R
R
IN
FF
and R
2
2
–20
– 40
– 60
0
20
Figure 15. Notch Response with f
R C
FF2
0
10k
R C
and Q.
Q
–
+
IN1
DC GAIN = H
1
1
can replace the input resistor R
IN
N
value when f
R
1
< f
GAIN
O
N
FREQUENCY (Hz)
f
f
N
O
2
2
100k
1562 F14
f
f
Q = 1
HIGH FREQ GAIN = 0dB
O
N
V
OUT
= 100kHz
= 50kHz
N
1
approaches f
HIGH FREQ
GAIN = H
1562 F15
N
N
< f
1M
0
0
(for f
IN
0
/f
of a
N