Datasheet

22
LTC1562
1562fa
APPLICATIONS INFORMATION
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The configuration is robust against tolerances in the C
IN1
value when f
N
approaches f
0
(for f
N
/f
0
≤ 1.4, as a rule of
thumb) which is attractive in narrow transition-band fil-
ters, because of the relative cost of high accuracy capaci-
tors. Further application details appear in Part 1 of the
series of articles.
Feedforward Notches for f
N
< f
0
Just as feedforward around an inverting bandpass section
yields a notch at the section’s f
0
(Figure 11 with C
IN1
= 0),
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
IN1
in Figure 11 moves
the notch frequency up from the center frequency f
0
. In
Figure 14, the inverting lowpass output (V2) of the Opera-
tional Filter block is summed, at a virtual ground, with a
fed-forward input signal. Capacitor C
IN1
shifts the result-
ing notch frequency, f
N
, up from zero, giving a low
frequency notch with a highpass shape (Figure 15). The
H
BR
(s) response parameters are now:
ƒ=ƒ


















=
NO
QININ
N
GAIN
FF
R
R
C
C
R
R
H
R
R
1
121
111
2
–
The constraint required for exact cancellation of the two
paths (i.e., for infinite notch depth) becomes:
R
R
RC
RC
IN
FF
QIN2
2
11
1
=
R1 and C are the internal precision components (in the
LTC1562, 10k and 159pF respectively) as described above
in Setting f
0
and Q.
The configuration of Figure 14 is most useful as a lower
stopband notch in a bandpass filter, because the resistors
R
IN2
and R
FF2
can replace the input resistor R
IN
of a
bandpass section made from a second Operational Filter
block, as in Figure 6a. The configuration is robust against
tolerances in the C
IN1
value when f
N
approaches f
0
(for f
0
/
f
N
≤ 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.
FREQUENCY (Hz)
10k
–60
GAIN (dB)
–40
–20
20
100k 1M
1562 F15
0
f
O
= 100kHz
f
N
= 50kHz
Q = 1
HIGH FREQ GAIN = 0dB
f
N
2
f
O
2
DC GAIN = H
N
()
HIGH FREQ
GAIN = H
N
Figure 15. Notch Response with f
N
< f
0
Figure 14. Feedforward Notch Configuration for f
N
< f
O
INV V1
2nd ORDER
1/4 LTC1562
V2
R21R
Q1
R
IN1
R
IN2
R
GAIN
I
O
R
FF2
C
IN1
V
IN
V
OUT
1562 F14
VIRTUAL
GROUND
–
+