LTC1562 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1562' PDF : 28 Pages View PDF
LTC1562
APPLICATIONS INFORMATION
robust against tolerances in the CIN1 value when fN ap-
proaches f0 (for fN/f0 ≤ 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 applica-
tion details appear in Part 1 of the series of articles.1
Feedforward Notches for fN < f0
Just as feedforward around an inverting bandpass section
yields a notch at the section’s f0 (Figure 11 with CIN1 = 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 CIN1 in Figure 11 moves
the notch frequency up from the center frequency f0. In
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 CIN1 shifts the resulting
notch frequency, fN, up from zero, giving a low frequency
notch with a highpass shape (Figure 15). The HBR(s)
response parameters are now:
ƒN = ƒO
1
–
R1
RQ1
C
CIN1
R21
RIN1
HN
=
RGAIN
RFF2
The constraint required for exact cancellation of the two
paths (i.e., for infinite notch depth) becomes:
RIN2 = RQ1CIN1
RFF2 R1C
R1 and C are the internal precision components (in the
LTC1562, 10k and 159pF respectively) as described above
in Setting f0 and Q.
The configuration of Figure 14 is most useful as a lower
stopband notch in a bandpass filter, because the resistors
RIN2 and RFF2 can replace the input resistor RIN of a
bandpass section made from a second Operational Filter,
as in Figure 6a. The configuration is robust against toler-
ances in the CIN1 value when fN approaches f0 (for f0/fN ≤
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.1
20
( ) 0
DC GAIN = HN
fN2
fO2
HIGH FREQ
GAIN = HN
– 20
– 40
– 60
10k
fO = 100kHz
fN = 50kHz
Q=1
HIGH FREQ GAIN = 0dB
100k
1M
FREQUENCY (Hz)
1562 F15
Figure 15. Notch Response with fN < f0
CIN1
RIN1
VIN
RQ1 R21 RIN2
IO
RGAIN
INV V1 V2
2nd ORDER
VIRTUAL
–
GROUND
1/4 LTC1562
RFF2
+
VOUT
1562 F14
Figure 14. Feedforward Notch Configuration for fN < fO
22
Share Link: 
