LTC1562 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1562' PDF : 28 Pages View PDF
LTC1562
APPLICATIONS INFORMATION
Note that the depth of the notch depends on the accuracy
of this resistor ratioing. The virtual-ground summing
point in Figure 11 may be from an op amp as shown, or in
a practical cascaded filter, the INV input of another Opera-
tional Filter block. The transfer function in Figure 11 with
CIN1 = 0 is a “pure” notch (fN = f0) of the HBR(s) form above,
and the parameters are:
ƒN = ƒO
HN =
RGAIN
RFF2
Because fN = f0 in this case, the gain magnitude both at DC
and at high frequencies (f >> fN) is the same, HN (assuming
that the op amp in Figure 11 adds no significant frequency
response). Figure 12 shows this. Such a notch is ineffi-
cient as a cascaded part of a highpass, lowpass or bandpass
filter (the most common uses for notches). Variations of
Figure 11 can add a highpass or lowpass shape to the
notch, without using more Operational Filter blocks. The
key to doing so is to decouple the notch frequency fN from
the center frequency f0 of the Operational Filter (this is
shown in Figures 13 and 15). The next two sections
summarize two variations of Figure 11 with this highpass/
lowpass shaping, and the remaining section shows a
different approach to building notches.
Feedforward Notches for fN > f0
When CIN1 ≠ 0 in Figure 11, the notch frequency fN is above
the center frequency f0 and the response has a lowpass
shape as well as a notch (Figure 13). CIN1 contributes
phase lead, which increases the notch frequency above
the center frequency of the 2nd order Operational Filter
section. The resistor constraint from the previous section
also applies here and the HBR(s) parameters become:
ƒN = ƒO
1
1– RIN1CIN1
RQ1C
HN
=
RGAIN
RFF2
ƒO2
ƒN2
C is the internal capacitor value in the Operational Filter (in
the LTC1562, 159pF).
The configuration of Figure 11 is most useful for a stopband
notch in a lowpass filter or as an upper stopband notch in
a bandpass filter, since the two resistors RIN2 and RFF2 can
replace the input resistor RIN of either a lowpass section
(Figure 5) or a resistor-input bandpass section (Figure 6a)
built from a second Operational Filter. The configuration is
0
– 20
– 40
– 60
– 80
fN = fO = 100kHz
HN = 1
Q=1
– 100
10
100
FREQUENCY (kHz)
1000
AN54 • TA18
Figure 12. Notch Response with fN = fO
20
( ) DC GAIN = HN
fN2
fO2
0
– 20
HIGH FREQ
GAIN = HN
– 40 fO = 100kHz
fN = 200kHz
Q=1
DC GAIN = 0dB
– 60
10
100
FREQUENCY (kHz)
1000
1562 F13
Figure 13. Notch Response with fN > fO
21
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