LTC1562 Linear Technology, LTC1562 Datasheet - Page 21

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

APPLICATIONS
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
C
and the parameters are:
Because f
and at high frequencies (f >> f
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 f
the center frequency f
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.
IN1
H
N
= 0 is a “pure” notch (f
N
R
–100
N
R
O
– 40
– 60
– 80
–20
GAIN
FF
= f
Figure 12. Notch Response with f
0
10
2
0
f
H
Q = 1
N
in this case, the gain magnitude both at DC
N
= f
= 1
O
= 100kHz
U
FREQUENCY (kHz)
0
of the Operational Filter (this is
INFORMATION
U
N
100
= f
N
0
) is the same, H
) of the H
W
AN54 • TA18
BR
N
1000
(s) form above,
= f
N
O
(assuming
U
N
from
Feedforward Notches for f
When C
the center frequency f
shape as well as a notch (Figure 13). C
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 H
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 R
replace the input resistor R
(Figure 5) or a resistor-input bandpass section (Figure 6a)
built from a second Operational Filter. The configuration is
H
N
N
IN1
O
– 60
–20
– 40
R
20
R
Figure 13. Notch Response with f
0
GAIN
0 in Figure 11, the notch frequency f
FF
10
1
DC GAIN = H
f
f
Q = 1
DC GAIN = 0dB
2
O
N
–
= 100kHz
= 200kHz
R C
IN
R C
1
Q
1
1
2
O
N
2
0
N
IN
FREQUENCY (kHz)
and the response has a lowpass
1
f
f
N
O
2
2
N
BR
IN
100
> f
(s) parameters become:
of either a lowpass section
0
HIGH FREQ
GAIN = H
1562 F13
N
IN2
LTC1562
IN1
N
> f
1000
O
and R
contributes
N
is above
21
FF2
can