LTC1562 Linear Technology, LTC1562 Datasheet - Page 23

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

APPLICATIONS
R-C Universal Notches
A different way to get 180 phase shift for a notch is to use
the built-in 90 phase difference between the two Opera-
tional Filter outputs along with a further 90 from an
external capacitor. This method achieves deep notches
independent of component matching, unlike the previous
techniques, and it is convenient for cascaded highpass as
well as lowpass and bandpass filters.
The V2 output of an Operational Filter is a time-integrated
version of V1 (see Figure 3), and therefore lags V1 by 90
over a wide range of frequencies. In Figure 16, a notch
response occurs when a 2nd order section drives a virtual-
ground input through two paths, one through a capacitor
and one through a resistor. Again, the virtual ground may
come from an op amp as shown, or from another Opera-
tional Filter’s INV input. Capacitor C
the 90 difference between V1 and V2, producing a
wideband 180 phase difference, but frequency-depen-
dent amplitude ratio, between currents I
frequency where I
becomes zero and a notch occurs. This gives a net transfer
function from V
with parameters:
H
N
N
2
R
R
GAIN
IN
R C R C
1
N N
IN
1
to V
C
R
U
C
N
1
and I
OUT
Figure 16. The R-C Universal Notch Configuration for an Operational Filter Block
INFORMATION
U
in the form of H
V
C
IN
have equal magnitude, I
R
IN1
INV
N
W
1/4 LTC1562
2nd ORDER
adds a further 90 to
V1
R
R
BR
Q1
and I
V2
(s) as above,
R21
U
C
. At the
R
C
N
N
I
I
R
C
O
I
O
R1 and C are the internal precision components (in the
LTC1562, 10k and 159pF respectively) as described above
in Setting f
Unlike the notch methods of Figures 11 and 14, notch
depth from Figure 16 is inherent, not derived from compo-
nent matching. Errors in the R
frequency, f
Also, the notch frequency, f
center frequency f
than or lower than f
without changing the configuration. The chief drawback of
Figure 16 compared to the previous methods is a very
practical one—the C
(and therefore the high frequency gain). Capacitor values
are generally not available in increments or tolerances as
fine as those of resistors, and this configuration lacks the
property of the previous two configurations that sensitiv-
ity to the capacitor value falls as f
GROUND
VIRTUAL
DC Gain
2
O
N
2
High Frequency Gain
0
–
+
N
and Q.
, rather than the degree of cancellation at f
R
DC Gain
R
R
GAIN
GAIN
IN
0
1
, so f
0
N
(Figures 12, 13 or 15, respectively)
capacitor value directly scales H
R
R
N
1562 F16
21
N
N
V
can freely be equal to, higher
, is independent of the section’s
OUT
N
or C
N
R C
R C
N
N N
21
approaches f
values alter the notch
LTC1562
0
23
.
N
N
.