LTC1562 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1562' PDF : 28 Pages View PDF
LTC1562
APPLICATIONS INFORMATION
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 CN adds a further 90° to
the 90° difference between V1 and V2, producing a
wideband 180° phase difference, but frequency-depen-
dent amplitude ratio, between currents IR and IC. At the
frequency where IR and IC have equal magnitude, IO
becomes zero and a notch occurs. This gives a net transfer
function from VIN to VOUT in the form of HBR(s) as above,
with parameters:
ƒN =
1
2π RNCNR1C
HN
=
RGAIN
RIN1
CN
C
DC
Gain
=
RGAIN
RIN1
R21
RN
ƒO2 = High Frequency Gain = RNCN
ƒN2
DC Gain
R21C
R1 and C are the internal precision components (in the
LTC1562, 10k and 159pF respectively) as described above
in Setting f0 and Q.
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 RN or CN values alter the notch
frequency, fN, rather than the degree of cancellation at fN.
Also, the notch frequency, fN, is independent of the section’s
center frequency f0, so fN can freely be equal to, higher
than or lower than f0 (Figures 12, 13 or 15, respectively)
without changing the configuration. The chief drawback of
Figure 16 compared to the previous methods is a very
practical one—the CN capacitor value directly scales HN
(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 fN approaches f0.
RIN1
VIN
RQ1 R21 RN IR
IO
RGAIN
INV V1 V2
VIRTUAL
–
GROUND
2nd ORDER
1/4 LTC1562
CN IC
+
VOUT
1562 F16
Figure 16. The R-C Universal Notch Configuration for an Operational Filter Block
23
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