16
LTC1060
1060fb
Modes 2, 2a, and 2b have a notch output which frequency,
f
n
, can be tuned independently from the center frequency,
f
0
. For all cases, however, f
n
<f
0
. These modes are useful
when cascading second order functions to create an
overall elliptic highpass, bandpass or notch response. The
input amplifier and its feedback resistors (R2/R4) are now
part of the resonant loop. Because of this, mode 2 and its
derivatives are slower than mode 1’s.
In mode 3 (Figure 11), a single resistor ratio (R2/R4) can
tune the center frequency below or above the f
CLK
/100
(or f
CLK
/50) ratio. Mode 3 is a state variable configuration
since it provides a highpass, bandpass, lowpass output
through progressive integration; notches are obtained by
summing the highpass and lowpass outputs (mode 3a,
Figure 12). The notch frequency can be tuned below or
above the center frequency through the resistor ratio
(R
h
/R
i
). Because of this, modes 3 and 3a are the most
versatile and useful modes for cascading second order
sections to obtain high order elliptic filters. Figure 20
shows the two sections of an LTC1060 connected in mode
3a to obtain a clock tunable 4th order sharp elliptic
bandpass filter. The first notch is created by summing
directly the HP and LP outputs of the first section into the
inverting input of the second section op amp. The indi-
vidual Q’s are 29.6 and the filter maintains its shape and
performance up to 20kHz center frequency (Figure 21).
For this circuit an external op amp is required to obtain the
2nd notch. The dynamics of Figure 20 are excellent be-
cause the amplitude response at each output pin does not
exceed 0dB. The gain in the passband depends on the ratio
of (R
g
/R
h2
) • (R22/R
h1
)• (R21/R11). Any gain value can be
obtained by acting on the (R
g
/R
h2
) ratio of the external op
amp, meanwhile the remaining ratios are adjusted for
optimum dynamics of the LTC1060 output nodes. The
external op amp of Figure 20 is not always required. In
Figure 22, one section of the LTC1060 in mode 3a is
cascaded with the other section in mode 2b to obtain a 4th
order, 1dB ripple, elliptic bandreject filter. This configura-
tion is interesting because a 4th order function with two
different notches is realized without requiring an external
op amp. The clock-to-center frequency ratio is adjusted to
200:1; this is done in order to better approximate a linear
R,C notch filter. The amplitude response of the filter is
shown in Figure 23 with up to 1MHz clock frequency. The
0dB bandwidth to the stop bandwidth ratio is 9/1. When
the filter is centered at 1kHz, it should theoretically have a
44dB rejection with a 50Hz stop bandwidth. For a more
narrow filter than the above, the unused BP output of the
Figure 19. Cascading the Two Sections of the LTC1060 Connected in Mode 1c to Obtain a Clock Tunable 4th Order
1dB Ripple Bandpass Chebyshev Filter with (Center Frequency)/(Ripple Bw) = 20/1.
0.9kHz
–15dB
–10dB
–5dB
0dB
1.1kHz
–25dB
–20dB
1kHz
50Hz
f
CLK
= 40kHz
18kHz
–15dB
–10dB
–5dB
0dB
22kHz
TLC1060 • CMO01b
–25dB
–20dB
20kHz
19kHz
21kHz
1kHz
f
CLK
= 800kHz
LTC1060 • CM01
V
IN
R31
R21
R52
R32
1
2
3
4
5
6
7
8
9
10
20
19
18
17
16
15
14
13
12
11
V
+
= 5V
V
OUT
T
2
L OR CMOS CLK IN
R51
R22
R62 R12
LP
A
BP
A
N
A
INV
A
S1A
S
A/B
V
A
+
V
D
+
LSh
CLK
A
LP
B
BP
B
N
B
INV
B
S1B
AGND
V
A
–
V
D
–
50/100
CLK
B
LTC1060
R61
R11
5V
PRECISE RESISTOR VALUES
R11 = 149.21k
R21 = 4.99k
R31 = 149.12k
R51 = 2.55k
R61 = 2.49k
R12 = 45.14k
R22 = 5.00k
R32 = 142.64k
R5 = 2.49k
R62 = 4.29k
V
–
= –5V
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