7
LTC1264
1264fb
ODES OF OPERATIO
W
U
optimum mode for high Q designs with f
CLK
to f
CUTOFF
(or
f
CENTER
) ratios greater than 20:1.
Please refer to the Maximum Frequency of Operation
paragraph under Applications Information for a guide to
the use of capacitor C
C
.
–
+
Σ
∫ ∫
AGND
R1
N
BP
LP
V
IN
1264 F04
+
–
S
1/4 LTC1264
R2
R3
C
C
f
i
= ; f
O
= f
i
; f
n
= f
O
Q = ; H
ON
= – ; H
OBP
= –
H
OLP
= H
ON
R2
R1
R3
R1
R3
R2
f
CLK
20
Figure 4. Mode 1, 2nd Order Filter Providing Notch,
Bandpass and Lowpass Outputs
–
+
Σ
∫ ∫
AGND
R1
N
BP
LP
V
IN
1264 F05
+
–
S
NOTE: R5 ≤ 5k
R2
R3
C
C
R5R6
f
i
= ; f
O
= f
i
; f
n
= f
O
Q = ; H
ON
= – ; H
OBP
= –
H
OLP
= –
R2
R1
R3
R1
R3
R2
f
CLK
20
√
R6
(R6 + R5)
R2
R1
R6 + R5
R6
1/4 LTC1264
√
R6
(R6 + R5)
()
Figure 5. Mode 1b, 2nd Order Filter Providing Notch,
Bandpass and Lowpass Outputs
Mode 3
In Mode 3, the ratio of the external clock frequency to the
center frequency of each 2nd order section can be
adjusted above or below 20:1. Figure 6 illustrates Mode 3,
the classical state variable configuration, providing high-
pass, bandpass, and lowpass 2nd order filter functions.
Mode 3 is slower than Mode 1. Mode 3 can be used to
make high order all-pole bandpass, lowpass, and high-
pass filters.
Please refer to the Maximum Frequency of Operation
paragraph under Applications Information for a guide to
the use of capacitor C
C
.
Mode 2
Mode 2 is a combination of Mode 1 and Mode 3, shown in
Figure 7. With Mode 2, the clock-to-center frequency ratio,
f
CLK
/f
O
, is always less than 20:1. The advantage of Mode
2 is that it provides less sensitivity to resistor tolerances
than does Mode 3. As in Mode 1, Mode 2 has a notch
output which depends on the clock frequency, and the
notch frequency is therefore less than the center fre-
quency, f
O
.
Please refer to the Maximum Frequency of Operation
paragraph under Applications Information for a guide to
the use of capacitor C
C
.
–
+
Σ
∫ ∫
AGND
R1
HP
BP
LP
V
IN
1264 F06
+
–
S
1/4 LTC1264
R2
R3
C
C
R4
f
i
= ; f
O
= f
i
;
Q = 1.005
f
CLK
20
R3
R2
√
R2
R4
R3
6.42•R4
( )
1
1 –
( )
√
R2
R4
H
OHP
= – ; H
OBP
= – ;
R2
R1
R3
R1
R3
6.42•R4
( )
1
1 –
R4
R1
H
OLP
= –
Figure 6. Mode 3, 2nd Order Section Providing
Highpass, Bandpass and Lowpass Outputs
