Datasheet
LTC1068 Series
13
1068fc
MODES OF OPERATION
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 the parts nominal ratio. Figure 6
illustrates Mode 3, the classical state variable configuration,
providing highpass, 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
highpass filters.
Please refer to the Operating Limits paragraph under Applica-
tions 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 the part’s nominal ratio.
The advantage of Mode 2 is that it provides less sensitivity
to resistor tolerances than does Mode 3. Mode 2 has a
highpass notch output where the notch frequency depends
solely on the clock frequency and is therefore less than
the center frequency, f
O
.
Please refer to the Operating Limits paragraph under Applica-
tions Information for a guide to the use of capacitor C
C
.
Figure 5. Mode 1b, 2nd Order Filter Providing Notch,
Bandpass and Lowpass Outputs
Figure 6. Mode 3, 2nd Order Section Providing
Highpass, Bandpass and Lowpass Outputs
Figure 7. Mode 2, 2nd Order Filter Providing Highpass
Notch, Bandpass and Lowpass Outputs
–
+
Σ
AGND
R1
N
BP
LP
V
IN
1068 F05
+
–
S
R2
R3
C
C
R5R6
f
O
= ; f
n
= f
O
Q = ; H
ON
= – ; H
OBP
= –
H
OLP
= –
R2
R1
R3
R1
R3
R2
f
CLK
RATIO
√
R6
(R6 + R5)
R2
R1
R6 + R5
R6
√
R6
(R6 + R5)
( )
DEVICE
LTC1068
LTC1068-200
LTC1068-50
LTC1068-25
RATIO
100
200
50
25
–
+
Σ
AGND
R1
HP
BP
LP
V
IN
1068 F06
+
–
S
1/4 LTC1068
R2
R3
C
C
R4
f
O
=
f
CLK
RATIO
R3
R2
√
R2
R4
R3
(RATIO)(0.32)(R4)
( )
1
1 –
R3
(RATIO)(0.32)(R4)
( )
1
1 –
( )
√
R2
R4
H
OHP
= – ; H
OBP
= –
R2
R1
R3
R1
R4
R1
; H
OLP
= –
; Q = 1.005
DEVICE
LTC1068
LTC1068-200
LTC1068-50
LTC1068-25
RATIO
100
200
50
25
–
+
Σ
AGND
R1
HPN
BP
LP
V
IN
1068 F07
+
–
S
R2
R3
C
C
R4
f
O
= ; f
n
=
f
CLK
RATIO
f
CLK
RATIO
√
R2
R4
1 +
Q = 1.005
R3
R2
( )
√
R2
R4
1 +
R3
(RATIO)(0.32)(R4)
( )
1
1–
R3
(RATIO)(0.32)(R4)
( )
1
1–
H
OHPN
= – (AC GAIN, f >> f
O
); H
OHPN
= –
R2
R1
R2
R1
R2
R1
1
R2
R4
1 +
( )
1
R2
R4
1 +
( )
(DC GAIN)
H
OBP
= –
R3
R1
; H
OLP
= –
DEVICE
LTC1068
LTC1068-200
LTC1068-50
LTC1068-25
RATIO
100
200
50
25