LTC1068 Datasheet, PDF (12/28 Pages) Linear Technology – Clock-Tunable, Quad Second Order, Filter Building Blocks

LTC1068 Series
W
U
MODES OF OPERATION
C_C
C_C
R1
V_IN
R6
R5
R3
R2
N
S
BP
LP
–
+
–
Σ
∫
∫
+
DEVICE
RATIO
AGND
LTC1068
LTC1068-200
√ ^fO
=
f_CLK
RATIO
(R6R+6R5;)f ^{n = fO}
LTC1068-50
LTC1068-25
√ Q = R3
R2
(R6R+6R5);
H_ON
=
–
RR21;
H_OBP
=
–
R3
R1
( ) ^HOLP
=
–
R2
R1
R6 + R5
R6
100
200
50
25
1068 F05
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 ad-
justed above or below the parts nominal ratio. Figure 6
illustrates Mode 3, the classical state variable configura-
tion, 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 Appli-
cations 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 de-
pends solely on the clock frequency and is therefore less
than the center frequency, f_O.
Please refer to the Operating Limits paragraph under Appli-
cations Information for a guide to the use of capacitor C_C.
R1
V_IN
AGND
R4
R3
R2
HP S
BP
LP
–
+
–
Σ
∫
∫
+
1/4 LTC1068
√ ^{( )√} _{( )} ^fO
=
f_CLK
RATIO
R2
R4
;
Q
=
1.005
R3
R2
R2
R4
1
1
–
R3
(RATIO)(0.32)(R4)
( ) ^HOHP
=
–
R2
R1
;
H_OBP
=
–
R3
R1
1
1
–
R3
(RATIO)(0.32)(R4)
;
H_OLP
=
–
R4
R1
DEVICE
RATIO
LTC1068
100
LTC1068-200 200
LTC1068-50 50
LTC1068-25 25
1068 F06
Figure 6. Mode 3, 2nd Order Section Providing
Highpass, Bandpass and Lowpass Outputs
C_C
R4
R3
R1
V_IN
AGND
R2
–
+
HPN S
+
–
Σ
√ ^fO
=
f_CLK
RATIO
1
+
R2 _;
R4
f_n
=
f_CLK
RATIO
( ) _{√ ( )} ^Q
=
1.005
R3
R2
1 + ^R2
R4
1
1–
R3
(RATIO)(0.32)(R4)
BP
LP
∫
∫
DEVICE
RATIO
LTC1068
100
LTC1068-200 200
LTC1068-50 50
LTC1068-25 25
1068 F07
( ) ^HOHPN
=
–
R2
R1
(AC
GAIN,
f
>>
f_O);
H_OHPN
=
–
R2
R1
1
1
+
R2
R4
(DC GAIN)
( ) ( ) ^HOBP
=
–
R3
R1
1
1–
R3
(RATIO)(0.32)(R4)
;
H_OLP
=
–
R2
R1
1
1 + ^R2
R4
Figure 7. Mode 2, 2nd Order Filter Providing Highpass
Notch, Bandpass and Lowpass Outputs
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