LTC1164_02 Datasheet, PDF (10/16 Pages) Linear Technology – Low Power, Low Noise, Quad Universal Filter Building Block

LTC1164
U
ODES OF OPERATIO
Mode 3
Mode 3 is the second of the primary modes. 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 50:1 or 100:1. Side D of the LTC1164 can only be
connected in Mode 3. Figure 7 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, highpass and notch
filters.
When the internal clock-to-center frequency ratio is set at
50:1, the design equations for Q and bandpass gain are
different from the 100:1 case. This was done to provide
speed without penalizing the noise performance.
R1
V_IN
R4
R3
R2
–
+
AGND
C_C
HP
S
+–
Σ∫
1/4 LTC1164
BP
LP
∫
MODE 3 (100:1):
f_o
=
f_CLK
100
R2; _{Q =} R3
R4 R2
R2
R4
;
H_OHP
=
–
R2/R1;
H_OBP = – R3/R1; H_OLP = – R4/R1
MODE 3 (50:1):
f_o
=
f_CLK
50
R2;
R4
Q
=
1.005 (√R2/R4)
(R2/R3) – (R2/16R4);
H_OLP
=
–
R2/R1;
H_OBP
=
–
1
–
R3/R1 _;
(R3/16R4)
H_OLP
=
–
R4/R1
NOTE: THE 50:1 EQUATIONS FOR MODE 3 ARE DIFFERENT FROM THE EQUATIONS
FOR MODE 3 OPERATION OF THE LTC1059, LTC1060 AND LTC1061. START WITH
f_o, CALCULATE R2/R4, SET R4; FROM THE Q VALUE, CALCULATE R3:
R2
R3 =
1.005 _{R2 + R2}
Q R4 16R4
; THEN CALCULATE R1 TO SET
THE DESIRED GAIN
LTC1164 • MOO02
SECONDARY MODES
Mode 1b
Mode 1b is derived from Mode 1. In Mode 1b, Figure 8, two
additional resistors R5 and R6, are added to alternate the
amount of voltage feedback from the lowpass output into
the input of the SA (or SB or SC) switched capacitor
summer. This allows the filter clock-to-center frequency
ratio to be adjusted beyond 50:1 or 100:1. Mode 1b
maintains the speed advantages of Mode 1.
Mode 2
Mode 2 is a combination of Mode 1 and Mode 3, as shown
in Figure 9. With Mode 2, the clock-to-center frequency
ratio, f_CLK/f_O, is always less than 50:1 or 100:1. 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 frequency, f_O.
When the internal clock-to-center frequency ratio is set at
50:1, the design equations for Q and bandpass gain are
different from the 100:1 case.
R1
V_IN
R3
R2
–
+
R6
R5
N
S
+–
Σ∫
BP
LP
∫
AGND
f_o
=
f_CLK
100(50)
R6 _;
R5 + R6
f_n
=
f_o;
Q
=
R3
R2
R6 _;
R5 + R6
( ) ^HON1 ^{(f → 0) = H}ON2
f
→
f_CLK
2
=–
R2
R1
;
H_OLP
=
– R2/R1
R6/(R5 + R6) ^;
H_OBP
=
–
R3
R1
;
(R5//R6)
<
5kΩ
LTC1164 • MOO03
Figure 7. Mode 3: 2nd Order Filter Providing Highpass,
Bandpass, Lowpass
10
Figure 8. Mode 1b: 2nd Order Filter Providing Notch,
Bandpass, Lowpass
1164fa