LTC1064 Linear Technology, LTC1064 Datasheet - Page 9

• Manufacturer Part Number: LTC1064
• Description: Low Noise/ Fast/ Quad Universal Filter Building Block
• Manufacturer: Linear Technology

W
Mode 3
Mode 3 is the second of the primary modes. In Mode 3, the
ratio of the external clock frequency to the center fre-
quency of each 2nd order section can be adjusted above or
below 50:1 or 100:1. Side D of the LTC1064 can only be
connected in Mode 3. 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, 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.
V
IN
R3 =
MODE 3 (100:1):
MODE 3 (50:1):
NOTE: THE 50:1 EQUATIONS FOR MODE 3 ARE DIFFERENT FROM THE EQUATIONS
FOR MODE 3 OPERATIONS OF THE LTC1059, LTC1060 AND LTC1061. START WITH
f
O
ODES OF OPERATIO
, CALCULATE R2/R4, SET R4; FROM THE Q VALUE, CALCULATE R3:
Figure 6. Mode 3: 2nd Order Filter Providing Highpass,
Bandpass and Lowpass
R1
1.005
Q
R2
R2
R4
AGND
+
–
+
16R4
f
H
f
H
R2
O
O
OBP
OHP
=
=
R4
R3
R2
f
f
100
CLK
CLK
50
= –
; THEN CALCULATE R1 TO SET
THE DESIRED GAIN.
= –
R3
R2
R1
R1
R2
R2
R4
R4
; H
; H
; Q =
; Q =
OLP
OBP
= –
= –
1.005
HP
R2
R3
R3
R2
1/4 LTC1064
C
+
R4
R1
1 –
C
–
16R4
R2
R4
R3
R1
16R4
R2
R3
R2
R4
–
; H
S
;
; H
U
OHP
OLP
= –
= –
R2
R1
R4
R1
;
BP
1064 F06 Eq
1064 F06
LP
SECONDARY MODES
Mode 1b
Mode 1b is derived from Mode 1. In Mode 1b, Figure 7, two
additional resistors R5 and R6 are added to alternate the
amount of voltage fed back from the lowpass output into
the input of the SA (or SB or SC) switched-capacitor
summer. This allows the filter’s 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 8. With Mode 2, the clock-to-center frequency
ratio f
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 fre-
quency and the notch frequency is therefore less than the
center frequency f
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 .
V
IN
R1
Figure 7. Mode 1b: 2nd Order Filter Providing Notch,
Bandpass and Lowpass
CLK
/f
O
f
H
H
AGND
O
ON1
OBP
=
is always less than 50:1 or 100:1. The
100(50)
–
+
(f
= –
f
CLK
O
0) = H
R3
R1
R3
R2
.
; R5 R6 5k
R5 + R6
ON2
R6
f
; f
f
n
CLK
2
= f
N
R6
1/4 LTC1064
O;
+
= –
Q =
R2
R1
R3
R2
–
S
; H
R5
OLP
R5 + R6
R6
= –
LTC1064
R5 + R6
;
1064 F07 Eq
R2
R1
R6
BP
;
1064 F07
9
LP