LTC1064
10
1064fb
f
O
= 1 + ; f
n
= ; H
OHP
f→ =
H
ON
(f = f
O
) = Q H
OLP
– H
OHP
; Q
=
1064 F09Eq
f
O
= ; f
n
= ; H
OHP
= ; HOBP =
f
CLK
100
R2
R4
R
H
R
L
MODE 3a (100:1):
NOTE: THE 50:1 EQUATIONS FOR MODE 3A ARE DIFFERENT FROM
THE EQUATIONS FOR MODE 3A OPERATION OF THE LTC1059,
LTC1060 AND LTC1061. START WITH f
O
, CALCULATE R2/R4, SET R4;
FROM THE Q VALUE, CALCULATE R3:
√
R2
R4
√
√
f
CLK
100
R2
R4
R3 = ; THEN CALCULATE R1 TO
SET THE DESIRED GAIN.
+
R2
1.005
Q
√
R2
16R4
R2
R1
–
R2
R3
R2
16R4
R2
R4
1.005
√
–
R3
R1
H
OBP
= – ; H
OLP
(f = 0) = Q =
R3
16R4
1 –
R4
R1
– ;
R3
R1
– ;
R3
R2
R4
R1
f
CLK
2
()()() ()()
R
H
R
L
√
R2
R4
√
f
CLK
50
f
CLK
50
MODE 3a (50:1):
R2
R1
– ;
f
CLK
2
()
()
H
OLP
= – ; H
ON1
(f→ 0) = ; H
ON2
f→ = ;
R4
R1
R2
R1
R
G
R
L
R
G
R
L
R
G
R
H
R
G
R
H
–
+
∫ ∫
LP
+
Σ
AGND
HP S
1/4 LTC1064
–
BP
R1
R2
V
IN
R3
R4
1064 F09
C
C
R
L
R
H
R
G
NOTCH
EXTERNAL OP AMP OR INPUT
OP AMP OF THE LTC1064,
SIDE A, B, C, D
–
+
1064 F08Eq
f
O
= 1 + ; f
n
= ; Q = 1 + ; H
OLP
= – ;
f
CLK
100
R2
R4
R3
R2
MODE 2 (100:1):
NOTE: THE 50:1 EQUATIONS FOR MODE 2 ARE DIFFERENT FROM THE EQUATIONS
FOR MODE 2 OPERATION OF THE LTC1059, LTC1060 AND LTC1061. START WITH
f
O
, CALCULATE R2/R4, SET R4; FROM THE Q VALUE, CALCULATE R3:
√
R2
R4
√
R2
R4
√
f
O
= 1 + ; f
n
= ; Q = ; H
OLP
= – ;
f
CLK
50
f
CLK
50
f
CLK
50
MODE 2 (50:1):
R2
R3
R2
16R4
R2
R4
1.005 1 +
√
–
R3
R1
H
OBP
= – ; H
ON1
(f→ 0) = – ; H
ON2
= f→ =
R3
16R4
1 –
R2
R4
R3 = ; THEN CALCULATE R1 TO SET THE DESIRED GAIN.
1 + +
R2
1.005
Q
√
R2
16R4
R2
R1
R2
R4
1 +
R2
R1
–
R2
R1
R2
R4
1 +
R2
R1
R2
R4
1 +
f
CLK
2
()
f
CLK
2
()
H
OBP
= – ; H
ON1
(f→ 0) = – ; H
ON2
f→ = –
R3
R1
R2
R1
R2
R1
R2
R4
1 +
–
+
∫ ∫
LP
1064 F08
+
Σ
AGND
NS
1/4 LTC1064
–
R1
R2
V
IN
R3
R4
BP
ODES OF OPERATIO
U
W
Figure 8. Mode 2: 2nd Order Filter Providing Notch, Bandpass and Lowpass
Mode 3a
This is an extension of Mode 3 where the highpass and
lowpass outputs are summed through two external resis-
tors R
H
and R
L
to create a notch. This is shown in Figure 9.
Mode 3a is more versatile than Mode 2 because the notch
frequency can be higher or lower than the center fre-
quency of the 2nd order section. The external op amp of
Figure 9 is not always required. When cascading the
sections of the LTC1064, the highpass and lowpass
outputs can be summed directly into the inverting input of
the next section. The topology of Mode 3a is useful for
elliptic highpass and notch filters with clock-to-cutoff
frequency ratios higher than 100:1. This is often required
to extend the allowed input signal frequency range and to
avoid premature aliasing.
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
.
Figure 9. Mode 3a: 2nd Order Filter Providing Highpass, Bandpass, Lowpass and Notch
