Datasheet
MAX7490/MAX7491
Dual Universal Switched-Capacitor Filters
______________________________________________________________________________________ 11
Mode 1
Figure 2 shows the MAX7490/MAX7491s’ configuration
of Mode 1. This mode provides 2nd-order notch, low-
pass, and bandpass filter functions. The gain at all
three outputs is inversely proportional to the value of
R1. The center frequency, f
O
, is fixed at f
CLK
/100. High-
Q bandpass filters can be built without exceeding the
bandpass amplifier’s output swing (i.e., H
OBP
does not
have to track Q). The notch and bandpass center fre-
quencies are identical. The notch output gain is the
same above and below the notch center frequency.
Mode 1 can also be used to make high-order Butter-
worth lowpass filters, low Q notches, and multiple-order
bandpass filters obtained by cascading identical
switched-capacitor sections.
Mode 1 Design Equations
Mode 1B
Figure 3 shows the configuration of Mode 1B. R5 and
R6 are added to lower the feedback voltage from the
lowpass output to the summing input. This allows the
clock-to-center frequency to be adjusted beyond the
nominal value. This mode essentially has the same
functions and speed as Mode 1 while providing a high-
Q with f
CLK
/f
O
ratios greater than the nominal value.
Mode 1B Design Equations
Mode 2
Figure 4 shows the configuration of Mode 2. Mode 2 is
a combination of Mode 1 and Mode 3. In this mode,
f
CLK
/f
O
is always less than the part’s nominal ratio.
However, it provides less sensitivity to resistor toler-
ances than does Mode 3. It has a highpass notch out-
put where the notch frequency depends solely on the
clock frequency.
f
f
R
RR
ff
Q
R
R
R
RR
H
R
R
RR
R
H
R
R
HasfHz
R
R
Hatff
R
R
O
CLK
nO
OLP
OBP
ON
ON CLK
=
+
=
=
+
=
+
=
→=
==
−
−
−
−
100
6
65
3
2
6
65
2
1
65
6
3
1
0
2
1
2
2
1
1
2
()
(/)
f
f
ff
Q
R
R
H
R
R
H
R
R
HasfHz
R
R
Hatff
R
R
O
CLK
notch O
OLP
OBP
ON
ON CLK
=
=
=
=
=
→=
==
−
−
−
−
100
3
2
2
1
3
1
0
2
1
2
2
1
1
2
()
(/)
Σ
∫ ∫
LPBPSN
R3
R2
R1
V
IN
C
C
COM
+
-
Figure 2. Mode 1, 2nd-Order Filter Providing Notch, Bandpass,
and Lowpass Outputs
Σ
∫ ∫
LPBPSN
R3
R6 R5
R2
R1
V
IN
C
C
COM
COM
+
-
Figure 3. Mode 1B, 2nd-Order Filter Providing Notch, Bandpass,
and Lowpass Outputs