MAX7490 Maxim, MAX7490 Datasheet - Page 13

• Manufacturer Part Number: MAX7490
• Description: Dual Universal Switched-Capacitor Filters
• Manufacturer: Maxim

Figure 6 shows the configuration of Mode 3. This mode
is a sampled time (Z transform) equivalent of the classi-
cal 2nd-order state variable filter. In this versatile mode,
the ratio of resistors R2 and R4 can move the center
frequency both above and below the nominal ratio.
Mode 3 is commonly used to make multiple-pole
Chebyshev filters with a single clock frequency. This
mode can also be used to make high-order all-pole
bandpass, lowpass, and highpass filters.
Mode 3 Design Equations
Figure 7 shows the configuration of Mode 3A. Similar to
Mode 2, this mode adds an external op amp. See
Table 3 for op amp selection ideas. This op amp cre-
ates a highpass notch and lowpass notch by summing
Figure 7. Mode 3A, 2nd-Order Filter Providing Highpass Notch or Lowpass Notch Outputs
Dual Universal Switched-Capacitor Filters
V
IN
______________________________________________________________________________________
Q
H OHP
H OLP
H OBP
f O
R1
=
COM
=
R
R
f CLK
100
3
2
=
=
=
−
−
R
R
−
R
R
R
R
R
R
R4
R3
R2
2
4
1
1
1
4
2
3
R
R
2
4
N/HP
+
Mode 3A
Σ
Mode 3
S
-
C
C
COM
R
the highpass and lowpass outputs through two external
resistors, R
adjusts the notch frequency, while R2 and R4 adjust
the bandpass center frequency, since the notch (zero
pair) frequency can be adjusted to both above and
below f
highpass elliptic or Cauer filters. In multipole elliptic fil-
ters, only one external op amp is needed. Use the
inverting input of the internal op amp as the summing
node for all but the final section of the filter.
Figure 6. Mode 3, 2nd-Order Section Providing Highpass,
Bandpass, and Lowpass Outputs
H
V IN
BP
R1
COM
O
. Mode 3A is suitable for both lowpass and
H
LP
and R
R4
R3
R2
R
L
COM
L
. The ratio of resistors R
HP
+
R
Σ
G
S
-
C C
COM
∫
LOWPASS
NOTCH
OUTPUT
BP
∫
H
and R
LP
13
L