Dual Universal Switched-Capacitor Filters

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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, fO, is fixed at fCLK/100. High-

Q bandpass filters can be built without exceeding the

bandpass amplifier’s output swing (i.e., HOBP 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 fCLK/fO 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,

fCLK/fO 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

Has f

Hz

R

R

Hat f

f

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

Has f

Hz

R

R

Hat f

f

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

∫

∫

LP

BP

S

N

R3

R2

R1

VIN

CC

COM

+

-

Figure 2. Mode 1, 2nd-Order Filter Providing Notch, Bandpass,

and Lowpass Outputs

Σ

∫

∫

LP

BP

S

N

R3

R6

R5

R2

R1

VIN

CC

COM

COM

+

-

Figure 3. Mode 1B, 2nd-Order Filter Providing Notch, Bandpass,

and Lowpass Outputs