Dual Universal Switched-Capacitor Filters

12

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Mode 2 Design Equations

Mode 2N

Figure 5 shows the configuration of Mode 2N. This

mode extends the topology of Mode 3A to Mode 2,

where the highpass and lowpass outputs are summed

through two external resistors, RH and RL, to create a

lowpass notch filter that has higher frequency than the

one in Mode 2. Mode 2 is most useful in lowpass elliptic

designs. When cascading the sections of the

MAX7490/MAX7491, the highpass and lowpass outputs

can be summed directly into the inverting input of the

next section. Only one external op amp is needed.

Mode 2N Design Equations

f

f

R

R

f

fR

R

Q

R

R

R

R

Hf

Hz

R

R

R

R

R

R

R

RR

O

CLK

n

CLK

H

L

ON

G

H

G

L

=+

=+

=+

→=

+

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎞

⎠

⎟

+

⎛

⎝

⎜

⎞

⎠

⎟

100

1

2

4

100

1

3

2

1

2

4

0

2

1

4

42

1()

f

f

R

R

f

f

Q

R

R

R

R

H

R

R

R

RR

H

R

R

Hf

Hz

R

R

R

RR

Hat f

f

R

R

O

CLK

n

CLK

OLP

OBP

ON

ON

CLK

=+

=

=+

=

+

⎛

⎝

⎜

⎞

⎠

⎟

=

→=

+

⎛

⎝

⎜

⎞

⎠

⎟

==

−

−

−

−

100

1

2

4

100

3

2

1

2

4

2

1

4

42

3

1

0

2

1

4

42

2

2

1

1

2

()

(/ )

Σ

∫

∫

LP

BP

S

HP/N

R3

R4

R2

R1

VIN

CC

COM

COM

LOWPASS

NOTCH

OUTPUT

+

-

RH

RL

RG

Figure 5. Mode 2N, 2nd-Order Filter Providing a Lowpass Notch Output

Σ

∫

∫

LP

BP

S

HP/N

R3

R4

R2

R1

VIN

CC

COM

+

-

Figure 4. Mode 2, 2nd-Order Filter Providing a Highpass

Notch, Bandpass, and Lowpass Outputs