Dual Universal Switched-Capacitor Filters

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13

Mode 3

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

Mode 3A

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

the highpass and lowpass outputs through two external

resistors, RH and RL. The ratio of resistors RH and RL

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 fO. Mode 3A is suitable for both lowpass and

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.

fO

fCLK R

R

Q

R

R

R

R

HOHP

R

R

HOLP

R

R

HOBP

R

R

=

=

=

=

=

−

−

−

100

2

4

3

2

2

4

2

1

4

1

3

1

Σ

∫

∫

LP

BP

HP

R3

R4

R2

R1

VIN

CC

COM

+

-

S

COM

Figure 6. Mode 3, 2nd-Order Section Providing Highpass,

Bandpass, and Lowpass Outputs

Σ

LP

BP

N/HP

R3

R4

R2

R1

VIN

CC

COM

COM

LOWPASS

NOTCH

OUTPUT

+

-

RH

RL

RG

S

COM

Figure 7. Mode 3A, 2nd-Order Filter Providing Highpass Notch or Lowpass Notch Outputs