8th-Order, Lowpass, Butterworth,

Switched-Capacitor Filter

6

_______________________________________________________________________________________

NAME

FUNCTION

1

COM

Common Input Pin. Biased internally at mid-supply. Bypass externally to GND with a 0.1µF capacitor. To

override internal biasing, drive with an external supply.

2

IN

Filter Input

PIN

3

GND

Ground

4

+5V Supply Input

8

CLK

Clock Input. To override the internal oscillator, connect to an external clock; otherwise, connect an external

7

SHDN

6

OS

Offset Adjust Input. To adjust output offset, bias OS externally. Connect OS to COM if no offset adjustment is

needed. Refer to

Offset and Common-Mode Input Adjustment section.

5

OUT

Filter Output

Pin Description

_______________Detailed Description

The MAX7480 Butterworth filter operates with a 100:1

clock-to-corner frequency ratio and a 2kHz maximum

corner frequency.

Lowpass Butterworth filters provide a maximally flat

passband response, making them ideal for instrumen-

tation applications that require minimum deviation from

the DC gain throughout the passband.

Figure 1 shows the difference between Bessel and

Butterworth filter frequency responses. With the filter

cutoff frequencies set at 1kHz, trace A shows the

Bessel filter response and trace B shows the

Butterworth filter response.

Background Information

Most switched-capacitor filters (SCFs) are designed

with biquadratic sections. Each section implements two

filtering poles, and the sections are cascaded to pro-

duce higher-order filters. The advantage to this

approach is ease of design. However, this type of

design is highly sensitive to component variations if any

section’s Q is high. An alternative approach is to emu-

late a passive network using switched-capacitor inte-

grators with summing and scaling. Figure 2 shows a

basic 8th-order ladder filter structure.

A switched-capacitor filter such as the MAX7480 emu-

lates a passive ladder filter. The filter’s component sen-

sitivity is low when compared to a cascaded biquad

design, because each component affects the entire fil-

ter shape, not just one pole-zero pair. In other words, a

mismatched component in a biquad design will have a

concentrated error on its respective poles, while the

same mismatch in a ladder filter design results in an

error distributed over all poles.

-100

-60

-80

-20

-40

0

20

0.1

0.5

1

0.2

A

B

25

10

FREQUENCY (kHz)

Figure 1. Bessel vs. Butterworth Filter Frequency Response

L3

L5

L7

C8

R2

C4

C2

+

-

L1

R1

C6

Figure 2. 8th-Order Ladder Filter Network