5th-Order, Lowpass,

Switched-Capacitor Filters

_______________________________________________________________________________________

9

Pin Description

_______________Detailed Description

The MAX7418/MAX7421/MAX7422/MAX7425 elliptic

lowpass filters provide sharp rolloff with good stopband

rejection. The MAX7419/MAX7423 Bessel filters provide

low overshoot and fast settling responses, and the

MAX7420/MAX7424 Butterworth filters provide a maxi-

mally flat passband response. All parts operate with a

100:1 clock-to-corner frequency ratio.

Most switch capacitor filters (SCFs) are designed with

biquadratic sections. Each section implements two

pole-zero pairs, and the sections can be cascaded to

produce 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. The MAX7418–MAX7425 use an

alternative approach, which is to emulate a passive net-

work using switched-capacitor integrators with sum-

ming and scaling. The passive network may be

synthesized using CAD programs, or may be found in

many filter books. Figure 1 shows a basic 5th-order lad-

der filter structure.

Typical Operating Characteristics (continued)

0

0.5

1.5

1.0

2.0

2.5

2.5

3.5

3.0

4.0

4.5

5.0

5.5

DC OFFSET VOLTAGE

vs. SUPPLY VOLTAGE

SUPPLY VOLTAGE (V)

0

1.0

0.5

2.0

1.5

2.5

3.0

-40

10

-15

356085

DC OFFSET VOLTAGE

vs. TEMPERATURE

TEMPERATURE (

°C)

Filter Output

OUT

5

Offset Adjust Input. To adjust output offset, connect OS to an external supply through a resistive voltage-

divider (Figure 4). Connect OS to COM if no offset adjustment is needed. See the Offset and Common-Mode

Input Adjustment section.

OS

6

SHDN

7

CLK

8

a 0.1µF capacitor.

4

Ground

GND

3

PIN

Filter Input

IN

2

Common Input Pin. Biased internally at midsupply. Bypass COM externally to GND with a 0.1µF capacitor.

To override internal biasing, drive COM with an external supply.

COM

1

FUNCTION

NAME