(VDD = +5V for MAX7418–MAX7421, VDD = +3V for MAX7422–MAX7425; fCLK = 2.2MHz; SHDN = VDD; VCOM = VOS = VDD / 2;

TA = +25°C; unless otherwise noted.)

Detailed Description

The MAX7418/MAX7421/MAX7422/MAX7425 elliptic

lowpass filters provide sharp rolloff with good stopband

rejection. The MAX7419/MAX7423 Bessel filters pro-

vide 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 sen-

sitive to component variations if any section’s Q is high.

The MAX7418–MAX7425 use an alternative approach,

which is to emulate a passive network using switched-

capacitor integrators with summing and scaling. The pas-

sive network may be synthesized using CAD programs, or

may be found in many filter books. Figure 1 shows a basic

5th-order ladder filter structure.

PIN

NAME

FUNCTION

1

COM

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.

2

IN

Filter Input

3

GND

Ground

4

VDD

Positive Supply Input: +5V for MAX7418–MAX7421, +3V for MAX7422–MAX7425. Bypass VDD to GND with a

0.1μF capacitor.

5

OUT

Filter Output

6

OS

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. The Offset and Common-Mode

Input Adjustment section.

7

SHDN

Shutdown Input. Drive low to enable shutdown mode; drive high or connect to VDD for normal operation.

8

CLK

Clock Input. Connect an external capacitor (COSC) from CLK to ground. To override the internal oscillator,

connect CLK to an external clock: fC = fCLK /100.

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MAX7418–MAX7425

5th-Order, Lowpass,

Switched-Capacitor Filters

Typical Operating Characteristics (continued)

0

1.0

0.5

2.0

1.5

2.5

3.0

-40

10

-15

35

60

85

DC OFFSET VOLTAGE

vs. TEMPERATURE

TEMPERATURE (°C)

VDD = 5V

VDD = 3V

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)

Pin Description