_______________Detailed Description

Lowpass Butterworth filters such as the MAX291/

MAX295 provide maximally flat passband response, making

them ideal for instrumentation applications that require mini-

mum deviation from the DC gain throughout the passband.

Lowpass Bessel filters such as the MAX292/MAX296

delay all frequency components equally, preserving the

shape of step inputs, subject to the attenuation of the high-

er frequencies. They also settle faster than Butterworth fil-

ters. Faster settling can be important in applications that

use a multiplexer (mux) to select one signal to be sent to

an analog-to-digital converter (ADC)—an anti-aliasing filter

placed between the mux and the ADC must settle quickly

after a new channel is selected by the mux.

The difference in the filters’ responses can be observed

when a 3kHz square wave is applied to the filter input

(Figure 1, trace A). With the filter cutoff frequencies set at

10kHz, trace C shows the MAX291/MAX295 Butterworth

filter response and trace B shows the MAX292/MAX296

Bessel filter response. Since the MAX292/MAX296 have a

linear phase response in the passband, all frequency

components are delayed equally, which preserves the

square wave. The filters attenuate higher frequencies of

the input square wave, giving rise to the rounded edges at

the output. The MAX291/MAX295 delay different frequen-

cy components by varying times, causing the overshoot

and ringing shown in trace C.

The MAX291/MAX295 give more attenuation outside the

passband. The phase and frequency response curves in

the

Typical Operating Characteristics reveal the differences

between the two types of filters.

MAX291/MAX292/MAX295/MAX296 phase shift and gain

do not vary significantly from part to part. Typical phase

shift and gain differences are less than 0.5% at the corner

Corner Frequency and Filter Attenuation

The MAX291/MAX292 operate with a 100:1 clock to corner

frequency ratio and a 25kHz maximum corner frequency,

where corner frequency is defined as the point where the

filter output is 3dB below the filter’s DC gain. The

MAX295/MAX296 operate with a 50:1 clock to corner fre-

quency ratio with a 50kHz maximum corner frequency.

The 8 poles provide 48dB of attenuation per octave.

Background Information

Most switched-capacitor filters are designed with biqua-

dratic sections. Each section implements two filtering

poles, and the sections can be cascaded to produce high-

er-order filters. The advantage to this approach is ease of

design. However, this type of design can display poor sen-

sitivity if any section’s Q is high.

An alternative approach is to emulate a passive network

using switched-capacitor integrators with summing and

scaling. The passive network can be synthesized using

CAD programs, or can be found in many filter books.

Figure 2 shows the basic ladder filter structure.

A switched-capacitor filter that emulates a passive ladder

filter retains many of its advantages. The filter’s com-

ponent sensitivity is low when compared to a cascaded

biquad design because each component affects the entire

filter shape, not just one pole pair. That is, a mismatched

component in a biquad design will have a concentrated

8th-Order, Lowpass,

Switched-Capacitor Filters

6

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_____________________Pin Description

Filter Input

14

8

Inverting Input to the uncommit-

ted op amp. The noninverting op

amp is internally tied to ground.

6

4

Filter Output

11

5

Ground. In single-supply oper-

ation, GND must be biased to

the mid-supply voltage level.

12

6

Positive Supply pin. Dual sup-

plies: +2.375V to +5.500V. Single

supplies: +4.75V to +11.0V.

13

7

Uncommitted Op-Amp Output

5

3

Negative Supply pin. Dual

supplies: -2.375V to -5.500V.

Single supplies: V- = 0V.

4

2

8-PIN

Clock Input. Use internal or

external clock.

3

1

No Connect

1, 2, 7,

8, 9, 10,

15, 16

FUNCTION

16-PIN

IN

OP IN-

OUT

GND

V+

OP OUT

V-

CLK

N.C.

NAME

A

B

C

A: 3kHz INPUT SIGNAL

TIME (200

µs/div)

Figure 1. Bessel vs. Butterworth Filter Responses