Dual Universal Switched-Capacitor Filters

8

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_______________Detailed Description

The MAX7490/MAX7491 are universal switched-capaci-

100:1. Operating modes use external resistors connect-

ed in different arrangements to realize different filter

functions (highpass, lowpass, bandpass, notch) in all of

the classical filter topologies (Butterworth, Bessel, ellip-

tic, Chebyshev). Figure 1 shows a block diagram.

Clock Signal

External Clock

The MAX7490/MAX7491 switched-capacitor filters are

designed for use with external clocks that have a 50%

±5% duty cycle. When using an external clock, drive

the external clock adjusts the center frequency of the

filter:

Internal Clock

When using the internal oscillator, drive the EXTCLK pin

between CLK and GND. The value of the capacitor

capacitance at CLK so that it does not affect the inter-

nal oscillator frequency. Varying the frequency of the

internal oscillator adjusts the filter’s center frequency by

a 100:1 clock-to-center frequency ratio. For example,

an internal oscillator frequency of 135kHz produces a

nominal center frequency of 1.35kHz.

NAME

PIN

FILTER A

FILTER B

FUNCTION

LP_

1

16

2nd-Order Lowpass Filter Output

BP_

2

15

2nd-Order Bandpass Filter Output

N_/HP_

3

14

2nd-Order Notch/Highpass Filter Output

INV_

4

13

Inverting Input of Filter Summing Op Amp

S_

5

12

Summing Input. The connection of the summing input, along with the other

resistor connections, determine the circuit topology (mode) of each 2nd-

order section. S_ must never be left floating.

SHDN

6

Shutdown Input. Drive SHDN low to enable shutdown mode; drive high or

GND

7

Ground Pin

8

µF capacitor to GND. A

low-noise supply is recommended. Input +5V for MAX7490 or +3V for

MAX7491.

CLK

9

ground to set the internal oscillator frequency. For external clock operation,

drive with a CMOS-level clock. The duty cycle of the external clock should be

between 45% and 55% for best performance.

EXTCLK

10

CLK externally. Connect to GND when using the internal oscillator.

COM

11

0.1

µF capacitor. To override the internal biasing, drive with an external low-

impedance source.

Pin Description