13

Maxim Integrated

Digitally-Programmed, Dual

2nd-Order Continuous Lowpass Filter

MAX270/MAX271

Applications Information

Power-Supply Configurations

The MAX270/MAX271 power supplies must be properly

bypassed. Best performance is achieved if V+ and V- are

bypassed to GND with 4.7µF electrolytic (tantalum is pre-

ferred) and 0.1µF ceramic capacitors in parallel. These

should be as close as possible to the chip supply pins.

Single supplies in the range of 4.75V to 16V may be

used to power the MAX270/MAX271 as shown in Figure

4. Digital logic may be referenced to V- (system ground),

but will not maintain TTL compatibility. CMOS (rail-to-

rail) logic recommended. For µP-mode operation with a

single supply, the MAX270 A0 pin must be configure with

a voltage divider (Figure 4).

Lowest quiescent current in shutdown mode is achieved

when A0 is either at V+ or V-.

Independent fC Programming Without a µP

Figure 6 shows how filter sections A and B may be pro-

grammed to different cutoff frequencies without the use

of a µP. The MAX690 µP supervisory circuit provides the

proper programming sequence when the circuit is pow-

ered up by controlling the 74HC373 data buffer and the

MAX270 addressing pin to load independent fC data for

filters A and B.

Figure 4. Power-Supply Configurations

V+

A0

(MAX270)

SINGLE-SUPPLY OPERATION

GND

V-

4.7kI

10kI

PIN-STRAP

MODE

µP

MODE

0V

+5V

+5V

10kI

4.7kI

4.7µF

0.1µF

0.1µF

4.7µF

+5V

MAX270

V+

BIPOLAR-SUPPLY OPERATION

GND

V-

4.7µF

0.1µF

0.1µF

4.7µF

+5V

-5V

MAX270

MAX271