Datasheet

AD790

REV. D

–8–

TTL

LEVEL

OUTPUT

400 *

GND

STANDARD

SCHOTTKY

DIODE

A RESISTOR UP TO 10k MAYBE USED TO

REDUCE THE SOURCE AND SINK CURRENT OF

THE DRIVER. HOWEVER, THIS WILL SLIGHTLY

LOWER THE MAXIMUM USABLE CLOCK RATE.

BIPOLAR

SIGNAL

INPUT

4.7V

0.3V

–

Ω

Figure 11. A Bipolar to CMOS TTL Line Receiver (N, Q

Package Pinout)

Bipolar to CMOS/TTL

It is sometimes desirable to translate a bipolar signal (e.g.,

± 5 V) coming from a communications cable or another section

of the system to CMOS/TTL logic levels; such an application is

referred to as a line receiver. Previously, the interface to the

bipolar signal required either a dual (± ) power supply or a refer-

ence voltage level about which the line receiver would switch.

The AD790 may be used in a simple circuit to provide a unique

capability: the ability to receive a bipolar signal while powered

from a single 5 V supply. Other comparators cannot perform

this task. Figure 11 shows a 1 kΩ resistor in series with the input

signal which is then clamped by a Schottky diode, holding the

input of the comparator at 0.4 V below ground. Although the

comparator is specified for a common mode range down to –V

(in this case ground) it is permissible to bring one of the inputs

a few hundred mV below ground. The comparator switches

around this level and produces a CMOS/TTL compatible swing.

The circuit will operate to switching frequencies of 20 MHz.