Datasheet AD790 (Analog Devices) - 8

AD790 Bipolar to CMOS/TTL + 5V – 5V
It is sometimes desirable to translate a bipolar signal (e.g.,
BIPOLAR 4.7V
±5 V) coming from a communications cable or another section
SIGNAL 0.3V
of the system to CMOS/TTL logic levels; such an application is
INPUT 400 *
Ω referred to as a line receiver. Previously, the interface to the
1 1k
Ω
8
bipolar signal required either a dual (± ) power supply or a refer-
2 5 TTL
ence voltage level about which the line receiver would switch.
STANDARD 7 LEVEL OUTPUT
The AD790 may be used in a simple circuit to provide a unique
SCHOTTKY 6 3 DIODE
capability: the ability to receive a bipolar signal while powered
4
from a single 5 V supply. Other comparators cannot perform this task. Figure 11 shows a 1 kΩ resistor in series with the input
GND
signal which is then clamped by a Schottky diode, holding the input of the comparator at 0.4 V below ground. Although the
A RESISTOR UP TO 10k * MAYBE USED TO
Ω comparator is specified for a common mode range down to –VS,
REDUCE THE SOURCE AND SINK CURRENT OF
(in this case ground) it is permissible to bring one of the inputs
THE DRIVER. HOWEVER, THIS WILL SLIGHTLY LOWER THE MAXIMUM USABLE CLOCK RATE.
a few hundred mV below ground. The comparator switches around this level and produces a CMOS/TTL compatible swing. Figure 11. A Bipolar to CMOS TTL Line Receiver (N, Q The circuit will operate to switching frequencies of 20 MHz. Package Pinout) –8– REV. E