AN-22| Application Note
Integrated Circuits for Digital Data Transmission
Integrated Circuits for Digital Data Transmission
Introduction
It is frequently necessary to transmit digital data in a high-noise environment where ordinary integrated logic circuits cannot be used because they do not have sufficient noise immunity. One solution to this problem, of course, is to use high-noise-immunity logic. In many cases, this approach would require worst case logic swings of 30V, requiring high power-supply voltages. Further, considerable power would be needed to transmit these voltage levels at high speed. This is especially true if the lines must be terminated to eliminate reflections, since practical transmission lines have a low characteristic impedance. A much better solution is to convert the ground referred digital data at the transmission end into a differential signal and transmit this down a balanced, twisted-pair line. At the receiving end, any induced noise, or voltage due to ground-loop currents, appears equally on both ends of the twisted-pair line. Hence, a receiver which responds only to the differential signal from the line will rej
ect the undesired signals even with moderate voltage swings from the transmitter. Figure 1 illustrates this situation more clearly. When ground is used as a signal return as in Figure 1, the voltage seen at the receiving end will be the output voltage of the transmitter
National Semiconductor Application Note 22 April 1986
plus any noise voltage induced in the signal line. Hence, the noise immunity of the transmitter-receiver combination must be equal to the maximum expected noise from both sources. The differential transmission scheme diagrammed in Figure 1 solves this problem. Any ground noise or voltage induced on the transmission lines will appear equally on both inputs of the receiver. The receiver responds only to the differential signal coming out of the twisted-pair line and delivers a single-ended output signal referred to the ground at the receiving end. Therefore, extremely high noise immunities are not needed; and the transmitter and receiver can be operated from the same supplies as standard integrated logic circuits. This article describes the operation and use of a line driver and line receiver for transmission systems using twisted-pair lines. The transmitter provides a buffered differential output from a DTL or TTL input signal. A four-input gate is included on the input so that the circuit can also perform lo
gic. The receiver detects a zero crossing in the differential input voltage and can directly drive DTL or TTL integrated circuits at the receiving end. It also has strobe capability to blank out unwanted input signals. Both the transmitter and the receiver incorporate two independent units on a single silicon chip.
00718801
a. Single-Ended System
00718802
b. Difference System
FIGURE 1. Comparing Differential and Single-Ended Data Transmission
AN-22
2002 National Semiconductor Corporation
AN007188
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