Definición

Differential signaling has been around for many years. With newer, high-speed components operating at a lower voltage swing level, this technique is

becoming widely used as a means of minimizing power consumption and crosstalk coupling. Component technologies, such as LVDS, are now the preferred means of transporting very high data rate bit streams. Two areas of concern are mandatory for use with LVDS technology [3]: termination and impedance control. (Impedance control was discussed earlier in this chapter.) A transmission line requires proper termination to eliminate reflections and enhance signal integrity. Various termination methods have been described for the single-ended transmission mode. Differential signaling requires an

approach similar to the single-ended mode, with modifications.

Differential signaling requires that signal traces be routed on the same layer, either microstrip or stripline, and not a combination of both. This requirement is due to the velocity of propagation of the transmitted signal through the dielectric of the PCB. Signals routed microstrip will arrive at their destination faster than traces routed stripline. The longer the trace, the more significant this

requirement becomes, especially if clock skew and timing at the differential receiver are critical.

Two propagation modes are present with differential signaling: differential mode and common mode. Both modes must be considered at the same time,

according to the logic family selected. Differential-mode impedance is the value of the line-to-line resistor that will optimally terminate pure differential signals.

Common-mode termination is the value of the signal trace to chassis ground.

These types of configurations are illustrated in Fig. 4.32. Because a termination works for one propagation mode does not mean it will work for the other.

Signals propagated differentially will also have a common-mode component.

Sometimes both termination methods are required, depending on application and logic family.

Figure 4.32: Differential- and common-mode signaling

comparison.

The LVDS logic family requires a single differential-mode termination resistor, usually 100 ohms. Traces need to be approximately the same length to

minimize signal skew. Common-mode RF energy is not present, since this logic family does not rely on a ground reference between source and load. This is the main benefit of using this logic family [3].

If the need to terminate two traces is required for any signal driven into them, a combination of both termination methods is permitted (Fig. 4.32). This

ƒ Impedance matched lines are not required, including controlled transmission line environments.

ƒ The diodes may be able to replace termination resistors if the edge rate is slow.

ƒ Clamping action reduces overshoot and enhances signal integrity.

combination prevents reflections from propagating on the trace pair. To properly select a correct terminator, the following are recommended:

1. For common-mode signals, a line-to-line terminator (differential type) is invisible to the circuit. This means that no current flows through it.

Termination to the transmission lines must be provided as a single-ended circuit, discussed earlier in this section.

2. For differential-mode signals, the line-to-ground terminators appear in series with each other across the line ends. Consequently, the line-to-line terminator should be the only additional resistance required to drop the trace impedance down to the differential impedance.

3. Resistor array terminators prevent reflections due to either differential-mode or common-differential-mode signaling. In a true differential-differential-mode environment, common-mode energy will not exist. Because differential-drivers are not perfect in their manufacturing, a small amount of common-mode energy will always be present.

Table 4.3 cites the advantages and disadvantages of various termination methods for paired signaling.

Table 4.3: Comparison of Termination Types for Paired Signals Open table as spreadsheet

Terminator Type Advantages Disadvantages Differential resistor

(across the differential lines)

Terminates

differential signals.

Requires only one component.

Does not terminate

common-mode signaling or noise within the

transmission line.

Common-mode resistor (resistor from each line to ground)

Terminates common-mode signals.

Does not terminate differential signals.

Resistor array Terminates

differential, common-mode, or any mix of signals.

Requires three resistors.

May consume DC power.

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Table of Contents Chapter 4 4.1: CREATING TRANSMISSION LINES WITHIN A PCB 4.2: TOPOLOGY CONFIGURATIONS 4.3: PROPAGATION DELAY AND DIELECTRIC CONSTANT 4.4: CAPACITIVE LOADING OF SIGNAL TRACES

4.5: COMPONENT PLACEMENT 4.6: IMPEDANCE

MATCHING—REFLECTIONS AND RINGING

4.7: CALCULATING TRACE LENGTHS (ELECTRICALLY LONG TRACES)

4.8: TRACE ROUTING 4.9: ROUTING LAYERS 4.10: CROSSTALK 4.11: TRACE SEPARATION AND THE 3-W RULE

4.12: GUARD/SHUNT TRACES 4.13: TRACE TERMINATION REFERENCES

Chapter 4 - Clock Circuits, Trace Routing, and Terminations

Printed Circuit Board Design Techniques for EMC Compliance: A Handbook for Designers, Second Edition by Mark I. Montrose

IEEE Press © 2000 Recommend this title?

REFERENCES

[1] Montrose, M. 1999. EMC and the Printed Circuit Board Design—

Design, Theory and Layout Made Simple. Piscataway, NJ: IEEE Press.

[2] Kaupp, H. R. 1967, April. "Characteristics of Microstrip Transmission Lines: IEEE Transactions." Vol. EC-16, No. 2.

[3] National Semiconductor. 1996. LVDS Owner's Manual.

[4] IPC-D-317A. 1995, January. Design Guidelines for Electronic Packaging Utilizing High-Speed Techniques. Institute for

Interconnecting and Packaging Electronic Circuits.

[5] IPC-2141. 1996, April. Controlled Impedance Circuit Boards and High Speed Logic Design. Institute for Interconnecting and Packaging Electronic Circuits.

[6] Paul, C. R. 1984. Analysis of Multiconductor Transmission Lines.

New York: John Wiley & Sons.

[7] Paul, C. R. 1992. Introduction to Electromagnetic Compatibility. New York: John Wiley & Sons.

[8] Mardiguian, M. 1992. Controlling Radiated Emissions by Design.

New York: Van Nostrand Reinhold.

[9] Motorola, Inc. Transmission Line Effects in PCB Applications (#AN1051/D).

[10] Johnson, H. W., and M. Graham. 1993. High Speed Digital Design.

Englewood Cliffs, NJ: Prentice Hall.

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Table of Contents

5.4: LOCAL AREA NETWORK I/O LAYOUT

5.5: VIDEO 5.6: AUDIO REFERENCES