Capítulo 2. Descripción de la solución propuesta
2.5 Requisitos funcionales
Different types of grounding methods are commonly used. The word
"ground," by itself is vague and does not represent any specific function within electrical engineering. An adjective must precede the word "ground"
to distinguish the type of ground referenced for a particular application.
Different types of grounding methodologies include the following. All can be present simultaneously or only one at a time: digital, analog, signal,
common, noisy, quiet, isolated, earth, single-point, multipoint, hybrid, chassis, safety, frame, and so on. One grounding topology always forgotten is RF ground.
Figures 2.22 through 2.24 illustrate three grounding methods: single-point, multipoint, and hybrid.
Figure 2.22: Single-point grounding methods. Note: Inappropriate for
high-frequency operation.
Figure 2.23: Multipoint grounding.
Figure 2.24: Hybrid (frequency selective) grounding
configurations.
2.9.1 Single-Point Grounding
Single-point grounding is best when the speed of components, circuits, interconnects, and the like are in the range of 1 MHz or less. At higher frequencies, the inductance of the interconnect will increase the PCB impedance. At still higher frequencies, the impedance of the power planes and interconnect traces become noticeable. These impedances can be very high if the trace lengths coincide with odd multiples of a quarter-wavelength of the edge transitions of components with periodic signals.
Not only will these traces and ground conductors have large impedances, they can also act as antennas and radiate RF energy. At frequencies above 1 MHz, a single-point ground generally is not used. However, exceptions do exist if the design engineer recognizes the pitfalls and designs the product using highly specialized and advanced grounding
techniques.
Single-point grounds are usually formed with signal radials and are
commonly found in audio circuits, analog instrumentation, and 60-Hz and dc power systems, along with products packaged in plastic enclosures.
Although single-point grounding is commonly used for low-frequency products, it is occasionally found in extremely high-frequency circuits and systems.
Use of single-point grounding within a CPU motherboard, or an I/O adapter (daughter card), allows loop currents to be developed between the 0V-reference and chassis housing, if metal is used as chassis ground. Loop currents allow magnetic fields to exist. Magnetic fields create electric fields.
Both electric and magnetic fields will develop RF currents that will propagate as either radiated or conducted emissions. It is nearly
impossible to implement single-point grounding in personal computers and similar devices because different sub-assemblies and peripherals are grounded directly to the metal chassis in different locations. Induced currents due to electromagnetic fields may couple voltages into a circuit through the mechanism of the transfer, developing loop structures.
Multipoint grounding places these loops in regions where they are least likely to cause problems. (RF loop currents can be controlled and directed rather than allowed to transfer energy inadvertently to other circuits and systems susceptible to electromagnetic field disturbance.)
2.9.2 Multipoint Grounding
High-frequency designs generally require use of multiple chassis ground connections. Multipoint grounding minimizes ground impedance present in the power distribution system of the PCB by shunting RF currents from the planes to chassis ground [1]. This lower impedance is caused primarily by the lower inductance characteristic of large, solid copper planes versus that of a small PCB trace or wire. In very high-frequency circuits, the ground leads from components must be kept as short as possible to minimize lead inductance. Lead inductance allows a voltage potential to be developed across the interconnect wire. This voltage potential is one cause of common-mode current generation.
PCB traces add inductance to a circuit or transmission line at
approximately 12–20 nH per inch. This variable inductance value is based on two parameters: trace width and thickness. Inductance allows a
resonance to occur when both the lumped, distributed capacitance between planes and chassis ground form a tuned resonant circuit. The capacitance value, C, is sometimes known, within a specific tolerance range. Inductance, L, is determined by knowledge of the impedance of copper planes. The typical value of inductance for a copper plane, 10 × 10 square in. (25.4 × 25.4 square cm), is listed in Table 2.2. The equations for solving an exact value for the impedance of planes is complex and beyond the scope of this book.
A common architectural concept for many designs is to have a PCB secured to a metal mounting plate or chassis. A resonance is developed between the power and/or ground planes and chassis housing. Figure 2.25 illustrates the capacitance and inductance present between a PCB that is screw secured to a mounting panel or metallic enclosure. Capacitance and inductance will always be present regardless of topology or configuration.
Depending on the distance spacing between mounting posts, relative to the self-resonant frequency of the power and ground planes, loop currents will exist. These loop currents are identified as eddy currents and will couple,
Table 2.2: Impedance of a 10 by 10 in. (25.4 × 25.4 cm) Copper Metal Plane
Open table as spreadsheet
Frequency (MHz) Skin Depth (cm) Impedance (ohms/square)
1 MHz 6.6 × 10-3 0.00026
10 MHz 2.1 × 10-3 0.00082
100 MHz 6.6 × 10-4 0.0026
1 GHz 2.1 × 10-4 0.0082
either by radiation or conduction to other PCBs located nearby, the chassis housing, internal cables or harnesses, peripheral devices, I/O circuits and connectors, or into free space.
Figure 2.25: Resonance in a multipoint ground to
chassis.
In addition to inductance in the planes, physically long traces also act as small antennas when routed microstrip, especially for clock signals and other periodic data pulses. By minimizing trace inductance and removing RF currents or magnetic flux within the transmission line, significant
improvement in signal quality and RF suppression will occur. This means coupling RF currents within the transmission line to the 0V-plane or chassis ground.
Digital circuits must be treated as high-frequency circuits. A good
low-inductance power distribution network is necessary on any PCB containing many logic devices. The planes internal to the PCB generally provide a lower-inductance return for both power supply and signal currents. This lower inductance allows for creating an enhanced, constant impedance transmission line for signal interconnects. When making ground plane to chassis plane connection, provision must be made for removal of high-frequency RF energy present between the power and ground planes with bypass capacitors. High-frequency RF currents are created by both the resonant circuit of the power distribution network (planes) and their physical relationship to signal traces. It is common to find use of high-quality bypass capacitors, usually 0.1ˤF in parallel with 0.001 ˤF at each and every ground connection to remove RF eddy currents, as will be reiterated in Chapter 3.
Chassis grounds must also be frequently connected directly to the ground planes of the PCB to minimize RF voltages and currents that exist between board and chassis. If magnetic loop currents are small (1/20 wavelength of the highest RF generated frequency), RF suppression or flux cancellation or minimization is enhanced [1].
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Table of Contents
2.3: MAGNETIC FLUX AND CANCELLATION
2.8: RF CURRENT DENSITY DISTRIBUTION 2.13: SLOTS WITHIN AN IMAGE PLANE
Chapter 2 - Printed Circuit Board Basics
Printed Circuit Board Design Techniques for EMC Compliance: A Handbook for Designers, Second Edition by Mark I. Montrose
IEEE Press © 2000 Recommend this title?