EDNE February 2013 - (Page 14)

SIGNAL INTEGRITY BY howard johnson, phD Ground loops I design audio circuits. My mentor taught me to avoid ground loops at all costs, yet in digital products I see a solid ground plane holding hundreds of circuits. Between boards, I see a web of ground-referenced connections shooting off in all directions. How can this possibly work? —Silence Dogood, Analog rules Imagine two parts of an electronic structure. Single-point ground networks system. When device A sends to B, a provide isolation only when commusignal current flows between them. At nications remain localized to isolated the same time, an equal and opposite sections of the network. current, called the returning signal curSuppose the main trunk of the rent, flows back to A through the power ground system comprises 12 in. of #18 or ground system. Current always makes gauge wire having a dc resistance of 6.5 a loop in that way. A ground loop is a mΩ. Assume that an audio-frequency situation where there exists more than current of 1A (that’s 8W into an 8Ω one path for the flow of returning signal current. Audio designers like singlepoint ground networks. Such a network of ground connections C has the topology of a tree, with one main trunk and many branches and sub-branches. None of the branches touch, so it contains B no loops. In this type of network there exists only one ground path between any two devices. When A sends to B, the returning signal A current disturbs the ground along that path, affecting everything that touches that path or branches from it, but does not affect devices connected to other parts of the tree structure (Figure 1). That property provides a measure of isolation between devices. In Figure 1, what happens if A sends to C? In that case, returning Figure 1 Communication from A to B pollutes signal current must traverse a big all circuits that lie along the green path or section of the main trunk, pol- branch from it; circuit C remains unaffected. luting almost everything in the 14 EDN EUROPE | february 2013 speaker) traverses the main trunk. The ground noise observed from one end of the ground wire to the other equals 1A×6.5 mΩ, or 6.5 mV. Compared with an audio reference level of perhaps 4V, the headroom in the circuit, defined as the difference between the standard reference level and the ground noise, equals a mere 56 dB—enough possibly for cheap consumer-grade audio, but not within a factor of a thousand of acceptable performance for high-end audio. Good audio equipment uses a single-point ground system and keeps disparate circuits confined to isolated sections of the tree. Single-point grounding provides isolation between localized regions. In the digital world, the resistance measured from side to side across a solidcopper ground plane is on the order of 1 mΩ. A current of 25A induces groundvoltage differences on the order of only 25 mV or less. Digital circuitry easily tolerates that level of noise, so in most cases we simply do not need the complication of single-point grounding for ordinary digital logic. In addition, all electronic systems suffer mutual-inductive coupling whose severity grows in proportion to the bandwidth of the signals involved. Because digital systems operate millions of times faster than audio systems, they suffer a correspondingly increased degree of inductive coupling, which, if not checked by the low-inductance properties of a solid plane, would incapacitate most modern digital electronics. We must have solid planes to control inductive crosstalk in digital products; that is the planes’ main function.EDN Howard Johnson, PhD, of Signal Consulting, frequently conducts technical workshops for digital engineers at Oxford University and other sites worldwide. Visit his Web site at www.sigcon.com, or e-mail him at howie03@sigcon.com. www.edn-europe.com http://www.sigcon.com http://www.edn-europe.com

Table of Contents for the Digital Edition of EDNE February 2013

RS Components
Agilent Technologies
Maxim Integrated
Signal Integrity
Mesago SMT 2013
Test & Measurement World
Rohde & Schwarz
Mesago PCIM 2013
Mechatronics in Design
Hot technologies: trends to watch in 2013
Hot 100 products of 2012
Add LED intelligence to improve light quality, efficiency, and cost
Design Ideas
Product roundup
Tales from the Cube

EDNE February 2013