Read an Excerpt
The Circuit Designer's Companion
By Peter Wilson
Newnes
Copyright © 2012 Elsevier Ltd.
All right reserved. ISBN: 978-0-08-097147-6
Chapter One
Grounding and wiring
CHAPTER OUTLINE
1.1 Grounding 2 When to consider grounding 3 1.1.1 Grounding within one unit 4 1.1.2 Chassis ground 4 1.1.3 The conductivity of aluminum 5 Other materials 6 1.1.4 Ground loops 6 1.1.5 Power supply returns 8 Varying loads 9 Power rail feed 10 Conductor impedance 11 1.1.6 Input signal ground 11 Connection to 0 V elsewhere on the PCB 11 Connection to 0 V within the unit 11 External ground connection 11 1.1.7 Output signal ground 13 Avoiding the common impedance 13 1.1.8 Inter-board interface signals 14 Partitioning the signal return 15 1.1.9 Star-point grounding 16 1.1.10 Ground connections between units 17 Breaking the ground link 18 1.1.11 Shielding 19 Which end to ground for LF shielding 19 Electrostatic screening 20 Surface transfer impedance 21 1.1.12 The safety earth 21 1.2 Wiring and cables 22 1.2.1 Wire types 22 Wire inductance 22 Equipment wire 23 Wire-wrap wire 23 1.2.2 Cable types 24 1.2.3 Power cables 24 1.2.4 Data and multicore cables 25 Data communication cables 26 Structured data cable 26 Shielding and microphony 27 1.2.5 RF cables 28 1.2.6 Twisted pair 29 1.2.7 Crosstalk 30 Digital crosstalk 32 1.3 Transmission lines 33 Transmission line effects 34 Critical lengths for pulses 34 1.3.1 Characteristic impedance 34 1.3.2 Time domain 35 Forward and reflected waves 35 Ringing 36 The Bergeron diagram 36 The uses of mismatching 38 1.3.3 Frequency domain 38 Standing wave distribution versus frequency 38 Impedance transformation 39 Lossy lines 40 Understanding the transmission line impedance graphically 42
1.1 GROUNDING
A fundamental property of any electronic or electrical circuit is that the voltages present within it are referenced to a common point, conventionally called the ground. This term is derived from electrical engineering practice, when the reference point is often taken to a copper spike literally driven into the ground. This point may also be a connection point for the power to the circuit, and it is then called the 0 V (nought-volt) rail, and ground and 0 V are frequently (and confusingly) synonymous. Then, when we talk about a five-volt supply or a minus-twelve-volt supply or a two-and-a-half-volt reference, each of these are referred to the 0 V rail.
At the same time, ground is not the same as 0 V. A ground wire connects equipment to earth for safety reasons, and does not carry a current in normal operation. However, in this chapter the word "grounding" will be used in its usual sense, to include both safety earths and signal and power return paths.
Perhaps the greatest single cause of problems in electronic circuits is that 0 V and ground are taken for granted. The fact is that in a working circuit there can only ever be one point which is truly at 0 V; the concept of a "0 V rail" is in fact a contradiction in terms. This is because any practical conductor has a finite non-zero resistance and inductance, and Ohm's law tells us that a current flowing through anything other than a zero impedance will develop a voltage across it. A working circuit will have current flowing through those conductors that are designated as the 0 V rail and therefore, if any one point of the rail is actually at 0 V (say, the power supply connection) the rest of the rail will not be at 0 V. This can be illustrated with the example in Figure 1.1.
Now, after such a trenchant introduction, you might be tempted to say well, there are millions of electronic circuits in existence, they must all have 0 V rails, they seem to work well enough, so what's the problem? Most of the time there is no problem. The impedance of the 0 V conductor is in the region of milliohms, the current levels are milliamps, and the resulting few hundred microvolts drop doesn't offend the circuit at all; 0 V plus 500 µV is close enough to 0 V for nobody to worry.
The difficulty with this answer is that it is then easy to forget about the 0 V rail and assume that it is 0 V under all conditions, and subsequently be surprised when a circuit oscillates or otherwise doesn't work. Those conditions where trouble is likely to arise are:
• where current flows are measured in amps rather than milli- or microamps;
• where the 0 V conductor impedance is measured in ohms rather than milliohms;
• where the resultant voltage drop, whatever its value, is of a magnitude or in such a configuration as to affect the circuit operation.
When to consider grounding
One of the attributes of a good circuit designer is to know when these conditions need to be carefully considered and when they may be safely ignored. A frequent complication is that you as circuit designer may not be responsible for the circuit's layout, which is handed over to a layout draughtsman (who may in turn delegate many routing decisions to a software package). Grounding is always sensitive to layout, whether of discrete wiring or of printed circuits, and the designer must have some knowledge of and control over this if the design is not to be compromised.
The trick is always to be sure that you know where ground return currents are flowing, and what their consequences will be; or, if this is too complicated, to make sure that wherever they flow, the consequences will be minimal. Although the above comments are aimed at 0 V and ground connections, because they are the ones most taken for granted, the nature of the problem is universal and applies to any conductor through which current flows. The power supply rail (or rails) is another special case where conductor impedance can create difficulties.
1.1.1 Grounding within one unit
In this context, "unit" can refer to a single circuit board or a group of boards and other wiring connected together within an enclosure such that you can identify a "local" ground point, for instance the point of entry of the mains earth. An example might be as shown in Figure 1.2. Let us say that printed circuit board (PCB) 1 contains input signal conditioning circuitry, PCB2 contains a microprocessor for signal processing and PCB3 contains high-current output drivers, such as for relays and for lamps. You may not place all these functions on separate boards, but the principles are easier to outline and understand if they are considered separately. The power supply unit (PSU) provides a low-voltage supply for the first two boards, and a higher-power supply for the output board. This is a fairly common system layout and Figure 1.2 will serve as a starting point to illustrate good and bad practice.
1.1.2 Chassis ground
First of all, note that connections are only made to the metal chassis or enclosure at one point. All wires that need to come to the chassis are brought to this point, which should be a metal stud dedicated to the purpose. Such connections are the mains safety earth (about which more later), the 0 V power rail, and any possible screening and filtering connections that may be required in the power supply itself, such as an electrostatic screen in the transformer. (The topic of power supply design is itself dealt with in much greater detail in Chapter 7.)
The purpose of a single-point chassis ground is to prevent circulating currents in the chassis. If multiple ground points are used, even if there is another return path for the current to take, a proportion of it will flow in the chassis (Figure 1.3); the proportion is determined by the ratio of impedances which depends on frequency. Such currents are very hard to predict and may be affected by changes in construction, so that they can give quite unexpected and annoying effects: it is not unknown for hours to be devoted to tracking down an oscillation or interference problem, only to find that it disappears when an inoffensive-looking screw is tightened against the chassis plate. Joints in the chassis are affected by corrosion, so that the unit performance may degrade with time, and they are affected by surface oxidation of the chassis material. If you use multi-point chassis grounding then it is necessary to be much more careful about the electrical construction of the chassis.
1.1.3 The conductivity of aluminum
Aluminum is used throughout the electronics industry as a light, strong and highly conductive chassis material – only silver, copper and gold have a higher conductivity. You would expect an aluminum chassis to exhibit a decently low bulk resistance, and so it does, and is very suitable as a conductive ground as a result. Unfortunately, another property of aluminum (which is useful in other contexts) is that it oxidizes very readily on its surface, to the extent that all real-life samples of aluminum are covered by a thin surface film of aluminum oxide (Al2O3). Aluminum oxide is an insulator. In fact, it is such a good insulator that anodized aluminum, on which a thick coating of oxide is deliberately grown by chemical treatment, is used for insulating washers on heatsinks.
The practical consequence of this quality of aluminum oxide is that the contact resistance of two sheets of aluminum joined together is unpredictably high. Actual electrical contact will only be made where the oxide film is breached. Therefore, whenever you want to maintain continuity through a chassis made of separate pieces of aluminum, you must ensure that the plates are tightly bonded together, preferably with welding or by fixings which incorporate shakeproof serrated washers to dig actively into the surface. The same applies to ground connection points. The best connection (since aluminum cannot easily be soldered) is a force-fit or welded stud (Figure 1.4), but if this is not available then a shakeproof serrated washer should be used underneath the nut which is in contact with the aluminum.
Other materials
Another common chassis material is cadmium- or tin-plated steel, which does not suffer from the oxidation problem. Mild steel has about three times the bulk resistance of aluminum so does not make such a good conductor, but it has better magnetic shielding properties and it is cheaper. Die-cast zinc is popular for its light weight and strength, and ease of creating complex shapes through the casting process; zinc's conductivity is 28% that of copper. Other metals, particularly silver-plated copper, can be used where the ultimate in conductivity is needed and cost is secondary, as in RF circuits. The advantage of silver oxide (which forms on the silver-plated surface) is that it is conductive and can be soldered through easily. Table 1.1 shows the conductivities and temperature coefficients of several metals.
1.1.4 Ground loops
Another reason for single-point chassis connection is that circulating chassis currents, when combined with other ground wiring, produce the so-called "ground loop", which is a fruitful source of low-frequency magnetically induced interference. A magnetic field can only induce a current to flow within a closed loop circuit. Magnetic fields are common around power transformers – not only the conventional 50 Hz mains type (60 Hz in the US), but also high-frequency switching transformers and inductors in switched-mode power supplies – and also other electromagnetic devices: contactors, solenoids and fans. Extraneous magnetic fields may also be present. The mechanism of ground-loop induction is shown in Figure 1.5.
(Continues...)
Excerpted from The Circuit Designer's Companion by Peter Wilson Copyright © 2012 by Elsevier Ltd. . Excerpted by permission of Newnes. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
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