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The Circuit Designer’s Companion

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Contents

Introduction 1
Introduction to the second edition 2
Chapter 1
Grounding and wiring 3
1.1 Grounding 3
1.1.1 Grounding within one unit 4
1.1.2 Chassis ground 4
1.1.3 The conductivity of aluminium 6
1.1.4 Ground loops 7
1.1.5 Power supply returns 8
1.1.6 Input signal ground 11
1.1.7 Output signal ground 13
1.1.8 Inter-board interface signals 14
1.1.9 Star-point grounding 16
1.1.10 Ground connections between units 16
1.1.11 Shielding 18
1.1.12 The safety earth 20
1.2 Wiring and cables 21
1.2.1 Wire types 21
1.2.2 Cable types 23
1.2.3 Power cables 23
1.2.4 Data and multicore cables 24
1.2.5 RF cables 27
1.2.6 Twisted pair 28
1.2.7 Crosstalk 29
1.3 Transmission lines 32
1.3.1 Characteristic impedance 34
1.3.2 Time domain 34
1.3.3 Frequency domain 37
Chapter 2
Printed circuits 40
2.1 Board types 40
2.1.1 Materials 40
2.1.2 Type of construction 41
2.1.3 Choice of type 42
2.1.4 Choice of size 44
2.1.5 How a multilayer board is made 45
2.2 Design rules 45
2.2.1 Track width and spacing 47
2.2.2 Hole and pad size 50
2.2.3 Track routing 51
2.2.4 Ground and power distribution 52
2.2.5 Copper plating and finishing 55
2.2.6 Solder resist 55
2.2.7 Terminations and connections 56
2.3 Board assembly: surface mount and through hole 58
2.3.1 Surface mount design rules 60
2.3.2 Package placement 62
2.3.3 Component identification 63
2.4 Surface protection 63
2.4.1 Guarding 64
2.4.2 Conformal coating 65
2.5 Sourcing boards and artwork 67
2.5.1 Artwork 67
2.5.2 Boards 68
Chapter 3
Passive components 70
3.1 Resistors 70
3.1.1 Resistor types 70
3.1.2 Tolerancing 73
3.1.3 Temperature coefficient 74
3.1.4 Power 75
3.1.5 Inductance 76
3.1.6 Pulse handling 76
3.1.7 Extreme values 77
3.1.8 Fusible and safety resistors 79
3.1.9 Resistor networks 79
3.2 Potentiometers 80
3.2.1 Trimmer types 81
3.2.2 Panel types 82
3.2.3 Pot applications 82
3.3 Capacitors 85
3.3.1 Metallised film & paper 85
3.3.2 Multilayer ceramics 89
3.3.3 Single-layer ceramics 90
3.3.4 Electrolytics 91
3.3.5 Solid tantalum 93
3.3.6 Capacitor applications 94
3.3.7 Series capacitors and dc leakage 96
3.3.8 Dielectric absorption 97
3.3.9 Self resonance 97
3.4 Inductors 99
3.4.1 Permeability 99
3.4.2 Self-capacitance 101
3.4.3 Inductor applications 101
3.4.4 The danger of inductive transients 103
3.5 Crystals and resonators 105
3.5.1 Resonance 106
3.5.2 Oscillator circuits 107
3.5.3 Temperature 108
3.5.4 Ceramic resonators 108
Chapter 4
Active components 110
4.1 Diodes 110
4.1.1 Forward bias 110
4.1.2 Reverse bias 113
4.1.3 Leakage 113
4.1.4 High-frequency performance 114
4.1.5 Switching times 115
4.1.6 Schottky diodes 116
4.1.7 Zener diodes 117
4.1.8 The Zener as a clamp 120
4.2 Thyristors and triacs 121
4.2.1 Thyristor versus triac 122
4.2.2 Triggering characteristics 122
4.2.3 False triggering 123
4.2.4 Conduction 124
4.2.5 Switching 124
4.2.6 Snubbing 125
4.3 Bipolar transistors 127
4.3.1 Leakage 127
4.3.2 Saturation 128
4.3.3 The Darlington 129
4.3.4 Safe operating area 129
4.3.5 Gain 130
4.3.6 Switching and high frequency performance 132
4.3.7 Grading 133
4.4 Junction Field Effect transistors 134
4.4.1 Pinch-off 135
4.4.2 Applications 136
4.4.3 High impedance circuits 137
4.5 MOSFETs 139
4.5.1 Low-power MOSFETs 139
4.5.2 VMOS Power FETs 140
4.5.3 Gate drive impedance 141
4.5.4 Switching speed 144
4.5.5 On-state resistance 144
4.6 IGBTs 145
4.6.1 IGBT structure 145
4.6.2 Advantages over MOSFETs and bipolars 146
4.6.3 Disadvantages 147
Chapter 5
Analogue integrated circuits 148
5.1 The ideal op-amp 148
5.1.1 Applications categories 149
5.2 The practical op-amp 149
5.2.1 Offset voltage 149
5.2.2 Bias and offset currents 152
5.2.3 Common mode effects 153
5.2.4 Input voltage range 154
5.2.5 Output parameters 155
5.2.6 AC parameters 156
5.2.7 Slew rate and large signal bandwidth 157
5.2.8 Small-signal bandwidth 158
5.2.9 Settling time 159
5.2.10 The oscillating amplifier 159
5.2.11 Open-loop gain 162
5.2.12 Noise 163
5.2.13 Supply current and voltage 166
5.2.14 Temperature ratings 167
5.2.15 Cost and availability 168
5.2.16 Current feedback op-amps 169
5.3 Comparators 170
5.3.1 Output parameters 171
5.3.2 AC parameters 171
5.3.3 Op-amps as comparators (and vice versa) 173
5.3.4 Hysteresis and oscillations 174
5.3.5 Input voltage limits 176
5.3.6 Comparator sourcing 177
5.4 Voltage references 177
5.4.1 Zener references 178
5.4.2 Band-gap references 178
5.4.3 Reference specifications 180
5.5 Circuit modelling 181
Chapter 6
Digital circuits 183
6.1 Logic ICs 183
6.1.1 Noise immunity and thresholds 183
6.1.2 Fan-out and loading 186
6.1.3 Induced switching noise 188
6.1.4 Decoupling 189
6.1.5 Unused gate inputs 192
6.2 Interfacing 192
6.2.1 Mixing analogue and digital 192
6.2.2 Generating digital levels from analogue inputs 195
6.2.3 Protection against externally-applied overvoltages 196
6.2.4 Isolation 197
6.2.5 Classic data interface standards 200
6.2.6 High performance data interface standards 203
6.3 Using microcontrollers 207
6.3.1 How a microcontroller does your job 208
6.3.2 Timing and quantisation constraints 210
6.3.3 Programming constraints 214
6.4 Microprocessor watchdogs and supervision 214
6.4.1 The threat of corruption 214
6.4.2 Watchdog design 215
6.4.3 Supervisor design 219
6.5 Software protection techniques 222
6.5.1 Input data validation and averaging 222
6.5.2 Data and memory protection 223
6.5.3 Re-initialisation 224
Chapter 7
Power supplies 225
7.1 General 225
7.1.1 The linear supply 225
7.1.2 The switch-mode supply 225
7.1.3 Specifications 226
7.1.4 Off the shelf versus roll your own 227
7.2 Input and output parameters 227
7.2.1 Voltage 227
7.2.2 Current 228
7.2.3 Fuses 229
7.2.4 Switch-on surge, or inrush current 230
7.2.5 Waveform distortion and interference 232
7.2.6 Frequency 234
7.2.7 Efficiency 235
7.2.8 Deriving the input voltage from the output 235
7.2.9 Low-load condition 237
7.2.10 Rectifier and capacitor selection 238
7.2.11 Load and line regulation 239
7.2.12 Ripple and noise 241
7.2.13 Transient response 242
7.3 Abnormal conditions 243
7.3.1 Output overload 243
7.3.2 Input transients 244
7.3.3 Transient suppressors 246
7.3.4 Overvoltage protection 247
7.3.5 Turn-on and turn-off 248
7.4 Mechanical requirements 249
7.4.1 Case size and construction 249
7.4.2 Heatsinking 251
7.4.3 Safety approvals 251
7.5 Batteries 252
7.5.1 Initial considerations 252
7.5.2 Primary cells 255
7.5.3 Secondary cells 256
7.5.4 Charging 260
Chapter 8
Electromagnetic compatibility 262
8.1 The need for EMC 262
8.1.1 Immunity 263
8.1.2 Emissions 267
8.2 EMC legislation and standards 267
8.2.1 The EMC Directive 268
8.2.2 Existing standards 269
8.3 Interference coupling mechanisms 272
8.3.1 Conducted 272
8.3.2 Radiated 273
8.4 Circuit design and layout 275
8.4.1 Choice of logic 275
8.4.2 Analogue circuits 276
8.4.3 Software 277
8.5 Shielding 277
8.5.1 Apertures 279
8.5.2 Seams 281
8.6 Filtering 282
8.6.1 The low-pass filter 282
8.6.2 Mains filters 285
8.6.3 I/O filters 286
8.6.4 Feedthrough and 3-terminal capacitors 287
8.7 Cables and connectors 289
8.8 EMC design checklist 291
Chapter 9
General product design 292
9.1 Safety 292
9.1.1 Safety classes 293
9.1.2 Insulation types 294
9.1.3 Design considerations for safety protection 294
9.1.4 Fire hazard 296
9.2 Design for production 296
9.2.1 Checklist 297
9.2.2 The dangers of ESD 298
9.3 Testability 300
9.3.1 In-circuit testing 300
9.3.2 Functional testing 301
9.3.3 Boundary scan and JTAG 302
9.3.4 Design techniques 305
9.4 Reliability 307
9.4.1 Definitions 307
9.4.2 The cost of reliability 308
9.4.3 Design for reliability 308
9.4.4 The value of MTBF figures 312
9.4.5 Design faults 313
9.5 Thermal management 313
9.5.1 Using thermal resistance 314
9.5.2 Heatsinks 317
9.5.3 Power semiconductor mounting 321
9.5.4 Placement and layout 324
Appendix
Standards 326
Bibliography 329
Index 333