The Ground Myth Bruce Archambeault, Ph.D. IBM Distinguished Engineer, IEEE Fellow barch@us.ibm.com 18 November 2008 IEEE
Introduction Electromagnetics can be scary Universities LOVE messy math EM is not hard, unless you want to do the messy math Goal: Intuitive understanding Understand the basic fundamentals Understand how to read the math November 2008 Bruce Archambeault, PhD 2
November 2008 Bruce Archambeault, PhD 3
Overview What does the derivative mean? What does integration mean? Weird vector notation In the beginning Faraday and Maxwell Inductance Ground Primary cause of EMI problems on PCBs November 2008 Bruce Archambeault, PhD 4
Derivative How fast is something changing? d dt d dx [ something] [ something] Changing with respect to time Changing with respect to position (x) November 2008 Bruce Archambeault, PhD 5
Partial Derivative How fast is something changing for one variable? t [ something( t, x) ] Changing with respect to time (as x is constant) x [ something( t, x) ] Changing with respect to position (x) (as time is constant) November 2008 Bruce Archambeault, PhD 6
Integration Simply the sum of parts (when the parts are very small) Line Integral --- sum of small line segments Surface Integral -- sum of small surface patches Volume Integral -- sum of small volume blocks November 2008 Bruce Archambeault, PhD 7
Line Integral (find the length of the path) piece of E field dl V = stop start ( E dl) November 2008 Bruce Archambeault, PhD 8
Line Integral -- Closed Circumference = = path around box x= l x= 0 dx + y= w y= 0 dy + x= 0 x= l dx + y= 0 y= w dy y x November 2008 Bruce Archambeault, PhD 9
Line Integral -- Closed Closed line integrals find the path length And/or the amount of some quantity along that closed path length November 2008 Bruce Archambeault, PhD 10
Surface Integral (find the area of the surface) Area = Area = da = dx dy da dx dy As dx and dy become smaller and smaller, the area is better calculated November 2008 Bruce Archambeault, PhD 11
Volume Integral (find the volume of an object) Volume = Volume = dv dv = dx dy dz [ dx dy dz] November 2008 Bruce Archambeault, PhD 12
Electromagnetics In the Beginning Electric and Magnetic effects not connected Electric and magnetic effects were due to action from a distance Faraday was the 1 st to propose a relationship between electric lines of force and time-changing magnetic fields Faraday was very good at experiments and figuring out how things work November 2008 Bruce Archambeault, PhD 13
Maxwell Maxwell was impressed with Faraday s ideas Discovered the mathematical link between the electro and the magnetic Scotland s greatest contribution to the world (next to Scotch) November 2008 Bruce Archambeault, PhD 14
Maxwell s Equations Maxwell s original work included 20 equations! Heaviside reduced them to the existing four equations Heaviside refused to call the equations his own Hertz is credited with proving they are correct November 2008 Bruce Archambeault, PhD 15
Maxwell s Equations are NOT Hard! D H = J + t B E = t November 2008 Bruce Archambeault, PhD 16
Maxwell s Equations are not Hard! Change in H-field across space Change in E-field (at that point) with time Change in E-field across space Change in H-field (at that point) with time (Roughly speaking, and ignoring constants) November 2008 Bruce Archambeault, PhD 17
Current Flow Most important concept of EMC Current flow through metal changes as frequency increases DC current Uses entire conductor Only resistance inhibits current High Frequency Only small part of conductor (near surface) is used Resistance is small part of current inhibitor Inductance is major part of current inhibitor November 2008 Bruce Archambeault, PhD 18
Skin Depth High frequency current flows only near the metal surface at high frequencies δ = 1 π fμσ Frequency Skin Depth Skin Depth 60 Hz 260 mils 8.5 mm 1 KHz 82 mils 2.09 mm 10 KHz 26 mils 0.66 mm 100 KHz 8.2 mils 0.21 mm 1 MHz 2.6 mils 0.066 mm 10 MHz 0.82 mils 0.021 mm 100 MHz 0.26 mils 0.0066 mm 1 GHz 0.0823 mils 0.0021 mm November 2008 Bruce Archambeault, PhD 19
Inductance Current flow through metal => inductance! Fundamental element in EVERYTHING Loop area first order concern Inductive impedance increases with frequency and is MAJOR concern at high frequencies X = 2πfL L November 2008 Bruce Archambeault, PhD 20
Current Loop => Inductance Courtesy of Elya Joffe November 2008 Bruce Archambeault, PhD 21
Inductance Definition Faraday s Law E dl = B t ds For a simple rectangular loop V B Area = A B t November 2008 Bruce Archambeault, PhD 22 V = A The minus sign means that the induced voltage will work against the current that originally created the magnetic field!
Self Inductance Isolated circular loop Isolated rectangular loop L 8a μ ln 2 0a r0 L = 2μ a π ln p + 1+ p 1+ 2 2 0 2 1 1+ Note that inductance is directly influenced by loop AREA and only less influenced by conductor size! November 2008 Bruce Archambeault, PhD 23 + 1 p 1 p + p 2 length of side p = wire radius
Partial Inductance Simply a way to break the overall loop into pieces in order to find total inductance L2 L1 L3 L4 L total =L p11 + L p22 + L p33 + L p44-2l p13-2l p24 November 2008 Bruce Archambeault, PhD 24
Important Points About Inductance Inductance is everywhere Loop area most important Inductance is everywhere November 2008 Bruce Archambeault, PhD 25
Decoupling Capacitor Mounting Keep as to planes as close to capacitor pads as possible Inductance Depends on Loop AREA Via Separation Height above Planes November 2008 Bruce Archambeault, PhD 26
Via Configuration Can Change Inductance SMT Capacitor The Good The Bad The Ugly Via Capacitor Pads Best Better Really Ugly November 2008 Bruce Archambeault, PhD 27
Comparison of Decoupling Capacitor Impedance 100 mil Between Vias & 10 mil to Planes 1000 100 1000pF 0.01uF Impedance (ohms) 10 1 0.1uF 1.0uF 0.1 0.01 1.0E+06 1.0E+07 1.0E+08 1.0E+09 1.0E+10 Frequency (Hz) November 2008 Bruce Archambeault, PhD 28
Comparison of Decoupling Capacitor Via Separation Distance Effects Via Separation 0603 Typical Minimum Dimensions 10 mils November 2008 Bruce Archambeault, PhD 29
Connection Inductance for Typical Capacitor Configurations Distance into board to planes (mils) 0805 typical/minimum (148 mils between via barrels) 0603 typical/minimum (128 mils between via barrels) 0402 typical/minimum (106 mils between via barrels) 10 1.2 nh 1.1 nh 0.9 nh 20 1.8 nh 1.6 nh 1.3 nh 30 2.2 nh 1.9 nh 1.6 nh 40 2.5 nh 2.2 nh 1.9 nh 50 2.8 nh 2.5 nh 2.1 nh 60 3.1 nh 2.7 nh 2.3 nh 70 3.4 nh 3.0 nh 2.6 nh 80 3.6 nh 3.2 nh 2.8 nh 90 3.9 nh 3.5 nh 3.0 nh 100 4.2 nh 3.7 nh 3.2 nh November 2008 Bruce Archambeault, PhD 30
Ground Ground is a place where potatoes and carrots thrive! Earth or reference is more descriptive Original use of GROUND Inductance is everywhere X = 2πfL L November 2008 Bruce Archambeault, PhD 31
What we Really Mean when we say Ground Signal Reference Power Reference Safety Earth Chassis Shield Reference November 2008 Bruce Archambeault, PhD 32
Ground is NOT a Current Sink! Current leaves a driver on a trace and must return (somehow) to its source This seems basic, but it is often forgotten, and is most often the cause of EMC problems November 2008 Bruce Archambeault, PhD 33
Grounding Needs Low Impedance at Highest Frequency Steel Reference Plate 4 milliohms/sq @ 100KHz 40 milliohms/sq @ 10 MHz 400 milliohms/sq @ 1 GHz A typical via is about 2 nh @ 100 MHz Z = 1.3 ohms @ 500 MHz Z = 6.5 ohms @ 1000 MHz Z = 13 ohms @ 2000 MHz Z = 26 ohms November 2008 Bruce Archambeault, PhD 34
Where did the Term GROUND Originate? Original Teletype connections Lightning Protection November 2008 Bruce Archambeault, PhD 35
Ground/Earth Teletype Receiver Teletype Transmitter November 2008 Bruce Archambeault, PhD 36
Ground/Earth Teletype Receiver Teletype Transmitter November 2008 Bruce Archambeault, PhD 37
FIG 7 Lightning striking house Lightning November 2008 Bruce Archambeault, PhD 38
Lightning effect without rod November 2008 Bruce Archambeault, PhD 39
Lightning effect with rod Lightning Lightning rod November 2008 Bruce Archambeault, PhD 40
What we Really Mean when we say Ground Signal Reference Power Reference Safety Earth Chassis Shield Reference D A Circuit Ground Chassis Ground Digital Ground Analog Ground November 2008 Bruce Archambeault, PhD 41
November 2008 Bruce Archambeault, PhD 42
Schematic with return current shown Signal trace currents IC1 IC2 IC3 Return currents on ground November 2008 Bruce Archambeault, PhD 43
Actual Current Return is 3-Dimensional IC Signal Trace Ground Vias BOARD STACK UP: IC Signal Trace Ground Via CURRENT LOCATION: Signal Trace Ground Layer Ground Layer Ground Layer November 2008 Bruce Archambeault, PhD 44
Low Frequency Return Currents Take Path of Least Resistance Driver Receiver Ground Plane November 2008 Bruce Archambeault, PhD 45
High Frequency Return Currents Take Path of Least Inductance Driver Receiver Ground Plane November 2008 Bruce Archambeault, PhD 46
PCB Example for Return Current Impedance Trace GND Plane 22 trace 10 mils wide, 1 mil thick, 10 mils above GND plane November 2008 Bruce Archambeault, PhD 47
PCB Example for Return Current Impedance Trace GND Plane Shortest DC path For longest DC path, current returns under trace November 2008 Bruce Archambeault, PhD 48
MoM Results for Current Density Frequency = 1 KHz November 2008 Bruce Archambeault, PhD 49
MoM Results for Current Density Frequency = 1 MHz November 2008 Bruce Archambeault, PhD 50
U-shaped Trace Inductance PowerPEEC Results 0.6 0.55 0.5 0.45 inductance (uh) 0.4 0.35 0.3 0.25 0.2 0.15 0.1 1.0E+03 1.0E+04 1.0E+05 1.0E+06 1.0E+07 1.0E+08 Frequency (Hz) November 2008 Bruce Archambeault, PhD 51
Traces/nets over a Reference Plane Microstrip Transmission Line Signal Trace Reference Planes Dielectric Stripline Transmission Line November 2008 Bruce Archambeault, PhD 52
Traces/nets and Reference Planes in Many Layer Board Stackup Signal Traces Reference Planes (Power, Ground, etc.) November 2008 Bruce Archambeault, PhD 53
Microstrip Electric/Magnetic Field Lines (8mil wide trace, 8 mils above plane, 65 ohm) Electric Field Lines Vcc Courtesy of Hyperlynx November 2008 Bruce Archambeault, PhD 54
Microstrip Electric/Magnetic Field Lines Common Mode 8 mil wide trace, 8 mils above plane, 65/115 ohm) Electric Field Lines Vcc Courtesy of Hyperlynx November 2008 Bruce Archambeault, PhD 55
Microstrip Electric/Magnetic Field Lines Differential Mode 8 mil wide trace, 8 mils above plane, 65/115 ohm) Electric Field Lines Vcc Courtesy of Hyperlynx November 2008 Bruce Archambeault, PhD 56
Electric/Magnetic Field Lines Symmetrical Stripline GND Vcc Courtesy of Hyperlynx November 2008 Bruce Archambeault, PhD 57
Electric/Magnetic Field Lines Symmetrical Stripline (Differential) GND Vcc Courtesy of Hyperlynx November 2008 Bruce Archambeault, PhD 58
Electric/Magnetic Field Lines Asymmetrical Stripline Vcc GND Courtesy of Hyperlynx November 2008 Bruce Archambeault, PhD 59
Electric/Magnetic Field Lines Asymmetrical Stripline (Differential) Courtesy of Hyperlynx November 2008 Bruce Archambeault, PhD 60
What About Pseudo-Differential Nets? So-called differential traces are NOT truly differential Two complementary single-ended drivers Relative to ground Receiver is differential Senses difference between two nets (independent of ground ) Provides good immunity to common mode noise Good for signal quality/integrity November 2008 Bruce Archambeault, PhD 61
Pseudo-Differential Nets Current in Nearby Plane Balanced/Differential currents have matching current in nearby plane No issue for discontinuities Any unbalanced (common mode) currents have return currents in nearby plane that must return to source! All normal concerns for single-ended nets apply! November 2008 Bruce Archambeault, PhD 62
Pseudo-Differential Nets Not really differential, since more closely coupled to nearby plane than each other Slew and rise/fall variation cause common mode currents! November 2008 Bruce Archambeault, PhD 63
Differential Voltage Pulse with Skew 1 Gbit/sec with 95 psec rise/fall time 1.2 1 Voltage 0.8 0.6 0.4 0.2 Complementary -- Line1 Complementary -- Line 2 Skew=2ps Skew=6ps Skew = 10ps Skew = 20ps Skew = 30ps Skew =40ps Skew =50ps Skew =60ps 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (nsec) November 2008 Bruce Archambeault, PhD 64
0.6 Common Mode Voltage From Differential Voltage Pulse with Skew 1 Gbit/sec with 95 psec rise/fall time Voltage 0.4 0.2 0 Balanced Skew=2ps Skew=6ps Skew =10ps Skew =20ps Skew =30ps Skew =40ps Skew =50ps -0.2-0.4-0.6 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (nsec) November 2008 Bruce Archambeault, PhD 65
100 Common Mode Current From Differential Voltage Pulse with Skew 1 Gbit/sec with 95 psec Rise/fall Time 80 Level (ma) 60 40 20 0-20 Balanced Skew=2ps Skew=6ps Skew =10ps Skew =20ps Skew =30ps Skew =40ps Skew =50ps Skew =60ps -40-60 -80-100 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (nsec) November 2008 Bruce Archambeault, PhD 66
150 Common Mode Current From Differential Voltage Pulse with Skew 1 Gbit/sec with 95 psec Rise/fall Time Level (dbua) 140 130 120 110 100 90 Skew=2ps Skew=6ps Skew =10ps Skew =20ps Skew =30ps Skew =40ps Skew =50ps Skew =60ps 80 70 60 50 1.E+08 1.E+09 1.E+10 1.E+11 Frequency (Hz) November 2008 Bruce Archambeault, PhD 67
Differential Voltage Pulse with Rise/Fall Variation/Unbalance 1 Gbit/sec with 95 psec Nominal Rise/Fall Time 1.2 1 Level (volts) 0.8 0.6 Original Pulse rise=95ps Complementary Pulse Rise=90ps Complementary Pulse Rise=80ps Complementary Pulse Rise=105ps Complementary Pulse Rise=115ps 0.4 0.2 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (ns) November 2008 Bruce Archambeault, PhD 68
0.2 Common Mode Voltage From Differential Voltage Pulse with Various Rise/Fall Unbalance 1 Gbit/sec with 95 psec Nominal Rise/Fall Time 0.15 0.1 0.05 Voltage 0-0.05-0.1-0.15 Complementary Pulse Rise=90ps Complementary Pulse Rise=80ps Complementary Pulse Rise=105ps Complementary Pulse Rise=115ps -0.2 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (ns) November 2008 Bruce Archambeault, PhD 69
60 Common Mode Current From Differential Voltage Pulse with Various Rise/Fall Unbalance 1 Gbit/sec with 95 psec Nominal Rise/fall Time 40 20 Current (ma) 0-20 -40-60 Complementary Pulse Rise=90ps Complementary Pulse Rise=80ps Complementary Pulse Rise=105ps Complementary Pulse Rise=115ps 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (ns) November 2008 Bruce Archambeault, PhD 70
90 Common Mode Current From Differential Voltage Pulse with Various Rise/Fall Unbalance 1 Gbit/sec with Nominal 95 psec Rise/fall Time 85 80 75 Complementary Pulse Rise=90ps Complementary Pulse Rise=80ps Complementary Pulse Rise=105ps Complementary Pulse Rise=115ps Level (dbua) 70 65 60 55 50 1.E+08 1.E+09 1.E+10 1.E+11 Frequency (Hz) November 2008 Bruce Archambeault, PhD 71
Antenna Structures Dipole antenna Non-Dipole antenna PCB GND planes November 2008 Bruce Archambeault, PhD 72
Board-to-Board Differential Pair Issues PCB Plane 2 Microstrip Connector Microstrip PCB Plane 1 November 2008 Bruce Archambeault, PhD 73 V Ground-to-Ground noise
Example Measured Differential Individual Signal-to-GND 500 mv P-P (each) Individual Differential Signals ADDED Common Mode Noise 170 mv P-P November 2008 Bruce Archambeault, PhD 74
Measured GND-to-GND Voltage 205 mv P-P November 2008 Bruce Archambeault, PhD 75
Pin Assignment Controls Inductance for CM signals 37.17 nh 25.21 nh (a) (b) 16.85 nh 20.97 nh (c) (d) Signal Pin Related Ground Pins November 2008 Bruce Archambeault, PhD 76
Different pins within Same Pair may have Different Loop Inductance for CM Ground pins Differential pair 4 3 pin 1 -- 26.6nH 2 1 pin 2 -- 23.6nH pin 3 -- 31.8nH pin 4 -- 28.8nH November 2008 Bruce Archambeault, PhD 77
Pseudo-Differential Net Summary Small amounts of skew can cause significant common mode current Small amount of rise/fall time deviation can cause significant amount of common mode current Discontinuities (vias, crossing split planes, etc) and convert significant amount of differential current into common mode current November 2008 Bruce Archambeault, PhD 78
Return Current vs. Ground For high frequency signals, Ground is a concept that does not exist The important question is where does the return current flow? November 2008 Bruce Archambeault, PhD 79
Referencing Nets (Where does the Return Current Flow??) Microstrip/Stripline across split in reference plane Microstrip/Stripline through via (change reference planes) Mother/Daughter card November 2008 Bruce Archambeault, PhD 80
Microstrip/Stripline Across Split in Reference Plane Don t Cross Splits with Critical Signals!!! Bad practice Stitching capacitor required across split to allow return current flow must be close to crossing must have low inductance limited frequency effect --- due to inductance Major source of Common Mode current! November 2008 Bruce Archambeault, PhD 81
Splits in Reference Plane Power planes often have splits Return current path interrupted Consider spectrum of clock signal Consider stitching capacitor impedance High frequency harmonics not returned directly November 2008 Bruce Archambeault, PhD 82
Split Reference Plane Example PWR GND November 2008 Bruce Archambeault, PhD 83
Split Reference Plane Example With Stitching Capacitors PWR GND Stitching Capacitors Allow Return current to Cross Splits??? November 2008 Bruce Archambeault, PhD 84
Capacitor Impedance Measured Impedance of.01 uf Capacitor 100.0 10.0 Impednace (ohms) 1.0 0.1 1.E+06 1.E+07 1.E+08 1.E+09 Frequency (Hz) November 2008 Bruce Archambeault, PhD 85
Frequency Domain Amplitude of Intentional Current Harmonic Amplitude From Clock Net 160 140 120 level (dbua) 100 80 60 40 0 200 400 600 800 1000 1200 1400 1600 1800 2000 freq (MHz) November 2008 Bruce Archambeault, PhD 86
MoM Microstrip Model Current Distribution Example November 2008 Bruce Archambeault, PhD 87
MoM Microstrip Model Current Distribution Example November 2008 Bruce Archambeault, PhD 88
Emissions From Board Far field emissions not important unless it is an unshielded product Near field emissions above board ARE important Example of emissions from board with critical net crossing split reference plane November 2008 Bruce Archambeault, PhD 89
Near Field Radiation from Microstrip on Board with Split in Reference Plane 120 Comparison of Maximum Radiated E-Field for Microstrip With and without Split Ground Reference Plane 110 100 Maximum Radiated E-Field (dbuv/m) 90 80 70 60 50 No-Split Split 40 30 20 10 100 1000 Frequency (MHz) November 2008 Bruce Archambeault, PhD 90
With Perfectly Connected Stitching Capacitors Across Split 120 Comparison of Maximum Radiated E-Field for Microstrip With and without Split Ground Reference Plane and Stiching Capacitors 110 100 Maximum Radiated E-Field (dbuv/m) 90 80 70 60 50 40 No-Split Split Split w/ one Cap Split w/ Two Caps 30 20 10 100 1000 Frequency (MHz) November 2008 Bruce Archambeault, PhD 91
Stitching Caps with Via Inductance 120 Comparison of Maximum Radiated E-Field for Microstrip With and without Split Ground Reference Plane and Stiching Capacitors 110 100 Maximum Radiated E-Field (dbuv/m) 90 80 70 60 50 40 30 No-Split Split Split w/ one Cap Split w/ Two Caps Split w/one Real Cap Split w/two Real Caps 20 10 100 1000 Frequency (MHz) November 2008 Bruce Archambeault, PhD 92
25 Example of Common-Mode Noise Voltage Across Split Plane Vs. Stitching Capacitor Distance to Crossing Point 20 Gap Voltage 15 10 5 100MHz 200MHz 300MHz 400MHz 500MHz 600MHz 700MHz 800MHz 900MHz 1000MHz 0 0 200 400 600 800 1000 1200 1400 1600 1800 2000 Distance (mils) November 2008 Bruce Archambeault, PhD 93
Are Stitching Capacitors Effective??? YES, at low frequencies No, at high frequencies Need to limit the high frequency current spectrum Need to avoid split crossings with ALL critical signals November 2008 Bruce Archambeault, PhD 94
Pin Field Via Keepouts?? d s Return Current must go around entire keep out area --- just as bad as a slot Return current path deviation minimal Recommend s/d > 1/3 November 2008 Bruce Archambeault, PhD 95
Changing Reference Planes Six-Layer PCB Stackup Example Signal Layer Signal Layer Plane Signal Layer Signal Layer Plane November 2008 Bruce Archambeault, PhD 96
Microstrip/Stripline through via (change reference planes) Via Trace November 2008 Bruce Archambeault, PhD 97
How can the Return Current Flow When Signal Line Goes Through Via?? What happens to Return Current in this Region? Return Current November 2008 Bruce Archambeault, PhD 98
How can the Return Current Flow When Signal Line Goes Through Via?? Current can NOT go from one side of the plane to the other through the plane skin depth Current must go around plane at via hole, through decoupling capacitor, around second plane at the second via hole! Use displacement current between planes November 2008 Bruce Archambeault, PhD 99
Return Current Across Reference Plane Change What happens to Return Current in this Region? Reference Planes Displacement Current Return Current Trace Current November 2008 Bruce Archambeault, PhD 100
Return Current Across Reference Plane Change With Decoupling Capacitor Decoupling Capacitor Displacement Current Return Current Reference Planes November 2008 Bruce Archambeault, PhD 101
Return Current Across Reference Plane Change With Decoupling Capacitor (on Top) Decoupling Capacitor Common-Mode Current Displacement Current Return Current Reference Planes November 2008 Bruce Archambeault, PhD 102
Location of Decoupling Capacitors (Relative to Via) is Important! One Decoupling Capacitor at 0.5 Two Decoupling Capacitors at 0.5 Two Decoupling Capacitors at 0.25 November June 20072008 Bruce Archambeault, PhD 103 103
RF Current @ 700 MHz with One Capacitor 0.5 from Via November June 20072008 Bruce Archambeault, PhD 104 104
RF Current @ 700 MHz with One Capacitor 0.5 from Via (expanded view) November June 20072008 Bruce Archambeault, PhD 105 105
RF Current @ 700 MHz with Two Capacitors 0.5 from Via November June 20072008 Bruce Archambeault, PhD 106 106
RF Current @ 700 MHz with One Capacitor 0.5 from Via (Expanded view) November June 20072008 Bruce Archambeault, PhD 107 107
RF Current @ 700 MHz with Two Capacitors 0.25 from Via November June 20072008 Bruce Archambeault, PhD 108 108
RF Current @ 700 MHz with Two Capacitors 0.25 from Via (expanded view) November June 20072008 Bruce Archambeault, PhD 109 109
RF Current @ 700 MHz with One REAL Capacitor 0.5 from Via November June 20072008 Bruce Archambeault, PhD 110 110
RF Current @ 700 MHz with Two REAL Capacitors 0.5 from Via November June 20072008 Bruce Archambeault, PhD 111 111
RF Current @ 700 MHz with Two REAL Capacitors 0.25 from Via November June 20072008 Bruce Archambeault, PhD 112 112
Possible Routing Options Six-Layer Board Bad Signal Layer Signal Layer Signal Layer Signal Layer Reference Plane Reference Plane Bad Signal Layer Signal Layer Signal Layer Signal Layer Reference Plane Reference Plane Good Signal Layer Signal Layer Reference Plane Signal Layer Reference Plane Signal Layer November 2008 Bruce Archambeault, PhD 113
Compromise Routing Option for Many Layer Boards Good Compromise Vcc1 Reference Plane Gnd Lot s of Decoupling caps near ASIC November 2008 Bruce Archambeault, PhD 114
Typical Driver/Receiver Currents V DC IC driver V CC switch Z 0, v p IC load C L logic 0-to-1 GND logic 1-to-0 IC driver V CC charge IC load IC driver V CC discharge IC load Z 0, v p V CC Z 0, v p 0 V GND GND November 2008 Bruce Archambeault, PhD 115
Suppose The Trace is Routed Next to Power (not Gnd) V cc1 TEM Transmission Line Area Fuzzy Return Path Area V cc1 Return Path Options: -- Decoupling Capacitors -- Distributed Displacement Current Fuzzy Return Path Area November 2008 Bruce Archambeault, PhD 116
Suppose The Trace is Routed Next to a DIFFERENT Power (not Gnd) V cc1 TEM Transmission Line Area Fuzzy Return Path Area V cc2 Fuzzy Return Path Area Return Path Options: -- Decoupling Capacitors??? May not be any nearby!! -- Distributed Displacement Current Increased current spread!!! November 2008 Bruce Archambeault, PhD 117
Via Summary Route critical signals on either side of ONE reference plane Drop critical signal net to selected layer close to driver/receiver Many decoupling capacitors to help return currents Do NOT change reference planes on critical nets unless ABSOLUTELY NECESSARY!! Make sure at least 2 decoupling capacitors within 0.2 of via with critical signals November 2008 Bruce Archambeault, PhD 118
Mother/Daughter Board Connector Crossing Critical Signals must be referenced to same plane on both sides of the connector November 2008 Bruce Archambeault, PhD 119
Mother/Daughter Board Connector Crossing Signal Path Connector GND PWR Signal Layers November 2008 Bruce Archambeault, PhD 120
Return Current from Improper Referencing Across Connector Displacement Current Decoupling Capacitors Signal Path Connector Return current GND PWR Signal Layers November 2008 Bruce Archambeault, PhD 121
Return Current from Proper Referencing Across Connector Signal Path Connector GND PWR Return current Signal Layers November 2008 Bruce Archambeault, PhD 122
How Many Ground Pins Across Connector??? Nothing MAGICAL about ground Return current flow! Choose the number of power and ground pins based on the number of signal lines referenced to power or ground planes Insure signals are referenced against same planes on either side of connector November 2008 Bruce Archambeault, PhD 123
Think about Return Currents!! Reference plane should be continuous under all critical traces When Vias are necessary make sure there are two close decoupling capacitors When crossing a connector to a second board, make sure the critical trace is referenced to the same reference plane as the primary board November 2008 Bruce Archambeault, PhD 124
Ground-Reference Plane Noise (Voltage Difference Across Plane) Connection of large PC ground planes to chassis important ESD current can result in voltage difference across ground plane Looks like input pulse to circuits More connection to chassis will reduce this voltage difference November 2008 Bruce Archambeault, PhD 125
Connection to Chassis Good connection in I/O area important for emissions control!! PCB gnd plane Chassis Screw post Connection to chassis away from I/O area NOT important for emissions control November 2008 Bruce Archambeault, PhD 126
Connection to Chassis for ESD Control PCB gnd plane Chassis Screw post Distributed Connection to chassis away from I/O area very important for ESD control November 2008 Bruce Archambeault, PhD 127
Contacts for Chassis Connection Screw head contact pad on top of PC Board Want this! Screw head NOT this! Copper pad Vias to Ground plane November 2008 Bruce Archambeault, PhD 128
Model for Current Simulations Trace Source Screw post PCB gnd plane ESD Voltage Between Chassis and gnd plane Trace Load Chassis November 2008 Bruce Archambeault, PhD 129
Comparison of Trace Load Noise Voltage for 1 Kv ESD Pulse from PCB GND to Chassis 2 1.5 1 No Connection to Chassis One connection to Chassis (Near I/O) Four Connections to Chassis (Near I/O) Eight Connections to Chassis 16 Connections to Chassis 20 Connections to Chassis Load Voltage (volts) 0.5 0-0.5-1 -1.5 0 0.5 1 1.5 2 2.5 3 3.5 Time (ns) November 2008 Bruce Archambeault, PhD 130
2.5 Comparison of Trace Load Noise Voltage for 1 Kv ESD Pulse from PCB GND to Chassis 2 Load Voltage (volts) 1.5 1 0.5 0-0.5-1 -1.5 No Connection to Chassis One connection to Chassis (Near I/O) Four Connections to Chassis (Near I/O) Eight Connections to Chassis 16 Connections to Chassis 20 Connections to Chassis Eight Connections to Chassis (4 @ each end) -2-2.5-3 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Time (ns) November 2008 Bruce Archambeault, PhD 131
Current Flow w/one Screw Post November 2008 Bruce Archambeault, PhD 132
Current Flow w/eight Screw Posts November 2008 Bruce Archambeault, PhD 133
Current Flow w/20 Screw Posts November 2008 Bruce Archambeault, PhD 134
Current Flow w/eight Screw Posts (4 each end) November 2008 Bruce Archambeault, PhD 135
Number ONE Problem Intentional signal return current November 2008 Bruce Archambeault, PhD 136
Where to Go for More? Limited selection of EMC design books Beware of some popular books!!! PCB Design for Real-World EMI Control (good choice) Bruce Archambeault EMC experts Experience is important Again, beware ---- ask questions and understand WHY Cookbooks do not work! Every case is special and different November 2008 Bruce Archambeault, PhD 137