The Ground Myth Dr. Bruce Archambeault IBM Distinguished Engineer - - PowerPoint PPT Presentation
The Ground Myth Dr. Bruce Archambeault IBM Distinguished Engineer IEEE Fellow IBM Research Triangle Park, NC Barch@us.ibm.com October 2007 IEEE IEEE Outline Electromagnetics Skin Effect Inductance Ground
IBM Distinguished Engineer IEEE Fellow
IBM Research Triangle Park, NC Barch@us.ibm.com October 2007
IEEE IEEE
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In the Beginning
connected
‘action from a distance’
relationship between electric lines of force and time-changing magnetic fields
– Faraday was very good at experiments and ‘figuring out’ how things work
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impressed with Faraday’s ideas
mathematical link between the “electro” and the “magnetic”
contribution to the world (next to Scotch)
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A difference in Magnetic Field across a small piece of space A difference in Electric Field across a small piece of space A change in Electric Flux Density with respect to time A change in Magnetic Flux Density with respect to time
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in E-field (at that point) with time
in H-field (at that point) with time
constants)
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frequency increases
– Uses entire conductor – Only resistance inhibits current
– Only small part of conductor (near surface) is used – Resistance is small part of current inhibitor – Inductance is major part of current inhibitor
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metal surface at high frequencies
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
µσ π δ f 1 =
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Resistance is determined by the area of the copper conductor actually used at each frequency!
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S21 Skin Effect Loss ONLY-- Comparison for Microstrip (1 cm Long) Various Widths -- 50 ohms (FR4, 4.2, 0.021) Trace Thickness = 0.6mil
0.1 1 10 100 Frequency (GHz) Loss (dB) 1 Mil Wide uStrip 2 Mil Wide uStrip 4 Mil Wide uStrip 6 Mil Wide uStrip 8 Mil Wide uStrip
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S21 Dielectric Loss ONLY-- Comparison for Microstrip (1 cm Long) Various Widths -- 50 ohms (FR4, 4.2, 0.021) Trace Thickness = 0.6mil
0.1 1 10 100 Frequency (GHz) Loss (dB) 1 Mil Wide uStrip 2 Mil Wide uStrip 4 Mil Wide uStrip 6 Mil Wide uStrip 8 Mil Wide uStrip
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12 S21 Loss Comparison Between Stripline and Microstrip (1 cm Long) 6 mil wide -- 50 ohms (FR4, 4.2, 0.021) Trace Thickness = 0.6mil
0.1 1 10 100 Frequency (GHz) S21 Mag (dB) 6 mil Wide uStrip 6 mil Wide StripLine
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13 S21 Loss Comparison for 6 Mil Wide Microstrip Loss (1 cm Long) Various Dielectric Loss -- 50 ohms (FR4, 4.2) Trace Thickness = 0.6mil
0.1 1 10 100 Frequency (GHz) S21 Mag (dB) Skin Only Diel (.010) Only Diel (.021) Only Diel (.029) Only Diel (.021) + Skin
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present
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inductance!
frequency and is MAJOR concern at high frequencies
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Courtesy of Elya Joffe
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⋅ ∂ ∂ − = ⋅ S d t B dl E
V B Area = A
The minus sign means that the induced voltage will work against the current that
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− ≈ 2 8 ln r a a L µ
+ − + − + + + + =
2 2
1 1 2 1 1 2 1 1 ln 2 p p p p p a L π µ
Note that inductance is directly influenced by loop AREA and only less influenced by conductor size!
radius wire side
length p =
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into pieces in order to find total inductance
L3 L4 L2 L1
L total=Lp11+ Lp22 + Lp33 + Lp44 - 2Lp13 - 2Lp24
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pads as possible
Height above Planes Via Separation
Inductance Depends
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trace and must return (somehow) to its source
forgotten, and is most often the cause of EMC problems
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‘Grounding’ Needs Low Impedance at Highest Frequency
– 4 milliohms/sq @ 100KHz – 40 milliohms/sq @ 10 MHz – 400 milliohms/sq @ 1 GHz
nH
– @ 100 MHz Z = 1.3 ohms – @ 500 MHz Z = 6.5 ohms – @ 1000 MHz Z = 13 ohms – @ 2000 MHz Z = 26 ohms
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this impossible!
makes this impossible!
your system can stand…...
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GND via
Board A Board B Metal Enclosure
GND via GND via GND via GND via GND planes
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Reality Overcomes Single-Point Ground Intentions
GND via
Board A Board B Metal Enclosure
GND via GND via GND via GND via GND planes
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Ground/Earth Teletype Receiver Teletype Transmitter
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Ground/Earth Teletype Receiver Teletype Transmitter
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Lightning striking house
FIG 7 Lightning
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Lightning effect without rod
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Lightning effect with rod
Lightning rod Lightning
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Circuit “Ground” Chassis “Ground” Digital “Ground”
D
Analog “Ground”
A
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IC1 IC2 IC3 Return currents on ground Signal trace currents
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Actual Current Return is 3-Dimensional
Ground Layer Signal Trace IC Ground Vias Ground Layer Signal Trace IC Ground Via BOARD STACK UP: Ground Layer Signal Trace CURRENT LOCATION:
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Ground Plane Driver Receiver
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Ground Plane Driver Receiver
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Microstrip Transmission Line Stripline Transmission Line
Dielectric Reference Planes Signal Trace
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Signal Traces Reference Planes (Power, “Ground”, etc.)
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(8mil wide trace, 8 mils above plane, 65 ohm)
Electric Field Lines
Vcc
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Common Mode 8 mil wide trace, 8 mils above plane, 65/115 ohm)
Electric Field Lines
Vcc
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Differential Mode 8 mil wide trace, 8 mils above plane, 65/115 ohm)
Electric Field Lines
Vcc
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Symmetrical Stripline
Vcc GND
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Symmetrical Stripline (Differential)
Vcc GND
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Asymmetrical Stripline
Vcc GND
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Asymmetrical Stripline (Differential)
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differential
– Two complementary single-ended drivers
– Receiver is differential
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matching current in nearby plane
– No issue for discontinuities
have return currents in nearby plane that must return to source!
– All normal concerns for single-ended nets apply!
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coupled to nearby plane than each other
mode currents!
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0.2 0.4 0.6 0.8 1 1.2 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (nsec) Voltage Complementary -- Line1 Complementary -- Line 2 Skew=2ps Skew=6ps Skew = 10ps Skew = 20ps Skew = 30ps Skew =40ps Skew =50ps Skew =60ps
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55 Common Mode Voltage From Differential Voltage Pulse with Skew 1 Gbit/sec with 95 psec rise/fall time
0.2 0.4 0.6 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (nsec) Voltage Balanced Skew=2ps Skew=6ps Skew =10ps Skew =20ps Skew =30ps Skew =40ps Skew =50ps
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56 Common Mode Current From Differential Voltage Pulse with Skew 1 Gbit/sec with 95 psec Rise/fall Time
20 40 60 80 100 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (nsec) Level (ma) Balanced Skew=2ps Skew=6ps Skew =10ps Skew =20ps Skew =30ps Skew =40ps Skew =50ps Skew =60ps
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57 Common Mode Current From Differential Voltage Pulse with Skew 1 Gbit/sec with 95 psec Rise/fall Time
50 60 70 80 90 100 110 120 130 140 150 1.E+08 1.E+09 1.E+10 1.E+11 Frequency (Hz) Level (dBuA) Skew=2ps Skew=6ps Skew =10ps Skew =20ps Skew =30ps Skew =40ps Skew =50ps Skew =60ps
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58 Differential Voltage Pulse with Rise/Fall Variation/Unbalance 1 Gbit/sec with 95 psec Nominal Rise/Fall Time
0.2 0.4 0.6 0.8 1 1.2 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (ns)
Level (volts)
Original Pulse rise=95ps Complementary Pulse Rise=90ps Complementary Pulse Rise=80ps Complementary Pulse Rise=105ps Complementary Pulse Rise=115ps
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59 Common Mode Voltage From Differential Voltage Pulse with Various Rise/Fall Unbalance 1 Gbit/sec with 95 psec Nominal Rise/Fall Time
0.05 0.1 0.15 0.2 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (ns) Voltage Complementary Pulse Rise=90ps Complementary Pulse Rise=80ps Complementary Pulse Rise=105ps Complementary Pulse Rise=115ps
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60 Common Mode Current From Differential Voltage Pulse with Various Rise/Fall Unbalance 1 Gbit/sec with 95 psec Nominal Rise/fall Time
20 40 60 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Time (ns) Current (ma) Complementary Pulse Rise=90ps Complementary Pulse Rise=80ps Complementary Pulse Rise=105ps Complementary Pulse Rise=115ps
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61 Common Mode Current From Differential Voltage Pulse with Various Rise/Fall Unbalance 1 Gbit/sec with Nominal 95 psec Rise/fall Time
50 55 60 65 70 75 80 85 90 1.E+08 1.E+09 1.E+10 1.E+11 Frequency (Hz) Level (dBua) Complementary Pulse Rise=90ps Complementary Pulse Rise=80ps Complementary Pulse Rise=105ps Complementary Pulse Rise=115ps
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significant common mode current
can cause significant amount of common mode current
etc) and convert significant amount of differential current into common mode current
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concept that does not exist
the return current flow?”
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reference plane
reference planes)
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– Bad practice – Stitching capacitor required across split to allow return current flow
– Major source of Common Mode current!
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impedance
returned directly
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PWR GND
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With Stitching Capacitors
PWR GND
Stitching Capacitors Allow Return current to Cross Splits ???
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Measured Impedance of .01 uf Capacitor
0.1 1.0 10.0 100.0 1.E+06 1.E+07 1.E+08 1.E+09 Frequency (Hz) Impednace (ohms)
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Frequency Domain Amplitude of Intentional Current Harmonic Amplitude From Clock Net
40 60 80 100 120 140 160 200 400 600 800 1000 1200 1400 1600 1800 2000 freq (MHz) level (dBuA)
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is an unshielded product
important
critical net crossing split reference plane
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Near Field Radiation from Microstrip on Board with Split in Reference Plane
Comparison of Maximum Radiated E-Field for Microstrip With and without Split Ground Reference Plane
20 30 40 50 60 70 80 90 100 110 120 10 100 1000 Frequency (MHz) Maximum Radiated E-Field (dBuv/m) No-Split Split
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With “Perfectly Connected” Stitching Capacitors Across Split
Comparison of Maximum Radiated E-Field for Microstrip With and without Split Ground Reference Plane and Stiching Capacitors
20 30 40 50 60 70 80 90 100 110 120 10 100 1000 Frequency (MHz) Maximum Radiated E-Field (dBuv/m) No-Split Split Split w/ one Cap Split w/ Two Caps
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Stitching Caps with Via Inductance
Comparison of Maximum Radiated E-Field for Microstrip With and without Split Ground Reference Plane and Stiching Capacitors
20 30 40 50 60 70 80 90 100 110 120 10 100 1000 Frequency (MHz) Maximum Radiated E-Field (dBuv/m) No-Split Split Split w/ one Cap Split w/ Two Caps Split w/One Real Cap Split w/Two Real Caps
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5 10 15 20 25 200 400 600 800 1000 1200 1400 1600 1800 2000 Distance (mils) Gap Voltage
100MHz 200MHz 300MHz 400MHz 500MHz 600MHz 700MHz 800MHz 900MHz 1000MHz
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spectrum
critical signals
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Return Current must go around entire keep out area --- just as bad as a slot Return current path deviation minimal s d Recommend s/d > 1/3
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Six-Layer PCB Stackup Example
Signal Layer Signal Layer Signal Layer Signal Layer Plane Plane
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Trace Via
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What happens to Return Current in this Region?
Return Current
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plane to the other through the plane
– skin depth
through decoupling capacitor, around second plane at the second via hole!
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GND PWR
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With Decoupling Capacitor (on Top)
Return Current Decoupling Capacitor Reference Planes Displacement Current
Common-Mode Current
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(expanded view)
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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 Signal Layer Signal Layer Reference Plane Reference Plane
Six-Layer Board
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Compromise Routing Option for Many Layer Boards
Good Compromise
Reference Plane Gnd Vcc1
Lot’s of Decoupling caps near ASIC
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switch IC load IC driver VDC GND CL VCC Z0, vp GND IC load IC driver VCC VCC charge
logic 0-to-1
Z0, vp IC load IC driver GND VCC 0 V discharge
logic 1-to-0
Z0, vp
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Vcc1 Vcc1 “Fuzzy” Return Path Area “Fuzzy” Return Path Area Return Path Options:
TEM Transmission Line Area
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Return Path Options:
Vcc1 Vcc2 “Fuzzy” Return Path Area “Fuzzy” Return Path Area TEM Transmission Line Area
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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
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same plane on both sides of the connector
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Connector
Signal Path
GND PWR Signal Signal PWR GND Signal Signal
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Connector
De-cap Signal Path
GND PWR Signal Signal PWR GND Signal Signal
Return Path De-cap Displacement Current
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PWR GND Signal Signal
Connector
Signal Path
GND PWR Signal Signal
Return Path
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C
n e c t
P C B P l a n e 1 PCB Plane 2 M i c r
t r i p Microstrip V Ground-to-Ground noise
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Individual Differential Signals ADDED Common Mode Noise 170 mV P-P 500 mV P-P (each)
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205 mV P-P
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Dipole antenna PCB GND planes Non-Dipole antenna
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Signal Pin Related Ground Pins 37.17 nH 25.21 nH 16.85 nH 20.97 nH (a) (b) (c) (d)
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pins based on the number of signal lines referenced to power or “ground” planes
same planes on either side of connector
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– Avoid split reference planes & changing reference planes
mode, and must follow normal EMC rules
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IEEE IEEE