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Grounding in EMC Engineering
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Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
1©Copyright 2009
Fundamentals of Grounding,Fundamentals of Grounding,Fundamentals of Grounding,Fundamentals of Grounding,from Circuit to Systemfrom Circuit to Systemfrom Circuit to Systemfrom Circuit to System
Elya B. JoffePresident: IEEE EMC Society
2008-2009e-mail: [email protected]
Speaker
“’Ground’ is where carrots and potatoes thrive”
Dr. Bruce Archambeault
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
2©Copyright 2009
About the Instructor: Elya B. JoffeAbout the Instructor: Elya B. JoffeAbout the Instructor: Elya B. JoffeAbout the Instructor: Elya B. JoffeJOFFE, Elya B., K.T.M. Project Engineering, Kfar-Sava, Israel, and Senior EMC engineering Specialist and consultant.
Mr. Joffe has over 25 years of experience in government and industry, in EMC/E3, Electromagnetic Compatibility/Electromagnetic Environmental Effects, for electronic systems and platforms, in particular aircraft and aerospace. He is actively involved in the EMC design of commercial and defense systems, from circuits to full platforms.
His work covers various fields in the discipline of EMC, such as NEMP and Lightning Protection design, as well as numerical modeling for solution of EMC Problems.Mr. Joffe has authored and co-authored over 30 papers in the IEEE Transactions on EMC and Broadcasting, as well as in the proceedings of International EMC Symposia. He is Senior Member of IEEE, President of the IEEE EMC Society (2008-2009), member of the Board of Directors of the IEEE EMC Society and the Product Safety Engineering Society, and Chairs several Committees. He is also the Immediate Past Chairman of the Israel IEEE EMC Chapter and has served as a "Distinguished Lecturer" of the IEEE EMC Society.
Mr. Joffe has received several awards and recognitions from the IEEE and EMC Society for his activities. In particular, he is a recipient of the prestigious "Lawrence G. Cumming Award of the IEEE EMC Society for outstanding service", 2002, the "Honorary Life Member Award" of the IEEE EMC Society, 2004, and the IEEE EMC Society "Technical Achievement Award". He is also a recipient of the IEEE "Third Millennium Medal".
Mr. Joffe is also a member of the " dB Society".
The biography of Elya Joffe has been published numerous times in the Marquis “Who’s Who In The World” .
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
3©Copyright 2009
• The Grounds for Grounding
• Basic Grounding Topologies
• The “Grounding Tree”
• Understanding and Precluding “Ground Loops”
• Grounding on PCBs
• Summary
Seminar OutlineSeminar OutlineSeminar OutlineSeminar Outline
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
4©Copyright 2009
Grounding Grounding Grounding Grounding in EMC Engineeringin EMC Engineeringin EMC Engineeringin EMC Engineering
• “Grounding” is probably among the most important, yet more confusing aspect of electrical/electronic system design, often considered as "black magic“
Not easy to understand intuitively
No straightforward definition, modeling or analysis
Many uncontrolled factors affect its performance
• Grounding forms an inseparable part of all electronic and electrical designs, from circuit through system up to installation design
Implemented for EMC and ESD protection, for safety purposes, for lightning and surge protection, etc.
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
5©Copyright 2009
• A clustered system with remote AC outlets may cause ground loops due to large potential differences between the outlets, even if connected to the same power and ground bus, thus Single-ended interfaces should be avoided
Ground Ground Ground Ground –––– A Case Study A Case Study A Case Study A Case Study Grounding Requirements by System LayoutGrounding Requirements by System LayoutGrounding Requirements by System LayoutGrounding Requirements by System Layout
Distributed System ExampleDistributed System ExampleDistributed System ExampleDistributed System Example
Control Center
Fuel Tank
External Lightning Protection System
Good Grounding
Surge propagating on Data Lines
Ungrounded cable penetration the
facility
50 kV
The ungrounded cable penetrating the fuel tank, caused a potential difference between the cable and the facility’s structure, and thus - caused its explosion 50 kA
Good Grounding
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
6©Copyright 2009
Fundamental ConceptsFundamental ConceptsFundamental ConceptsFundamental Concepts
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
7©Copyright 2009
““““Path of Least ImpedancePath of Least ImpedancePath of Least ImpedancePath of Least Impedance”””” Principle Principle Principle Principle Visualize Return CurrentsVisualize Return CurrentsVisualize Return CurrentsVisualize Return Currents
• Currents always return;
To ground??
To battery negative??
• Where are they?
They are all here… flowing to their source!!
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
8©Copyright 2009
““““Path of Least InductancePath of Least InductancePath of Least InductancePath of Least Inductance”””” PrinciplePrinciplePrinciplePrinciple
Which path will the return current follow?Which path will the return current follow?Which path will the return current follow?Which path will the return current follow?
• Currents always take the path of least ;
Distance? Resistance? Impedance!!!
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
9©Copyright 2009
Equivalent Circuit
““““Path of Least InductancePath of Least InductancePath of Least InductancePath of Least Inductance”””” Principle Principle Principle Principle Which path will the return follow?Which path will the return follow?Which path will the return follow?Which path will the return follow?
Frequency (ω)(ω)(ω)(ω)
S
1
I
I
Cu
rren
t R
ati
o
-3dB
SC
S
R
Lω = 5 S
S
R
Lω =
Asymptotic
Actual
or:-
1( ) 0S S SI R j L I j Mω ω⋅ + − ⋅ =
4( ), Hy/m
2S
HL Ln
d
µπ
= ⋅SL M=
1
S S
S S
I j L
I R j L
ωω
=+
1 1, Sg S
S
RI I I I
Lω<< → ⇔ >>
SS g
S
RI I
Lω>> ⇔ >>
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
10©Copyright 2009
““““Path of Least InductancePath of Least InductancePath of Least InductancePath of Least Inductance”””” PrinciplePrinciplePrinciplePrincipleWhich path will the return follow?Which path will the return follow?Which path will the return follow?Which path will the return follow?
• At LOW FREQUENCIESLOW FREQUENCIES, the current will follow the path of LEAST LEAST RESISTANCERESISTANCE, via ground (IG)
1 /
S
S S
jI I
R L j
ωω
= ⋅+
0
| | @
| | @ S S S
S
S S S
Z R R jZ R j M
Z L L R
Lω
ωω
ωω
→ = + ⋅ =
≈ ⋅ ⋅ >>
≈ >> ⋅
M
Source Cable Load
RS
LS RL
Ig
I1
IS
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
11©Copyright 2009
• At HIGH FREQUENCIESHIGH FREQUENCIES, the current will follow the path of LEAST LEAST INDUCTANCEINDUCTANCE, via the return conductor (IS)
| | @
|
| @ S S S
S S
S
S
Z R R j
Z L LM
R
LZ R j
ω
ω ωω
ω→∞
≈ ⋅ ⋅ >>
≈ >> ⋅= + ⋅ =
““““Path of Least InductancePath of Least InductancePath of Least InductancePath of Least Inductance”””” PrinciplePrinciplePrinciplePrincipleWhich path will the return follow?Which path will the return follow?Which path will the return follow?Which path will the return follow?
1 /
S
S S
jI I
R L j
ωω
= ⋅+
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
12©Copyright 2009
• Definition of Total Loop Inductance
• For I=constant, ΦΦΦΦ min implies S min
min min min:
S
B ds
LI I
Thus L S
φ
φ
⋅
= ≈
⇒ ⇒
∫
““““Path of Least InductancePath of Least InductancePath of Least InductancePath of Least Inductance”””” PrinciplePrinciplePrinciplePrincipleWhen is Inductance Minimized?When is Inductance Minimized?When is Inductance Minimized?When is Inductance Minimized?
,B Φ
Current I
Magnetic Flux
X X X X X
X X X X X
X X X X X
LI
Φ≜
Loop Area, S
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
13©Copyright 2009
Circuit Model for Simulation(Simulation run on Agilent Technologies "Momentum" 3D Planar EM Simulator; Courtesy of Alexander Perez, Agilent Technologies)
High Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHow Does The Return Current FlowHow Does The Return Current FlowHow Does The Return Current FlowHow Does The Return Current Flow
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
14©Copyright 2009
Circuit Return Current Simulation @ F=1 Hz(Simulation run on Agilent Technologies "Momentum" 3D Planar EM Simulator; Courtesy of Alexander Perez, Agilent Technologies)
High Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHow Does The Return Current FlowHow Does The Return Current FlowHow Does The Return Current FlowHow Does The Return Current Flow
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
15©Copyright 2009
Circuit Return Current Simulation @ F=5 GHz(Simulation run on Agilent Technologies "Momentum" 3D Planar EM Simulator; Courtesy of Alexander Perez, Agilent Technologies)
High Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHow Does The Return Current FlowHow Does The Return Current FlowHow Does The Return Current FlowHow Does The Return Current Flow
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
16©Copyright 2009
The Grounds for GroundingThe Grounds for GroundingThe Grounds for GroundingThe Grounds for Grounding
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
17©Copyright 2009
Purposes for GroundingPurposes for GroundingPurposes for GroundingPurposes for Grounding
• Safety: Prevention of shock hazard to personnel
Due to lightning strokes or power line short circuits to enclosure
Traditionally
• Functionality: Path for return current in particular vehicles (e.g., aircraft)
Vehicle serves as return conductor
• Reduction of EMI in equipment
Due to EM fields, common impedance or other forms of EMI coupling
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
18©Copyright 2009
A A A A ““““GroundGroundGroundGround”””” ---- What is it ???What is it ???What is it ???What is it ???• To the circuit designer: the circuit voltage circuit voltage referencereference oror
current return pathcurrent return path
• To the system designer: the cabinet or rack cabinet or rack chassischassis
• To the electrician: the safetysafety ground or ground or earth connectionearth connection
One of the problems with grounding is the term itself... One of the problems with grounding is the term itself... One of the problems with grounding is the term itself... One of the problems with grounding is the term itself... One of the problems with grounding is the term itself... One of the problems with grounding is the term itself... One of the problems with grounding is the term itself... One of the problems with grounding is the term itself...
itititititititit’’’’’’’’s too vagues too vagues too vagues too vagues too vagues too vagues too vagues too vague
Too many termsToo many termsToo many termsToo many termsToo many termsToo many termsToo many termsToo many terms……………………; Insufficiently defined; Insufficiently defined; Insufficiently defined; Insufficiently defined; Insufficiently defined; Insufficiently defined; Insufficiently defined; Insufficiently defined……………………
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
19©Copyright 2009
Ideal Case Real Case
Therefore, in reality, Therefore, in reality, ““floatedfloated”” systems may systems may compromise safetycompromise safety
AC AC
L
B C
VI VC
R Xω= ≅
+
RB
C
/230 10 300 2 50 0.2L V pF m m HzI V C mAω π= ⋅ ⋅ ≅ ⋅ ⋅ ⋅ ⋅ =
Rational for GroundingRational for GroundingRational for GroundingRational for GroundingElectrical Shock HazardsElectrical Shock HazardsElectrical Shock HazardsElectrical Shock Hazards
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
20©Copyright 2009
• AC power distribution is governed by national codes
• One requirement: With each outgoing phase and neutral wire theremust be a safety ground
Rational for GroundingRational for GroundingRational for GroundingRational for GroundingElectrical Shock HazardsElectrical Shock HazardsElectrical Shock HazardsElectrical Shock Hazards
230V
Phase
0V
Neutral
to Return Ground
230V
Phase
0V
Neutral
to Safety GND
Equipment Enclosure Equipment EnclosureAccidental
Short
Accidental
Short
GND @
Service
EntryGND @
Service
Entry
No Safety Ground: HazardHazard Safety Ground Protection: SafeSafe
The safety ground shunts the fault currents to the power return,The safety ground shunts the fault currents to the power return, bypassing bypassing (and saving) the person(and saving) the person
230230
1,00075 !!!L
B
V VI m mAA
R≈ ≅ = >>
Ω
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
21©Copyright 2009
Conflicts between Safety and EMI Conflicts between Safety and EMI Conflicts between Safety and EMI Conflicts between Safety and EMI Control Grounding ConsiderationsControl Grounding ConsiderationsControl Grounding ConsiderationsControl Grounding Considerations
Conflicts due to EMI FiltersConflicts due to EMI FiltersConflicts due to EMI FiltersConflicts due to EMI Filters
• In case of broken Safety Ground connection, leakage current through the Filter’s capacitors to the case (CM Caps) will flow through the human body
• For safety purposes, the caps must be limited to about 1µµµµF, limiting the leakage current
Thus CM (line to earth) filtering using Caps is limited for IB ≤ 5mA
230V
Phase
0V
Neutral
Small current
through body
Equipment EnclosureBroken Safety
Ground
Connection
GND @
Service
Entry
CM Filter
Capacitors
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
22©Copyright 2009
Circuit #1 (Sensitive )
Circuit #2 (Noisy)
ZL2
ZL1
ES1
ES2
I2
I1
VL2
+VN2
VL1
+VN1
ZR
- EN +
I1+I
2Signal Reference "Ground"
So, with A So, with A So, with A So, with A ““““PracticalPracticalPracticalPractical”””” Ground...Ground...Ground...Ground...What Do WE Do???What Do WE Do???What Do WE Do???What Do WE Do???
Noise from circuit #2 (Noisy) may couple into Circuit #1 Noise from circuit #2 (Noisy) may couple into Circuit #1 (sensitive)(sensitive)
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
23©Copyright 2009
• Lower the impedance of the common return path
Reduces the ground voltage drop below the sensitivity levels of the
victim circuits (a.k.a. Bonding, not to be discussed in this presentation)
• Limit other currents I ≠≠≠≠ IX circulating in the return path used for circuit X
Isolating currents from difference circuits, reducing coupling between
currents flowing in the same path
• Design a noise tolerant system
Using differential circuits with high common mode rejection, for
instance
• The choice of each technique (or their combination) depends on feasibility, system/circuit size, cost, frequency and safety aspects
So, We Have A So, We Have A So, We Have A So, We Have A ““““PracticalPracticalPracticalPractical”””” Ground...Ground...Ground...Ground...What Do WE Do???What Do WE Do???What Do WE Do???What Do WE Do???
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
24©Copyright 2009
Basic Grounding TopologiesBasic Grounding TopologiesBasic Grounding TopologiesBasic Grounding Topologies
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
25©Copyright 2009
So, We Have A So, We Have A So, We Have A So, We Have A ““““PracticalPracticalPracticalPractical”””” Ground...Ground...Ground...Ground...There is another wayThere is another wayThere is another wayThere is another way…………
Circuit #1 (Sensitive )
Circuit #2 (Noisy)
ZL2
ZL1
ES1
ES2
I2
I1
VL1
+VN1
I1+I
2ENG
Signal Reference "Ground"
Vi
ZS2
ZS1
ZG
Circuit #1 (Sensitive )
Circuit #2 (Noisy)
ZL2
ZL1
ES1
ES2
I2
I1
Signal Reference "Ground"
ZS2
ZS1
ZG
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
26©Copyright 2009
It isIt is
O MAGIC!!!O MAGIC!!!
• Grounding within equipment is intended to realize:
Signal return
Power return
Electrical safety connection
• Grounding from the EMI standpoint is intended to:
Implementation of the above functions, with minimum common mode noise
Establishment of a path for diverting interference currents from susceptible circuits
• Therefore, the grounding system topology must be designed to ensure a well controlled current flow in the different paths
The objective: Decreasing ground currents flowing into “critical paths”
The technique:
Proper segregation of ground paths
Careful location of ground nodes
Elimination of “ground loops”
Optimizing Ground System DesignOptimizing Ground System DesignOptimizing Ground System DesignOptimizing Ground System DesignAvoiding A Common Impedance PathAvoiding A Common Impedance PathAvoiding A Common Impedance PathAvoiding A Common Impedance Path
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
27©Copyright 2009
Optimizing Ground System DesignOptimizing Ground System DesignOptimizing Ground System DesignOptimizing Ground System DesignGoals of Equipment and System Level Goals of Equipment and System Level Goals of Equipment and System Level Goals of Equipment and System Level
Grounding SystemGrounding SystemGrounding SystemGrounding System• The grounding scheme inside a system must accomplish the
following goals:
Analog, low level circuits must have extremely noiseless dedicated returns;
typically wires are used, dictating a single point, “star” grounding scheme
Analog, high frequency circuits (RF, video, etc.) must have low
impedance, noise free return circuits, generally in form of planes or their
extensions (e.g., coaxial cables)
Digital, logic circuit returns, especially high speed digital circuit returns, must have low impedance over the entire bandwidth (determined by the “edge rates” ), as power and signal returns share the same paths
Returns of powerful loads (e.g., solenoids, motors, relays, lamps, etc.) should be separated from all the above, even if they end up at the same power supply output terminal
For signals that communicate between parts of the equipment or system, the grounding scheme must provide a common reference with minimum ground shift (low common mode noise) between system parts
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
28©Copyright 2009
Ground System Topologies Ground System Topologies Ground System Topologies Ground System Topologies A A A A ““““FloatingFloatingFloatingFloating”””” SystemSystemSystemSystem
∆V
EIA RS-422
S
System #1 System #2 System #3
ESGC
)Safety Ground(
ESGC
(Safety Ground)
ES1 ES3
Signal Reference
Structure
Safety
Signal
Power
Reference
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
29©Copyright 2009
Ground System Topologies Ground System Topologies Ground System Topologies Ground System Topologies Single Point Ground (SPG)Single Point Ground (SPG)Single Point Ground (SPG)Single Point Ground (SPG)
““““Daisy ChainDaisy ChainDaisy ChainDaisy Chain”””” Single Point Ground (DCSingle Point Ground (DCSingle Point Ground (DCSingle Point Ground (DC----SPG)SPG)SPG)SPG)
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
30©Copyright 2009
A GREAT method, except for... its DISADVANTAGES!A GREAT method, except for... its DISADVANTAGES!
Most sensitive circuit
V I I I ZA= + + ⋅( )
1 2 3 1
V I I I Z I I Z I ZC = + + ⋅ + + ⋅ + ⋅( ) ( ) ( )1 2 3 1 2 3 2 3 3
Ground System Topologies Ground System Topologies Ground System Topologies Ground System Topologies Single Point Ground (SPG)Single Point Ground (SPG)Single Point Ground (SPG)Single Point Ground (SPG)
““““Daisy ChainDaisy ChainDaisy ChainDaisy Chain”””” Single Point Ground (DCSingle Point Ground (DCSingle Point Ground (DCSingle Point Ground (DC----SPG)SPG)SPG)SPG)
S
System #1 System #2 System #3
Signal Reference
Structure
Safety
Signal
Power
Ground BusZ
3Z
2Z
1
I1
I2
I3I
2+I
3I1+I
2+I3
I3
A B C
GRP
Signal
Source
ES1
ℓℓℓℓ
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
31©Copyright 2009
V
EIA RS -422
Ground System Topologies Ground System Topologies Ground System Topologies Ground System Topologies Single Point Ground (SPG)Single Point Ground (SPG)Single Point Ground (SPG)Single Point Ground (SPG)
Parallel (Parallel (Parallel (Parallel (““““StarStarStarStar””””) Single Point Ground (P) Single Point Ground (P) Single Point Ground (P) Single Point Ground (P----SPG)SPG)SPG)SPG)
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
32©Copyright 2009
• At higher frequencies, where the length of the ground conductors approaches λλλλ/4, the SPG is ineffective
Distance along GD Conductor
λ/4
ZS
0
This circuit should be grounded every This circuit should be grounded every λλλλλλλλ/20!/20!
Ground System Topologies Ground System Topologies Ground System Topologies Ground System Topologies Single Point Ground (SPG)Single Point Ground (SPG)Single Point Ground (SPG)Single Point Ground (SPG)
SSignal Reference
Structure
GRP="0V"
Vx
Ix
Vx=0
=0V
Ix=0
=Imax
x=λ/4λ/4λ/4λ/4
Vx=λλλλ/4
=Vmax
Ix=λλλλ/4
=0A
x
Vx
, Ix
inZ →∞x=λ/4λ/4λ/4λ/4
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
33©Copyright 2009
∆V
EIA RS-422
Ground System Topologies Ground System Topologies Ground System Topologies Ground System Topologies MultiMultiMultiMulti----Point Ground (MPG)Point Ground (MPG)Point Ground (MPG)Point Ground (MPG)
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
34©Copyright 2009
• When a system comprises of several types of circuits, a composite grounding topology may be used
Single point grounding, for low frequencies (d ≤ λ/20 MHz)
Multi-point grounding for high frequencies (d > λ/20 MHz)
Ground System Topologies Ground System Topologies Ground System Topologies Ground System Topologies Composite Grounding TopologyComposite Grounding TopologyComposite Grounding TopologyComposite Grounding Topology
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
35©Copyright 2009
The The The The ““““Grounding TreeGrounding TreeGrounding TreeGrounding Tree””””
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
36©Copyright 2009
EquipmentEquipmentEquipmentEquipment----Level Level Level Level ““““Ground TreeGround TreeGround TreeGround Tree””””Design ProcessDesign ProcessDesign ProcessDesign Process
• Identify circuits
• Define Chassis Ground connections at the circuit level (heat-sink and RF Ground)
• Define PCB-level signal returns (ground) requirements
• Identify isolation requirements
• Define local ground connections
• Define CGP/SPG location
• Connect GNDs from circuits and Power Supply to CGP
• Identify “special cases” (GND System Violations) and potential ground loops
• Incorporate “isolation measures” (transformers, optocouplers, balanced interfaces, e.g.RS-422 and Isolated Ground Connections)
• Define special power supply outputs and connect returns to the CGP when applicable; define special isolated outputs
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
37©Copyright 2009
Video
Processor
Main CPU
I/O Circuit
Aux. Rx
Active
Antenna
Tx/Rx
Power Supply
5VD
15VA
5/3.3VD
15VA
5VA
5VD
5VD/RF
15VA/RF
15VA/RF
28VA/RF
CGP
DC/DC Module
5VD
3.3VD
15VA
5VA
5VD/RF
15VA/RF
5VD/RF
15VA/RF
15VA/RF
28VA/RF
Tree
Diagram
3.3V/5VD 15VA
5VA
15VA/RF
5VD/RF
15VA/RF
28VA/RF
5VD
LOOP???
Hig
h C
MR
R lin
k
Aux. Rx
Tx/Rx
Main CPU
5VD
15VA
Video
Processor
15VA
5VDI/O Ckt.
EquipmentEquipmentEquipmentEquipment----Level Level Level Level ““““Ground Ground Ground Ground TreeTreeTreeTree”””” Design Design Design Design ProcessProcessProcessProcess
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
38©Copyright 2009
Understanding and PrecludingUnderstanding and PrecludingUnderstanding and PrecludingUnderstanding and Precluding““““Ground LoopsGround LoopsGround LoopsGround Loops””””
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
39©Copyright 2009
A model for illustrating the effect of grounding topology on system performance
CA
d= ⋅ε ε π0
91 36 10= × F m/C=Capacitance of PCB to Ground
““““Ground LoopsGround LoopsGround LoopsGround Loops””””SPG vs. MPGSPG vs. MPGSPG vs. MPGSPG vs. MPG
Circuit #1 Circuit #2
ICM#1
ICM#2
VSRS
=VCM
Transmission Line
VL
C d
A
C
A
d
VS
ZS
Z2
Z1
ZCM
R1
R2
ZL
h
S
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
40©Copyright 2009
““““Ground LoopsGround LoopsGround LoopsGround Loops””””SPG vs. MPGSPG vs. MPGSPG vs. MPGSPG vs. MPG
Longitudinal Conversion Loss factor, LCLLongitudinal Conversion Loss factor, LCL:
Constant
20'
CMdB
DMVo
VLCL Log
V=
= ⋅
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
41©Copyright 2009
““““Ground LoopsGround LoopsGround LoopsGround Loops””””SPG vs. MPGSPG vs. MPGSPG vs. MPGSPG vs. MPG
• At Low Frequencies
Capacitances, C, are dominant
Circuit impedance reduces with Frequency (f)
CM current increases with f
DM voltage increases with f
• At High Frequencies
Low Pass Characteristics of the transmission line are dominant
Circuit impedance increases with f
Termination impedance limits line currents
Both sides floated
Floated Both Ends
Frequency [Hz]
Lo
ad
DM
Vo
ltag
e
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
42©Copyright 2009
““““Ground LoopsGround LoopsGround LoopsGround Loops””””SPG vs. MPGSPG vs. MPGSPG vs. MPGSPG vs. MPG
• At Low Frequencies
Circuit series impedance, due to the capacitances, C, is reduced by a factor of 2 (6 dB)
CM current (and DM voltage) increases by 6 dB
• At High Frequencies
No change from previous case
One side grounded
Floated One End
Frequency [Hz]
Lo
ad
DM
Vo
ltag
e
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
43©Copyright 2009
““““Ground LoopsGround LoopsGround LoopsGround Loops””””SPG vs. MPGSPG vs. MPGSPG vs. MPGSPG vs. MPG
• At Low Frequencies
Circuit series impedance, is independent of capacitances, C
Circuit impedance determined by wiring & Load resistance (R)
CM current (and DM voltage) independent of f
• At High Frequencies
No change from previous cases
Both sides grounded
Grounded Both Ends
Frequency [Hz]
Lo
ad
DM
Vo
ltag
e
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
44©Copyright 2009
Ground System Topologies Ground System Topologies Ground System Topologies Ground System Topologies SPG vs. MPGSPG vs. MPGSPG vs. MPGSPG vs. MPG
• Low frequency circuits Single point grounding only
Floating provides marginal improvement and increased risk
Low frequency performance is strongly dependent on the circuit grounding
topology
Low frequency performance significantly degraded with multipoint
grounding
• High frequency circuits Multipoint grounding only
High frequency performance independent of grounding topology
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
45©Copyright 2009
““““Ground LoopsGround LoopsGround LoopsGround Loops””””Techniques for Opening Techniques for Opening Techniques for Opening Techniques for Opening ““““Ground LoopsGround LoopsGround LoopsGround Loops””””
Isolation TransformerIsolation TransformerIsolation TransformerIsolation Transformer
Common Mode decoupling Common Mode decoupling of 100of 100--140 dB can be 140 dB can be achieved @ f=1kHzachieved @ f=1kHz
SSignal Reference
Structure
EG
ZGS
ZGL
VN
, V
L
ZS Z
LB
ZG
ES
ZLA
CP
L1
L2
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
46©Copyright 2009
Common Mode decoupling Common Mode decoupling exceeding 80exceeding 80--100 dB can be 100 dB can be
achieved @ highachieved @ high--ff’’ss
““““Ground LoopsGround LoopsGround LoopsGround Loops””””Techniques for Opening Techniques for Opening Techniques for Opening Techniques for Opening ““““Ground LoopsGround LoopsGround LoopsGround Loops””””
BALUNsBALUNsBALUNsBALUNs (Common Mode Chokes)(Common Mode Chokes)(Common Mode Chokes)(Common Mode Chokes)
Alternative Symbols
SSignal Reference
Structure
EG
ZGS
ZGL
VN
, V
L
ZS Z
LB
ZG
ES
ZLA
CP
IS
ICM2
ICM1
L2
L1
M
CM
Current
Signal DM
Current
Core
Hi µ−
CM-Generated
Flux
DM-Generated
Flux
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
47©Copyright 2009
““““Ground LoopsGround LoopsGround LoopsGround Loops””””Techniques for Opening Techniques for Opening Techniques for Opening Techniques for Opening ““““Ground LoopsGround LoopsGround LoopsGround Loops””””
Optical Isolator/OptocouplerOptical Isolator/OptocouplerOptical Isolator/OptocouplerOptical Isolator/Optocoupler
Common Mode decoupling Common Mode decoupling of 60of 60--80 dB can be achieved80 dB can be achieved
SSignal Reference
Structure
EG
ZGS
ZGL
VN
, V
L
ZS Z
LB
ZG
ES
ZLA
CP
IS
IS
Light Emitting
Diode (LED)
phototransistor
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
48©Copyright 2009
““““Ground LoopsGround LoopsGround LoopsGround Loops””””Techniques for Opening Techniques for Opening Techniques for Opening Techniques for Opening ““““Ground LoopsGround LoopsGround LoopsGround Loops””””
Buffer AmplifierBuffer AmplifierBuffer AmplifierBuffer Amplifier
Common Mode decoupling of 60Common Mode decoupling of 60--80 dB (*) can be achieved80 dB (*) can be achieved
(*) 120 dB in special applications(*) 120 dB in special applications
SSignal Reference
Structure
EG
ZGS
ZGL
VN
, V
L
ZS Z
LB
ZG
ES
ZLA
IS
IS
Input Stage Output Stage
+VI
-VI
+VO
-VO
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
49©Copyright 2009
Common Mode decoupling Common Mode decoupling of 60of 60--80 dB can be achieved80 dB can be achieved
““““Ground LoopsGround LoopsGround LoopsGround Loops””””Techniques for Opening Techniques for Opening Techniques for Opening Techniques for Opening ““““Ground LoopsGround LoopsGround LoopsGround Loops””””
Circuit BypassingCircuit BypassingCircuit BypassingCircuit Bypassing
SSignal Reference
Structure
EGZ
GSZ
GL
VN
, V
L
ZS Z
LB
ZG
ES
ZLA
ICM2
ICM1 I
S
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
50©Copyright 2009
Grounding on PCBsGrounding on PCBsGrounding on PCBsGrounding on PCBs
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
51©Copyright 2009
Simultaneous Switching Noise (SSN) in Simultaneous Switching Noise (SSN) in Simultaneous Switching Noise (SSN) in Simultaneous Switching Noise (SSN) in Power Distribution SystemsPower Distribution SystemsPower Distribution SystemsPower Distribution Systems
Switching NoiseSwitching NoiseSwitching NoiseSwitching Noise
Time Pattern of Switching Current of a 74LSXXX NAND Gate
5 nSec/div
7 m
A/d
iv
~ 40 mA
1.5 mA Steady State
VCC
DGND
DC
DC
DC?
IIN
[A]
IIN
IRTN
VIN
IIN
VIN
0 200 400 600 800 1000 1200 1400 1600 1800-100
-90
-80
-70
-60
-50
-40
-30
Frequency (MHz)
Powe
r bus
spe
trum
(dBm
)
VA
Power Bus Spectrum [dBm] of a Clock Driver IDT74FCT807
• Do we actually distribute “DC” in the PCB Power Distribution System?
BWt r
≈⋅
1
π
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
52©Copyright 2009
Simultaneous Switching Noise (SSN) in Simultaneous Switching Noise (SSN) in Simultaneous Switching Noise (SSN) in Simultaneous Switching Noise (SSN) in Power Distribution SystemsPower Distribution SystemsPower Distribution SystemsPower Distribution Systems
Power/Ground Bounce Noise Generation ModelPower/Ground Bounce Noise Generation ModelPower/Ground Bounce Noise Generation ModelPower/Ground Bounce Noise Generation ModelGate Switching from "Lo" ("0") to "Hi"
("1")
Gate 1
Gate 2
GND1
GND2
VS
VN
VOut
VCC
GND
LGND
C
VC
IC
LGND(PS)
LVcc(PS)
Gate 1
Gate 2
GND1
GND2
VOut
VCC
LGND
C
VC
VS
VN
IC
GND
LGND(PS)
LVcc(PS)
Gate Switching from "Hi" ("1") to "Lo" ("0")
( ) ( ) ( ) ( )2
2
D C
G!D G!D G!D
t t
dI t d V tV t L L C
dt dt= ⋅ = ⋅
Q1
Q2
Q3
Q4
R4
R2
R3
R1
Inputs
VCC
VOut
Gate 1
VEE
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
53©Copyright 2009
Simultaneous Switching Noise (SSN) in Simultaneous Switching Noise (SSN) in Simultaneous Switching Noise (SSN) in Simultaneous Switching Noise (SSN) in Power Distribution SystemsPower Distribution SystemsPower Distribution SystemsPower Distribution Systems
Consequences of Ground Bounce on the PCB Consequences of Ground Bounce on the PCB Consequences of Ground Bounce on the PCB Consequences of Ground Bounce on the PCB PerformancePerformancePerformancePerformance
"Ground Bounce" Interference Propagation in a Circuit
Ground Rail
Inductance
1
2
4
( )DI t
( )G!DV tPower Source Return
(0V Ground)
( )CV t( )CV t
VCC
VCC
LGND
3
VCC
GND3
GND4
DC Power
Source
VCC
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
54©Copyright 2009
Power Supply Distribution Power Supply Distribution Power Supply Distribution Power Supply Distribution SSN in Power Distribution SystemsSSN in Power Distribution SystemsSSN in Power Distribution SystemsSSN in Power Distribution SystemsConsequences of SSN on the PCB PerformanceConsequences of SSN on the PCB PerformanceConsequences of SSN on the PCB PerformanceConsequences of SSN on the PCB Performance
• Ground/Power bounce is exacerbated by a composition of several factors:
Load Capacitance
High-Q of discharge path
Short Discharge Current Transition
Time
Circuit Total Inductance
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
55©Copyright 2009
• Ground/Power bounce is exacerbated by a composition of several factors:
Reduce Load Capacitance (reduce fan-out)
High-Q of discharge path (add damping)
Short Discharge Current Transition Time
- add damping
- slow transition times
Circuit Total Inductance
- Use of planes
- Keep planes close adjacent
- Allocate power/return pins
- Use SMD technology
- Decouple power and return paths
G!D Pins
VCC
Int (5V) Pins
VCC
IO (3.3 or 5V) Pins
Legend
192-Pin PGA
U
T
R
P
M
L
K
J
H
G
F
E
D
C
B
A
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
EPM 7256E
CSTRAY
RD
VCC
Discharge Current
LG!D
Power Supply Distribution Power Supply Distribution Power Supply Distribution Power Supply Distribution SSN in Power Distribution SystemsSSN in Power Distribution SystemsSSN in Power Distribution SystemsSSN in Power Distribution SystemsConsequences of SSN on the PCB PerformanceConsequences of SSN on the PCB PerformanceConsequences of SSN on the PCB PerformanceConsequences of SSN on the PCB Performance
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
56©Copyright 2009
Power Supply Distribution Power Supply Distribution Power Supply Distribution Power Supply Distribution Ground Rules for Providing a Stable, LowGround Rules for Providing a Stable, LowGround Rules for Providing a Stable, LowGround Rules for Providing a Stable, Low----Z Voltage Z Voltage Z Voltage Z Voltage
SourceSourceSourceSource• Rule #1: Use low impedance return connections
between gates
• Rule #2: The impedance between power pins on anytwo gates should be just as low as the impedance between ground pins
• Rule #3: A low impedance path must be provided between power and ground
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
57©Copyright 2009
• Exposed traces over a PCB with no ground plane
• Traces routed as microstrip
• Traces routed as stripline
Crosstalk Reduction on PCBsCrosstalk Reduction on PCBsCrosstalk Reduction on PCBsCrosstalk Reduction on PCBsProper Design of Current Return PathProper Design of Current Return PathProper Design of Current Return PathProper Design of Current Return Path
Magnetic Flux opposes Flux from signal traces (Flux Cancellation)
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
58©Copyright 2009
W
ht
W
h
t
Zh
w t wr
0
60 4
0 67 0 8[ ] ln
. .Ω = ⋅
⋅ ⋅ +
FHG
IKJε π b g
Centered Stripline
T npd r≈ ⋅0 034. ,ε S / cm
Microstrip
Zh
w tr
0
87
1 414
5 98
0 8[ ]
.ln
.
.Ω =
+⋅
+FHG
IKJε
T npd r≈ ⋅ ⋅ +0 034 0 475 0 67. . . ,ε S / cm
High Speed Return Signals High Speed Return Signals High Speed Return Signals High Speed Return Signals Typical Transmission Line Topologies in PCBsTypical Transmission Line Topologies in PCBsTypical Transmission Line Topologies in PCBsTypical Transmission Line Topologies in PCBs
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
59©Copyright 2009
High Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHow Will The Return Current FlowHow Will The Return Current FlowHow Will The Return Current FlowHow Will The Return Current Flow
• The current distribution balances to opposing forces: If the return current is concentrated immediately below the trace, it
would have a higher inductance
• A skinny conductor has a higher inductance than a wide conductor
If the return current is spread farther apart from the trace, the loop inductance will increase
• Loop inductance is proportional to the current path loop area
( )i D
I
H D HA m( ) / = ⋅
+0
2
1
1π
H
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
60©Copyright 2009
High Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHigh Speed Return Signals on PCBsThe Case of the The Case of the The Case of the The Case of the ““““Trace in the Slotted Ground PlaneTrace in the Slotted Ground PlaneTrace in the Slotted Ground PlaneTrace in the Slotted Ground Plane””””
• A “Trace in the ground plane” diverts the return currents
• Signal+Return loop area increases
• Loop inductance increases
• Crosstalk, Radiation increase
•• Do not route high speed signal traces Do not route high speed signal traces above gaps in ground planeabove gaps in ground plane
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
61©Copyright 2009
• 1kV ESD injected onto PCB with and without split
• Noise coupled into a test circuit was measured
High Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHow do the Signal Return Currents FlowHow do the Signal Return Currents FlowHow do the Signal Return Currents FlowHow do the Signal Return Currents Flow………… with slots in with slots in with slots in with slots in
the ground plane?the ground plane?the ground plane?the ground plane?
Source: “ESD and EMI Effects in Printed Wiring Boards”, by Douglas C. Smith
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
62©Copyright 2009
• Increased current loop size increased noise coupling
• Violation of the Path of Least Inductance
High Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHigh Speed Return Signals on PCBsHow do the Signal Return Currents FlowHow do the Signal Return Currents FlowHow do the Signal Return Currents FlowHow do the Signal Return Currents Flow………… with slots in with slots in with slots in with slots in
the ground plane?the ground plane?the ground plane?the ground plane?
Source: “ESD and EMI Effects in Printed Wiring Boards”, by Douglas C. Smith3.7V Pk
170mV Pk
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
63©Copyright 2009
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal Systems Signal Systems Signal Systems Signal Systems Grounding in Combined Analog & Digital CircuitGrounding in Combined Analog & Digital CircuitGrounding in Combined Analog & Digital CircuitGrounding in Combined Analog & Digital Circuit
AMP AMP
I/O I/O
Analog Digital
Microprocessor
Xtal
C
RAM
C
RAM
CB
uffe
r
C
Bu
ffer
C
RAM
C
I/O
C
I/O
C
ADC
DAC
DGNDAGND
DGNDAGND
???
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
64©Copyright 2009
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal SystemsSignal SystemsSignal SystemsSignal Systems• Designing a high speed mixed (analog/digital) signal system without using a
proper ground is like trying to play basketball on a huge trampoline
• The analog nature of our physical world and the growing need for digital signal processing ⇒ need to design circuits which process both analog and digital signals
• Stringent performance demands on mixed-signal devices e.g., ADCs DACs and fast DSPs:
Increase in resolution
Drop in the signal voltage scale
Devices have become extremely vulnerable to noise
• Many opinions on the best method for grounding of ADCs, DACs and other mixed-signal circuits
Both analog and digital returns should remain at the same potential, but;
• Most data sheets provide little if any useful information
• Usually applies only to simple configurations containing only one converter
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
65©Copyright 2009
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal SystemsSignal SystemsSignal SystemsSignal SystemsSensitivity of Analog CircuitsSensitivity of Analog CircuitsSensitivity of Analog CircuitsSensitivity of Analog Circuits
• Analog ground plane noise voltages should be kept below the minimum analog signal level of concern
Depends on the sensitivity of the
analog input signal
• In ADCs (or DACs) the minimum resolvable signal level, or least significant bit (LSB) sets the limit
For an ADC, the weight of an LSB
equals the full-scale voltage range of
the converter divided by 2N, where N
is the converter's resolution
For instance, in a 12-bit ADC with a
unipolar full-scale voltage of 2.5V,
1LSB = 2.5V/212 = 610µV
Resolution (LSB) @1V
Number of Bits
59 nV24
1 µµµµV20
15 µµµµV16
60 µµµµV14
240 µµµµV12
1 mV10
4 mV8
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
66©Copyright 2009
• Consider:
A 24-bit ADC with an LSB of 59nV
A digital processor (e.g., DSPs, ASICs etc.) drawing a current surge of 10A through the common ground plane during state transition
⇒ The necessary impedance required to maintain the stray return path voltage at less than 59nV with 10A of switching current would be 5.9nΩ
⇒ Even with only 16 bit converters a common impedance of less than 0.152µΩ is still required
Practically unattainable
⇒ With 8 bit industrial measurement ADCs, an impedance of 39µΩ would be acceptable and could be achieved
• Sensitive analog circuitry must be provided a quiet return path
Digital
CircuitAnalog
Circuit
G!D-REF
ID
IA
ID
ID+I
A
VAV
D
+ +
VIn
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal SystemsSignal SystemsSignal SystemsSignal Systems““““To Split or not to Split (the Ground Plane)?To Split or not to Split (the Ground Plane)?To Split or not to Split (the Ground Plane)?To Split or not to Split (the Ground Plane)?””””, , , , PourquoiPourquoiPourquoiPourquoi ????
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
67©Copyright 2009
• The PCB should consist of separate analog and digital power and ground planes
• These return paths meet at a single system common reference point
A "star" or single-point ground system
• The digital and the analog return currents forced to flow directly to the system common reference point in otherwise galvanically isolated paths
• Splitting the plane intended to prevent the digital currents from flowing in the analog section of the ground plane
• Current return paths must consist of large planes exhibiting low impedance to high frequency currents
Digital
CircuitAnalog
Circuit
G!D-REF
ID
IA
ID
IA
VAV
D
+ +
VIn
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal SystemsSignal SystemsSignal SystemsSignal Systems““““To Split or not to Split (the Ground Plane)?To Split or not to Split (the Ground Plane)?To Split or not to Split (the Ground Plane)?To Split or not to Split (the Ground Plane)?””””, , , , PourquoiPourquoiPourquoiPourquoi ????
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
68©Copyright 2009
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal SystemsSignal SystemsSignal SystemsSignal SystemsElectrical Current Always flows in the Path of Least Electrical Current Always flows in the Path of Least Electrical Current Always flows in the Path of Least Electrical Current Always flows in the Path of Least
ImpedanceImpedanceImpedanceImpedance
Frequency of 1 kHz (Simulation run on Agilent Technologies "Momentum" 3D Planar EM Simulator)
Frequency of 1 GHz
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
69©Copyright 2009
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal SystemsSignal SystemsSignal SystemsSignal SystemsElectrical Current Always flows in the Path of Least ImpedanceElectrical Current Always flows in the Path of Least ImpedanceElectrical Current Always flows in the Path of Least ImpedanceElectrical Current Always flows in the Path of Least Impedance
• High frequency digital return currents, if not obstructed, tend to flow in the ground plane immediately beneath the signal trace
The path of least impedance
• The current slightly spreads out in the plane, but otherwise remains under the trace
PCB Trace at Height h above the Reference Plane, Carrying Current I0
Reference Plane Carrying the Signal Return Current
+d-d
JGP(d)
JGP(d) for an infinite plane
JGP(d) for a 20 mm wide plane
0
0(0)GP
IJ
hπ≈
PCB Dielectric Substrate
PCB Trace at Height h above the Reference Plane, Carrying Current I0
Reference Plane Carrying the Signal Return Current
+d-d
JGP(d)
JGP(d) for an infinite plane
JGP(d) for a 20 mm wide plane
0
0(0)GP
IJ
hπ≈
PCB Dielectric SubstratePCB Dielectric Substrate( )0
2
1( ) ,
1GP
IJ d A m
h d hπ≈ ⋅
+
97%20
94%10
87%5
70%2
Fraction of Current Density [%]
d/h
•• Why is it necessary, then, to physically split the ground plane Why is it necessary, then, to physically split the ground plane at the first at the first place?place?
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
70©Copyright 2009
• But; with the 24-bit ADC and the 10A DSP
The LSB of this ADC is equivalent to 59nV
• Assume a practical plane impedance of 40µΩµΩµΩµΩ 59nV equivalent to 0.15mA, approximately
≈≈≈≈0.15% of the digital switching current!
• The separation, d, between the digital and the analog traces must be increased so that 99.98% of the digital return current is contained within that distance
• Such separations are impractical
D
H
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal SystemsSignal SystemsSignal SystemsSignal Systems““““To Split or not to Split (the Ground Plane)?To Split or not to Split (the Ground Plane)?To Split or not to Split (the Ground Plane)?To Split or not to Split (the Ground Plane)?””””, My Reply, My Reply, My Reply, My Reply
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
71©Copyright 2009
• Split planes pose potential EMC problems
No traces can be routed across the split in the plane
No immediate return path available near the trace
Current must flow through a large loop ⇒ Increased EMI
Differential or galvanically-isolated interfaces required
• Or, interconnect the two ground planes at one point (a.k.a. "drawbridge") and route all the traces only above the bridge
An immediate return path is provided directly underneath each of the
traces
ID
Analog Return Plane
Digital Return PlaneG!D-REF
Analog Return Plane
Digital Return PlaneG!D-REF
Drawbridge
Moat
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal SystemsSignal SystemsSignal SystemsSignal Systems““““To Split or not to Split (the Ground Plane)?To Split or not to Split (the Ground Plane)?To Split or not to Split (the Ground Plane)?To Split or not to Split (the Ground Plane)?””””, My Reply, My Reply, My Reply, My Reply
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
72©Copyright 2009
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal SystemsSignal SystemsSignal SystemsSignal SystemsThe Mystery of A/D and D/A ConvertersThe Mystery of A/D and D/A ConvertersThe Mystery of A/D and D/A ConvertersThe Mystery of A/D and D/A Converters
• ADCs and DACs should be treated as analog devices and be grounded to the analog ground plane
Device return net often split internally into isolated analog and digital return nets
Stray capacitance exists between the nets
Problem resolved with dedicated AGND reference
AGND and DGND pins should be joined together with minimum lead lengths
Even if application notes suggest that AGND and DGND be connected separately, it is generally better to ignore this guidanceT
• Separate power supplies for analog and digital circuits are also highly desirable (with appropriate decoupling)
• Place a buffer latch adjacent to the converter to isolate converter's digital lines from any noise on the data bus
Grounded and decoupled to DGND
Use even if included internal to the Converter
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
73©Copyright 2009
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal SystemsSignal SystemsSignal SystemsSignal SystemsGrounding Scheme for a Single ADC/DAC on a Single PCBGrounding Scheme for a Single ADC/DAC on a Single PCBGrounding Scheme for a Single ADC/DAC on a Single PCBGrounding Scheme for a Single ADC/DAC on a Single PCB
Low Res (8-10 bits) : Solid Ground plane, common to
Analog and Digital Circuitry
High Res ADC: Split Ground plane between to Analog and Digital
Circuitry with “Drawbridge”
Solid Return Plane
Analog Zone Digital Zone
Digital
Circuits
Digital
Supply
Analog
Supply
DA
D DAAA
ADC
VDDV
AA
AG!D DG!D
Analog
Circuits
Latch/
Buffer
Digital
Circuits
Digital
Supply
Analog
Supply
DA
D DAAA
ADC
VDDV
AA
AG!D DG!D
Analog
Circuits
Latch/
Buffer
Analog Return Plane Digital Return Plane
Circuit
"Star Ground"
Bridge
Gap
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
74©Copyright 2009
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal SystemsSignal SystemsSignal SystemsSignal SystemsGrounding Scheme for a Single ADC/DAC on a Single PCBGrounding Scheme for a Single ADC/DAC on a Single PCBGrounding Scheme for a Single ADC/DAC on a Single PCBGrounding Scheme for a Single ADC/DAC on a Single PCB
• Typical width of the gap in the ground plane is 2 to 3 mm (or 80 to 120 mils) for practical PCB constructions (e.g., 1-oz copper and FR-4 dielectric)
• Narrow “drawbridge” between AGND and DGND
A relatively high impedance to HF digital return currents
A relatively low impedance to LF analog return currents
AGD DGD
ADC
AGD DGD
Digital
Interfaces
Analog
Interfaces
"Mickey Mouse" scheme:• “Dead End” for unintentional
currents in each “ear“
• No circulating currents
Digital
Circuits
Digital
Supply
Analog
Supply
DA
D DAAA
ADC
VDDV
AA
AG!D DG!D
Analog
Circuits
Latch/
Buffer
Analog Return Plane Digital Return Plane
Circuit
"Star Ground"
Bridge
Gap
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
75©Copyright 2009
Single-ended (chassis-referenced) Analog Driver Connection violates “Mickey Mouse” Scheme
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal SystemsSignal SystemsSignal SystemsSignal SystemsGrounding Scheme for a Single ADC/DAC on a Single PCBGrounding Scheme for a Single ADC/DAC on a Single PCBGrounding Scheme for a Single ADC/DAC on a Single PCBGrounding Scheme for a Single ADC/DAC on a Single PCB
• Single-ended (chassis-referenced) analog I/O complicate the situation:
AGND and DGND connected on the PCB beneath the
converter
AGND must also be connected to the chassis (single-
endedT)
• Stray noise currents flow through the ground loop
• Isolation achieved by the AGND-DGND gap defeated/violated
No longer constitute a "dead end“
• Balanced inputs (e.g., transformer) eliminate this problem
AGD DGD
ADC
AGD DGD
Digital
Interfaces
Analog
Interfaces
Chassis-Conection
at Analog Driver
Stray Noise Current
between the Two
Chassis ConnectionsDigital
Circuits
D DAAA
ADC
AG!DDG!D
Analog
Circuits
Latch/
Buffer
Analog Return Plane Digital Return Plane
Chassis-Referenced
Analog Driver
Chassis Connection of
the PCB at the Circuit
"Star Ground"
Stray Noise Current
between the Two
Chassis Connections
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
76©Copyright 2009
• Further complexity
Multiple return leads must all be somehow and somewhere tied together
AGNDs & DGNDs tied together under each converter ⇒⇒⇒⇒ numerous connections between
the two
Analog Zone
Digital ZoneDigital
Device
Analog Analog Analog
Digital
Device
Functional Partition
(No Gap in the Return Plane)
ADC ADC ADC
AGD Planelet 2
DGD Planelet
Digital
Device
Analog Analog Analog
Digital
Device
Partitioning Gap
in the Return Plane
ADC ADCADC
AGD Planelet 3AGD Planelet 1
A Properly Partitioned PCB Ground Plane with Multiple ADCs Acceptable Isolation
for Low-Res. (8-bit) Converters
A Properly Partitioned PCB Ground Plane with Multiple ADCs
Higher Noise Isolation for Moderate-Res. (10 to 12-bit) Converters
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal SystemsSignal SystemsSignal SystemsSignal SystemsGrounding Scheme for Multiple ADC/DACs on a Single PCBGrounding Scheme for Multiple ADC/DACs on a Single PCBGrounding Scheme for Multiple ADC/DACs on a Single PCBGrounding Scheme for Multiple ADC/DACs on a Single PCB
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
77©Copyright 2009
• In higher resolution systems (12 bits or more), stray digital currents are a serious concern
With a 5V reference level, the LSB in a 24-bit ADC is equivalent to 30nV, approx.
• Three lines of attack: Minimize level of the
interfering signal (current, waveform control)
Interrupt the interference pathway with gaps
• Single-ended chassis-referenced analog input signals result multiple "ground loops“
– Chassis connections
– PCB connections
• Differential and balanced analog I/Os solves this situation
Bypass GNDs through low-impedance shunts to a massive solid-metal sheet immediately underneath the PCB
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal SystemsSignal SystemsSignal SystemsSignal SystemsGrounding Scheme for Multiple ADC/DACs on a Single PCBGrounding Scheme for Multiple ADC/DACs on a Single PCBGrounding Scheme for Multiple ADC/DACs on a Single PCBGrounding Scheme for Multiple ADC/DACs on a Single PCB
Solid and Massive Metal
Plane underneath the PCB
Solid "Stitches" to the Shunt Metal Plane
(Recommended in HF and Digital
Circuits)
Analog
Connections
Digital
Connections
AGND Planelet 2
DGND Planelet
Digital
Device
Analog Analog Analog
Digital
Device
Partitioning Gap
in the Return Plane
ADC ADCADC
AGND Planelet 3AGND Planelet 1
Optional "Selective Stitches" to the Shunt
Metal Plane
(Recommended in Lower-Frequency
Analog Circuits)
Capacitors or 0 Ohm
Resistors
(Package 0402 or less)Optional Pairs of Pads for Bridging
the Gap in the Return Plane
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
78©Copyright 2009
• When high confidence does not exist provide bridging options to interconnect AGND and DGND
• Multiple mounting pads at both sides of the gap, providing the means for "stitching" the AGND and DGND together through short jumpers or 0ΩΩΩΩresistors
Virtually transforming the circuit back into a continuous (even if not solid) plane
• The mounting pads should be closely spaced (approximately 1 to 1.5 cm apart)
• The size of the jumpers should be kept to the absolute minimum ⇒⇒⇒⇒ minimize inductance
• Under no condition may any signal trace, particularly high-speed digital (but also analog), cross the gap in any layer, except over the drawbridge
Grounding in MixedGrounding in MixedGrounding in MixedGrounding in Mixed----Signal SystemsSignal SystemsSignal SystemsSignal SystemsGrounding Scheme for Multiple ADC/DACs on a Single PCBGrounding Scheme for Multiple ADC/DACs on a Single PCBGrounding Scheme for Multiple ADC/DACs on a Single PCBGrounding Scheme for Multiple ADC/DACs on a Single PCB
DGD Planelet
Analog
AGD Planelet 1
80 to 120 m
il
0 Ohm Resistor
(Package 0402 or smaller)
Mounting Pads protrude
into gap
ADC
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
79©Copyright 2009
Grounding Grounding Grounding Grounding in EMC Engineeringin EMC Engineeringin EMC Engineeringin EMC EngineeringSummarySummarySummarySummary
• “Grounding” is probably among the most important, yet more confusing aspect of electrical/electronic system design, often considered as "black magic“
• Grounding forms an inseparable part of all electronic and electrical designs, from circuit through system up to installation design
• Grounding is r5equired, primarily, for safety and for current return; It is NOT intended for EMI control, but if overlooked, may result in severe EMI
• Grounding is not magic and does make sense!!! It is founded on fundamental scientific theories of good good olol’’ Mike (Faraday) Mike (Faraday) and (J.C.) Maxand (J.C.) Max88
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
80©Copyright 2009
QUESTIONSQUESTIONSQUESTIONSQUESTIONS
Fundamentals of Grounding, from Circuit to System: South Africa Visit, Nov. 2009
81©Copyright 2009
Thank You for your Attention!!!Thank You for your Attention!!!Thank You for your Attention!!!Thank You for your Attention!!!