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What Is Ground?
• Earth Ground• Power Ground• Signal ground• Safety Ground• Return• Reference Level
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Ground Means any Reference Conductor Used for a Common Return
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Grounding Misuse and Myths
• Reasons for Grounding Are Not Clear
• The Word "Grounding" is Often Misusedwhen other Words are Meant, such as:
Connect to BondingReturn Path Earthing
• Many Myths Exist, such as:
"Lower Impedance is Always Better""Use Grounds for Digital-Circuit Reference""Use Separate Safety, Instrument & System Gnd.
• Reasons for Grounding Are Not Clear
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Grounding for EMC
• Ground Circuits, Equipments, Systems, Cables, Shields
• Common Mode and Differential Mode Coupling
• Avoid Ground Loops• More Grounds are not Better
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Interconnected Equipment Having 29 Questions = 229 = 500,000,000 Answers
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Effects of Shared Ground Impedance
• Common Source or Common Ground Impedance Coupling
• EMI and signal use same impedance• Shared impedance provides path for
EMI to couple from a source to a victim.• Minimizing the shared ground
impedane will help mitigate the problem.
• A single point ground may help.
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Power Source LoadCMC
CMC
EMI'
IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII IIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIIII
Metallic Structure
Figure 10. Common Ground Impedence Common Mode EMI
EMI
Common Ground Impedance Common Mode EMI
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Power Source Load
Figure 4. Illustration of Common Mode Currents
CMC 1
CMC 2
CMC
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Metallic Structure
Illustration of Common Mode Currents
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Power Source Load
DCM1
DCM2
Figure 3. Illustration of Differential Mode Currents
Illustration of Differential Mode Currents
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Power Source LoadCMC 1
CMC 2
CMC
Metallic Structure
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Figure 5. Illustration of Common and Differential Mode Currents
DMC 1
DMC 2
Illustration of Common and Differential Mode CurrentsIllustration of Common and Differential Mode Currents
Illustration of Common and Differential Mode Currents
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PRINCIPAL RADIATION SOURCES ON PRINTED CIRCUIT BOARD
Logic familiesclock rates
• Large single-layer board• PCB card cage with back plane• Multi-layer board
Radiation from ribbon cables
Radiation from IC dips
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Power Source Load
Radiated EMI
CMC 1
CMC 2
CMC
Metallic Structure
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Figure 11. Common Mode Radiated EMI
Common Mode Radiated EMI
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Ground-Loop CouplingConverts Common Mode Voltage
to Differential-Mode Voltage
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Using Ferrites to Absorb Common-Mode EMI
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Added Feed-Thru Capacitors -Help Reduce ESD and RF Susceptibility
Note: Max Cap Value
Must Support
Data Bandwidth
C < 1/ωR
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Coupling Rejection Offered by Twisting Wire Pair
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SHIELDING APPLIES TO ALL LEVELS
• Components
• Circuits
• Functional Stages
• Equipments
• Systems
• Cables
• Platforms
• Buildings
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CONCEPTUAL ILLUSTRATION OF FIELD INTENSITIES VS. SOURCE TYPE AND DISTANCE
High Current Corresponds toLow Impedance
Low Current Corresponds toHigh Impedance
High E Low ELoop
High HLow H
NearField
FarField
NearField
FarField
Hθ
Eθ
Hθ
Eθ
V
Monopole
V
I
Low - Impedance,Magnetic - Field Source and Wave
High-Impedance SourceElectric-Field Source and Wave
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FIELD IMPEDANCE AS A FUNCTION OF DISTANCE FROM SOURCE
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ELECTRIC FIELD VS SOURCE DISTANCE
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SUMMARY
Electric Fields
Z > 377 OhmsRadiated From High Impedance Sources
Magnetic Fields
Z < 377 OhmsRadiated from Low Impedance Sources
Near Field Far FieldPlane Waves
Are Generated for all
Source Impedances for
Distance Greater Than
Approximately
1/6 of a Wavelength
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REPRESENTATION OF SHIELDING PHENOMENAFOR PLANE WAVES
Outside World
Inside of Enclosure
Incident Wave
AttenuatedIncident
WaveInternal Reflecting
WaveMetalBarrier
ReflectedWave
Transmitted WaveA
B
Ey
Ey
HzEy
Ey
EyHz Hz
Hz
H
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SHIELDING EFFECTIVENESS (SE)
SEdB = 20 log10(Eoutside/Einside)SEdB = 20 log10(Houtside/Hinside)
where: E = Electric-field StrengthH = Magnetic-field Strength
SEdB = RdB + AdBwhere:
RdB = Reflection Loss in dBAdB = Absorption Loss in dB
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REFLECTION LOSS
σ ε ωfor σ
μ ω j ε ω j σ
μ ω j impedancebarrier Z
HE impedance wave Z
:Where
10 K ,Z 4
Z log 20
VSWR ZZ K,
4K1) K ( log 20 R
b
w
b
w10
b
W2
10dB
<<
=+
==
==
≥
≅
=+
=
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REFLECTION LOSS (RdB) OF PLANE WAVES VS FREQUENCY
Copper
Iron*
Hypernick*
0
50
100
150
200
0
50
100
150
200
1kHz 10kHz 100kHz 1MHz 10MHz 100MHz
3kHz 30kHz 300kHz 3MHz 30MHz 300MHz
Radio Frequency
Valid for thickness > 3 δδ = Skin Depth * Permeability assumed constant with frequency
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ABSORPTION LOSS, A
Current Density
t
mm σμ f
0.066 δrr MHz
=
copper to relative tyconductivi σcopper to relative typermeabili μ
MHz in frequencyfmm in thickness t where
σμft 131 δ t / 8.68 AdB
r
r
MHz
rr MHz
===
=
==
δ
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ABSORPTION LOSS VS FREQUENCY
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PRINCIPAL BOX SHIELDING COMPROMISES
Connectors
Fuse
Switch
PotentiometerPanel Meter
StatusIndicator
Lamp
Cover Platefor Access
Holes or Slotsfor Convection Cooling
Screw Spacing= Slot Radiation
Forced AirCooling
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SLOT AND APERTURE LEAKAGE
L
ht
t << h Shield Material
Log Frequency
•
•λ / 2
SE (d
B)
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Reducing Radiation Coupling by Shielding Cable Wires
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