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Importance Of Grounding Techniques
Case Shield 22%
Cable Shielding 18%
PCB Layout 6%
• Categories of solutions applied for compliance.
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• Grounding concepts take more time to understand than any other EMI concept because,
• Ground systems have diverse requirements and sometimes they appear conflicting.
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Can we connect signal ground to chassis?
Should you connect the cable shield to chassis?
You must avoid a Ground Loop.
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What Is Grounding?
• Connecting all grounds in the system in a manner such that all of the objectives are met.
• What is ground?
• Ground wire
• Zero volts
• Ground plane
• Signal ground
• Chassis ground
• Conductive paint
• A trace on PCB connecting chassis
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Two Approaches to Limit the Noise
Stop the noise escaping the system
Stop at the circuit
• Often you use combination of the two.
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Noise Generation In a Digital System
• In a digital system noise is generated by circuits.
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• Faraday cage is the quiet RF reference.
• The chassis can be the Faraday cage if-
It encloses the electronics.
It is several times thicker than the skin depth.
No conductor violates the Faraday cage.
Large openings are avoided.
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Printed Wiring Board
• A direct low impedance connection to chassis is important.
• A low cost approach is to use the mother board mounting pads.
• Capacitive connection can be made to the signal ground at several points if single point ground is to be maintained.
• The effective capacitor leads must be short.
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Connection To Chassis
Card edge guides
Connector bonded to system enclosure
• Another economical way to make chassis connection is by means of connector body- such as D connectors.
• DIN connectors are available with shield and spring contacts for easy and reliable contact to the chassis.
• Power line filter body should also be used for chassis connection.
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Wave Propagation Through A Shield
= Incident field strength
= Reflection loss at B
= 20 log (E
• A = Absorption loss
= 20 log (E
= Reflection loss at B
= 20 log (E
• Total shielding effectiveness
Eo = Incident wave
• = E
3 x A x R
2 x C m
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• Absorption loss occurs due to induced currents
• The field decays with distance (d) traveled
• The decay is exponential, and is dependent on skin depth δ
• Skin depth depends on
• µ= Permeability
• ω= Angular frequency of the wave
• δ= 2/(µωσ)
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• System performance: system must perform reliably.
• Safety of personnel: minimize electrical shock hazard.
• AF noise emissions and susceptibility.
• RF noise emissions and susceptibility.
• ESD immunity.
• Generally, the noise emission and noise susceptibility approaches are similar.
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Grounding For Personnel Safety
• The main concern is that the metal enclosure remain at “safe” potentials.
• So it should be connected to the green wire ground of the power cord by “reliable” means.
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Grounding For Safety
• Safety ground wire is connected to the chassis.
• EMI filter capacitors are limited by leakage current
-UL 1950 or IEC 950 - 3.5 mA
-Medical devices - in micro-amps.
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--- Based on Purpose
• General - Equipotential reference surface.
• EMC - Low effective impedance path for the return.
• ESD - Surface that can source or sink large amount of charge without changing its potential.
• Safety - Conductor providing a path for currents to flow during a circuit fault.
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Ground Design Objectives For EMC
• Minimize Cross- talk.
• Minimize Emissions.
• Minimize Susceptibility.
• One must consider signal characteristics as well as allowable noise levels when designing a grounding scheme.
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Ground System Considerations
• There are four important circuit characteristics to be considered during the design of ground system:
Frequency of signal: Digital signal is broadband.
Effective Impedance of path: not the resistance.
Current Amplitude: The voltage drop is proportional to the signal current.
Noise voltage threshold: The noise level that a circuit can withstand or generate.
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Avoid a Ground Loop
• If a ground connects point A to B, it should not have an alternate path.
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= Induced noise current
= I n
X R s
• Definition : A ground circuit allowing ground currents to flow in a loop causing two problems.
Induced noise voltage: magnetic coupling causes induced current resulting noise voltage.
The return current may take a path further away from the signal current and create a radiating loop.
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Low Frequency Grounds
-Separated According to Circuit Noise Levels
• Chassis ground normally carries no current.
• This arrangement avoids ground loops.
• Noise coupling by conduction is avoided.
• Chassis is connected to power ground for safety.
• It carries current only in fault condition.
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Typical Single Point Grounding
-for Low Frequency
• This grounding is inadequate for RF signals between the boards.
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Ground Systems For Signal Currents
• Single point ground
• Series or Parallel ground connection
• Multi- Point Ground
• When signal spectrum contains high frequency energy.
• Multi- Point AC Ground
• When low frequency and high frequency is present.
• These ground systems are selected based on the frequency of signal and noise.
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Series Ground Connection
• Question :
• when do you connect ground in this manner?
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Parallel Ground Connection
• Q: When do you connect ground in your system in this manner?
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Multi-point Ground Connection
Circuit 1 Circuit 2 Circuit 3 Circuit 4
Low impedance ground reference
• Definition: circuits are connected to a reference ground plane at several different points by low impedance connections.
• The low impedance, single reference ground replaces the SPG, when we add a ground plane on the PWB.
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Problem With SPG
It ignores signal connections!
• With the SPG, the signal circuit has magnetic loop coupling:
• These are formed by signal conductors and all ground paths returning through SPG. The coupling increases with frequency.
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• Provide ground paths close to the signal connections.
• This parallel path can be: (a) Twisted conductor with each signal
(b) coaxial cable shield or (c) a conductor in the ribbon cable.
• Should you worry about the ground loop? Not for RF designs.
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What Is The Return Current Path?
Multi-layer board with ground plane
• Return current takes path of the lowest resistance.
• Return current is distributed inversely proportional to the resistance of each path.
• Return current takes path of the lowest impedance.
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Return Current Division
• Current is divided : (1) shield and (2) ground plane.
Equivalent Circuit- Assuming ground resistance = 0
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Single Point Or RF Grounding
• Grounding scheme is chosen according to requirements. The RF and AF requirements are not contradictory.
• When low (audio) frequency and high (RF) frequency protection is required, use multi- point AC ground with only one DC connection.
• Separate grounds according to signal levels - since induced noise can affect signal only if ground loop is part of the signal circuit.
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Ground and Signal Go Together
• Keep ground with the signal when connecting different circuits.
• Ground is the return path for the signal and power current.
• This rule is very important - when we are breaking ground loop.
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• Distributed parameters, and characteristic Impedance.
• Reflections can be controlled by controlling the impedance.
• The transmission lines used in practice are not ideal.
For example, the distributed parameters include resistors attenuating the propagating signal.
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Layer Stacks For Four Layer PCB
• Would it help to put the ground layers on the outside surface?
• How useful are high frequency signals embedded into the ground and power planes?
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Large Loops In Signal Return
• Even with a ground plane in the PWB, a large loop in the signal path can exist.
• A return pin far away from signal pin will cause a loop.
• Large loops in signal return paths can be avoided by using distributed grounds.
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Layout Near Board Edge
Field lines Signal traces
• Fringing near edge changes the characteristic impedance of the signal.
• This can result in ringing and additional radiation for high frequency signals.
• The advantages of the ground plane may be lost completely, if traces are laid outside the ground plane boundary.
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Six Layer Board
Layer 1 = signal
Layer 3 = signal
Layer 4 = signal
Layer 6 = signal
• The ground layer is two and power plane is five.
• The distance between signal layers and the reference planes should be maintained constant, say X.
• The distance between layer three and four > 3X.
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• Chassis ground is important for RF.
• Consider Signal loop more important than ground loop – look at ground as return path.
• Transmission line is your goal when you add ground and power planes on PCB.
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