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					Grounding and Shielding
     (Continued)
       Presented by:
         NJ Putter
       A Groenewald
         2008 – 09 – 08
                    Topics to be discussed
   1.      Filtering
   2.      Shielding
   3.      Protection Against Electrostatic Discharge
   4.      General Rules for Design
   5.      Case Study

[1] Kim R Fowler, Electronic Instrument Design Architecting for the life cycle, Oxford
University Press


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                    1.Filtering
•       Frequency
•       Mode (common or differential)
•       Amplitude (surge suppression)




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            Frequency-selective filters
Low-pass filter




                          ω = 1/RC
                          f = 1/2пRC


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High-pass filter




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Decoupling and Bypass Capacitors

• Filtering based on frequency
• DC power of ICs
• Reduce impedance of power supply


            Z0 ≈ 0Ω = √ R2 + *ωL -(1/ωC)+2




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            Mode filters
Common-mode




            Line Filter


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Differential-mode




             Line Filter


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            Amplitude-selective filters
• Transient protection
• Spikes
      a)    Filter
      b)    Crowbar (thyristor)
      c)    Arching discharge
      d)    Voltage clamp (Zener)




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                           2. Shielding
• Prevents noise energy coupling
      • Magnetic flux – inductive shielding
      • Electric field – capacitive shielding
      • Electromagnetic wave propagation –
            electromagnetic shielding




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                Inductive Shielding
• Self inductance
• Mutual inductance
• Reduce or reroute magnetic flux
• Magnetic noise depends on loop area and current in
  the emitting and receiving circuits
• Minimize loop area, separate circuits, reduce current
  change
• Coaxial cable minimal loop
• Twisting signal and return conductors

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            Capacitive Shielding
• Reduce or reroute      • Reduce :
  electrical charge         • Noise voltage and
• Capacitive coupling         frequency
  provides a path for       • Signal impedance
  the injection of noise    • Floating metal
  charges                     surfaces

                          • Faraday shield
                          • Shielded transformer


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            Electromagnetic Shielding
• Reduce emissions       • Electromagnetic
                           interference (EMI)
• Emission sources:      • Begins conductive
   • Lightning           • Becomes radioactive
   • Discharges          • Ends conductive
   • Radio and TV        • Reduce EMI
     transmitters          – Reduced bandwidth
   • High-frequency        – Good layout
     circuits              – Shielded enclosures

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            Electromagnetic Shielding (2)
Shielded enclosure         Cable shield




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            3. Electrostatic discharge
• High voltages low current
• Integrated circuits is susceptible if not
  protected
• ESD stages: pickup, storage, discharge




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Protecting Against Electrostatic
           Discharge
Preventing Damage
Checklist for an ESD-safe work area
•   Wear a wrist ground strap
•   Discharge static before handling devices
•   Keep parts in original container
•   Minimize handling of components
•   Pickup devices by their bodies
•   Use antistatic containers for storage
•   Clear all plastic, vinyl, Styrofoam from work
    area
       General Rules of Design
• System Characterization
• Standards
• Procedure
       System Characterization
• Establish
• Possible coupling mechanism
• Topology of circuit paths
Standards
                Procedure
1) Reduce Frequency bandwidth
2) Balance currents
3) Route signals for self shielding
4) Add shielding only when necessary
                  Case Study
•   Enclosure Design
•   Enclosure Testing
•   Optimum Module Design
•   Circuit Suppression Design
•   Printed Circuit board
•   Power supply filtering
                   Summary
•   Grounding and shielding are very important
•   Define possible emission sources
•   Simple schematic of ground structure
•   Consider at the beginning of design
•   Know standards and regulations
Questions ?

				
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