Grounding systems I
Ground systems have diverse requirements and sometimes they appear conflicting.
Grounding concepts take more time to understand than any other EMI concept.
Ground system design takes effort and time.
However, once fully understood, a correctly implemented grounding scheme can help reduce noise and lower product cost and make the product more reliable.
• System performance: system must perform reliably.
• Safety of personnel: minimize electrical shock hazard.
• Audio frequency noise emission reduction.
• Radio frequency noise emission reduction.
• Noise susceptibility audio and RF.
• Communication security protect information.
• Generally, the noise emission and noise susceptibility approaches are similar.
Grounding for safety
Safety considerations are different than those for EMC. For safety purposes, the most important requirement is that the metal enclosure be reliably connected to the green wire ground of the power cord, which keeps it at safe voltage potentials
• This is to prevent electrical shock due to
1 Insulation breakdown in the system or
2 utility transformer breakdown because of line surge or lightning strike, etc.
Ground Design Objectives For EMC
• Minimize the noise voltages and current from different circuits coupling into each other. (Cross talk)
• Minimize electromagnetic energy escaping the system as radiated or conducted noise. (Emissions)
• Minimize electromagnetic energy entering the system as radiated or conducted noise. (Immunity)
One must consider signal characteristics as well as allowable noise levels when designing a grounding scheme.
Ground System Considerations
There are four important circuit characteristics to be considered during the design of ground system:
1 Frequency of signal: The signal and noise frequency constitutes an important factor in ground design. A digital signal has many harmonics.
2 Impedance of path: The total effective impedance of the path must be considered, not the resistance.
3 Current Amplitude: The potential drop developed across the signal path is directly proportional to the amplitude of the signal current.
4 Noise voltage threshold: The noise level that a circuit can withstand or generate.
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