Electromagnetic Interference and considerations

Sources of EMI:

1. Conducted EMI
2. Radiated EMI

Mediums of EMI:

1. Wires - conducted EMI
2. Air/ Space - Radiated EMI

EMI is usually due to 

• Near by RF transmitter for High frequency noise
• Switch Mode Power Supply (SMPS) for medium frequency switching noise
• Low frequency noise like from high voltage AC lines operating at 50Hz or so...

EMI noise is received into the circuit mainly because of 

• Long tracks
• Hanging wires

Compoenents like OPAMP may have the parameter EMIRR

• Electromagnetic Interference Rejection Ratio
• Better the EMIRR better is the EMI noise immunity of the compoenent
• There are ESD protection diode inside the OPAMP

FCC limits

Circuit board layout tips:

1. Keep all connectors on one side
2. Don't keep high speed circuitary between connectors
3. Poor decoupling
4. Short traces for 
5. Separate the low frequency (Analog signals) and high frequency (Digital signals) grounds i.e. place inductor or ferrite beads to connect grounds of low and high frequency plane

Follow link is a very good example of EMC

https://www.youtube.com/watch?v=g4YaSOKRklg&index=11&list=PL8h307Ml6hQqwqzYxiGntCiBVa-mcxdOO

Points to consider when designing a circuit board:

1. Design of ground plane
• Separation of analog and digital planes
• But care should be  taken between separting ground planes, because when the nearby connector  (analog and digital) have separate ground planes, this will introduce small potential difference between then, which cause connector cables emitting radiation
• Coaxial cable have one signal and one ground casing.
• Hence differential signals i.e. no need of ground plane is the best option to avoid high frequency noise on the ground plane
• So, selecting OPAMP with differential input is necessary when the input signal is very sensitive to noise from ground plane
• As C = Ae/d; A = common area of planes, d = distance between planes; so more widely spread power and ground planes, and closer the planes is the best for increasing the inherent capacitance between power and ground
• Read concept of "common impedance coupling"
2. Location of high speed circuitry
• Metal enclosure of product is always better than a plastic enclosure, as metal shorts out the potential difference between the various connectors.
• Eventually metal casing reduces the radiation to other circuitry, and increase the chances of clearing EMC requirements
• 
  
3. Analog signal current return path
• Dedicated analog return(ground) path for low frequency (analog) signal, and connecting it to ground plane at one point will solve the problem
• Read topic "Identifying current path"
4. Routing of clock traces
• Long distance clock traces are likely to emit EMI to other circuit. So minimize as far as possible
• Routing clock at the edge of board is a bad idea as it increases coupling of noise of other nearby circuits
5. Decoupling capacitors
• Decoupling capacitors should be close (local decoupling) to VDD of IC when the power and ground plane are separated by distance more than 1mm (Far placed)
• Decoupling capacitors can be placed away from the ICs when the power and ground plane are placed close as 0.25mm
• If power plane is farthest from the components, then decoupling capacitors should be placed close to VDD
• If ground plane is farthest from the components, then decoupling capacitors should be placed close to GND
6. Power Integrity
• This means adequate decoupling
7. Noise immunity
• Radiating emission structures on board are usually of lengths greater than 1/10th of the wavelength of signal they carry
8. Immunity to ESD and fast transients
• Filter io and power lines