Managing EMI in Class D audio applications

EMI is an unwanted disturbance caused in an electrical circuit by electromagnetic radiation emitted from an external source. The disturbance may interrupt, obstruct, or otherwise degrade the effective performance of the circuit.
In today's portable and consumer applications, space has become a premium, and engineers are often required to eliminate enclosures and shielding, and suppress EMI and noise through other means such as better segregation at the circuit level. Smaller space and higher functionality require high density PCBs, and the use of wafer-scale packaging with tiny PCB design rules makes EMI more of a concern.

EMI encompasses two aspects. Emissions refer to the scope to which equipment generates radiated noise. Susceptibility is the scope to which equipment is affected by emissions generated from other electromagnetic waves. The degree to which a designer controls unintended emissions may make the task of susceptibility easier. Emissions are generally classified as radiated and conducted emissions. Radiated emissions leave a circuit board, trace, or wire, and propagate through the air in the form of electromagnetic waves to interfere with a nearby receiver. It is important to note that a "receiver" refers to any circuit whose operation can be affected adversely by the reception of electromagnetic energy—such as a PCB trace or even the lead of an IC. Conducted emissions refer to energy which escapes, or is conducted, out of a circuit through wires or cables. Conducted emissions may cause problems directly or manifest themselves as radiated emissions.