The power supply EMC power-line filter provides the main protection to these transients. The CM line ground capacitors are normally the main devices used to shunt the transient currents to chassis and away from the internal circuitry. When this capacitance is limited by leakage current limits, the CM choke must provide more protection. This usually demands a special center tap CM choke, where the center tap is shunted to chassis via a capacitor that complies with the leakage current limits. It is important to use full ground and power planes, even for analog circuits, to limit high-frequency CM loops.
Most transient threats are high frequency and cause significant radiated energy to be present.
Often this energy comes in on analog leads, recouples to high-speed digital circuits, and causes them to respond adversely. By filtering all analog and digital leads, proper protection can be achieved. Post them in our Members' Discussion Forums .