Digital Circuit Boards: Mach 1 GHz

5.17 Electrostatic discharge

Electrostatic discharge (ESD) is a source of interference that must be addressed in every design. ESD is a potent pulse of electromagnetic energy that is characterized as being 5 A with a rise time of 1 ns. This pulse of current creates a field that can destroy nearby components. The rise-time frequency is about 300 MHz. The near-field/far-field interface distance is about 16 cm. Near the pulse, the wave impedance is low and the wave energy is dominated by the magnetic field. Circuit damage is most likely when a field couples to a loop area that involves an input gate.

In most logic structures, the loop areas are very small, so it is difficult to couple any significant voltage into a surface-mounted IC. The larger loop areas are apt to involve decoupling capacitors, connectors, or other component connections. Note that a coupled voltage of 0.3 V is all that is needed to cause a clamping diode to conduct, and this could result in a logic error or even component damage. If the current pulse were to flow in a ground trace then common impedance coupling could add voltages that would destroy a component.

The first line of defense is to keep the pulse from using conductors in the circuit board. If the pulse can hit a cable, then the cable must be shielded. If the pulse can follow a circuit conductor then some sort of diode protection is needed to divert the current so that it does not enter a signal path. As an example, keyboards should have a conducting ...