TABLE OF CONTENTS Because the microprocessor is a state machine, processor-based circuits are prone to corruption by fast transients which can cause the execution of false states. Great care is necessary to prevent any clocked circuit (not just microprocessor-based) from being susceptible to incoming interference. Analogue signals are more affected by continuous interference, which is rectified by non-linear circuit elements and causes bias or signal level shifts. The immunity of analogue circuits is improved by minimizing amplifier bandwidth, maximizing the signal level, using balanced configurations and electrically isolating I/O that will be connected to dirty external circuits.
Most of the critical interference in microprocessor circuits is ground-borne, whether it is common mode RF or transients, and the damage is done by conversion of common mode ground noise to differential mode noise at susceptible signal nodes. This occurs because of a high transfer impedance from common to differential mode due to poor PCB layout. Differential mode interference will not propagate far into the circuit from the external interfaces. Therefore, firstly you should concentrate on the best PCB tracking to minimize mode conversion that is, appropriate use of a ground plane; and secondly, lay out the mechanical design of the product to keep ground interference currents away from the logic circuits. Filter the I/O leads or isolate them, to define a preferential safe current path for interference. Radiated RF fields that generate differential mode voltages internally are dealt with in the same way...
