8 – Electromagnetic Interference

Publisher Summary This chapter discusses electromagnetic interference (EMI), which is broadly divided into two types: interference and susceptibility. It does not focus on interference generated by equipment, except where that interference is self-generated, but concentrates on susceptibility—what causes it and what can be done about it. The term EMI usually conjures images of high-frequency signals interfering with normal circuit operation. However, errors also can be caused by simple AC and DC circuits. The microprocessor ground is connected through its power supply back to the building ground. The sensor system is also connected to the building ground on a different circuit. When the AC load is operating, the voltage at the sensor circuit ground is different from the voltage at the microprocessor circuit ground. The result is a ground-loop current flowing through the ground in the cable connecting the sensor circuit to the microprocessor circuit. The connection to the pulse-width modulation (PWM) driver includes the control signals and the ground return. The problem with this approach is that there will be a current spike every time the PWM driver turns on. Motor grounding and noise issues often arise because the system aspects of grounding were ignored. In a large piece of equipment with subsystems designed by different teams of people, the grounding is often not consistent.