2 Sources of EMI and radiated emissions

EMC is the ability of an electric system to function properly in its intended environment in the presence of EMI, and to not be a source of interference to that electromagnetic environment beyond the limits as specified in the relevant standard [1].

EMI can be either radiated or conducted. Radiated interference travels in the form of radio waves, and is also called RF interference. Conducted interference comes from the magnetic field generated by current flow in cables carrying signals and power.

The focus of this article is on minimizing radiated emissions. On a printed circuit board (PCB) or inside an integrated circuit (IC) mounted on that PCB, some of the primary sources of radiated emissions include:

• Switching signals such as clocking signals, with rapid changes in voltage levels during digital signal transitions. This occurs because of the high-frequency components in the signals. Switching and clocking signals are essential for synchronizing the operation of various components within and between ICs.
• Switching regulators and other components, which cause rapid changes in current draw through power-supply lines.
• Input/output buffers, especially those associated with high-speed interfaces such as USB, HDMI or Ethernet, because of the high-speed signal transitions they handle.
• Harmonics created by nonlinear behavior in the IC’s internal circuits at frequencies higher than the fundamental signals.
• Parasitic capacitance, inductance and resistance in the IC’s interconnects and structures.
• Electrostatic discharge (ESD) events that trigger ESD protection circuits.

Figure 2 illustrates TI’s AMC131M03 galvanically isolated analog-to-digital converter (ADC) [8] and the predominant sources of radiated emissions resulting from its internal architecture and connections on the PCB. The ADC is used in a three-phase energy metering application, and Figure 2 shows the circuitry for one phase (phase A). The signal chain is designed to extract voltage and current measurements for energy monitoring [8]. ADC channel 0 measures the phase current using a shunt resistor, and channel 1 measures the phase voltage through a resistive divider [8]. The most relevant contributor to emissions is the internal switching DC/DC converter (a in Figure 1) that generates the isolated power supply on the high-voltage side [8]. The second-highest source of radiated emissions is the digital isolation (b in Figure 2), as it is implemented using high-frequency on/off keying transmission through a stacked capacitor barrier [8], [9]. Furthermore, clock signals emit radiation in a wide frequency range, such as the ADC modulator clock CLKIN (c in Figure 2) as well as the digital communication interface between the ADC and the microcontroller (d in Figure 2).