The sheer volume of electronic devices in use today, coupled with the constant reduction in the size of these devices, makes electromagnetic interference (EMI) a major problem for circuit designers. Circuits used for communications, computations and automation need to operate in close proximity [1]. Products must also comply with government electromagnetic compatibility (EMC) regulations. Virtually every country regulates the EMC of electronic products marketed or sold within its borders. In the United States, the Federal Communications Commission (FCC) regulates all commercial (nonmilitary) sources of electromagnetic radiation [2] and defines the radiated and conducted EMI test procedures in standards such as Standard C63.4 [3] from the American National Standards Institute (ANSI). Countries in the European Union (EU) regulate both electromagnetic emissions and the immunity of electronic devices; the Electromagnetic Compatibility Directive [4] basically states that equipment must comply with harmonized standards on EMC and be tested and labeled accordingly.

There are a large number of EMC standards pertaining to various types of equipment. For example, International Electrotechnical Commission (IEC) 61000 standards cover immunity requirements for most commercial products, while theComité International Spécial des Perturbations Radioélectriques (CISPR) 32 standard specifies limits on conducted and radiated emissions [5]. Table 1 lists CISPR, European Norm and FCC standards for the relevant product sector. Many other countries outside the U.S. and EU either specify compliance with FCC or EU EMC requirements or have their own requirements. Regulations in countries outside of the U.S. and Europe often resemble the FCC or EU requirements [6].

The need for low EMI becomes even more obvious when considering a specific type of equipment, for example in smart metering. Smart electricity meters are a significant part of the future of energy distribution. They provide real time data on usage to both utilities and end users, helping people monitor energy usage and eliminating meter reading visits. The majority of smart meters connect via wireless communications [7], such as Wireless M-Bus or ZigBee, or they connect to the cellular phone network (GSM, LTE cat NB1- NB2, 2G/3G/5G). As illustrated in Figure 1, a smart electricity meter contains a radio-frequency (RF) transmitter circuit, usually in the same housing with the energy-metering (metrology) circuit board. It is important to minimize radiated emissions from the metrology circuit in order to not disturb RF communication, which can operate at frequencies such as 800MHz, 900MHz, 1,800MHz, 2,100MHz or 2,700MHz. The metrology circuit also needs to be resistant in terms of electromagnetic susceptibility (the ability to withstand electromagnetic energy from wireless communication) in order to avoid billing errors from the injection of RF noise into the sensitive energy-measurement front end.

This article explains the sources of EMI – specifically radiated emissions – and present some techniques to minimize EMI for an analog signal chain, including detailed layout examples and measurement results.

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).