How EMI interferes with circuits through dielectrics

Electromagnetic interference (EMI) is a part of our lives. Over time, the generation of intentional and unintentional EMI radiation sources can wreak havoc on circuits. The signals from these radiation sources do not necessarily pollute the circuit, but the goal is to keep low-noise systems away from these hazards.

We can imagine a doctor using an ECG device to accurately diagnose the heart. Knowing that this is a high-precision measurement device, we will not worry about unwanted noise appearing in the diagnosis. This is a low-frequency measurement, and electronic devices will not exceed 1MHz. However, if an ECG device with poor EMI design is used, and the doctor is on a cell phone during the examination, then there is reason to worry about the diagnosis. See Figure 1.

Figure 1. Cardiac examination results of an ECG diagnostic device with the transmitter (f = 470 MHz, P = 0.5W) turned on and off at a distance of 1.5 feet.

In Figure 1, the system's cardiac input signal is approximately 0.25 mVp-p. This small signal requires an instrumentation amplifier gain of approximately 6000 V/V. Fortunately, the situation shown in Figure 1 does not represent the actual performance of medical ECG measurement equipment. This measurement was actually performed in an engineer's lab using the board shown in Figure 2.

Figure 2: Front view of the precision low-level ECG heart rate meter circuit board

Don't fall into this EMI trap. Build your board carefully and use components that are EMI-resistant, regardless of the bandwidth of the analog or digital circuit. When there is an EMI source near an application circuit, the radiation source may or may not affect it.

How does the radiated noise from the phone get into the measurement results (see Figure 1) when using this low-frequency board? Let's review and examine the entire EMI graph. When it comes to EMI, there are three factors at play: the radiating source, the coupling path through which the radiated signal propagates, and the radiating receptor. The radiating source in this case is obvious. However, the source of EMI signals can be propagated wirelessly through the air, or conducted through the PCB, and the radiated source is unknown.

EMI (also known as radio frequency interference, RFI) surrounds the receptor through direct conduction or various field propagation. These fields are directly coupled into circuit connections and PCB traces, converting into conducted RFI.

Three things are needed to create a force between two charges: electricity, magnetism, and electromagnetic fields (radiation). The electric field (volts/distance) describes the force created by an uneven distribution of charge between two physical points. To balance this charge distribution, a force is created between the charges.

Moving electric charges or electric currents form magnetic fields, which exert a force on all other electric charges around them. This field (or force) decreases rapidly with distance. Note that the electric and magnetic fields are related, and if one changes, the other changes as well.

Finally, the acceleration of electrons (or electric charges) creates an electromagnetic field. This electromagnetic field is the most common cause of EMI propagation.

Is there a way to solve this problem? Next time, we will discuss the characteristics of some of the radiation sources that cause EMI problems and introduce some tips to teach you how to minimize this radiation. Stay tuned.

References
• Hall, Kuehl, “EMI Rejection Ratio of Operational Amplifiers”, TI Application Report (SBOA128), August 2011.
• Wagt, Staveren, “Introduction to EMI-Resistant Operational Amplifier Specifications”, TI Application Note (SNOA497A), January 15, 2010