Skyworks: medical application note

EMI in the medical environment

Electromagnetic interference (EMI) is defined as any electromagnetic disturbance that interrupts, obstructs or otherwise degrades or limits the effective performance of electronic equipment. Unfortunately, EMI sources are plentiful and give rise to a seemingly endless combination of disturbance characteristics. For this reason, the industry categorizes different types of EMI by their characteristics, as summarized in Table 1.

Medical environments are electrically noisy; RF interference (RFI) generated by communications devices and local equipment can produce RF fields of 50 V/m or more. In addition, certain types of medical equipment use RF energy for diagnosis or treatment (e.g. MRI systems) or wireless communication (e.g. medical telemetry systems). Given these numerous and potent sources, EMI management in medical environments can be challenging.

EMI impact in medical applications

EMI can cause medical devices to malfunction with potentially catastrophic results. For example, errant signals induced by EMI can cause portable life support systems to malfunction, corrupt measurements in patient monitoring equipment and change patient intravenous medicine dosage levels. EMI is especially problematic in medical systems that acquire low-amplitude signals, such as electrocardiographs (ECGs), where signals collected from patients can range from 400 μV to 5 mVpk with 3 dB corner frequencies at 0.05 and 100 Hz. Looking forward, the trend toward higher-frequency, lower-power medical systems will complicate EMI management by emitting broader bandwidth RF noise at higher energy levels.

From a design point-of-view, EMI effects can be minimized by designing system circuitry for high EMI immunity and low emission. Traditional practices include proper printed circuit board (PCB) layout and grounding and limited trace lengths. Electronic components must be optimally placed on a PCB, and the system enclosure design, cable shielding and filtering must be adequate. The use of EMI-hardened semiconductor components (low-emission and high-immunity) is especially important in critical signal paths, including mixed-signal or wireless data transmission applications.

Isolation in medical systems

To ensure that medical electronic systems are immune to disturbances from localized fields and other phenomena, isolators are safety tested to a number of IEC 61000 standards using test limits specified by IEC 60601-1-2 as shown in Table 2. For example, electrostatic discharge (ESD) is tested to IEC 61000-4-2 and uses the test limits specified by IEC 60601-1-2. RF emissions and power line perturbations are tested using methods from CISPR11 test methodology, a subset of automotive specification J1750. (CISPR does not specify test limits; it is a test methodology standard only. Limits for emissions and power line sensitivities are specified in IEC 60601-1-2). The criteria for passing these tests are stringent: the system cannot exhibit component failures, parametric changes, configuration errors or false positives, and it cannot generate significant radiated or conducted emissions of its own.

Specifications published by various agencies place limits on conducted and radiated EMI. One common specification is FCC Part 15, covering circuit assemblies used in or near the home. Testing is conducted in an open-air environment using a 10 meter antenna positioned approximately 5 meters above the ground plane. Another specification, SAE J1752-3, is more IC-centric and recommends mounting the IC on a small shielded circuit board (a “TEM cell”, per CISPR11 test methodology) designed to measure only the radiated emissions from the isolator itself while operating within the actual application environment.

EMI-hardened silicon isolators

Many medical systems incorporate galvanic isolation to protect patients and equipment from hazardous voltages, to level shift signals between ground domains and/or to mitigate ground noise in highly sensitive circuit areas. Medical systems often use transformers and/or optocouplers for signal isolation, neither of which are optimal. Transformers generate EMI and are susceptible to corruption by external magnetic fields. Optocouplers offer low EMI emission and high immunity but suffer from poorer reliability and low common-mode transient immunity (CMTI), which can negatively impact data transmission integrity. Silicon isolators leverage advanced process technologies to improve EMI characteristics and gains in performance and reliability by fabricating insulating devices directly on the semiconductor die. Many implementations use either transformers or capacitors for the isolation barrier.

The silicon isolator of Figure 2A operates by encoding the logic state of each incoming digital edge, transmitting this data through isolation transformer T1, then decoding and storing the data in an output latch. Figure 2B shows the radiated EMI response, measured using CISPR methodology. The measurement was made with all isolator inputs low and a 90° rotation. The device generates resonant peaks as high as +20 dB between 100 and 300 MHz, assumed to be at least partially caused by T1’s structural size, inductance and Q.

The silicon isolator in Figure 3A operates by transmitting a carrier wave across a differential capacitive isolation barrier when the input is logic-high. The receiver asserts logic-high on the output when sufficient carrier energy is detected. Unlike the transformer design, there is no Q-dependent resonant peak to boost incoming EMI frequencies. Figure 3B shows a flatter, lower-amplitude radiated EMI response than the transformer implementation. As a result, this device passed FCC Class B Part 15 in a test using 6-channel differential capacitive isolators with all inputs tied high to maximize internal transmitter emissions.

Figure 4 shows an electric field immunity comparison between transformer- and capacitor-based silicon isolators. Outputs are monitored while the external RF field frequency is swept from DC to 10 GHz. Both isolators have grounded inputs to hold outputs low. The capacitive isolator remains low across DC to 10 GHz, whereas the transformer-based output is corrupted between 1 and 2 GHz. The capacitive isolator demonstrates high EMI immunity due to differential signal rejection and receiver selectivity.

Figure 5 shows magnetic field immunity. To meet IEC 61000-4-9, the isolator must operate normally while subjected to the flux density versus frequency curve (purple line). Values at or above the line are acceptable, while values below are failures. Both silicon isolators meet the criteria, but the capacitive isolator shows a higher degree of magnetic field immunity.

Summary

EMI can degrade the effective performance of electronic equipment and cause medical devices to malfunction with potentially catastrophic results. Galvanic isolation is required in many medical systems, and it is important that this isolation have high EMI immunity and not create emissions of its own. Silicon isolators offer many advantages over optocouplers and transformers. Transformer-based silicon isolators typically have lower immunity and higher emissions than capacitor-based isolators. Capacitive silicon isolators are therefore well-suited for EMI-hardened medical electronics.

References

UL 1577
IEC 60601-1, “General Requirements for Basic Safety and Essential Performance”, International Electrotechnical Commission.
IEC 60601-1-2, “International Standard for Medical Equipment”, International Electrotechnical Commission.

Related documents

Silicon Labs “Si86xx Digital Isolator Data Sheet”
Silicon Labs “CMOS Advanced Galvanic Isolators for Medical Electronics”
Silicon Labs “Competitive Analysis: Silicon Labs Digital Isolators vs. ADI and TI”