Low EMI Isolation for Medical Equipment Applications
Low EMI Isolation for Medical Equipment Applications
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 seemingly endless combination of disturbance characteristics.
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 V to 5 mVpk with 3 dB corner frequencies at 0.05 and 100 Hz. Looking forward, the trend towards 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. Obviously, the use of EMI-hardened semiconductor components should be used in critical signal paths. This is especially true for EMI issues that exist within the system itself, such as in 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 60101-1-2. 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 very stringent. The system cannot exhibit any component failures, parametric changes, configuration errors or false positives. In addition to external field immunity, the system under test cannot generate significant radiated or conducted emissions of its own.
Specifications published by various agencies place limits on conducted and radiated EMI. One of the more common specifications is FCC Part 15, which covers circuit assemblies used in or near the home. Testing to this specification is conducted in an open-air environment using a 10 meter antenna positioned approximately 5 meters above the ground plane. Another specification, SAEJ1752-3, is more IC-centric in its test methodology and recommends mounting the IC to be tested on a small shielded circuit board 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 voltage domains and/or to mitigate ground noise in highly sensitive circuit areas. Medical electronic systems often use transformers and/or optocouplers for signal isolation, neither of which are optimal. Transformers generate EMI and are highly susceptible to signal corruption by external magnetic fields. Optocouplers offer the benefits of low EMI emission and high immunity but suffer from poor reliability and low common-mode transient immunity (CMTI), the latter of which can negatively impact isolator data transmission integrity. As an alternative to transformers and optocouplers, silicon isolators leverage advanced process technologies to dramatically improve EMI characteristics and create significant gains in performance and reliability. These isolators fabricate insulating devices directly on the semiconductor die using process oxides or other native process materials.
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