Electromagnetic interference (EMI) refers to unwanted effects in a medical device due to electromagnetic energy present in the environment. EMI can prevent proper operation of a medical device. For example, certain electric wheelchairs and scooters would suddenly release their brakes and move unexpectedly. The FDA guidance letter based on a two-year investigation identified interference from radio signals as one of the culprits.
EMI paths are conductive (e.g. cables penetrating the device enclosure), radiated (e.g. through plastic enclosures and holes in metal enclosures), and near-field capacitive and inductive (e.g. proximity to large power lines). EMC compliance means interfering signals are prevented from entering or leaving your product.
Sometimes it takes multiple attempts to get a product through electromagnetic compatibility (EMC) testing. Here are five steps you can take to increase the likelihood of passing on your first or second iteration:
- Avoid unpredictability by designing devices to meet a specified level of EMC. A product meeting EMC standards will exhibit immunity – the ability to resist external EMI, and have low emissions –not create its own EMI and interfere with other products.
- Training and experience improve efforts at EMC compliance, both in the design and the testing phases. There are hundreds of references on the topic; many introductory treatments are found on semiconductor company websites. Two good examples are Analog Devices’ RFI and Shielding Concepts and National Semiconductor’s Understanding and Eliminating EMI in Microcontroller Applications.
- Design Processes like Design reviews for circuit boards can improve results. The circuit board review involves checking for good ground planes, controlled-impedance traces, power supply bypassing, absence of stub traces and minimizing the area of high-current loops.
- Conduct small assessments early in the design cycle to catch problems when they are easier and less expensive to fix. A typical pre-scan will look only at the device’s emissions, since usually a device that fails emissions also has poor immunity. Because you can’t see EMI, you need to measure it with instruments for an objective assessment of the design. Pre-scans usually take a day and identify areas of the spectrum that are over the limits. A complete EMC test to the IEC-61000 family of standards can take several days or longer. Don’t wait until the product design is complete before performing EMC testing. Deficiencies might be uncovered that require extensive redesign.
- Make temporary changes to your prototype, then re-test to check their effectiveness. Typical changes include: putting the device into different operational modes; depowering hardware sections or depopulating parts; re-routing internal cabling; and closing gaps with copper tape. Conduct multiple iterations of this measure – change – re-measure cycle are done to ensure one change does not invalidate another.
Follow these 5 Steps and you’ll likely launch a successful, EMC-compliant product on time. If you have additional tips or want to discuss EMC testing further, leave a comment below.