An MD&DI October 1998 Column
EMI FIELD NOTES
Addressing EMC issues early in the design process ensures the availability of a wide array of options at various prices compared with the limited selection that remains if this task is neglected until the last minute.
Although the medical device industry's awareness of electromagnetic interference and compatibility (EMI/EMC) issues has been increasing in recent years, many designers still wait until the late stages of the product development process to test their designs for EMC. Unfortunately, problems uncovered at that point are often difficult and expensive to fix. A much better approach is to consider EMC right from the start, by incorporating protection against EMI into a new product early in the design process. The earlier a designer addresses these issues, the more options are available and the less it costs to modify the design. For example, careful circuit board design can minimize or even eliminate the need for shielding in embedded control designs.
This article provides some guidelines on conducting an early-stage design review that focuses on three EMC issues: EMI threats, design constraints, and design strategies. A good time to perform such a review is after the initial circuit board layouts have been designed, so changes can be incorporated on subsequent revisions. It is also helpful to quickly revisit the issues whenever changes are made to the boards or enclosure, to be sure that EMC design features have not been violated.
The first issue to be addressed in an EMC design review is the device's susceptibility or immunity to such threats as electrostatic discharge (ESD), radio-frequency interference (RFI) from nearby transmitters and other sources, and power disturbances, all of which have the potential to harm sensitive components and cause device malfunctions. A secondary concern, but also an important EMI threat, is the device's own emissions. Both conducted and radiated interference that originates in electronic equipment can jam licensed communications sources, such as radios.
Several regulatory documents, including the European Union directive on EMC and FDA reviewer guidances, identify immunity tests that should be performed to ensure that products operate safely. The goal of these regulations is to set meaningful test limits that are representative of the anticipated electronic environment, which for a medical device can range from homes and physicians' offices to hospital operating rooms and intensive-care units and even land and air ambulances. When a device passes the appropriate tests, there is a high confidence that it will operate normally in the field. Sometimes, however, the standard test limits may not be adequate. For example, equipment to be used in ambulances may be subjected to higher than normal RFI levels from onboard VHF or UHF transmitters. At the same time, their emission levels may need to be lower than usual to prevent interference with sensitive communications receivers. In addition, any equipment operating from vehicle power sources is subject to a wide range of vehicular power disturbances. Thus, equipment that will be used in ambulances should be tested to the EMI levels specified in automotive or avionics regulations as well as to those applicable to medical devices.
After the potential EMI threats have been assessed, the next step in the EMC review process is to determine design constraints. These can include general project goals, such as cost, time to market, size and weight, and production volume, as well as issues unique to medical devices. Those systems that are patient connected and/or lifesaving or life-supporting have special, more-stringent EMC requirements.
The project's general constraints usually suggest the basic design approach. For example, with a high-volume, low-cost product, the designers can take a cost-aggressive approach, while a low-volume, high-cost device will call for a conservative design approach, which may mean overdesigning for EMC. In either case, it's important to take a hard look at the numbers. It's not cost-effective to try to save $2 per circuit board on an anticipated volume of 10,000 units ($20,000) and then spend more than $30,000 in extra EMC testing to optimize the design.
Patient-connected circuits complicate EMC design because the leakage current limits of such systems severely restrict common-mode capacitance. In addition, patient-connected monitoring circuits usually operate at low signal levels. This combination of factors often results in severe RFI immunity problems that are very difficult to resolve. Filtering techniques that work well on industrial controls (or non-patient-connected medical equipment) can't be used because of the leakage current limitations, so such alternative techniques as internal shielding or special circuit features may be necessary.
Life-supporting devices such as patient monitors and drug infusion pumps also require a conservative design approach, regardless of cost or volume. In the event of an EMI-induced malfunction, such devices must go to a safe condition and/or sound appropriate alarms to notify caregivers of a potential loss of function. In other words, they must be designed to be fail-safe.
Many of the design features used for EMC hardening are inexpensive (or even free) to implement when they are designed in early in the product development process. A few well-placed decoupling capacitors or ferrite beads can work wonders. The paragraphs that follow describe several areas to consider during an EMC design review and offer suggestions for designing in protection against EMI.
Circuits. Since ultimately all EMC problems begin and end at a circuit, these components are a good place to start this phase of a design review. There are four types of circuits that deserve special attention: clocks, reset circuits, low-level analog circuits, and voltage regulators.
Clocks are major sources of emissions because of their harmonics. Filtering may be necessary to slow down clock edges, so it is a good idea to provide pads for series ferrites or resistors and shunt capacitors early in the design process. Since many emissions are caused by power pulses on circuit power, designers should ensure that each clocked device has high-frequency decoupling capacitors located next to the integrated circuits.
Reset circuits are very vulnerable to spike-type interference, such as ESD and electrical fast transient (EFT) pulses. Thus, it's desirable to position high-frequency capacitors (1000 pF typical) at all inputs to these circuits. This same precautionary measure should be taken for nonmaskable interrupts, which also can be adversely affected by spikes.
Both low-level analog circuits and voltage regulators are vulnerable to RFI. The primary failure mode in these cases is rectification, so the best design strategy is to prevent such interference from reaching these critical circuits. The inputs and outputs should be protected by ferrite beads and shunt capacitors (1001000 pF) that provide RF filtering. Designers need to remember that even circuits that operate at low frequencies can be affected by high-frequency interference.
Circuit Boards. The next step in an EMC design review is to examine circuit board construction and layout. Key issues include component placement, the use of multiple layers, and separation of analog and digital circuits.
Circuit board components need to be grouped by both function and speed. During the EMC review, designers should look for high-speed circuits (and traces) that inadvertently have been located next to low-speed circuits or input/output (I/O) circuits. It is helpful to highlight critical traces such as clocks or reset circuits on a schematic and then determine their locations on the circuit boards. Warning signs of potential EMI problems are clock or reset traces near I/O ports and clock or reset traces running next to other traces for several inches or more.
From an EMC standpoint, the use of multilayer circuit boards is well worth their higher cost. It's not unusual to see emission reductions or immunity improvements of one or two orders of magnitude when a two-layer board is replaced by a multilayer board. Even with multilayer boards, however, protection against EMI can be compromised by poor EMC design practices. Designers should look for discontinuities, such as cuts for "extra" traces or missing metal around connector areas. Both can result in significant decreases in multilayer board performance.
Many medical devices have both analog and digital circuitry, and since analog circuits are sensitive to digital noise, using separate board areas for such circuits is a common design strategy. When implementing such separation, designers need to pay attention to the analog/digital connections and make sure that the respective circuit traces remain in the appropriate part of the board. The latter concern is particularly critical when multilayer boards are usedthe traces should be kept over their own planes. In addition, the analog and digital power and ground planes should not overlap.
I/O Circuits. Since I/O circuits go off the circuit boards, they deserve special attention during an EMC design review. Designers should look at the internal interconnections as well as the connections to the external world. In both cases, cable length is a key issue. Anything longer than 1/20 wavelength (6 in. at 100 MHz, 2 in. at 300 MHz, and <1 in. at 1 GHz) is a suspect antenna for both RF emissions and RFI.
If the device enclosure is shielded, the internal interconnections are not as critical as they would be without shielding. However, designers should check to see whether adequate ground returns are provided. For digital signals with edge rates >5 nanoseconds, a ratio of five signals to one return is a good rule of thumb. (For edge rates <5 nanoseconds, designers should consider adding more ground returns.)
For external connections and internal interconnections in an unshielded enclosure, it is desirable to provide some high-frequency filtering at the connectors. Series ferrites and/or shunt capacitors (1001000 pF) are good insurance against EMI energy entering or exiting circuit boards at the I/O ports. Because low-level analog I/O circuits are very vulnerable to external RFI, those circuits must have such RF filtering.
Power Supply. Unless the product development project includes designing a device-specific power supply, a standard off-the-shelf module with the appropriate safety marks will be used. Although many commercial modules are tested for emissions below 30 MHz, designers must take into consideration that these modules can still contribute to radiated emissions above 30 MHz, and they may also be vulnerable to external disturbances like RFI, ESD, and EFTs. To protect the device electronics, additional EMI filters may be needed on the input power lines. Because improper installation is a common cause of problems, it is critical that the EMI filters are solidly mounted to the chassis and that the input and output leads are separated. The best method is to mount an EMI power filter directly at the chassis boundary, assuming the device enclosure is metal.
Enclosure. Many medical devices are housed in a metallic enclosure, which provides shielding against both external RFI and internally generated emissions. Since only a thin metallic layer is needed to provide protection against high frequencies, the enclosure may be plastic that is coated with metallic paint or other conductive substance.
When examining enclosures for EMI performance, designers should pay particular attention to the seams and other openings, which can act as unwanted slot antennas in the shield. The same criterion that is used for cables also applies to these openingsa length of greater than 1/20 wavelength becomes suspicious. However, even a small opening can be a problem if something conductive passes through it, such as a power or signal wire or a cable shield. In those cases, filters and proper shield termination are very important. Shield termination is an especially important concern, since even a small discontinuity, such as a loose shield or a shield pigtail connection on a drain wire, can severely degrade high-frequency performance.
If the device does not have a shielded enclosure, internal shielding may be required on critical circuits. As explained earlier, low-level analog circuits are particularly vulnerable to RFI, so shielding for these important circuits is usually essential. At a minimum, designers should provide for the possibility of adding local shielding later, if it is found to be needed.
DESIGN REVIEW CHECKLIST
EMC design reviews need not be complicated, but it's important that designers take a few hours early in the product development cycle to step back and concentrate on EMC issues. The checklist in the accompanying box can provide a framework for the review process.
Submitting a new product to an EMC design review is a bit like going to the doctor for a physical examination: even if the exam reveals that there is nothing wrong, it's time well spent. And if preventive steps can be taken to protect the device against common EMI problemssuch as inoculating reset circuits against RFI by using capacitorsthe end result should be a healthy design.
Daryl Gerke and William Kimmel are principals in Kimmel Gerke Associates, Ltd., an electrical engineering consulting firm specializing in EMI/EMC issues with offices in Phoenix and St. Paul, MN. They can be reached at www.emiguru.com.
Illustration by Brad Hamann