17.4 Electromagnetic Interference and Compatibility

Electromagnetic interference can wreak havoc on medical devices, potentially endangering patients. This section explores how to shield equipment from unwanted signals and ensure devices play nice together in busy hospital environments.

We'll look at different types of electromagnetic emissions and ways to reduce them. You'll learn about shielding, filtering, and grounding techniques that keep medical devices running safely and reliably.

Electromagnetic Interference (EMI) and Compatibility (EMC)

Defining EMI and EMC

Top images from around the web for Defining EMI and EMC

• 

  Basics of Near Field RF Probes - Electronics-Lab View original

  Is this image relevant?

• 

  Electromagnetic Energy | Chemistry View original

  Is this image relevant?

• 

  3.2 The Electromagnetic Spectrum | Analytical Methods in Geosciences View original

  Is this image relevant?

• 

  Basics of Near Field RF Probes - Electronics-Lab View original

  Is this image relevant?

• 

  Electromagnetic Energy | Chemistry View original

  Is this image relevant?

1 of 3

Top images from around the web for Defining EMI and EMC

• 

  Basics of Near Field RF Probes - Electronics-Lab View original

  Is this image relevant?

• 

  Electromagnetic Energy | Chemistry View original

  Is this image relevant?

• 

  3.2 The Electromagnetic Spectrum | Analytical Methods in Geosciences View original

  Is this image relevant?

• 

  Basics of Near Field RF Probes - Electronics-Lab View original

  Is this image relevant?

• 

  Electromagnetic Energy | Chemistry View original

  Is this image relevant?

1 of 3

• Electromagnetic Interference (EMI) unwanted disturbance generated by an external source that affects electrical circuits through electromagnetic induction, electrostatic coupling, or conduction
• Electromagnetic Compatibility (EMC) ability of electrical equipment and systems to function acceptably in their electromagnetic environment without introducing intolerable disturbances to other equipment
• EMC standards define acceptable levels of EMI and provide guidelines for designing devices that can operate reliably in their intended electromagnetic environments (IEC 60601-1-2, MIL-STD-461)
• EMC testing involves subjecting devices to various electromagnetic disturbances to ensure they meet the required standards and can function properly in their intended environments
  • Includes tests for radiated and conducted emissions, as well as immunity to external disturbances (electrostatic discharge, power line transients)

Importance of EMC in Biomedical Devices

• Medical devices often operate in environments with multiple sources of electromagnetic disturbances (hospitals, clinics)
  • Sources include other medical devices, wireless communication systems, and power lines
• EMI can cause medical devices to malfunction or produce incorrect results, potentially leading to patient harm
  • Pacemakers and implantable cardioverter-defibrillators (ICDs) are particularly susceptible to EMI due to their sensitivity and critical function
• Ensuring EMC is crucial for the safety and reliability of biomedical devices
  • Manufacturers must design devices to minimize emissions and maximize immunity to external disturbances
  • Compliance with EMC standards is a regulatory requirement for medical devices in most countries (FDA, European Medical Device Regulation)

Types of Emissions

Radiated Emissions

• Radiated emissions electromagnetic energy emitted by a device through the air
  • Occurs when high-frequency currents flow through conductors or when high-speed digital circuits switch
• Can cause interference with other devices operating in the same frequency range (wireless communication systems, other medical devices)
• Radiated emissions testing involves measuring the electromagnetic field strength at various distances from the device using antennas and spectrum analyzers
  • Limits are specified in EMC standards based on the device's intended environment and frequency range (CISPR 11, FCC Part 15)

Conducted Emissions

• Conducted emissions electromagnetic energy emitted by a device through its power cord or other connected cables
  • Occurs when high-frequency currents are coupled onto the power lines or signal lines
• Can cause interference with other devices connected to the same power network or signal lines
  • Power line disturbances can affect the operation of sensitive medical devices (patient monitors, infusion pumps)
• Conducted emissions testing involves measuring the voltage and current levels on the device's power and signal lines using line impedance stabilization networks (LISNs) and spectrum analyzers
  • Limits are specified in EMC standards based on the device's intended environment and frequency range (IEC 60601-1-2, CISPR 22)

Mitigation Techniques

Shielding

• Shielding involves enclosing sensitive electronic components or circuits in conductive materials to reduce the coupling of electromagnetic energy
  • Materials include metal enclosures, conductive coatings, and shielded cables (coaxial cables, shielded twisted pair)
• Effectiveness of shielding depends on the material's conductivity, thickness, and coverage
  • Gaps or seams in the shielding can allow electromagnetic energy to leak through (aperture leakage)
• Shielding is particularly important for devices with sensitive analog circuits or high-speed digital circuits
  • Examples include electrocardiographs (ECGs), electroencephalographs (EEGs), and magnetic resonance imaging (MRI) systems

Filtering

• Filtering involves using passive or active electronic components to attenuate unwanted frequency components in a signal
  • Passive filters use combinations of resistors, capacitors, and inductors to create low-pass, high-pass, or band-pass characteristics
  • Active filters use operational amplifiers or other active devices to achieve higher-order filter characteristics
• Filters can be used to reduce conducted emissions by attenuating high-frequency noise on power lines or signal lines
  • Power line filters are commonly used in medical devices to reduce the coupling of electromagnetic disturbances from the power network
• Filters can also be used to improve a device's immunity to external disturbances by attenuating unwanted frequency components in the input signal
  • EMI filters are often used in the input stages of sensitive analog circuits (ECG amplifiers, EEG amplifiers) to reduce the effect of external electromagnetic disturbances

Grounding Techniques

• Grounding involves establishing a low-impedance path for electric current to flow between a device and a reference point (earth ground, chassis ground)
  • Helps to reduce the buildup of static charges and provides a path for fault currents to flow safely away from the device
• Proper grounding is essential for ensuring the safety and EMC of biomedical devices
  • Medical devices must have a reliable protective earth connection to prevent electric shock hazards
  • Grounding helps to reduce common-mode noise and improve the immunity of devices to external electromagnetic disturbances
• Techniques for improving grounding include using low-impedance grounding conductors, minimizing ground loop areas, and using equipotential grounding planes
  • Star grounding topology is often used in medical devices to minimize ground loops and reduce the coupling of electromagnetic disturbances between different parts of the system