EMI EMI

What is EMI?Electromagnetic interference: invisible

lines of force– Produced by combination of voltage

(electricity) and current flow (magnetism)– Signals may interfere with intended device

operation

The strength of EMI is proportional to the distance from the source. The closer you are, the more of an effect EMI will have.

The strength of EMI is proportional to the distance from the source. The closer you are, the more of an effect EMI will have.

What is EMI?

Electric fields– Produced by voltage

– Reduced in strength with increasing distance from the source

What is EMI?

Magnetic fields– Produced by flow of current

– Electric field also present

– Reduced in strength with increasing distance from the source

EMIEMI Interference in intentional receivers Noise produced in an AM radio by nearby lightning dischargesRadio not tuned to a particular transmitter frequency, receives it - reception of an unintended signal

Interference in unintentional receivers Strong transmission from an FM radio station or TV station -picked up by a digital computer – data corrupted or incorrect functioning of computerDigital computer – interference with a TV

How can EMI affect my device?• Implanted device may interpret EMI as rapid

signal from heart– Pacemaker: may withhold pacing– ICD: may deliver unneeded shock– An electric wheelchair with electronic

controls– Respirators also affected

• Effects of EMI are temporary• Does not usually harm device

What about magnets? Magnets do the following

things:– Attract certain materials

- such as iron, nickel, cobalt, certain steels and other alloys

– Have an effect on electrical conductors when the magnet and conductor are moving in relation to each other

Within six inches, a magnet may cause implanted devices to respond differently– Pacemaker will respond

by pacing at different, pre-set rate

– ICD will respond based on programming

– ICD may beep; move away from object causing beep, call doctor

PacemakersPacemakers ICDsICDs

To Use or Not Use – How do you know?

• Implanted devices are designed to work properly around most appliances and equipment.

• Most things you handle or work near every day will not cause a problem.

• Topics to be covered:– Personal Items– Kitchen, Tabletop, Household Items– Office, Shop and Yard Equipment– Entertainment Items– Travel/Environment– Dental and Medical Tests and

Procedures

Definitions• Safe under normal use:

– Only considered safe when used normally in accordance with their intended use.

– Check with your doctor for any additional restrictions that you may have for these items.

• Use precaution:– Getting too close to these items could

affect your device; when you are near any of these items, you should use precautions.

– Check with your doctor for detailed information before using these items.

• Do not use:– Strong electromagnetic and magnetic

fields may affect your device.– Talk to your doctor. For specific brand

items, consult the original manufacturer for any interaction with implantable devices.

Personal ItemsSafe under normal use

– Electric blankets, toothbrushes– Hair dryers– Heating pads– Pagers– Patient alert devices– Personal digital assistants

(PDAs, unless used as cell phone, see cell phone)

Use precaution– Cell phones

• Keep 6 inches from device

• Keep 12 inches from device if transmits more than 3 watts

• Hold phone to ear on the opposite side of body from device

• Do not carry phone in breast pocket or belt within 6 inches of device

– Cordless phones, Electric razors• Safe as long as not placed directly

over device

– Hand-held massagers• Safe as long as not placed directly

over device

Dental and Medical Tests and ProceduresSafe under normal use•Most medical and dental

procedures will not affect your device. Some examples include:– Dental drills and cleaning

equipment– Diagnostic x-rays– Electrocardiogram – Mammography

• Inform technician you have device to ensure device does not get compressed

– Diagnostic ultrasound

Use precaution• Be especially careful with the

following procedures: – CT Scans– Electrocautery used in

surgery– External defibrillators– Radiation therapy, lithotripsy– TENS unit

Boston Scientific. A Closer Look. Computed Tomography (CT) Scanning and Implantable Pacemakers and Defibrillators) August 18, 2008.

Do Not Use• Personal Items

– Body fat measuring scales (hand-held)– Magnetic mattresses or chairs

• Office, Shop and Yard Equipment– Jackhammers

• Dental and Medical Tests and Procedures

– MRI Scans– Diathermy

• Travel Entertainment– Stun guns

System is Electromagnetically compatible (EMC) with its environment if it satisfies three criteria:1. It does not cause interference with other systems2. It is not susceptible to emissions from other systems3. It does not cause interference with itself

Sources of InterferenceSources of Interference

• Ignition Systems• Charging Systems• Voltage Regulators• Electric Motors• Gauges and Instruments• Propeller Shaft(s)• Video Displays• Electronic Equipment• Fluorescent Lights

• Ignition Systems• Charging Systems• Voltage Regulators• Electric Motors• Gauges and Instruments• Propeller Shaft(s)• Video Displays• Electronic Equipment• Fluorescent Lights

Three ways to prevent interference:1. Suppress the emission at its source2. Make the coupling path as inefficient as possible3. Make the receptor less susceptible to the emission

General Classes of EMI Problems

1.Radiated2.Conducted3.Electrostatic discharge (ESD)

Routes for EMI Interference to Equipment

Prevention of EMI

Proper Grounding Shielding EMI Filtering PCB Design

Electromagnetic Shielding• It is the process of reducing EM field in

a space by blocking the field with barriers made of  conductive  and/or magnetic materials

• Shielding is typically applied (1)to enclosures to isolate electrical

devices from the 'outside world' and (2)to cables to isolate wires from the

environment through which the cable runs

Cont.

• Shielding can reduce the coupling of Radio waves, Electromagnetic fields & Electrostatic fields

• Amount of reduction depends - the material used, its thickness, the size of the shielded volume and the frequency of the fields of interest and the size, shape and orientation of apertures in a shield to an incident electromagnetic field

Materials used for EM Shielding

• Materials used for EM shielding – Sheet metal , metal screen, and metal foam.

• Any holes in the shield or mesh must be significantly smaller than the wavelength  of the radiation that is being kept out or the enclosure will not effectively approximate an unbroken conducting surface

Example of EM shield: Shielded cable

• Shielding in the form of a wire mesh surrounding an inner core conductor

• Shielding impedes the escape of any signal from the core conductor, and also signals from being added to the core conductor

Example of EM shield• Another commonly used shielding method-

with electronic goods housed in plastic enclosures - coat the inside of the enclosure with a metallic ink - ink consists of a carrier material loaded with a suitable metal- copper or nickel in the form of very small particles- sprayed on to the enclosure - once dry, produces a continuous conductive layer of metal, which can be electrically connected to the chassis ground of the equipment, thus providing effective shielding

EMI Filter• One of the best EMI solutions to filter out

EMI - before they hit the receiver front end - affects the desired signals only minimally

• Several different types of filter - used in reducing interference depending on circumstance

1.High-pass filter 2.Low-pass filter 3.Bandpass filter 4.Wavetrap

Ferrite Bead Filter

• Ferrite bead - passive electric component to suppress high frequency noise in electronic circuits

• It is a specific type of electronic choke

• Also called as ferrite blocks, ferrite cores, ferrite rings, ferrite EMI filters, ferrite chokes or ferrous beads

Ferrite Bead

A Ferrite bead at the end of a USB Cable

Preventing EMI in ECG Equipment

Transmission paths for EMI into a Sensor System

• Three principal transmission paths:

1.Penetration

2.Leakage

3.Conduction

Routes for EMI Interference to Equipment

Two Common Solutions for EMI in Sensor Systems

Filtering Shielding

Solution for EMI - Filtering

Solution for EMI - Shielding

References

•Introduction to Biomedical Equipment Technology by Joseph J Carr & John M Brown