electromagnetic noise reduction techniques part 1

8/9/2019 Electromagnetic Noise Reduction Techniques Part 1

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Purpose:

This course discusses techniques that can be applied to reduceproblems in embedded control systems caused by electromagneticnoise

Objectives:

Gain a basic knowledge about noise that affects embeddedsystems

Learn approaches and design methods for minimizing the noise

generated by microcontroller based embedded systems Get details about techniques chip designers use to decrease the

amount of noise a microcontroller produces

Obtain basic insights for handling noise problems during the systemdesign and development cycle

Content:19 pages3 questions

Learning Time:30 minutes

Course Introduction


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Noise = Unwanted electrical signals that produce undesirableeffects in the circuits of control systems in which they occur.

Two types of noise:

Noise Can Cause Big Problems

Electromagnetic Compatibility (EMC) issues encompass bothtypes

Noise reduction approaches:

- Techniques for reducing EMI (Electromagnetic Interference)

Cutting the noise emitted by a specific system, circuit or devicethat causes other devices/circuits to operate incorrectly

- Techniques for decreasing EMS (Electromagnetic Susceptibility) minimizing the effect that external noise has on the operation ofa system, circuit or device

Noise reduction: a goal common to both microcontroller (MCU)designers and the system engineers who apply those devices


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Many significant Ls and Cs arehidden impedances in the circuit

Noise is the undesirable result of

unintentional interaction due tostray capacitances andinductances

Parallel signal paths allow morecoupling and interaction, more

noise- Capacitance is proportional to the areaofelectrodes and inversely proportional to

the

distance between the electrodes- Inductance exists even in straight wires,

and mutual inductance between adjacentsignal lines causes electromagneticcoupling

Signal

A

SignalB

Hidden C and L

Effect of Hidden Impedances

Parts mountingsurface

Soldering surfaceHidden C and L

Cross-section ofPCB

Wiring on PCB

Printed Circuit Board

MCU Other

Device


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Signal interconnections on a circuit board haveresistance

Example: Impedance of a copper-foil trace 35m thick and0.5mm wide is about 0.01/cm at DC

Microcontrollers can have fast clock speeds and

signalsthat contain very high frequency components

- The frequencies in the spectrum of a digital signalincrease

rapidly as rise and fall times became shorter

Example: As the rise time decreases from10nsec/5V to 2 to 5nsec/5V, a signal willcontain frequency components >100MHz

Interconnection resistance increases assignal frequency components rise

Example: Impedance at 100MHz for the tracedescribed above is

Design PCBs to handle high frequencies

- Shorten the wiring traces that connect

bypass capacitors to Vcc and groundand make the traces wider, etc.

Impedance of Interconnects

Z 6/cm [~600 times larger than at DC!]~~

Vcc x 0.8

Vcc x 0.2GND

Vcc

10nsec

Vcc x 0.8

Vcc x 0.2

GND

Vcc2 to 5nsec

Spectrum hasfrequencycomponentsat 100MHz


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Ideal capacitor:

Example: Impedance of 10pF capacitor at 100MHz is

Frequency characteristics of

actual capacitors vary by type

and capacitance value

- Choices for bypass capacitors:

Z =C1

2 x 100 x 106 x 10 x 10-12Z =

1170~~

10pF 170

Impedance(I

I)

4.7FTantalum

Electrolytic

100FAluminumElectrolytic

0.05FCeramic

0.1FMylar

0.05FFeed-through

Ideal0.05F

Capacitor

Bypass capacitor working area

1k0.01

Frequency (Hz)

5mmWire

0.1

1

10

100

1000

10k 100k 1M 10M 100M 1G

Application Typical Design

Choice

Small capacity Ceramic capacitor(0.01 - 0.1F); install Small capacitance, but

between Vcc/GND of IC good frequencyresponse

High capacity Tantalum capacitor(0.1 - 10F); install Smaller capacitancethanbetween Vcc/GND aluminum electrolytic,butof PCB better frequency

response

Extra high capacity Aluminumelectrolytic(1 - 100F); install Poor frequency

Impedance of Capacitors


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Is the following statement true or false? Click Done when you are finished.

A ceramic capacitor is a good choice for a small-capacity bypass

capacitor because it has good frequency response.

True

False

Done

Question


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Typical Causes of Noise

Signal A

Signal B

Signalinterference

Signal A Signal A Signal A

Waveform distortion during signal

transmission

Vcc

GND

Sudden change of powercurrent

GND

Narrow ground

line

Electromagneticinduction(from power line, powerdevices)Signal A


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Review values of componentsin the clock oscillator circuit

Optimize C1, C2 and R2 toprevent unwanted emission and

allow circuit to oscillate atoptimum amplitude

Techniques for Reducing EMI - 1

Internal Memory

Adjust C1,C2, and R2

Vcc

GND

Vcc

GND

C1

R1R2

C2

Xtal

Change device package typefrom a DIP to a QFP

Adopt a package with shorterlead lengths to decreaseunwanted emissions fromantenna effect

Use a single-chip solutioninstead of a MCU and externalmemory

Choose an MCU with on-chipmemory to prevent unwantedemissions caused by driving anexternal bus line

MCU

Memory


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Typical: Bus-mode

Activates bus line whenaccessing peripheral IC andMCU (with on-chip memory + I/O).

Better: Port-mode

Activates bus line onlywhen MCU isaccessing peripheralIC.

Peripheral IC

Use port mode, not bus mode

Depending on the type of MCU,the signal on a general-purposeport may change voltage much

less frequently than the signal onthe bus. If this isthe case, use the port to reducethe amount of noise generated

Peripheral IC


Use serial damping resistorsInsert a damping resistor (100 to150 Ohms) in signal lines toreduce emissions caused bysignal reflection. A resistance ofabout 5k is OK if some signaldegradation is acceptable

Peripheral IC

MCU

MCU MCU

Vcc=5V

Vcc=3.3V

Reduce power supplyvoltage

If possible, reduce Vcc from5V to 3.3V to reduce the noiselevel

Damping

Resistors


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Revise system timing, if possible

Try to stagger significant signal-driving events, even just slightly, todiffuse the noise generated byswitching

Insert L-C filters on each power andground line

The filters help prevent switching noisefrom entering the Vcc and GND lines,from which they might otherwise beradiated

Ferrite beads

Vcc

GND

MCU

Use a multi-layer circuit board andapply best-practice layout methods

Develop effective sheet patterns forthe Vcc and GND planes; makeinterconnect wiring as short aspossible; interleave noisy signal lineswith GND and Vcc layers to shield the

line; etc.

Through hole

Vcc

GND

EventA

EventB

EventC Time

Event A

Event B

Event C


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Good designPoor designGeneratedelectro-magneticfield


-

-

-

-

-

-

-

+

+

+

+

+

+

+

Vcc

GND

Use a half-area pattern onthe circuit board whenwhole-area pattern cannotbe used

To the greatest extentpossible, make symmetricalpatterns for Vcc and GND onthe facing layers of a double-sided PCB

-

+

+

+

+

+

+

+

-

-

-

-

-

-

VccGND

Select a microcontroller thatproduces low levels of EMI

Consider the MCUs in the M16Cseries, devices that were designedfrom the outset to minimize noiseproblems in embedded systems

Man

yothe

r

type

sof

MC

Us

Noisy!

M16C

seriesMCUs

Quiet!


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Which statements about reducing EMI in embedded systems arecorrect? Select all that apply and then click Done.

Choose a microcontroller with on-chip memory to preventunwanted emissions caused by driving an external bus line.

To reduce emissions caused by signal reflections, insert a serialmatching resistor of 100 to 150 Ohms in signal lines.

Use bus mode, not port mode, to reduce the number of voltagechanges and, therefore, to reduce the unwanted noise.

If you have a two-sided circuit board, always try to make

symmetrical patterns for Vcc and GND on the facing layers.

Done

Question


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Change the drivingcapability of the clockoscillation circuit

Use high-drive mode to get

oscillator started reliably; low-drive mode to sustainoscillation

Synchronizedswitching

Time-sharedswitching

Outputterm

inal

Designing Low-EMI MCUs - 1

Use an optimum output bufferconversion speed (through rate)

Dont set the speed any higherthan necessary

Startoscillation(Highdrive)

Steady-stateoscillation(Low drive)

Vcc

GND

t1 t2t1

Change the timing of the

output buffersEliminate the simultaneousswitching of all channels,if possible

L

HA

B

B

A


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Use an innovative layout,enhanced with added internalcapacitance

Add capacitance to reduce the total

impedance of the power supplydistribution system thats internal tothe microcontroller

Designing Low-EMI MCUs - 3

GNDN+ P+ N+P+ N+ P+

P-

PN

IN OUTVcc

Vcc

GND

IN OUT

Vcc

GND

IN OUT

Vcc

GND

IN OUT

Noisepropagatio

nto outside

IC chip

Vcc

GND

Noiseabsorption ascommon mode

Parasitic capacitorWiring inductance

Pin layoutsimplifiesputting bypasscapacitor atbest position

Vcc

GND

Arrange terminals for easymounting of bypass capacitorsacross Vcc and GND terminals

Also, use parallel arrangement ofsignal lines inside package andon the chip to achieve extrafiltering

Optimize the driving capability ofthe internal buffer transistors

Design very small transistors withlarge drive capability; adjust buswiring, too

IN OUT

Vcc

GND


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MCUs in the M16C familyhave an innovative pinlayout

Arrangement of pins eases

the design of Vcc and GNDwiring on the circuit board andaids the placement of bypasscapacitors

Pattern for Vcc, GND, Oscillator

AVSS

VREF

AVCC

BYTE

CNVSS

XCIN

XCOUT

/RESET

XOUT

VSS

XIN

VCC

/NMI

VCC

VSS

Reset

IC

GND VCC

Oscillation capacitors(symetrically placed)

Bypasscapacitor(~0.01F)

Tantalum capacitor

Capacitor for RESET(1000pF)

Oscillation capacitors(symetrically placed)

Sheet pattern of GND

Sheet pattern of VCC

Bypass capacitor

(~0.01F)

M16CMicrocontroller

Bypass capacitor(~0.01F)


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Its very important toimplement noise measures

at initial stage of designwork

If a problem is discovered in thelater stages of the system

development process, the noisemeasures

required to solve the problemwill end up being far more costly

than expected

Basic Design Insights on Noise

Investigations of noiseproblems take time and

are costly

Because its difficult to simulatea noise-oriented malfunction, alot of work will be necessary to

identifythe root cause of the problem

Noise problemssometimes require a

fundamental solution,such as a re-design

Unless noise measures are taken atthe initial design stage, a

malfunction that occurs later mightbe extremely difficult to eliminate

using superficial correctionmethods; a re-design

may be necessary to correct

the problem

Attempts to eliminateall possible EMI/EMS

problems typically lead tounnecessarily high costs

The optimum design approachis to take pinpoint measures inkey areas that require solutions

Noise


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Match each item to the most appropriate MCU design advice by

dragging the letters on the left to the correct locations on the right. Click

Done when you are finished.

Through rate

Pass-through currentB A

Done ResetShow

Solution

A Adjust locations to make it easyto install a bypass capacitorD

C Signal from buffer

Set no higher than necessary

CReduce amplitude and slow downthe rise time

D Vcc and GND terminals B

Eliminate by changing the timing of

the N- and P-channel transistors

Questions


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Noise problems

EMI and EMS

System-level EMI reduction measures

Techniques for reducing EMI from microcontrollers

Design insights

Course Summary

electromagnetic noise reduction techniques part 1

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