Grounding - Curt White - 3-30-11

Copyright 2011 by Curtis B. WhiteAll rights reservedAll rights reserved

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One of many books

i covering subject

1056 1056 pages

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A low resistance connection to A low resistance connection to planet Earth (correct definition)p ( )

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Z i d Zero impedance common reference bus or planereference bus or plane

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Provide a common voltage reference location for Provide a common voltage reference location for systems, subsystems, units, PWB’s and circuits

P id “ i k” f t d i l i Provide a “sink” for unwanted signals more noises (return location for EMC filters)

P id “ i k” f bl d i hi ldiProvide a “sink” for cable and wire shielding

Meet electrical codes

Provide discharge path for lightning

P id di h th f ESDProvide discharge path for ESD

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C i d t t ( d d Common impedance structures (ground and power bus) generally needed for proper function of productsp

Conduction and distribution of EMI

Safety/ shock concerns

Lightning and ESD threatsLightning and ESD threats

Others

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O ll tOverall systemSubsystemUnit or boxPWBCircuitComplement (problems exist within ICs)Complement (problems exist within ICs)

Carefully consider each level in your design process (they must work together as well)process (they must work together as well)

A critical day one design task!y g

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Unit size, weight and cost impactsLayout restrictions usually involved (push backs)Layout restrictions usually involved (push backs)Lightning vs. safety (“green wire”)EMC vs. safety (“green wire”)

hOthers

Changes in ground design can be very costly if Changes in ground design can be very costly if initial opportunities are missed

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ffDiffering grounding philosophies can be usedSeveral philosophies exist (reviewed later)Pros and cons need to be consideredMixed approaches often used

Differing approaches can conflict and cause problemsMixtures need careExample – grounded vs. floatingp g g

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Zero impedance is desired and often assumed (even when it should not be!)

Unfortunately, this ideal never really exists (causes problems for designers)p g )

Finite impedance of real structures and materials is a critical, design driving problem, g g p

Impedance and RF issues increase with frequencies used within systems and products (digital)y p ( g )

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Supply side generally has similar or i d/ idsame issues as ground/return side

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Single point ground

Multiple single point ground

M l i l i d (i l di i l l Multiple point ground (including single large planes)

Floating (no ground to “outside world”)

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I till b th ll!I still remember them well!

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Low resistance and isolation were key design issues

l h hl dAnalog circuits (highly sensitive to ground noise voltages) were often design drivers

h l d dHigh current circuitry also drove grounding designs – big cross section conductors

l d d f h h fLittle concern needed for high‐frequency effects with many products

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Can no longer ignore RF effects even if we want to!Can no longer ignore RF effects even if we want to!

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Di i l l i i h l b i hi h Digital electronics inherently bring higher frequency signals (harmonics) into circulation

A general rule is digital signals require at least 100 harmonics of the clock/signal frequency100 harmonics of the clock/signal frequency(huge occupied bandwidth increase)

Designers must be knowledgeable of high frequency behaviors to minimize late in

f lprogram test failures16

Keep these in mind or you will likely have problems!p y y p

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Skin effect raises effective resistances

Finite impedance (inductance and capacitance) of real geometries in addition to resistance raises impedancesin addition to resistance raises impedances

Transmission line and resonance effects at higher frequencies cause non‐constant voltages and currents along structures

Unintentional antennas, inherently present in structures, provide radiation and pickup issues

Crosstalk issues (not covered tonight) provide undesired signal coupling

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SKIN DEPTH IN MILS OF ALUMINUM

100 00

1000.00

SKIN DEPTH IN MILS OF ALUMINUM

10.00

100.00

INC

H)

1.00MIL

S (1

E-3

0.01

0.10

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1E+02 1E+03 1E+04 1E+05 1E+06 1E+07 1E+08 1E+09 1E+10

HERTZ

1E+02

1E+03

SAMPLE WIRE PARAMETERS VS FREQUENCY

Ski ff t

1E+00

1E+01

1E+02

MS Rac

Skin effect dominates

1E-03

1E-02

1E-01OH

M

X (OHMS)

Z (OHMS)Inductance dominates

1E-05

1E-04

1E+02 1E+03 1E+04 1E+05 1E+06 1E+07 1E+08

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FREQUENCY IN HERTZ

For a given path length

L i t i il i l Low resistance primarily requires large conductor cross‐section

Low inductance primarily requires wide conductor (low conductor length to width ratio)

Your applications may need both parameters addressed

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Wires, traces, planes, housings all have multiple resonances!multiple resonances!Resonances produce non‐uniform impedances voltages and currents (standing impedances, voltages and currents (standing wave patterns)Resonances make structures more efficient Resonances make structures more efficient antennas for EMI pickup and radiation

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QUARTER WAVELENGTH FREQUENCY

10000

QUARTER WAVELENGTH vs. FREQUENCY

Remember resonances Formulas available

1000

Hz

will repeat as frequencygoes higher

available

100

FREQ

UEN

CY

IN M

H

10

PWB System

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11 10 100 1000

LENGTH IN INCHES

FIELD TO WIRE COUPLING INTO LOOP OVER GROUND PLANE DIFFERENTIAL MODE CASE

160 00170.00180.00

FIELD TO WIRE COUPLING INTO LOOP OVER GROUND PLANE DIFFERENTIAL-MODE CASE

Multiple resonances

100 00110.00120.00130.00140.00150.00160.00

40 0050.0060.0070.0080.0090.00

100.00

dBuV VZ1 dBuV

VZ2 dBuV

0.0010.0020.0030.0040.00

1E+04 1E+05 1E+06 1E+07 1E+08 1E+09

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FREQUENCY IN HERTZ

Classic view based upon circuit networks operating at relatively low frequencies and higher power levels (developed over decades)

b d h h d d lMore recent view based upon high‐speed digital electronics operating at low power levels

Preferences largely based upon users experiencesexperiences

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Return resistance and (to some degree) return impedance as well as crosstalk seen as pdominant grounding design concerns

Current in return and supply structures results in noise voltage coupling into g p gneighboring circuits

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fUse of large conductor cross‐sectional areas to handle larger currents

Use of single point sources and returns to avoid sharing of currents among circuitsg g

Use of sectionalized return planes on PWBs to id i l ti f i t i f provide isolation of various categories of power

and signals (control crosstalk)

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Common Impedance couplingA real problem in many designs

Ckt A Ckt B

I

Bus Z1Bus Z2

ICA + ICBICA ICB

VZ2 = ICB * Z2VZ1 = (ICA + ICB )* Z1

Both circuits see VZ1 – so Ckt B sees noise voltage from Ckt ABoth circuits see VZ1 so Ckt B sees noise voltage from Ckt Adue to finite bus impedance

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The supply side as well as the return side of power sources are subject to common impedance coupling

Si l d d b bj h diff Signal and data bus are subject to somewhat different design rules

Crosstalk issuesTransmission line technologies as frequencies rise

Keep in mind that bandwidths of at least 100 times titi t d d f l ti l d l repetition rate are needed for relatively good pulse

waveforms

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Addition of broadband digital signals

Co‐existence of additional differing circuit types in Co existence of additional differing circuit types in same products and on same PWBs

Analog ‐ often very sensitive to low level signalsDigital addition of signals using much wider bandwidthsDigital – addition of signals using much wider bandwidthsPower – now serving multiple types of circuitry

E l li i i i l l h l PWB Early limitation to single layer, then two layer PWBs, then many layer PWBs later

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Current levels are very low in most casesCurrent levels are very low in most cases

Frequencies involved are much higher in eque c es o ed a e uc g evirtually all cases

Supply and return impedances are a dominant design characteristics of concerng

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Extreme emphasis on very low impedance supply side and return side power bus structure impedances not just resistancejust resistance

Use of common source and return planes in lieu of trace or wire buses (usually whole PWB) to achieve trace or wire buses (usually whole PWB) to achieve lower impedances

Extreme use of distributed decoupling of source and Extreme use of distributed decoupling of source and return planes (over 100 SMT capacitors on some PWBs)

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S l d t t t ft b d l d Supply and return structures can often be modeled and simulated to improve designs

Need to consider full frequency range of interestq y gTransmission line models usually needed in VHF and aboveEstimation of parameters usually possible (inductance, capacitance and resistance per unit length)capacitance and resistance per unit length)

Higher frequencies/speeds and higher currents likely to dominate design

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"Stiff" (low Z) power

Circuit Circuit Circuit

Single point ground (and source)

powersupply

Do not forget to consider supply (high) side as well

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Works well at lower frequencies and analog circuitry

Works well for higher currents

Crosstalk (stray coupling) reduces isolation and effectiveness of this approach as frequencies rise (especially VHF and above)(especially VHF and above)

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SPG 1 (local to circuit region)


SPG 2 (local to circuit region)

Often used for larger products and systems

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Often used for larger products and systems

U f l d ft d t f l Useful and often advantageous for larger products

Tends to help higher frequency performance (upper HF and lower VHF range) especially as structures become largerstructures become larger

Helpful where higher currents are present

Crosstalk reduces effectiveness as frequencies rise (VHF and above)

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Concept is to utilize best available large t t th d f ( i f structure as the ground reference (airframe, housing, large plane on PWB etc.)

Grounds are frequently made to selected t t ff i l i d th structures offering lower impedance than wires or planes

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f fImproved performance can often be obtained where higher frequencies are involved

Quality of multiple point grounds can vary d lwidely

f l d f l l dCareful design of multiple point grounds is essential to achieve minimal impedance

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C f b l h d ( h d ff l )Can often be accomplished (with difficulty)

Fl i i i i “ d” i l i Floating circuitry requires “ground” isolation to be fully maintainedPower sources usually employ shielded isolation transformersIsolation of all circuitry I/O (optical, transformer, high impedance required

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Technique used to mitigate lightning or ESD

Potential advantages for lightning and similar intense ground seeking (usually relatively low frequency) threatsthreats

Stray capacitance and crosstalk reduce isolation effectiveness as frequency riseseffectiveness as frequency rises

Can be very costly to implement

Single missed path to “ground” can totally defeat efforts

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Often a complicated issue in and of itself

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Horizontal isolation on PWBsCircuitry grouped by type as “islands” on PWBsI l d PWB d b “ ”Islands on PWBs separated by “moats”Moats only to be crossed with “bridges”

Vertical isolation on PWBs (multi‐layer)Power and return layers (planes)Signal layers (x and Y directions)Signal layers (x and Y directions)Overlapping of islands using vertical spacing and shielding (usually power and return planes)

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Moats (see Montrose books for more information)

Digital circuitry andg yreturn sectionSwitching

mode power supply and

return section

Analog circuitry and return section

HORIZONTAL PWB PARTITIONING FOR ISOLATION

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Di it l i it

Analog circuitry

Analog circuitry

Digital circuitryy

Vertical PWB partitioning for isolation

Shield or powerplane(s)

Additional signal traces toconnect two analog areas

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More engineering issues to consider

Unshielded cases covered here with shielded cases covered later

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Usually form large pickup and emission antennas

Suggested remediesAvoid external loopsProvide additional wire(s) to minimize loop areas (avoiding structure returns for currents – common mode currents)“Break” connecting loops using transformers, optical isolators or high impedancesExpect significant filtering and transient suppression requirement to deal with external loop issuesrequirement to deal with external loop issuesTwist pairs or groups if possible (big help with magnetic field coupling)Add shielding for electric field and plane wave protectionAdd shielding for electric field and plane wave protection

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L f i l lLow‐frequency signals onlyGenerally grounded both ends of the shieldOccasionally single and ground may work betterOccasionally, single and ground may work better

High‐frequency signals or wiring exposed to high g q y g g p gfrequenciesGround both ends of shield peripherally to the back h ll fshells of connectorsAdditional shield grounds may improve performance particularly if spacing is randomizedparticularly if spacing is randomized

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Presenters opinions( ll h i C fi ld)(as well as many others in EMC field)

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Grounding design is a day one issue! Grounding design is a day one issue! (later changes difficult to make)

Orient yourself to your products key EMC parameters before Orient yourself to your products key EMC parameters before proceeding with ground design efforts

Types of circuitry involvedFrequencies and bandwidths being used/occupiedq g / pExpected emission frequency ranges and levelsExpected sensitive frequencies for susceptibility issuesExpected sensitivity thresholds for analog and digital circuitry for

b lsusceptibility issues

Keeping this information in mind will help make your design process go faster and with fewer issuesprocess go faster and with fewer issues

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Remember grounding is a design issue requiring customizing and optimizing (no simple set of rules customizing and optimizing (no simple set of rules can be easily applied)

What is your EMC problem history and lessons learned

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System levelSubsystem levelSubsystem levelUnit or box levelCircuit levelCircuit levelPerhaps even component level

ll l l d lAll levels do not apply in every case

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A lAnalogNon‐clocked digital (switch only occasionally at very low rate – alarms etc.)yClocked digitalSwitching mode power suppliesVery high power non switching circuitryVery high power non switching circuitryVery high power switching circuitryAdditional specialized circuit types unique to your productyour product

Each type has unique requirements which yp q qshould be addressed in your grounding design

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V hi h iti it t d d b i d iVery high sensitivity to ground and power bus carried noises

Particularly sensitive at lower frequencies in most cases

Frequent susceptibility problems in VHF region when using some operational amplifiers (audio rectification)

Classic grounding design approaches tend to support this type of circuitry wellyp y

Low resistance buses

SPGs56

Large amounts of high‐speed broadband noise virtually everywhere in these circuit regions

E l h d fi ( lk d )Extremely hard to confine (crosstalk due to strays)

Power source must be physically close to using circuitry (and stored local energy) to minimize bus noisestored local energy) to minimize bus noise

Virtually any inductance will have a significant effect (1 nH, such as a via, will have about 12 ohms at 1 ns rise time)Physical distance causes in delays in power reaching circuitry Physical distance causes in delays in power reaching circuitry (7.5 cm can be quarter wave at 1 ns rise time) (definitely in transmission line region at these speeds!)

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Use of power and return planes (lowest inductance practical)Use of power and return planes (lowest inductance practical)

Limited physical size of circuitry

Use of extensive distributed capacitance between planesDistributed SMT capacitors (beware of via inductance –impedance) (some folks use over 100 per PWB)p ) ( p )Buried capacitance (substrates having high r) between the PWB supply and return planes

Unbroken planes – breaks (slots and holes) add inductance (common impedance)

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A major source of high frequency broadband noise which A major source of high frequency broadband noise which must be contained

Power levels provide significant hard to confine magnetic Power levels provide significant hard to confine magnetic fields and noise currents

Very effective filtering required on all input and output leadsVery effective filtering required on all input and output leads

Noise created is often low impedance and hard to filter (very low impedance power wiring is critical in most cases)p p g

Often require special isolation and shielding

C ld b ti t t b k b it lfCould be an entire text book or course by itself

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C b f EMI i iCan be source of EMI emissions

Often a conductor of EMI from another locations

Very low resistance conductors critical

Careful isolation is often required

Special shielding may be requiredSpecial shielding may be required

Avoid common ground connection with lower glevel circuitry if possible

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Can be source of extreme EMI emissionsCan be source of extreme EMI emissions

Often a conductor of EMI from another locations as llwell

Very low impedance conductors critical in most cases

Careful isolation is usually required

S i l hi ldi ll i dSpecial shielding usually required

Avoid common ground connection with lower level i it if iblcircuitry if possible

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Proper design of source and return structures is challenging design problem for most products

Start with a good understanding of the frequency spectrum being used within your product and to which your product will be subjected

Analytic modeling techniques are available and may Analytic modeling techniques are available and may help determine the best compromise design earlier in the program minimizing cost and schedule impacts

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Resistances of various conductorsSkin effectInductanceStray capacitanceResonant lengths of wiresResonance frequencies of PWBsR f i f h iResonance frequencies of housingsOthers

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F d t i hi h d l For products using high power and low frequencies only, the classic approach is recommended

For products using high speed digital circuitry only the more recent approaches are only, the more recent approaches are recommended

For products between these extremes consider For products between these extremes, consider sectionalizing and use appropriate mixture of techniquesq

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Early design implementation criticalGrouping/sectionalizing by circuit type is also

la critical stepAnalysis techniques may help process achieve b lbetter resultsCareful oversight of implementation is critical

f h f l h fas temptations for harmful short cuts often abound – design reviews!

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Feel free to speak up

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All recommendedAll recommended

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EMC ReferencesEMC ReferencesRed type entries used for this class

Bogatin, Eric. Signal Integrity – Simplified. Upper Saddle River, N.J. Prentice Hall 2004.Celozzi, Araneo and Lovat. Electromagnetic Shielding. Hoboken, N.J: John Wiley and Sons, 2008.Joffe and Lock. Grounds for Grounding. Hoboken, N.J: John Wiley and Sons, 2010. Johnson, Howard and Graham, Martin. High Speed Digital Design. Upper Saddle River, N.J. Prentice Hall 2003.Kaiser, Kenneth L. Electromagnetic Compatibility handbook. Boca Raton, FL: CRC Press, 2005.a se , e et lect o ag et c Co pat b l ty a dbook oca ato , C C ess, 005Keiser, Bernhard. Principles of Electromagnetic Compatibility, Third Edition. Norwood, MA: Artech House, Inc., 1987.Montrose, Mark I. Printed Circuit Board Design Techniques for EMC Compliance. Piscataway, NJ: IEEE Press, 1996.Morrison, Ralph. Grounding and Shielding Techniques, Fourth Edition. Hoboken, N.J: John Wiley and Sons, 1998.Morrison, Ralph. Grounding and Shielding Techniques, Fifth Edition. Hoboken, N.J: John Wiley and Sons, 2007.Nave, Mark J., Power Line Filter Design for Switched‐Mode Power Supplies. New York, NY, Van Nostrand , , g f pp , ,Reinhold,1991.Paul, Clayton R. Introduction to Electromagnetic Compatibility, Second Edition. Hoboken, N.J: John Wiley and Sons, 2006.Paul, Clayton R. Analysis of Multiconductor Transmission Lines. Hoboken, N.J: John Wiley and Sons, 1994.Pozar, David M. Microwave Engineering, Second Edition. Hoboken, N.J: John Wiley and Sons, 1998.Schlicke, Heinz M. Electromagnetic Composibility, Second Edition. New York, N.Y: Marcel Dekker, 1982.S i h Alb A J C li f E l El i Fi ld T i i Li S d Edi i I f Smith, Albert A. Jr. Coupling of External Electromagnetic Fields to Transmission Lines Second Edition. Interference Control Technologies, 1989.Tsaliovich, Anatoly. Electromagnetic Shielding Handbook for Wired and Wireless EMC Applications. Boston, MA: Kluwer Academic Publishers, 2001.Weston, David A. Electromagnetic Compatibility Principles and Applications. New York, N.Y: Marcel Dekker, 1991Weston, David A. Electromagnetic Compatibility Principles and Applications, Second Edition. New York, N.Y: Marcel Dekker 2001Marcel Dekker, 2001

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Grounding - Curt White - 3-30-11

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