What means .3.2 interpret .3.3 Methods measure .3.8 Influences compone

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Noise Interference
What means .3.2 interpret .3.3 Methods measure .3.8 Influences components PCBs .3.17 Cables .3.33
Contributing Authors: Jan-Hein Broeders Mark Meywes Bonnie Baker
Noise Interference
past EMC, otherwise known Electromagnetic Compatibility, referred variety guidelines used industry describe electrical/electronic appliances radiated influence and/or susceptibility increasingly rich electronics environment. Each country different requirements electronic products, spurring inconsistency regulations around world. Consequently, when machine exported from country, made according guidelines that country necessarily comply with regulations country that product exported 1992 European Economic Community took step forward standardization these specifications adopting common specification, 89/336/EEC, mandating compliance January, 1996. January 1996, electronic equipment that purchased within European Union's borders must comply with this guideline. Additionally, international trading from outside European borders into Europe must comply with IEC801 standards. company brings device machine market, they responsible testing their equipment according standards international specification IEC801. device complies with these guidelines, will have CE-mark product. This CE-mark indicates that product meets fundamental environment safety rules deemed necessary European Economic Community. This does give full technical qualification device, only indicates that product essential requirements. adaptation this regulation sweeping effect design products world-wide. This seminar module will discuss basics guidelines, methods measure EMC, possible sources solutions problems.
Electromagnetic Interference
Level
Source
Coupling Path
Victim
Immunity level Emission level
Frequency
Electromagnetic Interference (EMI) defined influence unwanted signals devices systems, making operation device difficult impossible. device EM-compatible, viewed having EMIproblem. three elements shown this slide used describe understand disturbance problems. electromagnetic signal must have "Source" origin. source then needs "coupling path" facilitate transmission disturbance signal "Victim". these three elements removed from system, disturbance problem solved. coupling path between source victim does have conducting medium such electric conductor dielectric. coupled through atmosphere well. most instances, coupling path combination conduction radiation. technique that used minimize EMI-problems keep disturbance signals from source across coupling path below certain level.
Equipment that Requires
Creates emissions susceptible Creates electromagnetic energy susceptible electromagnetic energy
guidelines, 89/336/EEC IEC801, applicable devices that emit disturbance signals devices that sensitive disturbances from electromagnetic energy. Some examples equipment would portable tools, computer peripherals, industrial equipment Radio telecommunication equipment separated possible exceptions some elements guidelines. second group equipment complies with only half guidelines. This kind equipment split groups. Equipment that only able disturb environment equipment that only sensitive environment disturbances. Examples devices that only disturb environment commutator motor bi-metal switch. example device that only sensitive environmental disturbances would signal-conditioning equipment. Devices well systems installations controlled this guideline. system combination many devices such computer system comprising computer, monitor, keyboard, drives, etc. There also group electrical devices that fall under guidelines. These kind devices capable disturbing electromagnetic environment. Light bulbs, squirrel-cage motors watches would into this category devices. Additionally, many electronic components that used consumer required meet guidelines. Finally, hybrids, power supplies computer modules assembly also excluded.
Wave-Length
When Frequency approach
Source Load
108(m/s)/
Must true
first step minimizing EMI-problems determine whether system operating high frequencies. system defined operating frequencies length transmission lines less equal tenth length maximum signal wave length where =3x108(m/s)/f (where maximum signal frequency). These formulas derived from examining frequency behavior system. system current flowing from supply load, magnitude accurrent change point function time. When this changing current noticeable particular point within time (where speed light), circuit defined frequency circuit. This small current fluctuation phenomena also described quasistationary state, where current circuit real DC-current where frequency large enough cause interference. above relationship holds true, system identified frequency system modeled with network resistors, capacitors inductors. high frequency systems, other issues such propagation delays, reflections radiation longer negligible.
Common-Mode Differential-Mode
Differential-Mode Voltage (VDM) Common-Mode Voltage (VCM) VREF Reference Voltage (VREF)
Within system, relationship between various signals create potential source disturbance. simple understand combinations examine effects pairs signals terms their common-mode voltage differential-mode voltage relationships. Although, disturbances caused current sources, this voltage example helpful clarifying definitions terminology. This simplistic approach will identify majority disturbance sources circuit. These signals viewed three ways: independent voltages voltage VDM. This difference voltage between also called differential-mode voltage. voltage VCM. difference voltage between middle reference VREF. ideal voltage VREF zero. Differential currents flow from When connection between ideal, which nearly impossible, common-mode currents occur.
Current Coupling Paths
ANTENNA
Emission
Common-Mode Radiation
Differential-Mode Radiation
radiation from circuit come from different relationships between more current sources, common-mode radiation currents differentialmode currents. differential-mode radiation currents circuit natural consequence physical layout. They occur result separate traces with differing current densities. resulting phenomena from these currents differential-mode H-field radiation. Likewise, common-mode radiated currents created result losses cables traces printed circuit boards. Common-mode currents create radiated signals that predominately E-fields.
Measurement Current
SCOPE SOURCE LOAD
Transformer
Electric-field limit 100µV distance meters Common-mode current maximum
Common-mode radiated signals measured during design phase determine corrective action should taken, such change layout system configuration. emission most devices dependent magnitude high frequency currents cables traces. guideline commonly used maximum allowed common-mode current ICM<5µA (assuming meters). Conversely, general rule thumb used emission measurements radiation maximum 30µV/m. relationship between common-mode current radiated electrical field where, Electrical field strength (V/m) Common mode current Distance from common-mode current source Common-mode currents that couple signals into environment measured with set-up shown above. These emissions parasitic capacitive coupling between lines from line earth ground. differential measurements same equipment used turning wire time around transformer.
Reducing CM-Currents
ferrite beads clamps reject common mode currents
Common-mode current signals cause unwanted radiation. radiation reduced placing choke clamp path current. pass filter choke easy implement. pass filter designed using combination. this approach feasible, choke constructed winding wire turns around ferrite core bead. choke will restrict high frequency dI/dt signals through conductor. frequency line increases does influence choke. Digital ribbon cables also present problem terms radiated signal common-mode currents. When ferrite bead feasible ferrite clamp used instead. important note that differential-mode currents affected this technique radiated noise reduction.
Measurement Magnetic Field
Remove coax shield
PROBE
0.1m
chip resistor measuring system
coax cable make probe
3.10
Measure field with "coax" probe
magnetic H-field strength inversely proportional distance from source. relationship between magnetic field, electric field, calculated shown below with following assumptions: Distance from H-field 0.1m Distance from E-field (legal cable length consumer products) frequencies below 300MHz antenna probe built with coax cable used measure magnetic field. H-field induces voltage signal across resistor. This voltage increases with lower frequencies. frequency above 300MHz sensitivity probe becomes independent frequency. that point voltage will increase with frequency current does increase because larger than (where inductance probe). This resistor value sensor impedance added input impedance measurement system. Once H-field been measured with this technique, E-field estimated following formula: E(r=3m) (r=0.1m) With electrical field strength (V/m) magnetic field strength (A/m)
frequency
3.10
(Hz)
EMI-properties
Application will sometimes produce results normally expected These problems usually occur frequencies higher than bandwidth active components Keep your mind
Parasitic Effects
3.11
Electromagnetic interference that occurs application explained previous discussion have unexpected behavior. Many times possible explanation this unexpected behavior attributed components circuit. components operate higher frequencies, they usually primary contributors unexplainable high frequency disturbances. These high frequency disturbances magnified component parasitic inductances capacitances. Keep mind that passive components have parasitic characteristics well. next slides give some more information about parasitic properties passive components after some suggestions reduce EMIdependence circuit.
3.11
Work with Currents!
meet EMC-specifications There current without voltage-source Emissions Every current creates electromagnetic field Currents sensitive emission Susceptibility susceptibility With EMC, current dominates evaluation!!
3.12
Whenever current flows through conductor, EM-field created. Consequently, troubleshooting always starts ends with evaluation relationships currents circuit. possible have current flowing circuit without supply voltage. example such occurrence magnetic field which generates current toriod. Regardless whether voltage source present, Kirchhoff's laws still applied circuit throughout evaluation. stated Kirchhoff, summation currents node circuit always zero. When emissions occur device, radiated energy path modeled current vectors that intersect circuit node under analysis. case diagram above, represents radiated signal. Additionally, current "injected" into circuit shown above. This type circuit said susceptible EMI.
3.12
Passive Components
Parasitic components circuit reduced with parallel circuit Capacitors should short frequency EMI-problem Coils have many parasitic components: check resonance
3.13
equivalent circuit resistor trace also inductance capacitance model. Parallel circuits used minimize influence parasitic components high frequencies. Concurrently, recommended shorten leads much possible minimize inductive contribution implementation. Normally capacitors selected their characteristics over specific frequency range. that frequency range capacitor acts like short circuit. They particularly useful between power supply rails ground, which case they filter supply noise that circuit components incapable rejecting. same technique applied problem, where short circuit caused capacitor occurs specific frequency where problem occurring. Capacitors operate like short circuits higher frequencies, conversely inductors operate like open circuits. Normally inductor core order obtain relatively high induction relatively small coil size. behavior coil core vary over wide frequency range. coil will resonate specific frequency because capacitance from wire-to-wire wire-tometal housing can. Below this resonant frequency, coil will behave like inductor. Above this frequency appears capacitive.
3.13
Coupling Cross-Talk
Limit your frequency spectrum
Source
Short Coupling Path
Victim
3.14
problems have been defined having system with source, coupling path, victim. When length coupling path very small disturbed system described having "cross-talk" problems. Cross-talk occurs under different conditions. situation that fosters cross-talk problems where both systems have common impedance. This common impedance establishes coupling path from source victim causing disturbance system. Another situation where cross-talk problems occur when signal coupled inductively capacitively influence field. Knowledge cross-talk effects critical when designing multi-cable assembly.
3.14
Cross-talk Common Impedance
Signal Distortion dependence
3.15
uncommon common conductor cause cross-talk between systems. shown diagram above, currents separate circuits flow through conductor, common reference, which usually ground. order minimize effects changes voltage across impedance should low. evaluation signal distortion ratio this circuit easy start understand affects cross-talk with this simple circuit. Intuitively easy that signal-source will contribute ultimate voltage drop across impedance will same voltage drop across impedance common conductor, equal zero. look contribution voltage drop across assumptions that VS2=0 other impedances circuit. results final calculation VZL2 (ZS1 )(ZS2 Cross-talk dependent impedance common conductor, When zero cross-talk zero. effect cross-talk signal distortion ratio VZL2 WITH WITH SMALL [(ZS1 ZL1) calculated ratio shown below ZS1=10, ZL1=50, =0.1 VS1=0.01 15.5
3.15
Reducing Common Impedance
conductors instead Keep common impedance Keep current through
3.16
order reduce effects some options exercised. common line between circuits eliminated using second common line described circuit above. This feasible second option would attempt keep impedance, possible. This accomplished shortening lead length from node node dedicating layer achieve impedance reference path. final solution that implemented keep current between circuits with galvanic separator such isolation amplifier optocoupler, example.
3.16
Isolated Instrumentation Amplifier
+VSS REF1004C +2.5V +15V
ISO213 +VSS OPA130 -VSS
3.17
VOUT
-2.5V -VSS +VSS
isolated instrumentation amplifier used here galvanically separate bridge circuit. input ISO213 high impedance differential input with adjustable gains. Included package DC/DC converter that used power isolated (input) side isolation instrumentation amplifier external circuitry, such REF1004C OPA130. isolation product, barrier integrity paramount importance achieving high reliability. ISO213 uses miniature toroidal transformers designed give maximum isolation performance when encapsulated with high dielectric-strength material. internal component layout designed that circuitry associated with each side barrier positioned opposite ends package. Areas where high electric fields exist positioned center package. result that dielectric strength barrier typically exceeds 3kVrms. reference circuit, REF1004C (+2.5V) provides voltage power bridge. power, input OPA130 drives bottom bridge, providing linearity correction single element bridge.
3.17
Reject Noise Errors
50k/(R GND) SIGNAL NOISE) ERROR SIGNAL
50k/RG VOUT (VSIGNAL) SIGNAL
NOISE ERROR
NOISE ERROR
3.18
first figure, connected standard non-inverting gain configuration. Three sources error shown: External resistors meant gain, input signal, amplified 1+RF/(RIN+RGND) instead 1+RF/RIN, VNOISE represents noise pickup signal path, VERROR represents ground loop errors ground currents from other circuitry reacting with wiring impedance, RGND. With amp, effects error sources appear amplifier output, with gain applied. When classical instrumentation amplifier used, noise (VNOISE) other errors (VERROR) rejected. second figure shows instrumentation amplifier connected with same error sources shown previously. classical instrumentation amplifier comprised stages. first stage uses amplifiers which provide high impedance, differential input gain stage. second stage configured unity gain differential amplifier with four resistors, changing differential output first stage single output signal. Gain instrumentation amplifier single-ended external resistor, gain equation depends feedback resistors contained instrumentation amplifier With internal feedback resistors, input signal, VSIGNAL amplified 1+50k
3.18
Capacitive Cross-Talk
Coupling electric fields
Circuit Circuit Circuit VSOURCE Circuit
Coupling looks like high pass filter Cross-talk increases with increasing Voltages responsible coupling Signals phase
3.19
Cross-talk between circuits occur when there common conductor between systems. coupling path medium supports current voltage changes directly. Other problems arise from EM-fields well. discussed previously, currents cause electric magnetic fields. These "air born" EM-fields couple into other circuits within certain distances. This type cross-talk separated into categories, capacitive inductive cross-talk. Capacitive coupled cross-talk disturbances transmitted electric fields. diagram measuring set-up parallel circuits with equivalent circuit shown below. capacitive coupling occurs through capacitor CAB. value capacitance decreased, crosstalk problem also decreased. Using equivalent circuit following formula derived:
VLOAD VSOURCE 0.25 jZLCAB
From this formula following conclusions about cross-talk found: Capacitive coupling acts like High pass filter Cross-talk increases with increasing Cross-talk dependent ratio voltages. cross-talk signals phase with each-other.
3.19
Capacitance
Trace
Cross-Section
(typ 0.003mm)
width thickness length trace distance between dielectric constant dielectric constant
trace traces 8.85 10-12 substrate coating relative
3.20
Traces form parasitic capacitors with other traces. magnitude capacitance dependent ratio length traces distance between them. This first order calculation gives designer rough estimate stray capacitance that been designed into layout.
3.20
Reducing Capacitive Cross-Talk
Decrease adjacent surfaces Increase distance between circuits Create guard between circuits
Shield Circuit Circuit
3.21
Cross-talk conduct between traces layers print circuit board, contacts switches relay pins connectors leads components also source victim cross-talk. order decrease capacitive cross-talk, capacitor value should decreased. value capacitance will decrease with decreasing surface area increasing distance between conductors. When impossible change these variables, capacitor value changed mounting shields around conductors.
3.21
Electrostatic Coupling
Electrostatic Coupling
eOUT OPA131
eECM(s) eOUT
3.22
CM(s))
This slide models basic electric field coupling affect with inverting configuration, electric field noise source mutual capacitance (CM). source couples noise current through into amplifiers feedback network. That current flows through feedback resistor, produce output noise signal -(INE R2). practice, other mutual capacitances couple noise currents other points circuit, however, impedance these other points minimize effects currents. Switching differential input configuration produces balanced conditions that allow amp's reject noise signal (INE R2).
3.22
Using Reduce Electrostatic Coupling
Electrostatic Coupling
eOUT OPA131
3.23
This circuit operation uses simple method noise reduction. Adding balancing resistance series with amp's non-inverting inputs cancels noise coupled those inputs. Further examination reveals impedance balance that technique produces. From figure, impedance that drives from amp's inverting input also equals returns impedance source, returns impedance amplifier's output. Thus, impedance analysis purposes, these resistors effectively return ground appear parallel. Therefore, amp's rejects electric field coupling long circuit presents equal impedances inputs. resistors resemble those often added reduce offset produced amplifier input currents. These resistors will cancel coupled noise offsets caused input currents, however, this does preclude bypassing with capacitors across Bypassing would again cause imbalance impedances that drive amplifier inputs over frequency.
3.23
Inductive Cross-Talk
Coupling magnetic fields
Coupling looks like high pass filter Cross-talk increases with decreasing Changes current responsible coupling Signals phase
3.24
Magnetic-coupling dominates inductive cross-talk. This kind coupling compared transformer. circuit above factor, (mutual inductance), given inductive coupling coils. equivalent circuit formula below derived:
VLOAD VSOURCE 0.25
From this formula following conclusions about cross-talk found: coupling high pass filter characteristic. This same capacitive coupling case With same couple factor, cross-talk will increase decreasing ZLOAD Changes current responsible cross-talk undesirable voltages phase
3.24
Decrease Inductive Cross-Talk
Circuit
Circuit
Twisting Reduces H-Field Well
Increase distance (RX) between circuits Twist wires circuits counteract their fields
3.25
techniques that used reduce inductive cross-talk similar methods used capacitive cross-talk reduction. coupling factor between inductors should reduced much possible. This achieved increasing distance between circuits, causing field reduce with inverse square distance. Another approach would twist wires circuits. relationship distance between current conducting circuits strength magnetic field first order approximated with following calculations. this example, magnetic field point generated wires Circuit with current magnitude calculated This equation mathematically illustrates that magnetic field strength inversely proportional squared distance between circuits. second approach reducing coupling factor twist wires different circuits. H-fields will counteract each other, reducing cross-talk effect.
3.25
Magnetic Coupling Amps
OPA131
3.26
circuits, most confusing task inductive coupling reduction identifying pick-up loops. physical arrangement circuit's components form these loops several ways. circuit above shows loops non-inverting circuit. connections with source, feedback load form three loops. seems break these loops, amplifier's feedback action continues them. First, ground return resistor forms loop with coupled noise source This loop's noise signal eM1, drives inverting amplifier that provides gain -R2/R1. This gain potentially makes loop, serious noise source good choice area minimization. Next, consider loop coupled signal eM3. This less obvious loop exists because connect circuit's ground return, feedback extends signal path through amplifier. Signal drives input non-inverting amplifier. Feedback makes amplifier output respond continuing loop between resulting noise signal, eM3, appears load with unity gain. final loop, depends loop continuity conditions described resulting noise signal eM2, also appears circuit output with unity gain.
3.26
Difference Amplifier Reduces Noise Further
OPA131
3.27
(eM3 eM2)
Minimizing preceding loops reduces magnetically coupled noise, does eliminate Some finite loop areas magnetic coupling always remain. However, difference amplifier's reduces noise further. circuit above shows relevant loops coupled noise signals this amplifier. ground differential input acts center pick-up loop, splitting signal into equal parts. Unfortunately, these parts present opposite polarity signals differential input's, resistors. Now, instead common mode input, presents differential signal, which circuit's does reject. However, balanced structure difference amplifier still permits noise reduction matching loops These loops produce signals eM3, which tend cancel circuit output. This cancellation requires matching loops their distances from interfering magnetic source, their orientations relative that source. Matching these three features equalizes eM3, making their effect common mode signal amplifier's inputs. Matching loop areas distances equalizes magnitude Matching distances produces first order phase equalization. Accurate phase matching, which high common mode cancellation requires, also necessitates matched loop orientations relative magnetic source. Most often, this noise reduction technique aids rejection frequency noise, such power transformer interference.
3.27
Monolithic Difference Amplifier
+VIN -VIN Feedback Load Supply VOUT INA105
VOS/T Gain Nonlinearity
1000µV (max) 20µV/°C (max) 0.01% (max) 0.001% (max)
3.28
further precision difference amplifier, monolithic version available. Because monolithic difference amplifier thin film resistors chip, loop areas reduced minimum. Additionally, resistors trimmed precisely obtain better than average CMR.
3.28
Cross-Talk Reduction
Each Circuit should have conductors Keep conductors circuit close possible reduce loop area Place small resistor signal and/or supply lines only bandwidth that NEEDED designs double multi-layers create conductive shields
3.29
These points summarize discussion performance improvements when cross-talk problem.
3.29
Earth Reference
Create your application System Reference (SR) must IMPEDANCE Placement dependent
emission immunity conductors position components
3.30
Ground concept that used most engineers their daily work. Grounding circuit sometimes taken granted later proto-typing process present some most bewildering problems. Many EM-problems attributed poorly grounded circuit. EMC-applications ground earth-protection often referred system reference (SR). system reference defines central point circuit that used check other signals. often defined supply which usually connected earth ground. Keep mind, connection that will probably have many incoming outflowing currents. determination location system reference very important. impedance should possible signals area kept minimum. meet these requirements, knowledge should applied when designing layout. determination location dependent Circuit emissions Circuit immunity Circuit conductors Placement components
3.30
Effects Reference
System Reference
OPA2234
OPA2234
Input
ZLOAD
{(I1ZA) (I2+ I4)(ZA+ (I3ZC)}
3.31
effects reference (SR) EMC-application problem illustrated here. this example, dual, single supply amp, OPA2234, configured gain stage. represented symbol layout designed without EMC-considerations taken into account. results, terms signal-distortion ratio summarized following formula: I1ZA )(ZA where trace impedances. system reference this layout ideal, causing errors. order reduce cross-talk, layout should eliminate currents that flow through currents current through smoothing capacitor amplifier ground current aerial current input cable load current approach eliminating this problem make impedance system reference possible. This achieved using ground plane, grid strip.
3.31
Layout with impedance
OPA2234
Input Output
3.32
improved layout reduces risk. signal reference changed from trace strip, making impedance. This lowered impedance essentially eliminates impedances thereby improving cross-talk error. layer also makes possible place passive pass filters input output amplifiers further reject band noise.
3.32
Connecting More Than
MAINS PCB1 PCB2 Connector PCB1 PCB2
PCB3
PCB4
MAINS
PCB3
PCB4
RIGHT
WRONG
3.33
When multiple boards system, "star" configuration recommended better configuration supplies system references. This technique allows lower impedance system reference eliminates larger differential current loops (ground loops). Short cables connected side each board gives best performance. Lack planning create difficult loop problems down road. typical example shown above configure system. This circuit prone produce differences potentials individual board system references.
3.33
Component Placement
high
Digital
Buffer
frequency
High Frequency Components placed near connectors
Analog
Separate Digital Analog Sections Circuit
3.34
Many times circuit designs contain variety high speed frequency components analog digital components. Generally, best performance achieve these groupings components kept close proximity. example, analog devices positioned reside same ground plane high frequency components grouped separately from medium speed frequency components. Additionally, high frequency circuits should placed close connectors. This will shorten length interconnects, consequently reduce line inductances. Another good layout technique would separate ground planes digital analog sections. This technique could reduce considerable amount cross-talk between types components. These general guidelines offer designer starting point layout. There are, course exceptions these suggestions, such grounding practices with converter. case converter, both analog digital ground connections should connected analog ground plane.
3.34
Digital
Decreasing Antenna Effect
0.06m
Trace
corners should have angles
0.018m
Trace 16.6 m/s, length trace
3.35
previously discussed, connectors should close high speed section circuit possible. Shorter traces make layout less sensitive high frequency interference that happens signal bandwidth. traces above used illustrate implications these statements. first trace length 0.06m making perfect antenna frequencies that multiples 5GHz. trace length changed shown with second trace. this illustration, longest trace length 0.018m making that trace sensitive frequencies that multiples 16.6GHz.
3.35
Cables Connectors
Cables perfect antenna emissions susceptibility coupling between desirable signal circuit (undesirable) common-mode signal transfer impedance gives leakage cable
3.36
Cables connectors applications require special attention because they like antenna transmission receiving. most important parameter cable transfer impedance also known cable impedance, coupling mechanism used send signals from circuit circuit. transfer impedance well leakage characteristics cable provide mechanism susceptibility emission EM-environment. Many engineers assume that coax cable sufficiently shielded EMCapplications. This always case because coax cable have higher leakage than expected. This fact that coax shield consists only conductor. Higher quality coax cables have lower leakage lower line impedance, Once cable with selected, quality connections between cable board have great influence leakage issue. Glass fiber good alternative coax cables. true that actual fiber immune interference, however, aware that electronics both sides (usually high speed) must compatible.
3.36
Influences Cables
transfer-impedance cable
+15V INA118 -15V
will undesirable signal. cable impedance dependent value this signal.
Source
Cable
Load
3.37
this application, cable connection between Instrumentation amplifier (INA118) load sensitive emitted signals. These unwanted signals will added common-mode signal appear load. Before quantitative answer determined concerning magnitude interfering signal, inductance wires relation interfering signal require evaluation.
3.37
Global Transfer-Impedance
SOURCE LOAD
3.38
equivalent circuit INA-application shown this circuit. cable's inside outside impedance used explain influence connection between source load. impedance, serves coupling path undesirable signal coupling factor, facilitates inductive cross-talk. total effects coupling shown below: With Source Cable Load. Since cable specifications most interest this example, they separated from other circuit coupling paths. total impedance cable called global transfer impedance used this calculation. global transfer impedance describes entire connection. obtain global transfer impedance particular system, transfer impedance multiplied total length specific system. gives value leakage, derived from combination resistive, capacitive inductive factors. value equal leakage cable resulting common voltage. ZCABLE With Cable This approximation only applicable frequency applications where where L=length =wave length (m).
3.38
Total EM-Leakage
Signal
BEST
impedance between cable cover Choosing right configuration
WORST
3.39
Once EM-leakage determined using global cable impedance, cable connections should evaluated. addition contribution EMleakage these connections complete picture. total impedance calculated below: ZTCS ZTCL ZTCS ZTCL global cable impedance connection source impedance connection load impedance
When circuit impedance good connection made disturbance signals also low. impedance connection also made steering disturbance current cover device. best good approaches configuring grounds connector shown diagram above.
3.39
make Multi-cable Assembly
Separate signal power cables Separate primary secondary mains filter wires Separate Sensor cables from power digital cables Earth ground should have housing
3.40
Multi-cable assemblies unavoidable applications. cables inside machine other kind device, kind signals that they carrying, dictate proper groupings cables. these guidelines used, cross-talk disturbances problems reduced. combine wires with inductive loads such relays motors together with digital cables. When mains filter used, primary secondary wires filter same multi-cable assembly. High-frequency signal cables same multi-cable assembly cables transmitting sensor signals. wire earth protection multi-cable assembly. other cables disturbed earth-wire causing commonmode problems.
3.40
Shielding your application
impedance cover Make good connections between different parts cover Make many smaller holes instead conductive foil with plastic cover
Paint
smaller holes
3.41
When design constructed using guide-lines possible that still high. Further action taken reduce these signals with shielding (Faraday shielding) around housing. Shielding will help reject emission improve immunity. most cases metal housing device will work shield. case where housing made plastic possible paint inside housing with conductive coating place conductive foil against cover. Most covers need holes wires cooling lines. When device needs these holes housing many smaller holes better than hole. wave length unwanted signals that come through holes determined formula: When selecting shielding material magnetic coupling dealt with separately. frequency differences interference sources necessitate different shielding materials. lower frequencies, only ferromagnetic materials offer properties needed shields practical thickness. higher frequencies, decreases both shield thickness requirement magnetic response ferromagnetic materials make copper good alternative. Even copper layer ground plane becomes effective magnetic shield. wave length speed light (m/s) frequency (Hz)
3.41
Shielding Attenuates
effect E-field Damping decreases with increasing frequency Damping decreases with increasing distance between source shield effect H-field Damping increases with increasing frequency Damping increases with increasing distance between source shield
3.42
important know what kind interference signals need damping. signal's source creates electric-field require different shielding strategies than signal source creates magnetic-field. H-fields have different impedances. impedance E-dipole, higher impedance than H-dipole, near-field high impedance E-field impedance H-field E-fields damping factor E-field Damping decreases with increasing frequency Damping decreases with increasing distance between source shield. H-fields damping factor H-field: Damping increases with increasing field Damping increases with increasing distance between source shield. dielectric constant, permiability, distance from source sheild)
3.42
Connect Shield Reference
SHIELD SHIELD
Connect Shield System Reference
3.43
Shielding device have positive results well negative. Parasitic capacitance between shield circuit natural consequence. These capacitances, large enough create large enough feedback path critical node application. most simple direct solution this type problem connect shield impedance system reference.
3.43
Using Mains filter
Keep connection filter/ protecting-earth short possible. Reject cross-talk between output. Keep cables short possible. Don't bring input output cables together.
Filter
Equipment
ideal position filter
3.44
last resort, mains filter should employed. mains filter rejects small emissions noise enhances immunity. mains filter constructed mechanically, using shielding techniques electrically using capacitive bypass techniques filters. This filter necessarily enough meet requirements alone. design should carefully configured according guidelines then filter help reach required performance. Mains filters used with following guidelines: sure there good connection between reference filter protecting earth ground keeping connection short possible. Reject capacitive inductive cross-talk between input output filter. When mounting metal filter cover application, sure there good contact. Mounting painted metal anodized aluminum give problems. Keep cables from secondary filter side circuit short possible. Don't bring cables from filter together cable bundle. When possible avoid coupling between circuit PCB(s) filter input output cables, shield inserted between filter network cables.
3.44
rejection CM-signals
Galvanic isolation will reject CM-signals galvanically isolation devices have parasitic capacitances CISO 0.4pF 10pF SIGNAL INPUT SIGNAL OUTPUT
CISO
3.45
Often isolation device such isolation amplifier opto-coupler used galvanic separation and/or reduction common-mode voltages. With these devices, light source, magnetic field electric field used transmit circuit signal order obtain galvanic separation. There parasitic capacitance between input devices output. equivalent circuit shows parasitic capacitance isolated instrumentation amplifier, ISO175. ISO175 also isolation instrumentation amplifier, similar ISO213, except ISO175 does have on-board DC/DC converter. Additionally, isolation achieved with E-field transmission across barrier through embedded capacitors package. unreasonable expect emissions 1GHz EM-environment. When effects isolation amplifier's parasitic capacitance evaluated that frequency operates like impedance resistor. example
CISO (for ISO175) 1GHz CISO Similar total values isolation capacitance found circuits using optocouplers, where signal communication paths total optocouplers. this case, total equal summation isolation capacitance individual couplers.
3.45
Bottom Line
Consider Good Design Practices from Beginning Design Process Prevent Emission from Entering Device Traces Loops Perfect Antenna
3.46
give emitted signals coupling path into device. simplest paths entrance unwanted signals cables. Eliminate large traces loops which function antenna picking emitting high frequency signals.
3.46
Items
good design Keep your mind: Parasitic properties only bandwidth need Create impedance system reference Separate high frequency circuits Every trace antenna Create shield with small holes
3.47
items keep mind when designing print circuit board EMC-rule emission immunity Place cables connectors side close possible each other. Keep mind affects parasitics. only bandwidth need. Create impedance reference-layer side where cables connected board. This will reduce cross-talk. Each trace antenna disturbance-signals send receive). shield when required with small holes keep disturbance signals inside out, case
3.47

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