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PRTBDS1A.docPRINTED BOARDSMAR 2004TRANSMISSION MODES AND PATHSDIFFERENTIAL MODEUseful signals transmissionCOMMON MODEDisturbance signal transmissionCURRENT PATHSLow frequency currentshigh frequency currentsreturn pathreturn pathPOWER SUPPLY AND GROUNDSCOMMON MODE IMPEDANCEUcm: Icm ZcmDecrease Icm and Zcm in sensitive zonesSeparate noisy (digital) and sensitive (low level analog) circuits (Icm)Ground planes without slots (Zcm)For high frequency or fast digital signals a ground plane is necessary because paths show a higher ZNo slot in ground plane or supply plane , this would share the plane into two parts separated by an equivalent impedanceBetween boards , connectors show Z , so Z reduction is acheived by several groundpins spread over the whole connector width , this will also decrease the slot length .Smaller slotsLAY OUT AND DECOUPLINGAvoid large distance between PS line and ground line PS plane and ground plane is the best solution noise reduction: 2000 from previous caseThis is mandatory for fast digital circuits and very low level analog .Decoupling capacitor provide a close primary supply source for current inrush in ICs.These capacitors must be as close as possible to the IC .Their connections must be as short as possible SMD are recommended , direct link with planesDecoupling capacitors must be also installed on the board edge to decrease the slot at this point to limit radiation at board edges.For analog circuits:There must not be any variation between the supplies V+ and V-Supply path impedance must be reduced (wide and short)Lower noisy currents as much as possible in sensitive zones supplyStar supply: Each circuit has its own supply lineBut:The conductor length (Z) is not minimumThe noisy and sensitive stage supplies share a common pathThere is a large loop surface (supply and signal paths)Chain supply:Short supply links between successive stages ( Z and loop surface)The noisy stage is first suppliedsupplyDecoupling at each stageThis last disposition is prefered for analogcircuitry , the signal level increasing progressively along the chain . For digital circuits:There is no signal progression the equipotentiality must be global . For double sided boards:the minimum will be a meshed network up to the board edges .For multilayer boards:A ground plane and a supply planeare recommended (no slots)Internal planes do not offer the paths a comparable shielding asexternal planes . But cross talk between signal layers isnegligible .External planes shield the board butinternal lay out must be XY (cross talk)For mixed circuits:One must be able to distinguish directly analog parts and digital parts .Analog and digital ground are linked at ADC DAC level .The analog part must be supplied via the digital part .Power supply decoupling is necessary at converter levelFor multilayer boards if two ground planes are used (analog and digital) they must be connected together at converter level but the slot which is created there must not be crossed over by any path .One single ground plane is preferable but the two parts must be geometrically separatedConnector positions must take this into account .ELECTRICAL GROUND AND MECHANICAL GROUNDThe electronic 0V must be connected to the chassisFor digital and power boards there must be as many of these links as possible at connector level , at the board edges and below the hot zone at clock level and microprocessor level .For analog boards especially in case of very sensitive signals , there must not be several 0V-chassis links .For swiching power supply: a very short link must be set between 0V output and chassisSwitchingunitfilterIcmIn some applications this link is a capacitor The position of the SPS on the board must take this into account (close to mechanical contact).SENSITIVE PATHSCAPACITIVE CROSS TALKCapacity between pathsThe capacity depends on the distance and the width of the paths A screen path between the two conductors devides the cross talk by 5This screen must be connected to 0V at only one end On a bus a lot of screen lines must be used (one signal , one screen alternatively)A ground plane allows to decrease the mutual capacity if the distance between lines and GP is small compared with the distance between lines .INDUCTIVE CROSS TALKMutual inductance between pathsA magnetic field shorting loop must be set , this conductor must then be connected to 0V at both ends close to the victim and the disturbing conductorThe presence of a slot in a GP widens the loop of the active circuit The coupling with a neighbouring other loops is drastically increasedof course a bridge may be installed!When a slot is opened at one extremity it is 4 times as worse .Holes in a ground planeThey have only a small influence on Z , but some magnetic fields will not be intercepted by such a GP.There will be an increase of cross talk between layers , XY disposition is then recommended .Blind holes make things better .INOUTSUPPLY FILTERSTheir main problem is input output coupling (cross talk)Input and output should be at 180 anglePass through filters are the most efficientBOARD TO CHASSIS CAPACITIVE COUPLINGThe capacity of a board with regards to surrounding grounds allow any parasiticsource to use the board as an intermediate path , making trouble on sensitive pathsThis capacity always exist either with the chassis or with remote grounds .The capacity of a path at the edge of a board is much higher than that of a path inside .(edge effect)To avoid this effect a guard ring (0V) must be placed as a belt arround the board . Its width: 3 paths .The ring must be closed and linked with the ground plane by pass through holes . It is an electrostatic screen .GROUND FILLINGAfter routing paths , every empty zone should be filled with copper connected to the 0V (except special cases)INPUT OUTPUT FILTERINGGrouping the I/O cablesWhen a disturbance reaches the equipment by means of a cable , it leaves the board by the other cables and its parasitic capacityThese parasitic currents flow through the board , so to avoid this all the I/O cables must be grouped on one side on a reference ground .Filters implementationNo serial impedance between filter and potential reference (at least I/O local GP linked with chassis)No I/O cross talkNo I/O radiationSENSITIVE PATHS IMPLEMENTATIONSensitive paths are those supporting low level signals (generally input signals)which have a relatively high impedance load (and source)Of course sensitive paths must be identified first , this is a work between design and lay out people.Sensitive paths lay out must be routed manually first as much as possible at the center of the board to avoid edge effects , they must be as short as possible. This first state should not be modified .ELECTROSTATIC SCREENIn the case of plastic casing , the internal side must be covered with an electrostatic screen connected to the 0V through a low impedance link at I/O level . To decrease edge effects this screen must be a little larger than the board , a guard ring is very helpful .This screen effect may be obtained in multilayer circuits by laying the sensitive paths between supply and ground planes , very sensitive paths may be between two ground planes avoiding supply noise capacitive injection .GPGPPSPPSPELECTRIC FIELD TO WIRE COUPLINGIn presence of an electric field (voltage) badly protected and routed paths may collect currentsable to disturb electronic circuits , sometimes high components may also undergo this problem in RF.Equivalent surface of paths must be reducedGround plane cancells tangent electic field High components must be set parallel to the ground planeMAGNETIC FIELD TO LOOPS COUPLINGIn presence of a magnetic field (current) circuit loops develop voltages which may be disturbing .Loop surface reduction ( ground plane without slot) (SMD)Ground plane cancell normal magnetic field (top ground plane )Avoid high sockets for electronic components .ELECTRONIC CIRCUITS RADIATIONSLOOPS Currents in circuit loops radiate fieldsThe radiated fields strength is proportional to the loop surface conversely proportional to the distanceproportional to the frequency squaredproportional to the currentnoisy loops size must be taken into account for lay outfast switching circuitry must be laid far from the openings in the board centerdo not use faster switching circuits than necessary .CABLESCurrents in cables radiate fieldsThe radiated field strength is proportional to the frequency (Ll/2)proportional to the length (Ll/2)conversely proportional to the distanceproportional to the currentfit cables with several 0V wires to cancell signal radiationconnect 0V to chassis close to hot zone (currents return directly to the source)connect 0V to chassis close to cables connection (to avoid 0V noise on cables)filter every cables at I/O level , set cables against conductive enclosure (0 pick up )0V CHASSIS 0V CHASSIS CABLES AGAINST CHASSIS FILTERS THROUGH CHASSIS0V CHASSIS COMPONENTS IMPLANTATIONSeparate analog and digital zones , I/O on one side of the board Analog components are positioned starting with the most sensitive ones from the zone center far from edges .Digital components are positioned starting with the noisiest (fastest) ones preferably from zone center (0V / chassis connection).to limit edge effects , then according to decreasing noise order . Normally only interfacing circuitry is close to connectors (0V / chassis connection).CLOCKSThe clock must be the first to be routed after power supply , 0V must not be away from clock so as to limit loop surface (ground plane is best) . Clock path must be on signal layer immidiately after ground plane In case of cross talk a screen path (0V at each end) may be laid on each side of clock path If a clock signal is sent on a cable out of an enclosure , this should be in differential mode for radiation limitation .No use of IC sockets .Data transmission on bus: ground plane for direct return under each data line . At least one 0V line for eight data lines (near LSB).TRANSMISSION LINESFor high frequency (RF) and fast digital signal transmissions , paths behave in a different way than for low frequency and slow digital signals .For a 1ns rise time signal , the distance covered by this signal during its rise time is 30cm .It means that if the total path is longer than that , during at least 1ns , the generator does not de