INTRODUCTIONAnalogDevicesAD1845Parallel-Port,16-Bit,Sound-Port,StereoCodecandCrystalSemiconductorsCS4231 Parallel Interface, Multimedia Audio Codec arepin-for-pin compatible. Many customers have had dif-ficultiesusingthesecodecsinterchangeablybecausetheyhaveslightlydifferentexternalinterfacerequire-ments and the parts documentation recommend differ-ingpowersupplycircuitry.Thesedesigndetailscontribute to the challenges of PC OEMs trying to designacompatiblesocket(aPCBlayoutcompatiblewithboth codecs) for these highly popular audio codecs.Thisapplicationnoteshowsdesignershowtoputasocket in their PC motherboard or plug-in card designthat is compatible with both parts (with some minor as-semblydifferences).Inadditiontotherecommendeddesign (provides the highest performance, but requiresthemostassemblydifferences),thisnotedescribescost and performance tradeoffs that are available withcompromise components (reducing the number of as-sembly differences with compatible external circuitry).TableIliststheassemblydifferencesbetweenanAD1845andCS4231systemfortherecommendedcodec socket, shown in Figure 1.USING A CODEC ON YOUR PCBThis application note was inspired by the difficulties sev-eralAnalogDevicescustomersreportedwhenputtingan AD1845 into PC boards laid out using the specifica-tion in the Crystal Semiconductor CS4231 (AD1845 pin-compatiblecodec)datasheet.Reportedproblemsvariedfromreducedperformancetocompletepartbreakdown.Thisapplicationnoteexplainsthedesignissuesin-volved in designing a codec socket that provides thehighest performance from both parts. For simplicity, allfiguresinthisapplicationnoteuseAD1845/CS4231aAN-404APPLICATIONNOTEONETECHNOLOGYWAY P.O.BOX9106 NORWOOD,MASSACHUSETTS02062-9106 617/329-4700Considerations for Mixed Signal Circuit Board Design(How to Design a PCB Layout/Assembly Compatible with the AD1845and CS4231 Codecs)PLCCpackagepinnumbers,butthedesignprinciplescovered apply as well to other Analog Devices packagetypes.IncludingacodecinyourPCmotherboardorplug-incarddesign(andgettingreasonableperformancefrom the part) requires some effort. For the AD1845 andCS4231,asmallgroupofdesignconsiderationshaveaprofoundinfluenceontheperformanceofyourfinaldesign. The design considerations that relate to creating acompatiblecodecsocket(aPCBlayoutcompatiblewith both codecs) for these codecs include the following: Input Circuit DesignThis section describes input circuit design and assem-bly differences between the two codecs for the highestperformance, compatible socket (shown in Figure 1). Power Supply DesignThissectiondescribescompatiblecodecsocketpowersupplydesign(includingthetworecom-mendedpowersupplylayouts)andexplainswhatmakessomecodecvendorsrecommendedpowersupply design incompatible with the AD1845. Layout DesignThissectiondescribeslayoutprinciples(componentplacementprioritiesandgrounding)forthehighestperformance compatible codec socket. Cost/Performance TradeoffsThis section describes a compatible codec socket thatdoes not require any assembly differences for the twocodecs (at the expense of lower performance).The application circuits shown in this note are sugges-tionsonly.Youshouldchoosecomponentvaluesthatit the needs of your own design and fall with the specifi-cations of the AD1845 and CS4231 data sheets.SoundPort is a registered trademark of Analog Devices, Inc.2AD1845 PLCC(CS4231 PLCC)XTAL2133pF33pF33pF33pF16.9344MHz24.576MHzCS4231ONLY21172218XTAL20XTAL11XTAL10(SEE L_MIC CIRCUIT)R_MIC (RMIC)L_LINE (LLINE)R_LINE (RLINE)L_AUX1 (LAUX1)R_AUX1 (RAUX1)L_AUX2 (LAUX2)R_AUX2 (RAUX2)M_IN (MIN)2830273942384346+5V100k1F1F47kM_OUT (MOUT) 471874_245DIRGDATAL_MIC (LMIC)1k1F1000pFAD1845ONLY29R_FILT (RFILT)L_FILT (LFILT)VREF (VREF)VREF_F (VREFI)1F(1000pF)1F(1000pF)10F10F 0.1F26313233PWRDWN (PDWN)RESET (NC)R_OUT (ROUT)47k41L_OUT (LOUT)47k40RD (RD)CS (CS)ADR1 (A1)ADR0 (A0)WR (WR)XCLL0 (XCTL0)XCTL1 (XCTL1)ISA BUSSA 19:2AENSAISAOIOWCIORCD7D6D5D4D3D2D1D0DRQxDRQyDAKxDAKyIRQz23VCC(VA2, VA1)GNDA(AGND1, 2)34 37NC(TEST)55GNDD(DGND3, 4, 7, 8, 1, 2, NC, NC)53 20 16 8 2 64 25 44VDD(VD4, VD3, VD1, VD2, NC, NC)FERRITEBEAD+12V0.1F 1F 0.1F 10F36 35VOUTVIN+5VREGULATOR0.1F10F 0.1F+5VFERRITEBEAD54 450.1F 0.1F 0.1F 0.1F 0.1F7 1 15 19ANALOGGNDDIGITALGNDANALOG AND DIGITALGNDCONNECTED AT ONE POINTBENEATH CODEC24599106160565865666768345662631412131157ADDRESSDECODEDBEN (DBEN)DATA7 (D7)DATA6 (D6)DATA5 (D5)DATA4 (D4)DATA3 (D3)DATA2 (D2)DATA1 (D1)DATA0 (D0)DBDIR (BDIR)PDRQ (PDRQ)CDRQ (CDRQ)PDAK (PDAK)CDAK (CDAK)INT (IRQ)1F1FFigure 1. Highest Performance AD1845/CS4231 Codec System DiagramTable I. Assembly Differences Between AD1845 And CS4231 For Codec Socket (In Figure 1)Component Function For AD1845 Install . . . For CS4231 Install . . .Crystal oscillator and capacitors on XTAL2 input Not required 16.9334 MHz crystaland 33 pF (2)Antialiasing filter on L_MIC, R_MIC, L_LINE, R_LINE, L_AUX1, 1 k and 1000 pF Not required, (but canR_AUX1, L_AUX2, R_AUX2, and M_IN inputs be left installed)External filtering capacitors for L_FILT and R_FILT inputs 1 F (2) 1000 pF (2)3Input Circuit DesignFigure 2 shows a portion of the AD1845/CS4231 systemand highlights the differences between input circuit de-signs for an optimum performance codec socket. Thissectiondescribesthefollowinginputstructurediffer-ences between the codecs. Crystal oscillators Analog input filteringCrystal OscillatorAs shown in Figure 2, the CS4231 requires two crystalinputs, 24.575 MHz (XTAL1) and 16.9344 MHz (XTAL2).The AD1845 defaults to one crystal input (24.576 MHz),but also can use other frequency sources including the14.31818 MHz PC bus clock. The AD1845 uses its Vari-ableSampleFrequencyGeneratortogenerateanyof50,000selectablesampleratesfromthesinglecrystalinput.AD1845 PLCC(CS4231 PLCC)33pF33pF33pF33pF16.9344MHz24.576MHz1k1F1000pF1F(1000pF)1F(1000pF)10F10F 0.1FXTAL21L_MIC (LMIC)R_MIC (RMIC)L_LINE (LLINE)R_LINE (RLINE)L_AUX1 (LAUX1)R_AUX1 (RAUX1)L_AUX2 (LAUX2)R_AUX2 (RAUX2)M_IN (MIN)R_FILT (RFILT)L_FILT (LFILT)VREF (VREF)VREF_F (VREFI)CRYSTALOSCILLATORINPUTDIFFERENCESANALOGINPUTFILTERDIFFERENCESCS4231ONLYAD1845ONLY2122171826313233XTAL20XTAL11XTAL10Figure 2. AD1845/CS4231 Codec InputStructures DiagramInput FilteringAs shown in Figure 2, each of the AD1845s ADC analoginputs (MIC, LINE, AUX1, AUX2, & M_IN) require an ex-ternal low pass antialiasing filter (1 k and 1000 pF), andthe AD1845 uses 1F capacitors on the external filterpins to apply a 2.6 Hz high pass filter to the ADC.The CS4231 applies its internal low pass antialiasing fil-tering after the input multiplexer stage and uses the ex-ternal filter pins to attach 1000 pF capacitors for the lowpass filtering.Note that for a compatible codec socket the externallowpassantialiasingfilterrequiredfortheAD1845iscompletely compatible with the CS4231 inputs, but thecapacitors on the external filter pins MUST change forbest performance. If you are (for example) replacing aCS4231 with an AD1845 in your system, the two 1000 pFcapacitorsmustbereplacedwith1Fcaps.Ifthe1000pFcapsareleftin,theAD1845shighpassfilterbreak point moves from 2.6 Hz to 2.4 kHz seriously re-ducing the audio band frequency performance. This per-formancereductionincludesanonlineargainvs.frequencyresponseandanoverallreductioningain.The gain loss can be as much as 30 dB at 20 Hz.Optional Input Level ScalerThe two codecs have slightly different input impedancesand their data sheets provide differing designs for scal-ing 2 V rms line level inputs. (The AD1845 and CS4231codecs can handle 1 V rms signals.) Figure 3 shows anexample voltage divider circuit for use with 2 V rms linelevelinputsthatiscompatiblewithbothcodecs.Forotherapplicationrelatedcircuits,seetheAD1845andCS4231 Data Sheets.AD1845(CS4231)2k1F1000pFL_MIC (LMIC)R_MIC (RMIC)L_LINE (LLINE)R_LINE (RLINE)L_AUX1 (LAUX1)R_AUX1 (RAUX1)L_AUX2 (LAUX2)R_AUX2 (RAUX2)M_IN (MIN)2kTHE 2k RESISTORSACT AS A VOLTAGEDIVIDERFigure 3. AD1845/CS4231 Codec Input Structures withVoltage Dividers DiagramPower Supply DesignYourpowersupplydistributionstrategymustaccountforthemixedsignal(analog&digital)natureoftheAD1845 and CS4231 codecs. For power supply designconsiderations, think of these codecs as having a digitalsection (digital portions of ADC, DAC, and ISA bus driv-ers) and analog section (analog portions of ADC, DAC,multiplexer, and output mixer stages).This section presents two successful strategies for com-patible power supply design and explains what makesthepowersupplystrategydescribedbyothercodecvendors documentation incompatible with the AD1845.4Recommended Power Supply DesignFigure 4 shows the recommended power supply design.This method regulates the codecs +5 volt analog supplyfrom the PCs +12 volt supply and uses the PCs +5 voltsupply directly for the codecs digital supply. While thismethod does require more parts, the regulated analogsupplyprovidesbetternoiseisolationfortheanalogside of the chip and yields improved converter perfor-mance. The dynamic range of this design is 1.5dB betterthan the alternate power supply method shown in Fig-ure 5.VCC(VA2, VA1)GNDA(AGND1, 2)NC(TEST)GNDD(DGND3, 4, 7, 8, 1, 2, NC, NC)VDD(VD4, VD3, VD1, VD2, NC, NC)0.1F10F 0.1F+5VFERRITEBEADFERRITEBEAD+12V0.1F 1F 0.1F 10F+5VREGULATORANALOGGNDDIGITALGNDANALOG AND DIGITALGNDCONNECTED AT ONE POINTBENEATH CODECRECOMMENDED P/S FORAD1845 PLCC (CS4231 PLCC)53 20 16 8 2 64 55 25 44 34 3736 35 54 450.1F 0.1F 0.1F 0.1F 0.1F7 1 15 19VOUTVINFigure 4.AD1845/CS4231 Codec Recommended Power Supply DiagramAlternative Recommended Power Supply DesignFigure 5 shows an alternative power supply design. Thisdesign uses the PCs +5 volt supply for the codecs ana-loganddigitalsupplies,isolatingthesupplieswithsmall inductors (ferrite beads) to minimize stray noisecausing currents. The advantages of this supply designare its low part count and cost reduction; each at the ex-pense of a slightly lower dynamic range than the systemshown in Figure 4.+5V1F1VCC(VA2, VA1)GNDA(AGND1, 2)NC(TEST)GNDD(DGND3, 4, 7, 8, 1, 2, NC, NC)VDD(VD4, VD3, VD1, VD2, NC, NC)0.1F0.1FFERRITEBEAD1F0.1F 1FANALOG AND DIGITAL GNDCONNECTED AT ONE POINTBENEATH CODECALTERNATE P/S FORAD1845 PLCC (CS4231 PLCC)53 20 16 8 2 64 55 25 44 34 3736 35 54 450.1F 0.1F 0.1F 0.1F 0.1F7 1 15 190.1FFERRITEBEADFigure 5. AD1845/CS4231 Codec Alternate Power Supply Diagram5Avoid Vendor/Codec Specific Power Supply Design!A third strategy for power supply design, shown in Fig-ure 6, divides the codec into three sections (analog andtwodigital).Thesectionsareanalog,internaldigital,and external digital (ISA bus drivers). Unfortunately thispowersupplydesignstrategyyieldsavendor/codecspecificsystemandshouldbeavoidedifyouwanttodesign a compatible codec socket.Theproblemwiththisparticularpowersupplydesign(recommended in some codec vendors documentation)istheassumptionthatallcodecmanufacturersassigndigital power pins in the same manner. Tables II and IIIshowhowthesupplypinoutsfortheAD1845andCS4231 are (virtually) identical for a two supply design,but differ greatly for a three supply design.ANALOGSUPPLYANALOGRETURNINTERNALDIGITAL RETURNVENDOR/CODEC SPECIFIC TRIPLE P/SNOT RECOMMENDEDEXTERNALDIGITAL RETURNINTERNALDIGITAL RETURNEXTERNALDIGITAL RETURN1F 1F+5VFERRITEBEADFERRITEBEAD+12V0.1F 1F 0.1F 10F+5VREGULATORANALOG AND DIGITALGNDCONNECTED AT ONE POINTBENEATH CODEC0.1F 0.1F 0.1F 0.1FVOUTVINFigure 6. Vendor/Codec Specific Power Supply DiagramNot RecommendedTable II shows that in the dual supply design the codecspower supply pinouts are identical, and Table III showsthedifferencebetweenthetwocodecspowersupplypinouts in a triple supply design. If you use this triplesupplydesign,youaredesigningavendorspecificsystem.Suppose, for example, that you designed a triple supplysystemfortheAD1845.Insuchasystem,the+5Vregulators output is tied to all of the AD1845s internaldigitalpowerpins(15,45),andalloftheISAdriverpower pins (1, 7, 19, 54) are tied to the ISA supply. If youneeded to install the CS4231 in this system for some rea-son, you would find that the Crystal part will not workbecause socket Pin 19 is connected to the ISA supplyand Pin 15 to the regulator to support the AD1845. ThesetwopinsareconnectedtogetherintheCS4231codecandthetriplesupplydesignfortheAD1845usestheCrystal part as a short between the ISA supply and theregulators output. If there is any substantial differencebetween the ISA +5 V and the regulator +5 V, the CS4231will (at worst) burn out or (at best) the system will notachieve optimum performance because the digital noisefrom the ISA supply is coupled through the codec ontothe analog supply.Because triple power supply designs (recommended insome codec vendors documentation) tend to treat othervendorspartsasashort(eitherthroughthepartspower pins or substrate), these designs produce vendor/codec specific systems.Note: If you want to design a system that can use anyAD1845compatiblecodec,donotuseatriplepowersupply design.Table II. AD1845 vs. CS4231 Codec DualPower Supply PinoutCodec PowerSupply Line AD1845 Pinout CS4231 PinoutDigital +5 V 1, 7, 15, 19, 45, 54 1, 7, 15, 1945 and 54NC)Digital Ground 2, 8, 16, 20, 25, 44, 2, 8, 16, 20, 53,53, 64 64 (25 and 44NC)Analog +5 V 35, 36 35, 36Analog Ground 34, 37 34, 37Table III. AD1845 vs. CS4231 Codec TriplePower Supply PinoutCodec Power AD1845 CS4231 Supply Line* Pinout PinoutExternal Digital 1, 7, 19, 54 1, 7(ISA Bus Driver) +5 VExternal Digital 2, 8, 20, 53, 2, 8(ISA Bus Driver) Ground 64Internal Digital +5 V 15, 45 15, 19Internal Digital Ground 16, 25, 44 16, 20, 53, 64Analog +5 V 35, 36 35, 36Analog Ground 34, 37 34, 37*NOTE: Division of digital power among internal/external digital powerpins on Analog Devices codecs is subject to change without notice. DoNOT base power distribution designs on the information in Table III.6Layout DesignWhenlayingoutaPCBformixedsignaldevices,likecodecs,beawarethatasmallsetoflayoutgeometryissueshaveaprofoundeffectoncomponentperfor-mance.Thissectionexaminesthefollowingmixedsignal PCB layout issues. Effects of long-etch impedance (impedance associ-atedwithtracebetweenchippinandcapacitor)ontheperformanceofbypasscapacitors,VREFcapaci-tors, and antialiasing circuits. Effects of ground and supply plane geometry on per-formance of mixed signal components.This section concludes with the following guidelines forAD1845 codec PCB design. A recommended ground and supply plane geometry. Aprioritylistforcloseplacementofexternalcomponents. AsummaryofrecommendationsformixedsignalPCB layout.Effects of Long-Etch ImpedanceBypass capacitors on your PCB are suppose to reducenoise by acting as shorts for noise generated by digitalcomponents and the digital side of mixed signal compo-nents.For example, a codec generates noise as it oper-ates when its internal digital circuitry turns currents onand off.These current changes show up on the powerand ground pins for the associated section of the codec.For each change in power pin current, there is a changeingroundpincurrent.Abypasscapacitorplacedinclose proximity to the part couples stray power currentsbackintothecodecthroughthenearestgroundpin.Without bypass capacitors, these straycurrents moveovernearbypower/groundplanesandincreasethenoise of the PCB.Close placement of bypass and filter capacitors to thecodec is crucial for a low noise PCB. The need for closeplacementoftheseexternalcomponentsstemsfromthe effects of long etch length between capacitors andcodecpins.Attheoperatingfrequencyofthecodec,lengthsofetchactassmallinductors;thelongertheetchthegreatertheinductance.Figure7showsarepresentationofabypassingcircuitbetweenacodecpower pin and ground as a capacitor in series with aninductor. Note that the value of the inductor in Figure 7 isdirectly related to the etch length between the capacitorand the power pin.+5V0.1FCODEC POWER PIN"LONG-ETCH" INDUCTANCEEFFECT, APPROXIMATELY1nH/mmBYPASS CAPACITORFigure 7. Long-Etch Inductance Effect ModelA bypassing circuit is supposed to be a low impedancepoint for high frequency currents. Because the imped-anceofthebypassingcircuitisdependentonthedis-tancebetweenthecapacitorandthepowerpin,thelong-etchinductanceeffectcanforcestrayhighfre-quency currents on the power pin into the part when thepart become the path of least resistance to the groundplane.Attypicalcodecoperatingfrequenciesforexample, a bypass capacitor connected to a power pinwith a 20 mm (0.5 in) trace is actually a 3.55 MHz band-pass filter.Toavoidlong-etcheffects,usetheshortestpos-sibletracesforbypassandfiltercapacitors.Alsonotethatusing3to5mmwideetchesforcapacitorcon-nectionsreducesinductanceeffectsaswell(highlyrecommended).The AD1845 is available in PLCC or TQFP packages. Thesmall physical dimensions of these packages can make itchallenging to place all the required capacitors close tothe part. Because some pins have a much greater effectonperformancethanothers,usetheinformationinTable IV to prioritize component placement. Pins with anA priority in Table IV should be connected to their as-sociatedcapacitorswiththeshortesttracespossible.Figure 8 also shows the distribution of codec bypass andfilter capacitor locations and placement priorities.Table IV. AD1845 Compatible Codec Capacitor Placement PrioritiesPriority of Close Proximity to Chip PinSignal Description PLCC Package Pins Placement of Filter and Decoupling CapacitorsVoltage Reference (VREF) 32 AVoltage Reference Filter (VREF_F) 33 ADigital Supply Voltage, +5 V (VDD) 1, 7, 15, 19, 45, 54 BChannel Filters (L_FILT and R_FILT) 26, 31 BAnalog Supply Voltage, +5 V (VCC) 35, 36 CAnalog Signal Inputs (Filter and Decouple) 27, 28, 29, 30, 38, D(L_LINE, R_LINE, L_MIC, R_MIC, L_AUX1, 39, 42, 43, 46R_AUX1, L_AUX2, R_AUX2, & M_IN)7Effects of Ground and Supply Plane GeometryFigure 8 shows an example ground plane layout for anAD1845 PLCC package. This layout separates the analogand digital ground planes with a 2 to 3 mm gap and con-nects them at one point beneath the codec with a single3 to 4 mm wide link. The ground planes reduce boardnoise by shielding analog lines from digital interfer-ence. The link between the planes, as close to the codecaspossible,preventsanypotentialdifferenceduetoESDorfaultcurrents.Withoutthelink,thesecurrentscould flow through the codecs substrate degrading per-formance. You should try to avoid running any digital oranalog signal traces across the gap between the digitaland analog planes.9106160274443ABC ABBBBBBB C D D D D D DDD DAD1845COMPATIBLEPLCC FOOTPRINTDIGITAL GROUNDPLANEANALOGGROUNDPLANECODEC PIN NUMBERSPRIORITY LEVEL FOR CLOSEPROXIMITY PLACEMENTOF FILTER & DECOUPLINGCAPACITORS (A, B, & C)26 Figure 8. AD1845 Compatible Codec RecommendedGround Plane and Capacitor PlacementDuring PCB development, you may find it useful to pro-vide removable links between the ground planes in sev-eral PCB locations, to permit debugging and testing forground isolation.Another way to reduce PCB noise, in addition to groundplanes, is to include separate digital and analog powersupplyplanesdirectlyovertheirrespectivegroundplanesnooverlappingofsupplyplanes.Thesupplyand ground plane pairs should be separated by approxi-mately1mm.ThisrecommendationimpliesthatyouuseafourlayerPCB(atleast)withthegroundandpowerplanesformingahighcapacitivesandwich.This layout technique yields an extremely effective, lowESR/low ESL power distribution scheme.Foralayoutthathelpsreducenoise,locatealldigitalcomponents over the digital power/ground plane sand-wich and all analog components over the analog power/ground plane sandwich. Though this technique does noteliminate the need for bypass capacitors at the powerpins(mentionedabove),theimportanceofpower/ground planes in reducing overall PCB noise cannot beover emphasized.Digital noise coupled onto the analog portion of the chiphas three possible current return routes. The first returnpath is back through VCC and VDD where the currents arecapacitively coupled to their respective ground planes.The second possible path is through the componentssubstrate which has an 10 characteristic impedance.The final return path is through the external analog anddigital ground plane connection.By keeping the ground plane connection as close to thepartaspossible,thegroundconnectionbecomesthepathofleastresistanceandminimizestheamountofdigital current pushed into the substrate. If the groundconnectionisalongdistancefromthepart,returningcurrenttendstousethesubstrateconnectionincreasing signal noise in the part.Thiscurrentpathexampleissimplisticandisonlyamodel of how a PCBs layout can help reduce noise. Formore information on how noise coupling really works,see any of the texts listed in the References section.Codec PCB Layout Strategy SummaryThis section summarizes the layout suggestions for anAD1845compatiblecodecsocket.Togetthebestperformance from the codec in your system, apply thefollowing principles to your boards layout: Locatefilteranddecouplingcapacitorsascloseaspossible to their corresponding codec pins (Table IVand Figure 8 describe the specifics and priorities thisprocessentailsforanAD1845/CS4231socket).Close placement of capacitors to the chip pins helpstoavoidnoiserelatedtolong-traceinductanceeffects. Use a split (separate analog and digital) ground planeandmatchingnon-overlapping+5Vsupplyplanes.The power and ground planes should be separatedby approximately 1 mm (i.e., four layer PCB). A single3to4mmwidelinkunderthecodecconnectstheground planes (see Figure 8). Matching ground andpower planes provide a highly effective low ESR andlow ESL bypass capacitor for the system. Locate analog and digital components only over theirrespective ground planes and decouple their powerpins as closely to the chip pin as possible. Route ana-log and digital signal traces only over their respectiveground planes. These steps greatly reduce noise inthe analog section. Avoid using IC sockets for any analog components. Enable only a single oscillator on a PC board at a time(ifpossible).Thisisnotalwaysaproblem,butbeawarethatsomesystemproblemscanoccurasaresultofinterferencefrequenciesbetweenmultiplecrystaloscillatorsenteringthecodecthroughtheanalog or digital supplies or signal/reference pins.8Table V. Codec System Design Cost/Performance TradeoffsCost Reduction Design Choice Resulting Performance TradeoffUse .3 F capacitors on the external filter pins While this method eliminates an assembly difference betweenfor both the AD1845 and CS4231. using the codecs, the drawback of using .3 F capacitors is degradedperformance of both parts. A CS4231 in such a design has reducedhigh frequency performance and an AD1845 has reduced low fre-quency performance.Eliminate external antialiasing filter circuits. While it is possible to use the AD1845 without antialiasing filters (theCS4231 does not need them), the drawback of this design choice isthat codec performance can be dramatically affected if noise iscapacitively coupled into the ADCs inputs.Reduce the number of bypass capacitors While this method does reduces part count and cost, the placement ofthrough having common sectional power pins of the remaining capacitors becomes more difficult, bypassinguse the same capacitor for bypassing. efficiency is reduced, and the sound card has more noise problems. Minimizecapacitiveloadingondigitaloutputpins.Fordigitalsignals,drivinglongtraces,youmayhave to terminate the trace in its characteristic imped-ance (typically 100 ) to prevent over/undershoot andringing. Beawareoftheeffectsthatinductor/transformersexternalmagneticfieldsmayhaveonanalogcir-cuitry.Useelectrostaticandmagneticallyshieldedcomponentsasnecessary.RFdecouplingchokesmounted at right angles minimize mutual inductance.Mount power transformers off the board and orientthemwiththemostintenseareaoftheirexternalfields away from critical analog circuits. Use toroidalpower transformers to minimize external fields. Shield analog I/O lines by running shield traces be-tweenthemand/orrunningthemoverananalogground plane.COST VS. PERFORMANCE DESIGNSometimeslowercostismuchmoreimportantthanhigh performance. Table V lists methods for cutting sys-temscostsanddescribestheircorrespondingperfor-mance reduction.CONCLUSIONPlacing an AD1845/CS4231 compatible codec socketon your PC motherboard or plug-in card design is notdifficult. For optimum performance from each part, sucha socketusing a PCB layout common to both partsdoesentailminorassemblydifferencesbetweenthecodecs (see Table I and Figure 1). Using the guidelinesintheCostVs.PerformanceDesignsection,youcandesignacompatiblecodecsocketthatdoesnotre-quire any external component differences besides thecodec (with some loss in performance). For more infor-mation on application circuits for the AD1845, see theAD1845 data sheet and the reference documents listed.REFERENCESThe sources listed below contributed information to thisapplications note. They provide recommendations andtechniques for high-speed and mixed-signal design:Mixed Signal Processing Design Seminar,Analog De-vices, Inc., 1991, ISBN-0-916550-08-7.High Speed Design Seminar, Analog Devices, Inc., 1990,ISBN 0-916550-07-9.ApplicationsReferenceManual,AnalogDevices,Inc.,1993.Especially refer to collected Application NotesSection 24AN-214, AN-280, AN-282, AN-345, AN-346,AN-347, AN-353, AN-362.Noise Reduction Techniques in Electronic Systems, 2ndEd, Henry W. Ott, Wiley Interscience, 1988.Interfacing Techniques in Digital Design With Emphasison Microprocessors, Ronald L. Krutz, J ohn Wiley, 1988.Audio/VideoReferenceManual,AnalogDevices,Inc.,1992.Systems Application Guide, Analog Devices, Inc., 1993,ISBN 0-916550-13-3.High-Speed Digital Design, A Handbook of Black Magic,H.W.J ohnson,M.Graham,PTRPrenticeHall,1993.ISBN 0-13-395724-1.PRINTED IN U.S.A.E203958/95