1EE290C - Spring 2004Advanced Topics in Circuit DesignHigh-Speed Electrical InterfacesLecture 28Case StudiesOptical LinksBorivoje NikolicApril 29, 2004.2AnnouncementsNo quiz:Grading: 70% project, 30% homeworksNo lectures next weekProject discussion/help on Tu, May 4, 4-5:30Project presentationsTu, May 11, 2:30-5:30 in BWRC23OutlineFiber optic transcieversMaterial: B. Razavi, “Design of Integrated Circuits for Optical Communications,” McGraw-Hill 2003.E. Sackinger, Tutorial at VLSI Circuits’004Copyright © The McGraw-Hill Companies, Inc.Complete Optical System35Transimpedance AmplifiersResistive conversion of current into voltageTransimpedance gain: ZT= δVout/δIin= RLTradeoff between transimpedance gain, bandwidth and noise6TIA Specifications: NoiseWith PAM, BER is simple:BER = errfc(Q), Q = is,pp/2in,rmsis,ppFor BER = 10-12, Q = 7Typical specsOC-48 (2.5Gb/s) BER = 10-10OC-192 (10Gb/s) BER = 10-1247Receiver SensitivityReceiver sensitivityMinimum average optical power needed to achieve required BERSens = Q in,rms/R [dBm]R = responsivity of the photodetectorPIN photodiode R < 1 A/W, noise low, symmetricalTIA noise determines sensitivity Avalanche photodiode (APD) R < 10 A/W, noise high, non-symmetrical (2.5Gb/s)Both TIA and APD contribute noiseOptical preamp + PIN photodiode (10+ Gb/s), noise high, non-symmetricalPreamp noise dominates8Sensitivity Examples-29dBm-34dBmSensitivity, APD, 7.5A/W-19dBm-24dBmSensitivity, PIN, 0.75 A/W17uA5.6 uAInput p-p signal, BER = 10-121.2uA400 nATIA rms noise10 Gb/s2.5 Gb/s0.2 - 0.25 dB better sensitivity translates into 1km distance(1.55um SM fiber)59TIA BandwidthTIA sets the receiver bandwidthHigh receiver bandwidth – lots of noiseLow receiver bandwidth – ISITypical setting – BW = 2/3 bit rate10TIA CircuitsOpen-loop TIAsLow device countHigh input BW (low input impedance) + high gainCommon gateCommon base611Common Gate TIAsCG + current sourceCheck Razavi (or Gray and Meyer) for complete treatment of noise12Open-Loop TIAsTradeoff between broad band and high gain @ given noiseWorse at lower suppliesLow gain in the first stage – the noise contributed by the second stage adds upTight tradeoff between gain, bandwidth and noise713Copyright © The McGraw-Hill Companies, Inc. Feedback TIAFirst-order TIA( )sTRsACRRAAsZFDFFT+≈+++−=1111DFdBCRAf ≈−3• RFnoise is directly referred to the input• but the RFdoes not limit the headroom/biasing14Feedback TIASingle-pole amplifier (second-order TIA)( )22/1 TsQsTRsZFT++=DFACRAf221π>No peaking Q < 0.7amp poleTIA BWTransimpedance limit()DFCRAABW1221+=π221BWCAfAARTATπ+<815TIA ImplementationKhorramabadi, ISSCC’95Dynamic range ~ 1/RFInput-referred noise ~RFMn1 adapts the RFto the signal level16TIA ImplementationTanabe, ISSCC’98917TIA ImplementationMohan, JSSC’0018Main AmplifiersLimiting amplifier (LA)vinvoutvinvoutLess gain with higher signal envelopeVariable-gain amplifier (LA)1019Main Amplifier SpecsGainHas to bridge the gap between TIA signal level and CDR (>20dB)BandwidthLimit ISI (20% larger BW than TIA)Dynamic rangeSensitivity of MA for required BER (~few mV p-p)Maximum signal – limiting in VGAs (few V p-p)Noise figureMA adds to total noise budgetLimit the impact on overall sensitivity (e.g. <0.2dB)20Main Amplifier SpecsInput offsetReduces receiver sensitivity (limit to < 0.2dB)Example VOFF< 0.2mVAM-to-PM conversionVariations in amplitude may be converted to PM (jitter)Needs to limit ∆tp< 0.1UI1121MA CircuitsRequired GBW is larger than fT2.5Gb/s: GBW = 100GHz10Gb/s: GBW = 400GHz22MA ExampleGreshishchev, ISSCC’991223MA: CMOS VGATanabe, ISSCC’9824MA: CMOS LASackinger, ISSCC’001325CDRsTypical CDR26Hogge CDR1427Alexander CDR28DMD in FiberDifferential mode dispersionImpulse response1km MMF, VCSEL1529PMD in FiberPolarization Mode DispersionMeasured PMD in SMFSimulated PMD in SMF30EqualizationAzadet, JSSC’021631That’s All FolksThanks for the fun semester.See you next