Boosted Readout for CMOS APS PixelsStUASuat U. AyDepartment of Electrical and Computer Engineering, ggUniversity of Idaho, Moscow, ID 83844-1023Email: [email protected]• Introduction•Low Voltage CMOS APS ReadoutLow Voltage CMOS APS Readout• Supply Boosted CMOS APS Readout•Measurement ResultsMeasurement Results• Conclusion110/13/2011 @ ECE Colloquium, Moscow, IDAnatomy of CMOS APS Pixel Readout• 3T CMOS APS Pixel readout• 3 Pixel transistors M1 R t•M1: Reset•M3: Select• M2: Source Follower (SF)()• Column transistor (M4) as active load to pixel SFGi i l th it•Gain is less then unity • Pseudo-CDS readout• Read FD node voltage gfirst, reset FD node and read reset level•Subtract two level for2•Subtract two level for effective pixel voltage10/13/2011 @ ECE Colloquium, Moscow, IDCMOS APS Pixel Input (reset level)310/13/2011 @ ECE Colloquium, Moscow, IDCMOS APS Pixel Input (signal level)410/13/2011 @ ECE Colloquium, Moscow, IDCMOS APS Pixel Input/Output Range510/13/2011 @ ECE Colloquium, Moscow, IDCMOS APS Pixel Input/Output Range()6DSATTHAAPDVOV2VVΔV(SF)AΔV−−≅×=10/13/2011 @ ECE Colloquium, Moscow, IDCMOS APS Pixel Input/Output RangeDSATTHAAPDVOV2VVΔV(SF)AΔV−−≅×=7Can we improve the I/O range of Pixel SF buffer?10/13/2011 @ ECE Colloquium, Moscow, IDCMOS APS Pixel Input/Output RangeDSATTHAAPDVOV2VVΔV(SF)AΔV−−≅×=7Can we improve the I/O range of Pixel SF buffer? YES10/13/2011 @ ECE Colloquium, Moscow, IDMethod-1 : Reset Boosting8Boosting RST pulse at least VTHabove VAAlevel10/13/2011 @ ECE Colloquium, Moscow, IDMethod-1 : Reset Boosting (cont.)9Boosting RST pulse at least VTHabove VAAlevel10/13/2011 @ ECE Colloquium, Moscow, IDMethod-1 : Reset Boosting (cont.)DSATTHAAPDVOVV2VΔV(SF)AΔV−⋅−≅×=10Boosting RST pulse at least VTHabove VAAlevel10/13/2011 @ ECE Colloquium, Moscow, IDMethod-2 : PMOS Reset TransistorDSATTHAAPDVOVVVΔV(SF)AΔV−−≅×=11Use PMOS reset transistor (Still 3T CMOS APS!)10/13/2011 @ ECE Colloquium, Moscow, IDMethod-3 : Rail-to-Rail Pixel BuffersOth th d [56] l il bl•Other methods [5-6] also available. – Achieving rail-to-rail operation,– Not a 3T pixel anymore,– Fill factor reduced drastically, hamper scaling.[5] C. Xu, W. Zhang, M. Chan, “A 1.0 V VDD CMOS active pixel image sensor with complementary pixel architecture fabricated with a 0.25 μm CMOS process,” IEEE Int. Solid State Circuit Conf., ISSCC-2002, pp. 44–443.[6] H.-C. Chow, J.-B., Hsiao, "A new rail-to-rail readout circuit of CMOS image sensor for low power applications," in 48th Midwest Symposium on Circuits and Systems (MWSCAS 2005), pp.1019-1022 Vol. 2, 7-10 Aug. 2005.1210/13/2011 @ ECE Colloquium, Moscow, IDProposed Method : Supply BoostingPdMthdBti l lt (V)l•Proposed Method: Boosting supply voltage (VAA) along with select (SEL) and reset (RST) signals.1310/13/2011 @ ECE Colloquium, Moscow, IDProposed Method : Supply BoostingPdMthdBti l lt (V)lVAASupply BoosterVAA•Proposed Method: Boosting supply voltage (VAA) along with select (SEL) and reset (RST) signals.VAACBIGVAASupply BoosterRST/SELSupplyBoosterGlobalRSTM1VAABCBIG0RSTDriverIPDCPDM3M2FDDriverRow(i)VCOLCPDSELVAA0SELDriver13COLVLNM410/13/2011 @ ECE Colloquium, Moscow, IDProposed Method : Supply BoostingWh B ti ?•Why Boosting?– Reset boosting : extending input/output range–Select boosting : extend linear gain rangeggg– Supply boosting : extended input/output range• Different combinations possibleR t d l t b ti (RSB)–Reset and select boosting (RSB)– Reset, select, supply boosting (RSSB)1410/13/2011 @ ECE Colloquium, Moscow, IDProposed Method : Supply BoostingWh B ti ?•Why Boosting?– Reset boosting : extending input/output range–Select boosting : extend linear gain rangeggg– Supply boosting : extended input/output range• Different combinations possibleR t d l t b ti (RSB)–Reset and select boosting (RSB)– Reset, select, supply boosting (RSSB)Possible Implications SolutionspDevice ReliabilityUse lower system supply voltage (VAA=1.2V) than process supply (5V).Imager NoiseUse relaxed supply boosting, improving noise Imager Noise pp y g p gperformance = Supply Boosting Technique (SBT)Complicated Electronics SBT requires very simple supply boosterASliLbti it ddPiblffhi14Array ScalingLarger boosting capacitor needed. Possibly off chip.10/13/2011 @ ECE Colloquium, Moscow, IDReset-Select-Supply Boosting (RSSB) OperationVAACBIGVAABoosterVAABVAA0VAA+-RSTM1M2FD0IPDCPDSELM3VVCOLVLNM4VAA015Step1: Initial state10/13/2011 @ ECE Colloquium, Moscow, IDRSSB Operation (cont.)VAACBIGVAABoosterVAAVAA+-VAABVAARSTM1M2FDVTH+-IPDCPDSELM3VVCOLVLNM4VAA016Step1: Soft reset10/13/2011 @ ECE Colloquium, Moscow, IDRSSB Operation (cont.)VAACBIGVAABoosterVAAVAA+-VAABVAA2VAARSTM1M2FDVTH+-IPDCPDSELM3VVCOLVLNM4VAA017Step2: RST Boosting, Hard reset10/13/2011 @ ECE Colloquium, Moscow, IDRSSB Operation (cont.)VAACBIGVAABooster2VAAVAAVAA+-VAABVAA2VAARSTM1M2FDVTH+-IPDCPDSELM3VVCOLVLNM4VAA018Step3: Supply Boosting, Soft reset10/13/2011 @ ECE Colloquium, Moscow, IDRSSB Operation (cont.)VAACBIGVAABoosterVAAVAA+-VAAB0RSTM1M2FD0SELVIPDCPDM3VCOLVLNM4VAA019Step4: Integrate Charge10/13/2011 @ ECE Colloquium, Moscow, IDRSSB Operation (cont.)20Step5: Boost supply10/13/2011 @ ECE Colloquium, Moscow, IDRSSB Operation (cont.)21Step6: Boost select and sample signal10/13/2011 @ ECE Colloquium, Moscow, IDRSSB Operation (cont.)VAACBIGVAABoosterVAAVAA+-VAABVAA2VAARSTM1M2FDVTH+-IPDCPDSELM3VVCOLVLNM4VAA022Step7: RST Boosting, Hard reset10/13/2011 @ ECE Colloquium, Moscow, IDRSSB Operation (cont.)VAACBIGVAABooster2VAAVAAVAA+-VAABVAA2VAAICOLRSTM1M2FDVTH+-ICOLVIPDCPDM3SELV2VVCOLVLNM4VAAICOLSample ResetVAAVAA2VAA23Step 8: RSSB, Soft to softer reset(go to Step 4)10/13/2011 @ ECE Colloquium, Moscow, IDRSSB Input/Output RangeVAAVAACBIGBoosterVAARSTM1VAABIPDCPDM3M2FDVCOLVSELVAAVLNM4()SHCOLCOLCOLΔTINβ⋅⋅24()THPARBIGSHCOL,COLCOLAAIMPVCCV1-βΔV −+−⋅=RSSB versus RSB10/13/2011 @ ECE Colloquium, Moscow, IDSimulation : Select boosting–Select boosting improves the output impedance linearity of M325Select boosting improves the output impedance linearity of M3 (SEL), improving both gain and linearity of pixel SF amplifier.10/13/2011 @ ECE Colloquium, Moscow, IDSimulation : RSB and RSSB versus no boostingdVdVVdVV2610/13/2011 @ ECE Colloquium, Moscow, IDSimulation : RSB and RSSB versus no boosting2710/13/2011 @ ECE Colloquium, Moscow, IDSimulation : RSSB
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