ECE 1352 PresentationECE 1352 Presentation Active Pixel Imaging CircuitsActive Pixel Imaging Circuits

By :

Ashkan Olyaei

Outline:Outline:

Active Pixel Sensors (APS) Vs. Charge Coupled Devices (CCD)

APS Design Issues– Dynamic Range– Noise

Defining Some ConceptsDefining Some Concepts

Quantum Efficiency

QE = Electrical Energy / Radiant EnergyFill Factor

FF = Active Area in Pixel / Total Pixel Area– Microlenses improve the effective fill factor 2

or 3 times

APS Vs. CCDAPS Vs. CCD CCD

– requires specialized expensive processes; not easily integrable with CMOS

– has high Quantum Efficiency, high fill factor and low noise– lacks random access and fast readouts– needs multiple voltages on chip for efficient charge transfer

APS – is lower voltage and lower-power– achieves random access and faster readout– can yield low noise with peripheral circuitry– compatible with CMOS process

APS DesignAPS Design

A Simple Photodiode APS Cell– Described by Noble in

1968– Three transistors per

pixel– High quantum

efficiency (no overlying polysilicon)

E. R. Fossum, “CMOS image sensors: Electronic camera-on-a-chip,” in IEEE IEDM Tech. Dig., 1995.

Dynamic RangeDynamic Range

DRpixel (dB) = 20 log (Vmax/Vnoise)

Chen Xu, M. Chan “The Approach to Rail-to-Rail CMOS APS for Portable Applications ,” in IEEE Tencon. 2001.

Vdsat+Vtp

Vdsat+Vtn

+ = Vin

Vdsat

Vtp

Vtn

Vdsat

+ =

Vdsat

Vdsat

Vout

Wider output swing

Rail-to-rail input swing

Dynamic RangeDynamic Range

Parameters Rail to Rail Photodiode APS Architecture

Normal Photodiode APS Architecture

Technology 0.25 um 0.8 um

Operating Voltage 1.2 V 5 V

Pixel Size 12*10 um 16*16 um

Fill Factor 30% 35%

Dynamic Range 67 dB 68 dB

Noise AnalysisNoise Analysis Temporal Noise

– Time-dependent fluctuations in the signal level of fundamental origins. Pixel Noise

– Photon shot noise (Photon detection a Poisson process, Noise = N^0.5) – Reset (kT/C) noise associated with reset level – Dark current shot noise proportional to leakage current and exposure time– MOS device noise (flicker 1/f noise and Thermal noise)

Column Noise– Thermal kT/C noise associated with the sampling process– Thermal and 1/f noise of the column amplifier MOS devices

Spatial Noise (FPN) Refers to a non-temporal spatial noise and is due to device mismatches in

the pixels & color filters, and variations in column amplifiers.

Noise Reduction Noise Reduction Correlated Double Sampling (Level 1)

– Reduce FPN and Temporal noise in pixel– Reset Level Transferred to CR

– Signal Level Transferred to CS

CMOS Active Pixel Image Sensors for Highly Integrated Imaging SystemsMendis, S.K.; Kemeny, S.E.; Gee, R.C.; Pain, B.; Staller, C.O.; Quiesup Kim; Fossum, E.R.; Solid-State Circuits, IEEE Journal of , Vol. 32 Issue: 2 , Feb. 1997 Page(s): 187 -197

Noise Reduction Noise Reduction Correlated Double Sampling (Level 2)

AJ Blanksby, MJ Loinaz, “Performance Analysis of a Color CMOS Photogate Image Sensor,” IEEE Transactions on Electron Devices, Vol. 47, No. 1, Jan 2000.

Noise ReductionNoise Reduction

Conclusion Conclusion

Two generation of imagers: CCD and APSDynamic range critical as technology

scalesNoise an important impediment of APS

ReferencesReferences CMOS image sensors: electronic camera on a chip

Fossum, E.R.; Electron Devices Meeting, 1995., International , 10-13 Dec. 1995

The approach to rail-to-rail CMOS active pixel sensor for portable applications Chen Xu; Mansun Chan; Electrical and Electronic Technology, 2001. TENCON. Proceedings of IEEE Region 10 International Conference on , Volume: 2 , 19-22 Aug. 2001 Page(s): 834 -837 vol.2

CMOS Active Pixel Image Sensors for Highly Integrated Imaging SystemsMendis, S.K.; Kemeny, S.E.; Gee, R.C.; Pain, B.; Staller, C.O.; Quiesup Kim; Fossum, E.R.; Solid-State Circuits, IEEE Journal of , Vol. 32 Issue: 2 , Feb. 1997 Page(s): 187 -197

Performance analysis of a color CMOS photogate image sensor Blanksby, A.J.; Loinaz, M.J.; Electron Devices, IEEE Transactions on , Volume: 47 Issue: 1 , Jan. 2000 Page(s): 55 -64

Two generation of imagers: CCD and APS Dynamic range critical as technology