ECE 1352 Presentation Active Pixel Imaging Circuits

1
ECE 1352 Presentation Active Pixel Imaging
Circuits

• By
• Ashkan Olyaei

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Outline

• Active Pixel Sensors (APS) Vs. Charge Coupled
  Devices (CCD)
• APS Design Issues
• Dynamic Range
• Noise

3
Defining Some Concepts

• Quantum Efficiency
• QE Electrical Energy / Radiant Energy
• Fill Factor
• FF Active Area in Pixel / Total Pixel Area
• Microlenses improve the effective fill factor 2
  or 3 times

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APS 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

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APS 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.
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Dynamic Range

• DRpixel (dB) 20 log (Vmax/Vnoise)

• Rail-to-rail input swing

• Wider output swing

Chen Xu, M. Chan The Approach to Rail-to-Rail
CMOS APS for Portable Applications , in IEEE
Tencon. 2001.
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Dynamic Range
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Noise Analysis

• Temporal Noise
• Time-dependent fluctuations in the signal level
  of fundamental origins.
• Pixel Noise
• Photon shot noise (Photon detection a Poisson
  process, Noise N0.5)
• 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.

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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 Systems
• Mendis, 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

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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.

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Noise Reduction
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Conclusion

• Two generation of imagers CCD and APS
• Dynamic range critical as technology scales
• Noise an important impediment of APS

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References

• 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 Systems
• Mendis, 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