1. ECE 1352 Presentation Active Pixel Imaging Circuits
By :
Ashkan Olyaei

2. 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

4. 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

5. 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.

6. Dynamic Range DRpixel (dB) = 20 log (Vmax/Vnoise)
Rail-to-rail input swing
Vdsat+
Vtp
Vtn + =
Vin
Wider output swing
Vdsat
Vtp
Vtn + =
Vout
Chen Xu, M. Chan “The Approach to Rail-to-Rail CMOS APS for Portable Applications ,” in IEEE Tencon

7. Dynamic 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

8. Noise Analysis Temporal Noise Spatial Noise (FPN)
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.

9. 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 Page(s):

10. 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.

11. Noise Reduction

12. Conclusion Two generation of imagers: CCD and APS
Dynamic range critical as technology scales
Noise an important impediment of APS

13. References Two generation of imagers: CCD and APS
CMOS image sensors: electronic camera on a chip Fossum, E.R.; Electron Devices Meeting, 1995., International , Dec. 1995
The approach to rail-to-rail CMOS active pixel sensor for portable applications Chen Xu; Mansun Chan; Electrical and Electronic Technology, TENCON. Proceedings of IEEE Region 10 International Conference on , Volume: 2 , Aug Page(s): 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 Page(s):
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 Page(s):
Two generation of imagers: CCD and APS
Dynamic range critical as technology