Transcript ECE 1352 Presentation Active Pixel Sensors

ECE 1352 Presentation
Active Pixel Imaging Circuits
By :
Ashkan Olyaei
Outline:

Active Pixel Sensors (APS) Vs. Charge
Coupled Devices (CCD)

APS Design Issues
– Dynamic Range
– Noise
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
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
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.
Dynamic Range

DRpixel (dB) = 20 log (Vmax/Vnoise)

Rail-to-rail input swing
Vdsat
+
Vtp
+
=
Vin
Vdsat
+
Vtn
Wider output swing
Vdsat
Vtn
+
Vtp
Vdsat
=
Vdsat
Vout
Vdsat
Chen Xu, M. Chan “The Approach to Rail-to-Rail CMOS APS for
Portable Applications ,” in IEEE Tencon. 2001.
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
5V
Pixel Size
12*10 um
16*16 um
Fill Factor
30%
35%
Dynamic Range
67 dB
68 dB
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 = 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

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
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 Reduction
Conclusion

Two generation of imagers: CCD and APS
 Dynamic range critical as technology
scales
 Noise an important impediment of APS
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