CMOS IMAGE SENSORS

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• CMOS IMAGE SENSORS
• A 200dB Dynamic Range Irisless CMOS Image Sensor
  with Lateral Overflow Integration Capacitor and
  Current Readout Operation
• Nana Akahane, Rie Ryuzaki, Satoru Adachi, Koichi
  Mizobuchi, Shigetoshi Sugawa
• ISSCC Dig. Tech. Papers, Feb. 2006

Presented by Katayoun Zand Advanced VLSI Course
Class Presentation, Fall 2006
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Outline

• Introduction
• Imag Sensor Architecture
• Pixel circuits
• Non-idealities and Performance measures
• CMOS Active Pixel Image Sensors
• Using a Lateral Overflow Integration to Improve
  the Dynamic Range of the Sensor
• A 200dB Dynamic Range Iris less CMOS Image
  Sensor with Lateral Overflow Integration
  Capacitor and Current Readout Operation

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Introduction

• Mobile imaging, digital still and video cameras,
  Internet-based video conferencing, surveillance,
  and biometrics
• Over 230 million parts shipped in 2004
• Estimated annual growth rate of over 28

The Imaging System Pipeline.5
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IMAGE SENSOR ARCHITECTURES
A cross-section photograph of an image sensor.5
(a) Readout architectures of interline transfer
CCD and (b) CMOS image sensors.5
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Pixel Circuits
Passive Pixel Sensor.5
3- and 4-T active pixel sensor.5
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Pixel Circuits (Cont.)
Diagram pixel sensor.5
Logarithmic pixel.6
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Non-Idealities and Performance Measures

• Three important aspects of image sensor
  performance are
• SNR (signal to noise ratio) higher than 40 dB
  required
• DR (dynamic range) range of illumination that
  can be detected by the image sensor.
• Standard CMOS Image sensors have a DR of 40-60
  dB, While human eye exceeds 90 dB.
• Spatial Resolution determined by the Nyquist
  sampling theorem.

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CMOS Active Pixel Image Sensor
Timing for CMOS APS readout.3
.
Schematic of the circuit.3
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Using a Lateral Overflow Integration to Improve
the Dynamic Range of the Sensor
Pixel schematic diagram, Timing and Potential
diagram.2
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Using a Lateral Overflow Integration to Improve
the Dynamic Range of the Sensor ( Cont.)
System Block Diagram2
Signal Processing Diagram2
The gain of the signal S2 is matched with the
gain of S1 by multiplying the capacitance
(CFDCCS)/CFD ratio by S2.
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Using a Lateral Overflow Integration to Improve
the Dynamic Range of the Sensor ( Cont.)
Switching concept from S1 signal to S2 signal.2
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Using a Lateral Overflow Integration to Improve
the Dynamic Range of the Sensor Sample Image
Non-saturated signal (S1)
Over-saturated signal (S2)
Wide dynamic range signal
2
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A 200 dB Dynamic Range Iris-less CMOS Image
Sensor with Lateral Overflow Integration
Capacitor using Hybrid Voltage and Current
Readout Operation
Nana Akahane, Rie Ryuzaki, Satoru Adachi, Koichi
Mizobuchi, Shigetoshi Sugawa ISSCC Dig. Tech.
Papers, Feb. 2006
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• The sensor features a hybrid readout operation to
  improve DR
• voltage-readout operation based on the lateral
  overflow integration capacitor in the pixel.
• Current-readout operation based on current
  amplification and logarithmic compression

The voltage-readout circuit with a lateral
overflow integration capacitor in the pixel is
the same as the one described previously, leads
to an extension of the DR keeping a high
sensitivity and a high SNR.
The current-readout circuit achieves further
extension of the DR on the bright end of the
range by reading out the logarithmic compression
of the photocurrent amplified in each pixel and
column.
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Schematic of the pixel
Current readout operation Timing.1
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• The voltage-readout operation incorporates the
  electrical shutter operation, the electric
  shutter time is sequentially varied as 1/30s,
  1/500s, 1/8ks, and 1/30ks

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Photoelectric conversion characteristics
The hybrid operation of the voltage and the
current readout extends the DR over 200 dB
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The Sensor Block Diagram
Performance summary1
Sensor block diagram1
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Sample Images

• The Image sensor is capable of capturing various
  scenes with the incident light ranging from about
  10-2 to 108 lx.

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Summary and Conclusion

• An introduction CMOS Image sensors was provided,
  the APS structure was explained.
• It was shown that
• SNR, DR and Spatial resolution are the three main
  performance measures of image sensors.
• a lateral overflow integration capacitor would
  enhance the dynamic range of APS and improve the
  circuit sensitivity and linearity It integrates
  the overflowed charges and improves the DR at the
  bright end, and reduce noise so the dynamic range
  would increase at the dark end.
• using a hybrid readout operation of the voltage
  and current would increase the dynamic range
  because the current readout circuit achieves
  further extension of the DR on the bright end.
• A 64x64 pixel, 200dB dynamic range CMOS image
  sensor using 0.35 µm technology with lateral
  overflow integration capacitor using hybrid
  readout operation was described.

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Selected References

• 1 N.Akahane, et al.,A 200dB Dynamic Range
  Iris-less CMOS Image Sensor with Lateral Overflow
  Integration Capacitor using Hybrid Voltage and
  Current Readout Operation ISSCC Dig. Tech.
  Papers, 2006
• 2 N. Akahane, et al., A Sensitivity and
  Linearity Improvement of a 100dB Dynamic Range
  CMOS Image Sensor Using a Lateral Overflow
  Integration Capacitor, Symp. on VLSI Circuits,
  pp.62-65, 2005.
• 3 S.K.Mendis, et al. , A 128x128 CMOS Active
  Pixel Image Sensor for Highly Integrated Imaging
  Systems, IEEE IEDM Tech. Dig., 1993
• 4 E.R.Fossum, CMOS Image Sensors Electronic
  Camera-On-A-Chip, IEEE Trans. on Electron
  Devices, Vol. 44, No.10, Oct. 1997
• 5 A.E.Gamal, et al. CMOS Image Sensors, IEEE
  Circuits and Device Magazine, May. 2005
• 6 B.Choubey, et. al.,An Electronic-Calibration
  Scheme for Logarithmic CMOS Pixels, IEEE Sensors
  Journal, Vol.6, No. 4,August 2006