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An Electronic Calibration Scheme for Logarithmic CMOS Pixels

An Electronic Calibration Scheme for Logarithmic CMOS Pixels. Bhaskar Choubey, Satoshi Ayoma*, Stephen Otim, Dileepan Joseph**, Steve Collins, University of Oxford, UK *Now with Renessas Technology Corp, Japan ** Now with University of Alberta, Edmonton, Canada. Outline. Introduction

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An Electronic Calibration Scheme for Logarithmic CMOS Pixels

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  1. An Electronic Calibration Scheme for Logarithmic CMOS Pixels Bhaskar Choubey, Satoshi Ayoma*, Stephen Otim, Dileepan Joseph**, Steve Collins, University of Oxford, UK *Now with Renessas Technology Corp, Japan ** Now with University of Alberta, Edmonton, Canada

  2. Outline • Introduction • FPN reduction Techniques • High Gain Readouts to reduce effects of gain variations • Electronic Calibration Scheme • Experimental Results • Conclusion

  3. Introduction • Limited range of CCDs and CMOS APS • Linear high dynamic range pixels require large number of bits with complex circuitry and/or post processing • Logarithmic Pixels • Human Eye like response • Encode Contrast Information

  4. Logarithmic Pixel Circuit

  5. Fixed Pattern Noise • Device Variations cause Fixed Pattern Noise • FPN reduces the contrast sensitivity of the image sensor, resulting in inferior images, compared to linear sensors • Joseph and Collins Model of Logarithmic Pixel

  6. FPN Reduction Methods • In Pixel Techniques • Hot Electron Effects (Ref. Ricquer and others) • Gate Voltage Adjustment with feedback (Ref. Loose and others) • Off Pixel Technique • Subtraction of Uniform Scene Response (Ref. IMS Chips) • Convenience • Error Possibility • Off Chip Technique • On Chip current source for Double Sampling (Ref. Kavadias and others) • Offset only • Remaining FPN ~ 15 % of one decade response

  7. FPN Reduction Methods • Corrected Response • Error in Corrected response • Equivalent Percentage Error

  8. Electronic Calibration • Joseph and Collins used 24 images and an iterative scheme • Computation of thee parameters

  9. Circuit Implementation

  10. The Two Readouts

  11. Residual FPN

  12. Comparison of Residual FPN

  13. Conclusion • Offset only FPN correction can not produce high quality image. • Impact of gain variation can be reduced by having a high gain readout like differential readout. • FPN can be reduced to a contrast sensitivity of <2% by electronic calibration scheme, comparable to the human eye.

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