An Electronic Calibration Scheme for Logarithmic CMOS Pixels

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
Parameter Mean/SD SF Diff Amp
Offset (V) Mean SD 0.5524 0.0165 1.7430 0.0174
Gain (mV/decade) Mean SD -56.87 1.084 -66.7 1.023
Bias (fA) Mean SD 28.163 4.4974 26.188 6.3040
11
Residual FPN
Pixel Current (A) Residual FPN Contrast Sensitivity
3.55e-13 0.922 2.11
1.07e-12 0.694 1.62
3.91e-10 0.397 0.92
8.80e-10 0.440 1.10
3.25e-9 0.429 0.99
3.45e-8 0.501 1.16
1.57e-7 1.089 2.53
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
  lt2 by electronic calibration scheme, comparable
  to the human eye.