An Electronic Calibration Scheme for Logarithmic CMOS Pixels Bhaskar Choubey, Satoshi Ayoma*,...
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Transcript of An Electronic Calibration Scheme for Logarithmic CMOS Pixels Bhaskar Choubey, Satoshi Ayoma*,...
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
• FPN reduction Techniques
• High Gain Readouts to reduce effects of gain variations
• Electronic Calibration Scheme
• Experimental Results
• Conclusion
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
Logarithmic Pixel Circuit
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
c)(xbay log
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
FPN Reduction Methods
)/log( corrincorr xxby
• Corrected Response
• Error in Corrected response
• Equivalent Percentage Error
)/log(100
corrin xxb
bk
)/log( corrincorr xxby
Electronic Calibration
• Joseph and Collins used 24 images and an iterative scheme
• Computation of thee parameters
b
ayc
xx
yyb
dexp
)/log( 12
12 )log( 11 xbya
Circuit Implementation
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
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
Comparison of Residual FPN
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.