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.