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Sensors 2016, 16(5), 663;

Analysis of Subthreshold Current Reset Noise in Image Sensors

Research Institute of Electronics, Shizuoka University; 3-5-1 Johoku, Naka-ku, Hamamatsu 432-8011, Japan
Laboratory of Advanced Science and Technology for Industry, University of Hyogo; 1-1-2 Koto, Kamigori, Ako-gun, Hyogo 678-1205, Japan
Academic Editor: Eric R. Fossum
Received: 26 January 2016 / Revised: 13 April 2016 / Accepted: 4 May 2016 / Published: 10 May 2016
(This article belongs to the Special Issue Photon-Counting Image Sensors)
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To discuss the reset noise generated by slow subthreshold currents in image sensors, intuitive and simple analytical forms are derived, in spite of the subthreshold current nonlinearity. These solutions characterize the time evolution of the reset noise during the reset operation. With soft reset, the reset noise tends to m k T / 2 C P D when t , in full agreement with previously published results. In this equation, C P D is the photodiode (PD) capacitance and m is a constant. The noise has an asymptotic time dependence of t 1 , even though the asymptotic time dependence of the average (deterministic) PD voltage is as slow as log t . The flush reset method is effective because the hard reset part eliminates image lag, and the soft reset part reduces the noise to soft reset level. The feedback reset with reverse taper control method shows both a fast convergence and a good reset noise reduction. When the feedback amplifier gain, A, is larger, even small value of capacitance, C P , between the input and output of the feedback amplifier will drastically decrease the reset noise. If the feedback is sufficiently fast, the reset noise limit when t , becomes m k T ( C P D + C P 1 ) 2 2 q 2 A ( C P D + ( 1 + A ) C P ) in terms of the number of electron in the PD. According to this simple model, if CPD = 10 fF, CP/CPD = 0.01, and A = 2700 are assumed, deep sub-electron rms reset noise is possible. View Full-Text
Keywords: CMOS image sensor; 3-transistor scheme; reset noise; subthreshold current; hard reset; soft reset; feedback reset; tapered reset CMOS image sensor; 3-transistor scheme; reset noise; subthreshold current; hard reset; soft reset; feedback reset; tapered reset

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This is an open access article distributed under the Creative Commons Attribution License which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited (CC BY 4.0).

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Teranishi, N. Analysis of Subthreshold Current Reset Noise in Image Sensors. Sensors 2016, 16, 663.

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