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Proceedings Paper

A low light level sensor with dark current compensating pixels
Author(s): Mitchell Perley; Patrick Baxter; Jeffrey M. Raynor; David Renshaw
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Paper Abstract

In ultra-low light conditions the presence of dark current becomes a major source of noise for a CMOS sensor. Standard dark current compensation techniques, such as using a dark reference frame, bring significant improvements to dark noise in typical applications. However, applications requiring long integration times mean that such techniques cannot always be used. This paper presents a differential dark current compensating pixel. The pixel is made up of a differential amplifier and two photodiodes: one light shielded photodiode connected to the non-inverting input of the opamp and a light detecting photodiode connected to the inverting input of the opamp. An integrating capacitor is used in the feedback loop to convert photocurrent to voltage, and a switched capacitor network is present in parallel with the light shielded pixel, which is used to satisfy the output equation to compensate the dark current. The pixel uses 150 μm x 150 μm photodiodes and is fabricated in a standard 0.18 μm, 6M1P, CMOS process. The results show that the pixel is light sensitive and has a linear output as expected. However, the dark current is not predictably controlled. Further work will be carried out on the pixel design, and particularly the switched capacitor circuit, to determine the cause of the non-predictability of the pixel output.

Paper Details

Date Published: 27 September 2008
PDF: 12 pages
Proc. SPIE 7100, Optical Design and Engineering III, 71002E (27 September 2008);
Show Author Affiliations
Mitchell Perley, STMicroelectronics (United Kingdom)
The Univ. of Edinburgh (United Kingdom)
Patrick Baxter, STMicroelectronics (United Kingdom)
Jeffrey M. Raynor, STMicroelectronics (United Kingdom)
David Renshaw, The Univ. of Edinburgh (United Kingdom)

Published in SPIE Proceedings Vol. 7100:
Optical Design and Engineering III
Laurent Mazuray; Jean-Luc Tissot; Jeffrey M. Raynor; Rolf Wartmann; Andrew Wood, Editor(s)

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