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A CAPD based time-of-flight ranging pixel with wide dynamic range Host Publication: Finds and Results from the Swedish Cyprus Expedition: A Gender Perspective at the Medelhavsmuseet Authors: D. Van Nieuwenhove, W. van der Tempel, R. Grootjans and M. Kuijk Publisher: SPIE Publication Date: Apr. 2008 ISBN: 978-0-8194-7198-7
Abstract: An in depth view is given of time-of-flight distance sensor and read-out circuit implementations. The sensor is a current assisted photonic demodulator, using majority currents to accelerate generated minority carriers towards the detecting junctions, and combines an effcient mixer with a sensitive detector. Single ended and differential confgurations of the structure are illustrated. Measurements show a quantum effciency of 58%, a DC demodulation contrast of nearly 100% and a contrast of over 40% maintained up till 60 MHz. An improved embodiment, having an extra drain tap to tune sensitivity, is also presented. With this structure automatic gain control and improved dynamic range can be achieved. Measurements demonstrate a controllable sensitivity from maximal to below 1% by changing a sensor control voltage. Secondly design aspects of the read-out circuit are discussed. A circuit with improved background light suppression and dynamic range is explained, called variable transimpedance amplifier. Special low pass filters for in-pixel averaging are presented, having a tuneable cut-off frequency of 10 Hz to 10 kHz and using only 100 um2. Circuit simulations show a dynamic range of 87 dB, as well as a background light tolerance up to 10 uA. Combining the circuit with a drain tap sensor yields a dynamic range of 127 dB. A standard CMOS 0.35 um proof of principle prototype pixel, measuring 50 um x 50 um and having a 60% fill factor, is presented. Properties, as well as an example image, of a 32x32 CAPD imager are included.
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