Mr. Daniel Van Nieuwenhove Profile

Daniel Van Nieuwenhove

at Vrije Univ Brussel

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Proceedings Article | 25 April 2008 Paper

Analysis of non-ideal behavior of CAPD based time-of-flight pixels

Riemer Grootjans, Ward van der Tempel, Daniël Van Nieuwenhove, Maarten Kuijk

Proc. SPIE. 7000, Optical and Digital Image Processing

KEYWORDS: Capacitors, Modulation, Cameras, Sensors, Amplifiers, Phase shift keying, Integrated circuits, Phase measurement, Integrated circuit design, Phase shifts

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An overview is given of the output of two main pixels based on a Current Assistant Photonic Demodulator (CAPD) and their usability in a ranging camera based on the Time-Of-Flight (TOF) principle. The first structure is based on an active pixel with an integrating capacitor, while the second structure is based on a transimpedance amplifier. A measurement system capable of interfacing with a one-dimensional array of CAPD pixels has been designed and is briefly discussed. The linearity of both CAPD pixels is measured, compared to each other and discussed. Several sources of non-idealities present in the CAPD output have been investigated, including differences between rise and fall time and duty cycle imperfections of the high frequency binary correlating signal, as well as CAPD bandwidth. The possible influences of such non-idealities of the correlating wave on the measurements as well as on the calculated distance are discussed. Resulting non-idealities in the raw CAPD output values include clipping and non-linearity, yielding offsets and non-linearities in the calculated phase difference between the original and the reflected signal. Theoretic values are shown and compared against measurements. Finally, inter-pixel properties of a one-dimensional array of CAPD structures are presented, as well as a sample range image acquired by scanning.

Proceedings Article | 25 April 2008 Paper

A CAPD based time-of-flight ranging pixel with wide dynamic range

Daniel Van Nieuwenhove, Ward Van der Tempel, Riemer Grootjans, Maarten Kuijk

Proc. SPIE. 7000, Optical and Digital Image Processing

KEYWORDS: Modulation, Imaging systems, Sensors, Quantum efficiency, Amplifiers, Linear filtering, Demodulation, Tolerancing, Prototyping, Ranging

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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 efficient mixer with a sensitive detector. Single ended and differential configurations of the structure are illustrated. Measurements show a quantum efficiency 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 μm². Circuit simulations show a dynamic range of 87 dB, as well as a background light tolerance up to 10 μA. Combining the circuit with a drain tap sensor yields a dynamic range of 127 dB. A standard CMOS 0.35 μm proof of principle prototype pixel, measuring 50 μm x 50 μm 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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