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Time-to-Digital Converter using an Analogue Time Stretcher for 3D Time of Flight Camera
Luleå University of Technology, Department of Computer Science, Electrical and Space Engineering, Embedded Internet Systems Lab.
Department of Electrical Engineering, University of Oulu.
Department of Electrical Engineering, University of Oulu.
Department of Electrical Engineering, University of Oulu.
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2014 (English)Conference paper, Presentation (Refereed)
Abstract [en]

This paper describes an architecture and achievable performance of a time-to-digital converter by cascading a time stretcher and a gated ring oscillator based time-to-digital converter (GRO-TDC). The analogue time expansion, where the time interval to be measured is stretched by a factor k, is realized by charging a capacitor with a current I followed by discharging the capacitor by a current I/k . The currents are created by wide swing cascode current source/sinks with a current ratio of k. The time stretching method involves two conversions: time to charge and then charge to time. Whereas these are performed in each individualpixel, the final time to digital conversion is performed by the global GRO-TDC, where a multiphase gated ring oscillator is used to measure the stretched time interval by counting full clock cycles and storing the states of the ring oscillator within the clock period to obtain increased resolution. A block diagram of the proposed structure is shown paper.The nine phase single ended gated ring oscillator operates as an interpolator and as a clock during charge to time conversion. The layout of the gated inverters in the oscillator is designed by placing them in an order to compensate for the parasitic loading of capacitance for each stage and to minimize the effects of process variations. The clock of the 8-bit ripple counter is enabled at the start of the charge to time conversion by the Start-Stretch signal. The result of the counter will be ready after the clock of the counter is disabled to the counter by the Stop-Stretch timing mark from the comparator. Special attention is paid to the design of the hysteresis based comparator using positive feedback. The comparator is designed to achieve acceptable robustness against transistor mismatch, small power dissipation, offset voltage, linearity, speed, small area and good noise immunity. The digital flip-flops functioning as an interpolator will store the time interval measurement responses with in the clock cycle. By selecting an appropriate stretch factor and suitable clock frequency for the gated ring oscillator, measurement error of few cm in distance is achievable. To ensure reliable recording of the timing signals, the counter is synchronized by usinga dual edge synchronization scheme where one of the two flip-flops always has enough delay between data change and clock edge to avoidmetastability problems.

Place, publisher, year, edition, pages
Keyword [en]
3D time of flight camera, Time to digital converter, Single photon avalanche diode, open-loop integration, wide swing cascode cur- rent source/sink, Technology - Electrical engineering, electronics and photonics
Keyword [sv]
Teknikvetenskap - Elektroteknik, elektronik och fotonik
Research subject
Industrial Electronics
URN: urn:nbn:se:ltu:diva-39606DOI: 10.1117/12.2036539Local ID: e6f598c1-569e-44b5-8e44-3fe3164be1eaOAI: diva2:1013121
IS&T/SPIE Electronic Imaging 2014:Image Sensors and Imaging Systems 2014 : Image Sensors and Imaging Systems 2014 02/02/2014 - 06/02/2014
Känsligare avståndssensor baserad på TOF, Fas II, ESIS
Godkänd; 2014; 20121004 (tanmuh)Available from: 2016-10-03 Created: 2016-10-03Bibliographically approved

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Tanveer, MuhammadBorg, JohanJohansson, Jonny
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