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Sensors 2018, 18(10), 3413; https://doi.org/10.3390/s18103413

Mutually Coupled Time-to-Digital Converters (TDCs) for Direct Time-of-Flight (dTOF) Image Sensors

1
AQUA Laboratory, Delft University of Technology (TU Delft), 2628 CD Delft, The Netherlands
2
AQUA Laboratory, École Polytechnique Fédérale de Lausanne (EPFL), 2000 Neuchâtel, Switzerland
These authors contributed equally to this work.
This paper is an extended version of our paper published in: Ximenes, A.R.; Padmanabhan, P.; Charbon, E. Mutually Coupled Ring Oscillators for Large Array Time-of-Flight Imagers. In Proceedings of the International Image Sensor Workshop, Hiroshima, Japan, 30 May–2 June 2017.
*
Author to whom correspondence should be addressed.
Received: 21 August 2018 / Revised: 27 September 2018 / Accepted: 3 October 2018 / Published: 11 October 2018
(This article belongs to the Special Issue The International SPAD Sensor Workshop)
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Abstract

Direct time-of-flight (dTOF) image sensors require accurate and robust timing references for precise depth calculation. On-chip timing references are well-known and understood, but for imaging systems where several thousands of pixels require seamless references, area and power consumption limit the use of more traditional synthesizers, such as phase/delay-locked loops (PLLs/DLLs). Other methods, such as relative timing measurement (start/stop), require constant foreground calibration, which is not feasible for outdoor applications, where conditions of temperature, background illumination, etc. can change drastically and frequently. In this paper, a scalable reference generation and synchronization is provided, using minimum resources of area and power, while being robust to mismatches. The suitability of this approach is demonstrated through the design of an 8 × 8 time-to-digital converter (TDC) array, distributed over 1.69 mm2, fabricated using TSMC 65 nm technology (1.2 V core voltage and 4 metal layers—3 thin + 1 thick). Each TDC is based on a ring oscillator (RO) coupled to a ripple counter, occupying a very small area of 550 μ m2, while consuming 500 μ W of power, and has 2 μ s range, 125 ps least significant bit (LSB), and 14-bit resolution. Phase and frequency locking among the ROs is achieved, while providing 18 dB phase noise improvement over an equivalent individual oscillator. The integrated root mean square (RMS) jitter is less than 9 ps, the instantaneous frequency variation is less than 0.11%, differential nonlinearity (DNL) is less than 2 LSB, and integral nonlinearity (INL) is less than 3 LSB. View Full-Text
Keywords: ring oscillator; clock distribution; synchronization; low-jitter TDC; dTOF image sensor; frequency synthesizer ring oscillator; clock distribution; synchronization; low-jitter TDC; dTOF image sensor; frequency synthesizer
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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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Ronchini Ximenes, A.; Padmanabhan, P.; Charbon, E. Mutually Coupled Time-to-Digital Converters (TDCs) for Direct Time-of-Flight (dTOF) Image Sensors . Sensors 2018, 18, 3413.

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