Design of integrated time of flight system in cmos technologies

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Chowdhury, Asif Jahangir
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Electronic thesis
Electrical engineering
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Light detection and ranging (LiDAR) or Time-of-flight (TOF) distance measurement technique is a well-known apparatus used in various areas such as astronomy, geology, soil science, agriculture, law enforcement, surveying, transport, automotive vehicles, etc. Emerging application spaces include human occupancy sensing, pose estimation, and gesture recognition in elderly care facilities, day-cares, schools, hospitals, and anywhere people movement monitoring adds value. Moreover, in the current COVID-19 pandemic situation, when small businesses and commercial enterprises are suffering from an unwanted and extended period of closures, this technology could be utilized in social distancing and workplace safety monitoring. For the widespread deployment of such sensors, the primary requirements are low latency, low cost, reduced energy consumption, and a small form factor without sacrificing the measurement accuracy. While the commercial sensors are available, they are either intended for long-range automotive applications, where a high dynamic range, low noise, and high precision are needed. However, power consumption, monolithic integration, and production cost are not priorities. On the other hand, the low power, low-cost proximity sensors do not have enough distance range to fulfill the requirement of the indoor occupancy sensing. This thesis presents the design considerations, analysis, and measurement results for CMOS avalanche photodetector (APD) based TOF sensor receiver system. The underlying system architecture consists of a large area CMOS avalanche photodetector, an analog front end (AFE), and a Time-to-Digital converter (TDC). A fully integrated TOF AFE is designed in 0.35 µm CMOS technology with on-chip large area APD. Measurement results show the 300 µm× 300 µm APD achieves a responsivity of 0.29 A/W at 850nm and 0.12 A/W at 940 nm wavelength with an avalanche gain of 10 before a breakdown voltage of 10.1 V. The depletion capacitance of the APD is 30pF. The AFE achieves a transimpedance gain of 40 KΩ and a bandwidth of 50MHz. The total chip, including the APD, occupies an area of 1.48 mm2 and consumes 132mW power during operation. Direct illumination free space measurement is performed up to 1 m distance with no lens at the transmitter side and a 50mm focal length lens in front of the receiver. Additionally, the same 0.35 µm CMOS technology is used for the implementation of a novel multi-tier TDC chip for converting the time difference between the transmitted and reflected received signal into digital values. The maximum measurement range of the TDC is 2.56 µS, corresponding to a distance of 384m and measurement accuracy of ≈ 400 pS, corresponding to a distance value of 6.23cm. Minimizing the numbers of delay elements with multi-stage interpolation strategy through use of cyclic coarse and fine interpolators, the TDC could achieve an integral non-linearity as low as 0.33 LSB rms while maintaining a dynamic range of 2.56 µS. The area occupied by the TDC with pads is 3.74 mm2, and the power consumption is 165mW. The designed blocks in CMOS technology paves the way for low-cost and high accuracy TOF sensor solutions for personal and commercial indoor privacy-preserving sensing applications.
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Rensselaer Polytechnic Institute, Troy, NY
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