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

Area of Science:

• Electrical Engineering
• Computer Engineering
• Sensor Technology

Background:

• Direct time-of-flight (dTOF) image sensors need precise timing references for depth calculation.
• Traditional synthesizers like PLLs/DLLs are limited by area and power for large pixel arrays.
• Relative timing methods require constant calibration, unsuitable for dynamic outdoor environments.

Purpose of the Study:

• To develop a scalable, low-area, low-power, and robust timing reference generation and synchronization method for dTOF imaging.
• To demonstrate the feasibility of the proposed approach through a practical hardware implementation.

Main Methods:

• Designed an 8x8 time-to-digital converter (TDC) array using a ring oscillator (RO) and ripple counter.
• Achieved phase and frequency locking among ROs for synchronized timing references.
• Fabricated the array using TSMC 65 nm technology.

Main Results:

• Each TDC occupies 550 μm² and consumes 500 μW, with a 2 μs range and 125 ps LSB.
• Achieved 18 dB phase noise improvement, <9 ps RMS jitter, and <0.11% frequency variation.
• Demonstrated low nonlinearity with DNL <2 LSB and INL <3 LSB.

Conclusions:

• The proposed scalable timing reference generation is efficient in area and power for dTOF imaging systems.
• The design provides robust synchronization and high precision, suitable for demanding applications.
• This method overcomes limitations of existing techniques, enabling advanced dTOF sensor applications.