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

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 μm2, while consuming 500 μW of power, and has 2 μs 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.

Modeling and Analysis of a Direct Time-of-Flight Sensor Architecture for LiDAR Applications

“…Figure 4 shows a high-level block diagram of the subgroup within a shared TDC architecture. Two such subgroups share an always-on TDC where the subgroup size has been assumed based on the achievable activity rate for a given incoming photon flux and power efficiency [24]. Within every subgroup is a combination tree which propagates the incoming events down to the TDC based on a first-in-win-all scheme, thus providing the timestamp of every first event [8].…”

“…However, the overall SPAD dead time in a shared case reduces to ≈t d,spad /M, thus making t d,comb the dominant contributor. Consequently, the maximum TDC conversion rate in a shared architecture as this is given by the inverse of the tree dead time, ≈1/t d,comb , which can reach up to Gtimestamps/s [24].…”

Time Resolution Improvement Using Dual Delay Lines for Field-Programmable-Gate-Array-Based Time-to-Digital Converters with Real-Time Calibration

“…Time-to-digital converters (TDCs) are vital components in numerous scientific applications, such as positron emission tomography (PET) [1][2][3][4][5][6], time-of-flight (ToF) image sensors [3][4][5][6][7], and light detection and ranging (LiDAR) [8][9][10]. Especially in ToF imaging and LiDAR applications, high-resolution TDCs affect overall performance.…”

A Cyclic Vernier Two-Step TDC for High Input Range Time-of-Flight Sensor Using Startup Time Correction Technique

“…The work in [ 32 ] presents detailed circuit techniques for all digital TDC to achieve a high resolution of 10 ps, however, only a limited amount of measured data is available. Two works developed for TOF imager, which realizes the TDC using a ring oscillator and a ripple counter, present TDCs achieving a high input range [ 13 , 33 ]. By locking phase and frequency among the ring oscillators, the TDC in [ 33 ] achieves an input range of 2 μs with a resolution of 125 ps.…”