A Sub-100 $\mu$ m-Range-Resolution Time-of-Flight Range Image Sensor With Three-Tap Lock-In Pixels, Non-Overlapping Gate Clock, and Reference Plane Sampling

Abstract

This paper aims to propose a time-of-flight (TOF) range imager with high range resolution in the sub-100 µm range. The range imager employs a TOF measurement technique that uses an impulse photocurrent response and a three-tap lock-in pixel based on the lateral electric field modulation. To reduce the clock skew of the gating clocks, a column-parallel digital delaylocked loop (DLL) with a dual clock tree is implemented with a short calibration time of approximately 42 µs. A non-overlapping gate clock generation included in the skew calibration circuit effectively suppresses the photogenerated-charge partitioning for reduced nonlinearity for distance measurements. Reference plane sampling (RPS) is proposed to reduce low-frequency jitter that limits the range resolution in the light-pulse trigger. In this technique, a reference pixel array is embedded in the same focal plane of the main pixel array to sample and cancel the correlated jitter in the light trigger. The prototype range imager with 192 × 4 effective pixels is implemented in a 0.11-µm CMOS image sensor technology. Using the RPS, a range resolution of 64 µm rms has been achieved, corresponding to a 430-fs time resolution with a 25-mm range. The range resolution has a large column-to-column deviation due to the jitter induced by the column-parallel gating buffers. In noisy columns, jitter that acts as random telegraph noise (RTN) is observed. Index Terms-CMOS image sensor (CIS), delay-locked loop (DLL)-based skew calibration, high range resolution, lateral electric field modulator (LEFM), lock-in pixel, non-overlapping gate clock, photogenerated-charge partitioning, range imaging, reference plane sampling (RPS), skew calibration, time-offlight (TOF).

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

“…Unlike noise, the returning power of the signal is dependent on the distance, d and decreases with d 2 , following the inverse square law. Given a certain photon detection probability (PDP) for the SPAD and fill-factor, FF, will reduce the Equations (3) and (4) to…”

“…Instead of letting the sensor integrate events over the entire measurement window, t meas , imposing this time constraint, referred to as the "coincidence window", reduces the likelihood of acquiring noise events whose probability of occurence within that window is very low, thus, electrically enhancing the SBR. Coincidence may be implemented at the sensor level over clusters/groups of closely-spaced pixels, exploiting a "more-likely" fact that neighboring pixels may belong to similar target depths (and thus, TOFs), as depicted in Figure 6 for the object labelled [4].…”

“…However, due to the fact that the illuminator modulation frequency in an ITOF system is directly proportional to the detection range while being inversely proportional to the achievable precision, they are limited to short-distance ranging (typically <30 m) [1][2][3]. A class of ITOF sensors based on short-pulse modulation and multi-tap lock-in pixels is becoming an attractive candidate due to higher achievable range resolution [4], however, it is currently limited to distances under 10 m [5]. Another drawback of ITOF sensors is their limited ability to distinguish two nearby objects [6].…”

A synchronous driving approach based on adaptive delay phase-locked loop for stitching CMOS image sensor

“…Similar problems arise in time resolved CMOS image sensors and timeinterleaved ADC, the influence of clock skew on the accuracy of time resolution is very important. To solve this problem, column parallel skew calibration circuits have been proposed and demonstrated its effectiveness [26,27,28]. A statistics-based background calibration and digital calibration scheme for timing skew is employed in timeinterleaved flash ADC [29,30].…”

An improved segmented DAC for column readout circuit correction of large array CMOS image sensor

“…With the development of CMOS technology and hybrid integration technology, CMOS image sensor (CIS) has become the most popular photoelectric conversion solidstate image sensor device [1][2][3][4][5]. O Because CMOS image sensor has the characteristics of high integration, low power consumption and fast speed, the demand for CMOS image sensor is growing rapidly with the increasing application in recent years.…”