An 11-bit 2MS/s column parallel SAR-RAMP ADC for ToF imager readout

Abstract

Time-of-Flight (ToF) is an optical range detection technique that derives range from the round-trip time of light pulse between the imager and the target. Due to the simple hardware and distance extraction algorithm, ToF imagers are widely adopted in existing (automotive, consumer electronics, etc.) and emerging (AR/VR, etc.) applications requiring depth information. ToF is divided into Direct-ToF (D-ToF) and Indirect-ToF (I-ToF). Compared with the D-ToF system which directly measures the round-trip time of the laser beam, the I-ToF system indirectly measures the time from a phase measurement of the received modulated pulse. I-ToF provides a smaller detection range but it is less sensitive to jitter in the system and can achieve higher accuracy. Hence, I-ToF is used in short-range high precision applications such as face recognition.

This thesis describes the readout ADC design for the next-generation I-ToF imagers used for smartphone face recognition in collaboration with Infineon. A compact 11-bit 2 MS/s column-level ADC is developed in 65nm CMOS image technology. To balance between the available area and required speed, a hybrid SAR-RAMP ADC concept is proposed, which also embeds a threshold comparison phase to relax requirements on successive phases. The ADC has been verified with the post-layout extracted view of the column-level analog circuits and the schematic view of the central/digital circuits with an ideal ramp buffer, achieving a 1.1 V - 3.3 V input range with a 1.1 V reference, a signal-to-noise and distortion ratio (SNDR) of 64.3 dB, 45 µW power consumption, a Walden figure of merit (FOM$_W$) of 107 fJ/conv-step and an area of 7 µm $\times$ 815 µm.