An Indirect ToF Sensor With In-Pixel Adaptive ΔΣ-Scheme for Background Light Rejection and Floating Diffusion Mismatch Cancellation

Abstract

This article presents a 160 x 120 indirect time-of-flight (iToF) sensor with an in-pixel adaptive Delta Sigma-scheme that controls the number of sub-integration times adaptively to mitigate large background light (BGL) with alleviated reset noise penalty. A pixel-level charge amplifier is employed to suppress common-mode BGL charge, while the adaptive Delta Sigma-scheme automatically selects the number of charge-subtraction operations with respect to BGL intensity to reduce the noise contribution from the charge amplifier. Additionally, an in-pixel floating diffusion (FD) swapping technique is proposed to alleviate non-idealities due to FD mismatches, such as depth linearity degradation and residual BGL charge. The proposed iToF sensor, fabricated in a 110-nm backside illumination CMOS image sensor (CIS) process, acquires depth images from 0.3 to 2.9 m with less than 2.7% depth noise and 2.1% non-linearity in a 10-ms integration time, offering 30 fps with a 50-MHz modulation frequency. The proposed FD swapping improves the depth non-linearity from 3.8% to 1.5% for the depth range of 0.6-3 m. The proposed adaptive Delta Sigma-scheme supports BGL resilience up to a BGL optical power of 3.3 mW/cm(2) at 940 nm with a 30-nm bandwidth (BW) optical bandpass filter, and the prototype has been successfully demonstrated outdoors under more than 60-klx sunlight.