An Adaptive Multiple Sampling With Slope Calibration Scheme in Indirect Time-of-Flight Sensor for Depth Precision Enhancement

Abstract

This paper presents a 200x232 CMOS indirect time-of-flight (iToF) sensor with an adaptive multiple sampling scheme that adjusts the number of samplings depending on the signal level for suppressing depth noise. A 10-b column-parallel single-slope analog-to-digital converter (SS-ADC) with five folded ramps is proposed to improve signal quality in the digital domain by sampling pixel voltages multiple times. The appropriate ramp is selected based on the signal level, and the optimal number of samplings is proceeded, enhancing overall efficiency. As the number of folding in the ramps is doubled in consecutive order while the range of the subsequent ramp is half of the previous one, the conversion time of the proposed SS-ADC is constant regardless of the ramp choice. In addition, the sensor incorporates an on-chip foreground calibration to mitigate the nonlinearity stemming from the mismatch of multiple ramps. The calibration is achieved by modifying the capacitance of the ramp generators. The prototype iToF sensor fabricated in a 110 nm BSI process achieves depth images from 0.6 m to 6.0 m with a high depth precision of 0.72% at 25 MHz modulation frequency and 60 fps without any frame rate degradation. The foreground calibration is successfully demonstrated by improving DNL errors smaller than +/- 0.7 LSB.