Wun-ki Jung

CMOS image sensor with analog gamma correction using nonlinear single-slope ADC

A human eye has the logarithmic response over wide range of light intensity. Although the gain can be set high to identify details in darker area on the image, this results in saturation in brighter area. The gamma correction is essential to fit the human eye. However, the digital gamma correction degrades image quality especially for darker area on the image due to the limited ADC resolution and the dynamic range. This paper proposes a CMOS image sensor (CIS) with nonlinear analog-to-digital converter (ADC) which performs analog gamma correction. The CIS with the proposed nonlinear ADC conversion scheme was fabricated with a 0.35-mum CMOS process. The test results show the improved image quality than digital gamma correction

Ramp Slope Built-in-Self-Calibration Scheme for Single-Slope Column Analog-to-Digital Converter Complementary Metal-Oxide-Semiconductor Image Sensor

The conversion gain of a single-slope analog-to-digital converter (ADC) suffers from the process and frequency variations. This ADC gain variation eventually limits the performance of image signal processing (ISP) in a complementary metal–oxide–semiconductor (CMOS) image sensor (CIS). This paper proposes a ramp slope built-in-self-calibration (BISC) scheme for a CIS. The CIS with the proposed BISC was fabricated with a 0.35-µm CMOS process. The measurement results show that the proposed architecture effectively calibrates the ramp slope against the process and the clock frequency variation. The silicon area overhead is less than 0.7% of the full chip area.

A complementary metal-oxide-semiconductor image sensor with analog gamma correction using a nonlinear single-slope analog-to-digital converter

An image sensor has limited dynamic range while the human eye has a logarithmic response over a wide range of light intensity. Although the sensor gain can be set high to identify details in darker areas on an image, this high gain results in saturation in brighter areas. Therefore, gamma correction is essential to match the human eye response. However, the digital gamma correction degrades image quality, especially for darker areas on the image, due to the limited resolution and dynamic range of the analog-to-digital converter (ADC). In this paper, we propose a complementary metal–oxide–semiconductor (CMOS) image sensor (CIS) with a compact nonlinear ADC which performs analog gamma corrections that use the full dynamic range. A CIS with the proposed nonlinear ADC was fabricated with a 0.35-µm CMOS process. The test results show that the analog gamma correction provides a 2.2 dB peak-signal-to-noise-ratio (PSNR) improved image quality, which is better than conventional digital gamma corrections.