An Approach to Improve the Signal-to-Noise Ratio of Active Pixel Sensor for Low-Light-Level Applications

CMOS photodetectors are compact, cheap, and of low power, making them good candidates for many biomedical applications. However, many of these applications require the capability of detecting low-level light. Therefore, the noise in CMOS sensors must be carefully considered. This paper presents a detailed analysis of the signal and noise properties in active pixel sensor (APS) elements. An optimum signal-to-noise ratio (SNR) of 54 dB is achieved by varying the integration time. Based on a rigorous reset-time analysis of the APS, the dc level of the sense node is proposed as the new output signal, which is more sensitive to low-level light than existing APS techniques. By varying the reset time, an optimum SNR of 56 dB is achieved for a 30-ms integration time. This approach can achieve higher SNR for the same APS structure than the previous reports found in the literature