Noise characterisation in CMOS APS imagers for highly integrated imaging systems

The design of active pixel image sensors (APS) fabricated in traditional CMOS foundries has been a topic of renewed interest in the last several years. The noise reduction is a key issue and often defines the sensitivity or detection limit. In this paper, a thorough noise analysis is made of the expected performance of the APS imagers. White noise and low-frequency (LF) noise sets a fundamental limit on APS performance, especially for low-light applications. Therefore, a detailed theoretical analysis of the in-pixel amplifier and the readout circuit response to the LF noise is investigated. Some experimental LF noise results obtained at room temperature on N-channel MOSFETs fabricated using a 0.7 μm CMOS process are presented. We show that the LF noise spectra generated by small area MOSFETs are Lorentzian rather than pure 1/f shape chiefly for the weak inversion mode. Next, using PSPICE simulations, the noise due to the readout circuit during integration is carried out.