Suppression of Spatial and Temporal Noise in a CMOS Image Sensor

Abstract

This paper presents methodologies for suppressing the spatial and the temporal noise in a CMOS image sensor (CIS). First of all, it demonstrates by using a longer-length column bias transistor, both the fixed pattern noise (FPN) and temporal noise can be suppressed. Meantime, it employs column-level oversampling delta-sigma ADCs to suppress temporal noise as well as to facilitate the realization of the thermal compensation of dark signal non-uniformity (DSNU). In addition, the image pixels are re-configured as temperature sensors with inaccuracies within ±0.65 °C, between −20 and 80 °C. If the dark current and its non-uniformities are caused by thermal gradients, the obtained in-pixel thermal information can be employed to compensate for the measured dark current by 95 % and DSNU, up to 13 %. All the column-level 13 bit 2nd-order incremental delta-sigma ADCs are measured with SNR around 65 dB and INL around 1.5 LSB, when tested with a −8 dB input signal and sampling at 2 MHz with an oversampling ratio (OSR) of 128, when the full scale voltage is 2 Vp-p. The 4T Pinned Photodiode (PPD) CIS is measured to have a temporal noise of $34~\mu \text{V}$ rms (with an OSR of 128, or, an input referred temporal noise of 0.5 e− rms, with a conversion gain, CG, of $73~\mu \text{V}/ \text{e}^{-}$ ), a column gain FPN of 0.06 %, a dynamic range (DR) of 92 dB (with OSR = 512), as well as a linearity of 1 %. It has a measured DSNU of 3.2 %, after the thermal compensation using the in-pixel temperature sensors, a dark current of 290 pA/cm2 and 15 pA/cm2, measured at 60 °C, before and after the thermal compensation, respectively.