Noise analysis of a fully integrated CMOS image sensor

The read noise characteristics of a 3T photodiode-based CMOS active pixel image sensor IC is described. The sensor is fabricated in Hewlett Packard's standard 0.5 mu m and 3.3V mixed-signal process. The read noise characteristic of the analog signal path is theoretically estimated by adding together the noise contributions of the pixel, column amplifier and programmable gain amplifier (PGA). The read noise of the imager is then measured as a function of the on-chip programmable gain with a HP9494 mixed-signal production tester. An analysis of the measured read noise is performed to separate the noise contributions into pre-PGA and post-PGA components. The measured pre-PGA noise component is compared to the calculated estimate of the analog signal path noise. The measured pre-PGA noise is found to be much smaller than the calculated estimate. Consistency is substantially improved if pixel kTC reset noise is excluded from the calculated estimate.