On-focal-plane signal processing for current-mode active pixel sensors

On-focal-plane signal processing for current-mode active pixel sensors (APS), including fixed pattern noise (FPN) suppression and high-resolution analog-to-digital conversion (ADC), is presented. An FPN suppression circuit that removes the offset current variation between pixels by using a combination of an n-type and a p-type current copier cell is described. The FPN suppression circuit exhibits linear transfer characteristics in the input current range from 0 to 30 mu A. On-chip ADC is expected to improve imaging system performance and reliability, while reducing system size, weight, and cost. Operation and performance of a current-mode second-order incremental Delta-Sigma A/D converter with column parallel architecture and for high-resolution and medium-slow-speed applications is presented. A 12-bit resolution with +/-1.5 LSB accuracy at the conversion rate of 5.6 kHz was obtained. The LSB corresponds to less than twelve signal charges of current-mode 10-mu m pixel APS's. Based on the experimental results, a brief description of a possible image sensor with the on-chip signal processing is also described.