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-resol ution analog-to-digital conversion (ADC), is presented. An FPN suppres sion circuit that removes the offset current variation between pixels by using a combination of an n-type and a p-type current copier cell i s described. The FPN suppression circuit exhibits linear transfer char acteristics in the input current range from 0 to 30 mu A. On-chip ADC is expected to improve imaging system performance and reliability, whi le reducing system size, weight, and cost. Operation and performance o f a current-mode second-order incremental Delta-Sigma A/D converter wi th column parallel architecture and for high-resolution and medium-slo w-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 corr esponds to less than twelve signal charges of current-mode 10-mu m pix el APS's. Based on the experimental results, a brief description of a possible image sensor with the on-chip signal processing is also descr ibed.