P 24 Event-Driven Correlated Double Sampling for Pulse-Frequency-Modulation A / D Converters Integrated in Pixel-Parallel Image Sensors

We report a novel event-driven correlated double sampling (CDS) technique for pulse-frequencymodulation (PFM) analog-to-digital converters (ADCs). PFM-ADCs are promising for pixel-parallel 3-D integrated image sensors with excellent imaging performance. The developed ADC with CDS consists of comparators, capacitors, and timing control logic circuits that are designed to generate the triggered clocks to cancel kTC noise in a pixel. We confirmed that the prototype ADC showed noise reduction effects and also an excellent linearity with a wide dynamic range of 120 dB, which indicates the feasibility of high-quality pixelwise image sensors. Introduction Recent demands are escalating for higher resolution and frame rate of image sensors [1]. Pixel-wise signal processing combined with 3-D integration technology is a promising solution to meet future demands. We have developed a pixel-parallel 3-D integrated image sensor as shown in Fig. 1 [2-4]. Several silicon-on-insulator (SOI) layers are bonded and connected within every pixel by high-density embedded Au electrodes [5]. We adopted PFM-ADCs [6] for the sensor, whose basic configurations are shown in Fig. 2. The ADCs generate pulses whose frequencies are proportional to the illuminance. The ADC has advantages of small footprint, wide dynamic range, and high tolerance to noise outside pixel. However, kTC noise during resetting the PD causes a deviation of pulse frequency as noise per pixel. The conventional CDS is operated to cancel the kTC noise by periodical clocks for reset, clamp, and sample [7-8]. However, such periodical signals are not available for PFM-ADCs because the reset clock is identical to the output pulse and that the timing of which is not periodic but depends on illuminance. Pixel-parallel 3-D integrated image sensor Conventional 2-D image sensor Pixel-parallel signal processing Photodetector Pixel Signal processor 3-D integration Column-parallel signal processing Pixel Signal processor Photodetector Figure 1: Pixel-parallel 3-D integrated image sensor compared with the conventional 2-D sensor.