Low Power, High Dynamic Range CMOS Image Sensor Employing Pixel-Level Oversampling $\Sigma \Delta$ Analog-to-Digital Conversion

We present a theoretical analysis, design, and experimental characterization of a CMOS image sensor with pixel-level ΣΔ oversampling analog-to-digital conversion (ADC). The design employs five transistors per-pixel to implement a charge-based ΣΔ ADC at each pixel. In the current design a dynamic regenerative latch comparator is divided into an input transistor, which is contained within each pixel, and the remaining comparator structure shared among the pixels of each column. A charge feedback digital-to-analog converter (DAC) is implemented at each pixel with a three-transistor structure. As opposed to more traditional CMOS image sensors, this image sensor architecture is suitable for implementations in advanced low supply voltage CMOS technologies since its dynamic range is not affected by the reduction of the pixel reset voltage. In addition, similar to the readout methods in low power random access memory designs, this pixel readout architecture does not employ any active amplifiers which allows for low static power operation. Experimental characterization of a prototype fabricated in a 0.35 μm silicided CMOS technology is presented. The estimated power consumption of the fully integrated 128 × 128 imager including decimation filters and I/O interface is 60 nW/pixel at 30 frames per second for 8-bits per-pixel. A peak signal-to-noise ratio of 52 dB and intra-scene dynamic range of 74 dB were measured. The dynamic range was extended to 91 dB through control of the in-pixel DAC supply voltage over the range of 0.8 V-3.3 V.