A 640×512 CMOS image sensor with an 8b bit-serial Nyquist rate ADC per 4 pixels achieves 10.5 × 10.5 µm pixel size at 29% fill factor in 0.35 µm CMOS technology. Binary floating point output with measured dynamic range of of 65536 : 1 is achieved by multiple sampling at exponentially increasing exposure times.D. Yang, A. El Gamal, B. Fowler, and H. Tian 1 Dynamic range, defined as the ratio of the largest nonsaturating signal to the standard deviation of the noise under dark conditions, is a critical figure of merit for image sensors.The dynamic range of an image sensor is often not wide enough to capture scenes with both high lights and dark shadows. This is especially the case for CMOS sensors, which, in general, have lower dynamic range than CCDs. Several approaches have been proposed to enhance the dynamic range of a CMOS APS. In [1] dynamic range is enhanced by increasing well capacity one or more times during exposure time. Another approach, which achieves consistently higher SNR, is multiple sampling. Here the scene is imaged several times at different exposure times and the data is combined to construct a high dynamic range image.For this approach to work at reasonable capture times, readout must be performed at speeds much higher than normal APS speeds. In [2] an APS with two column parallel signal chains is presented. The sensor can simultaneously read out two images, one after a short exposure time T and the other after a much longer exposure time, e.g. 32T . Two images, however, may not be sufficient to represent the areas of the scene that are too dark to be captured in the first image and too bright to be captured in the second. It is difficult to extend the scheme to simultaneously capture more than two images, since more column parallel signal chains must be added at considerable area penalty.In this paper we demonstrate, using a 640×512 image sensor with Nyquist rate pixel level ADC implemented in a 0.35µm CMOS technology, how pixel level ADC enables a highly flexible and efficient implementation of multiple sampling. Since pixel values are available to the ADCs at all times, the number and timing of the samples as well as the number of bits obtained from each sample can be freely selected without the long readout time of APS. Typically, hundreds of nanoseconds of settling time per row are required for APS readout. In contrast, using pixel level ADC, digital data is read out at fast SRAM speeds.This demonstrates yet another fundamental advantage of pixel level ADC -the ability to programmably widen dynamic range with no loss in SNR.The 640 × 512 sensor employs the MCBS ADC technique described in [3]. Each 2 × 2 block of pixels share a 1-bit comparator/latch pair. The signals required to operate the ADCs are globally generated by off chip DAC and digital control circuitry. The ADC is bit D. Yang, A. El Gamal, B. Fowler, and H. Tian 2 serial and each bit is generated by peforming a set of comparisons between the pixel values and a RAMP signal. The bits are generated independently and ...
Bubble-based actuation in microfluidic applications is attractive owing to elementary microfabrication requirements. In the present study, the mechanical and chemical characteristics of electrochemically generated bubble valves were studied. By generating electrochemical bubbles as valves directly inside the channel, valves could be closed and opened in milliseconds. Whereas bubble inflation (or valve closing) rate increases with applied voltage, small microfluidic dimensions accelerate bubble deflation rates. It is found that bubbles need not collapse fully to restore full flow, and the channel opens when its hydraulic resistance equals that between the bubble and the wall--a process requiring only milliseconds. Since only picomoles of salt are needed to generate bubbles, pH gradients that are invariably associated with electrochemical reactions were readily suppressed by using a small amount of buffer, as visualized by a pH-sensitive fluorescent dye. A range of common laboratory reagents and electrolytes in varying concentrations, including weak to strong acids and bases, as well as nonaqueous/aqueous mixtures were successfully tested. Using such bubble valves, an eight-way multiplexer was fabricated and tested.
A CMOS 64 × 64 pixel area image sensor chip using Sigma-Delta modulation at each pixel for A/D conversion is described. The image data output is digital. The chip was fabricated using a 1.2µm two layer metal single layer poly n-well CMOS process. Each pixel block consists of a phototransistor and 22 MOS transistors. Test results demonstrate a dynamic range potentially greater than 93dB, a signal to noise ratio (SNR) of up to 61dB, and dissipation of less than 1mW with a 5V power supply.
The effect of various couplings on the switching field and coercivity in NiO-Co-Cu-based giant magnetoresistance ͑GMR͒ bottom spin valves is investigated. Bottom spin valves as well as different layer permutations that make up a bottom spin valve, viz., Co single films, Co/Cu/Co trilayers, and Co/NiO bilayers ͑deposited under similar growth conditions͒, were investigated for their magnetic, crystal, and interfacial structure. Asdeposited bottom spin valves exhibit a large GMR of Ϸ16.5%, and a small net ferromagnetic coupling ͑ϩ0.36 mT͒ between the ''free'' Co layer and the NiO-pinned Co layer. The high resolution transmission electron microscopy ͑HRTEM͒ and in situ scanning tunneling microscopy ͑STM͒ studies on spin valves and trilayers show that the average grain size in these films is Ϸ20 nm and average roughness Ϸ 0.3 nm.
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