The market for image sensors explored in the last few years shows an enormous increase in sales and developments of cameras. Imaging sensors are mainly classified into two types: complementary metal oxide semiconductor (CMOS) image sensors and charge couple device (CCD) sensors. Active pixel sensors (APS) are the emerging sensors for the replacement of existing and widely used charged couple device (CCD) sensors. Now a days, APS are extensively used in webcams, robotics, X-rays, computer based videos, smart toys, both still and video digital cameras, mobile phones cameras, automobiles, cinematography, spectrography,radiography, photogrammetric, and in many scientific applications. The above applications of APS are driving researchers to concentrate on achieving low power consumption, reduced size, increasing resolution, more sensitivity, marginal noise and more importantly fast operation. This work explores state-of-the-art research on APS by reviewing the concepts behind existing designs and new designs as well. Imaging sensors of different varieties are widely used in commercial and scientific applications. CMOS active pixel sensor (APS) imagers are fabricated in standard CMOS processes, which make it possible to integrate the timing and control electronics, sensor array, signal processing electronics, analog-to-digital converter (ADC) and full digital interface on one chip. llus helps to achieve a cost effectivehghly integrated and highly compact imaging system, i.e. camera-on-a-chip, by utilizing the same design techniques that have been developed over the years for lowpowerCMOS digital and analog circuits. 


APS Technology: The image sensor is photodiode-type CMOS active pixel sensor
imaging system on chip,designed by Jet Propulsion Laboratory and manufactured by a standard commercial CMOS production line. The imager is a 512 by 512 photodiode pixel array, which can randomly access any window in the array from 1 pixel by 1 pixel allthe way to 512 pixels by 512 pixels in any rectangular shape. The minimum interface consists of five wires: Vdd, Ground, Serial Data Input, Serial Data Output and Clock.The imager size is approximately 10 mm by 15.5 mm with

                        THEME: ACTIVE PIXEL SENSOR

            CCD Image Sensors were invented in 1969 by Bell Laboratories. At the time digital photography was the major field of application. Upon exposure of the sensor, the charge on the first row (row of pixels) is transferred to the read out register. The read out register signals are fed to an amplifier and then on to an analog-to-digital converter. Once the row has been read, the charges on the readout register row are deleted, the next row is transferred to the first row and this procedure is carried on till last row is read out.

CMOS sensors found their first applications in the detection of visible light and became widely spread in  consumer applications. We are now  developing this technology to meet the stringent requirements of scientific  applications. CMOS sensors can be efficiently used to detect a broad  spectrum of electromagnetic radiation and charged particles.  The dominant source of noise, the reset noise, can be reduced and, at low  illumination levels, noise in the range of 10  rms can be obtained,
without any correlated double sampling. Different types of analogue to  digital converter architectures allow to trade off between speed and  resolution required. We anticipate that the use of CMOS sensors for  scientific applications will expand in the next few years

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