The Design Of High Dynamic Image Sensor Used In Visual Prostheses

Visual prosthesis is an effective way to repair the person's visual function, it could help the blind to restore some visual function. the basic mechanism of visual prosthesis is as the following:the picture signal is collected and translated into electrical signal by the cmos image sensor, then the electrical is used to stimulate the corresponding optic nerve cells, it will be converted to biological signals by the visual cells, Then the picture signal was recognized by the brain. According to the difference stimulate parts, the visual prosthesis can be divided into retinal prosthesis, optic nerve prosthesis, visual cortex prosthesis. In this paper, based on the research of the visual prosthesis, the basic parameters of the image sensor were presented, and the image sensor was designed. the principal tasks are as follows:First, researched the image acquisition system of the visual prosthesis, analyzed the built-in image acquisition system of the visual prosthesis, discussed the image acquisition, transmission and energy supply of the built-in image acquisition system, and presented the basic parameters of the image sensor that used in the visual prosthesis.Then, analysis the classification and working principle of various structures of CMOS image sensors, established the spectre model of photodiode, analyzed the basic methods to improve dynamic range.Based on the application requirements of visual prosthesis, designed a high dynamic range image sensor chip. In the design of the pixel unit, the conditions overflow capacitor technology and multiple integral technology was used to improve the image sensor's dynamic range, in the same time, the image sensor keep high sensitivity and signal to noise ratio in the low light condition.After then, the readout circuit and programmable amplifier PGA was designed. In the readout circuit, dual-sampling techniques was used to reduce the fixed pattern noise of the image sensor, the output data was sent to the programmable amplifier through the buffer amplifier, the open-loop gain amplifier, using the capacitor structure, was chosen in the design of programmable amplifier circuit.During the design process, cadence software was used to simulate the designed circuit; the advanced semiconductor company's 0.35um process parameters were used in the simulation process. The simulation results show that the dynamic range of the new design image sensor is increased by about 62dB than the normal 4T active image sensor's(APS), at the same time, it keep the signal to noise ratio and sensitivity level in low light as the normal 4T APS.