Research On Wide Dynamic Range Image Sensor Based On CCPT

With the rapid development of the information technology and the continuous expansion of the multimedia market,the demand for image sensor applications and performance is also increasing.As an important way to obtain modern visual information,image sensor has experienced from chemical silver salt film to today's digital imaging,which can realize the expansion of information acquisition,storage,conversion and visual perception functions,and can provide intuitive,real and hierarchical Rich visual image information.CMOS image sensors occupy the market of similar products with obvious advantages such as high performance,low power consumption,low cost and high integration.For the typical CMOS image sensor,due to the limitation of the maximum potential well capacity of the photosensitive element in the pixel circuit,only the dynamic range of 70dB~80dB is much lower than the dynamic range of 180dB in the natural scene.Therefore,narrow dynamic range is an important factor limiting the development of image sensors.In this paper,the dynamic range of image sensor is studied as follows.Firstly,the basic research of CMOS image sensor is done in this paper.Several main performances of image sensor are analyzed.The influence of each performance on the imaging of image sensor and the relationship between each performance parameter is expounded.Several main noise sources affecting the performance of image sensors are introduced,including FPN and temporal noise,and the causes and elimination methods of various noises are analyzed in detail.The present situation of the dynamic range of the image sensor is analyzed,and several methods of expanding dynamic range are introduced,including well capacity expansion,multiple exposure,multi-detector technology,and linear-logarithmic response technology.In order to improve the dynamic range,designers usually modify the pixel structure greatly and add a lot of additional auxiliary circuits,which will certainly occupy a certain chip area,and increase the noise of the circuit,and there may be various irrational factors that restrict and affect other performance of the image sensor.Secondly,based on the analysis of the existing high dynamic range imaging technology,this paper proposed a charge compensation phototransistor(CCPT).Based on CCPT,two pixel structures are designed,which are based on 3T pixel architecture and 4T pixel architecture,and the image sensor systems are designed by using two pixel structures.The pixel structure of CCPT designed on the basis of the traditional 3T pixel architecture only adds a compensation voltage source formed by a P+doping region to the conventional structure.The new doped region forms a compensating diode,whose anode is connected to an externally adjustable compensating voltage source,and the cathode is connected to the cathode of the main photodiode to serve as the output node of the optoelectronic device,thus,the two back-to-back diodes form a phototransistor structure.When the incident light intensity is weak or the integration time is short,the compensation diode works in a reverse bias state,which is equivalent to parallel with the main photodiode,both of them work as ordinary photodiodes in integration mode.Therefore,the pixel has a good linear response under low light.With the extension of integration time or the enhancement of light intensity,the output point voltage decreases gradually,and then the voltage difference between the two ends of the compensating diode gradually exceeds its threshold voltage,and the compensating diode turns to the forward bias.After that,the compensation diode no longer generates photogenerated charge,but provides a positive charge to the main photodiode to counteract the photogenerated charge,avoiding the pixel output premature saturation.When the positive current generated by the compensating diode reaches a balance state with the photogenerated charge generated by the main photodiode,the output point voltage will not change,and the balance point is only related to the illumination intensity,regardless of the integration time.Since the forward current of diode is exponential to the output voltage,the pixel output voltage is logarithmically to the illumination intensity,thus greatly expanding the dynamic range.The pixel structure of CCPT based on 4T pixel architecture has a similar working principle.The structure designed in this paper combines the advantages of both linear and logarithmic modes.It has the same performance as the integrated mode sensor under low light,and has a good linear response.The response under strong light is logarithmic with light intensity,and the simulation results show that this structure can greatly expand the dynamic range.Thirdly,the image sensor system is designed based on the two CCPT pixel structures,including pixel array,readout circuits and timing design.The correlated double sampling technique is used in the readout circuit to effectively eliminate pixel reset noise and FPN noise.In the overall structure design,the encoder is used to directly select a certain pixel for output.This method makes the test more convenient and saves the chip area.The designed image sensor system chip was fabricated by a0.18?m standard commercial CMOS process.The fabricated chip areas were 4×4mm~2and 3×3mm~2,respectively,and the pixel resolutions were 210×280 and 160×200,respectively,and the pixel pitch was 10?m and 8?m,respectively,and the fill factor was about 36%and 31%,respectively.The performance of the fabricated chip was tested and analyzed by dark current,photoelectric response characteristics and noise.The test results show that the dynamic range of the two pixel structures is 167dB and169dB respectively.Finally,a variable gain switched capacitor amplifier is designed for the analog front-end of the readout circuit.In order to improve the accuracy of the entire VGA system,the operational amplifier,as the core circuit,adopts a fully differential two-stage structure to improve the open-loop gain.To ensure the stability of the system,the gain-bandwidth product of the op amp can vary with the VGA gain.The introduction of the error correction DAC can compensate the error caused by the dark current,and the compensation voltage of the error correction DAC increases with the VGA gain.The VGA is designed with the xfab 0.18?m CMOS process,and the results shows that the VGA circuit consumes 65mW and the gain dynamic range is-3dB~19 dB with a supply voltage of 3.3V.At the sampling frequency of 40MS/s,the effective bit accuracy is 14.9bit,the signal-to-noise ratio is 91.4dB,the spurious-free dynamic range is 97.9dB,and the error correction range is-507mV~507mV.Therefore,the amplification and correction function of the pixel output signal can be well realized.