Research On Nonlinear ADC Applied In CMOS Image Sensor

The appearance of lensless imaging system makes it possible for cell detection to be economic and portable.As a vital imaging part of this system,CMOS image sensor(CIS)should be demanded excellent performances for image acquisition.However,this system always presents a shitty resolution,and the gray of the particle cell image is relatively close to each other,which brings blurred image details.Since the linear ADC of CIS is always used to deal with the uniform signal range,but the signal in the low contrast region can not be well converted linearly.In order to solve this problem,it is necessary to use a nonlinear ADC circuit to handle uneven quantization ranges by researches on improvement of the cell image local contrast.So,using the 10-bit nonlinear ADC that composed of a SS ADC and a multi-frequency counter enlarges the local dynamic range of gray levels to increase the required information.The sub-module ramp generator is based on current-steering DAC and uses a 5+3+4 segmented structure to improve the current matching and reduce circuit area.For meet the requirement of system's accuracy and speed,the comparator adopts a regenerative comparator structure with cascade pre-amplifiers.And the counter has multiple reference frequencies to produce the fine quantified conversion at the essential part and the course quantified conversion at the unimportant part.Besides,the counter is configurable for different input types.The design and simulation of the nonlinear ADC's schematic and layout are completed by a 0.11-?m CMOS process.The simulation results show that the ENOB is 9.04-bit and the SNR is 57.8 dB at the linear mode.Then the nonlinear response curve is obtained from the A/D conversion data at the nonlinear mode.And the DNL of the nonlinear circuit is±0.34 LSB,and the INL is+0.7 8/-0.98 LSB.The whole circuit layout area is approximately 920 ?m×750 ?m,and when the supply voltage is 3.3 V/1.2 V,the circuit power consumption is 16.398mW.Finally,by MATLAB simulating the effect of cell image transaction between the digital contrast stretching algorithm and nonlinear ADC structure,Gray-scale Mean Gradient of the essential area is increased from 9.39×103 to 1.14×104.The comparison results proved that the details of the image process-ing by nonlinear ADC are more obvious.