Design And Optimization Of Readout Circuit For Low-noise CMOS Image Sensor

Low-noise CMOS image sensor has the advantages of low-power consumption,high integration,and low cost.Therefore,it is widely used in consumer electronics,machine vision,security surveillance,military reconnaissance and other fields.Under low illumination,the swing of the pixel output signal is usually relatively small,and this small swing signal may be buried in the larger noise of the image sensor.Correlated multiple sampling（CMS）technique can effectively reduce random noise.Therefore,in this paper,the theoretical analysis of noise and the design of low-noise readout circuit are implemented for low-noise CMOS image sensor,and a low-power conditional analog correlated multiple sampling（CACMS）circuit based on adaptive operation of pixel output swing is proposed.Firstly,this thesis analyzes the noise sources of CMOS image sensor and the dominant factors of signal to noise ratio（SNR）,and investigates the noise suppression effects of auto-zero technique,correlated double sampling technique and CMS technique.Then a novel passive switched capacitor（SC）CMS structure is proposed,which allows increasing the number of capacitors to reduce the readout time.In addition,according to the theoretical basis that the SNR of CMOS image sensor is dominated by temporal random noise in low light environment and by photon shot noise in strong light environment,a low-power CACMS technique is proposed.This technique determines whether to perform the CMS operation based on the magnitude of the pixel output swing.The CMS operation is performed for the small-swing output of the pixel under low light to reduce random noise.And the CMS operation is turned off for the large-swing output of the pixel under strong light to reduce the power consumption without affecting the noise performance of the CMOS image sensor.Finally,this paper completes the design of key circuit modules and readout circuits of low-noise CMOS image sensor,including pixel timing drive circuit,bias current source and the low-power CACMS readout circuit.This paper uses a 0.11μm CMOS process to design,simulate and layout the circuit.In the case of a multi-sample number of 8,the low-power CACMS circuit reduces the readout time by 16.7%using the novel SC CMS structure.The simulation results show that when the closed-loop gain of the preamplifier is 8,the low-power CACMS circuit performs the CMS operation with a multi-sample number of 8 for the small swing output of the pixel under low light,resulting in a reduction of the input-referred random noise from 155μV_rms to 66.8μV_rms.And the circuit turns off the CMS operation for the large swing output of the pixel under strong light.At this time,the input-referred random noise is 91.9μV_rms.If the equivalent photon shot noise under strong light is774μV_rms,the input-referred noise when CMS is executed and CMS is turned off are777μV_rms and 779μV_rms,respectively.Therefore,turning off the CMS operation under strong light has little effect on the noise performance of CMOS image sensor,and the power consumption of the readout circuit is dropped by 28%.