Research On Readout Circuit Of Quanta Image Sensors Based On CMOS Active Pixels

The quanta image sensor(QIS)has three characteristics including spatial oversampling,temporal oversampling and single photon counting to deal with the drop of dynamic range and signal-to-noise ratio in the CMOS image sensor due to device shrink and the decrease of supply voltage.The QIS has a wide-ranging application space: low light imaging,high speed imaging,high dynamic range imaging and high resolution imaging,etc.Much research progress has been made since 2011 when the QIS was first proposed.There are still many problems need to be resolved,and the design of the high-speed,low power and high-accuracy readout circuit is one of them.In this paper,detailed theoretical and modeling analyses are performed to determine the required precision of the readout circuit in the QIS.On the basis of the analysis,the design of the readout circuit is accomplished.Mathematical models and imaging models for the QIS are build up.The non-uniform of the quantization threshold is an important factor influencing the detection accuracy of photoelectrons,so this paper focuses on the influence of the non-uniform of the quantization threshold on bit error rate and image quality.Meanwhile,some other facts are also analysed,like quanta exposure,shot noise,readout noise in jots,spatial oversampling factor and equivalent full well capacity.The models show that the normalized standard deviation of the quantization threshold,which is also the input offset of the readout circuit should be lower than 0.15e-in the single-bit QIS to ensure bit error rate lower than 1%.On the basis of theoretical and modeling analyses,the readout circuit of the single-bit QIS is designed.The core of the circuit is a 1-bit analogto-digital converter(ADC)based on correlated double sampling charge transfer amplifier(CTA).The CTA employed in the ADC is robust and simple,and dissipates zero static power.The readout circuit also contains the precharging voltage production circuit,shift register,charge pump and timing and control circuit.The proposed 1-bit ADC is designed in 0.11?m CMOS process,and the column width of the layout is 3.9?m.When the threshold voltage is 500?V and the total gain of the gain stage is 400,the 1-bit ADC could operate at 1MS/s and 10MS/s dissipating only 1.17?W and 10.5?W.Monte Carlo simulations show that the input offset of the 1-bit ADC is 57?V,so it is able to distinguish whether a photoelectron is produced accurately.