| As a new photon detection device,signle photon avalanche diodes(SPAD)possesses lots of advantages like low time jitter,high detection sensitivity,fast response speed,and high integration.Therefore,the TOF technology based on SPAD detectors are widely used in some fields such as ranging and 3D imaging.The SPAD pixel circuit is a key component of imaging Li DAR,however,a lot of problems still extist in practical application which to be solved,such as mutual restriction between detection accuracy and detection distance,low speed of imaging,large area of pixel,and low threshold of human eye safety.In this thesis,the issues mentioned above are deeply studied and improved.(1)Calculation and analysis of human eye safety threshold for near infrared laser.Aiming at the problem of human eye safety in the field of military-civilian integration,the energy threshold analysis method is adopted to calculate the near-infrared laser threshold in this thesis.Due to the different working methods of the emitted laser,the single-pulse energy threshold and continuous pulse maximum irradiation threshold of the specific laser source are calculated respectively.The calculation results show that the laser energy threshold of human eye safety conditions could be affected by the laser wavelength,operating frequency,irradiation/radiation time,and laser working mode.The calculation uses an 808 nm near-infrared laser source with a heavy frequency of 8.8 MHz.If irradiation time is 10 s,the laser energy in single pulse mode must be less than 0.72 J/m~2,and the energy threshold in continuous pulse mode must be lower than 7.61 m J/m~2.If remaining conditions and reducing frequency to 8.8KHz,the energy threshold in continuous pulse mode must be lower than 42.81m J/m~2.The results of calculation and analysis provide a reference for specifying the laser energy threshold value which applied to the field of human eye safety.(2)Design and implement of wide dynamic range and high precision pixel circuit.For the purpose of solving the mutually restrictive problem between low speed of detection imaging,detection accuracy and pixel area,detection distance,a novel active gated quenching circuit is proposed,by which the quenching time is reduced to 1.6 ns.Furthermore,a large number of ambient light noise is effectively avoided owing to gating regulations,improving detection efficiency and solving the problem of slow imaging speed in the same time.Moreover,several structures are put forward for time-to-amplitude conversion(TAC)circuit,including charge transfer method,current mirror method,and high-impedance switching method.Especially for high-impedance switching method,a stable charge current is generated on the principle of gain boost by increasing the current internal resistance.Simulation results indicate that a good differential non-linearity(DNL)is obtained and the power consumption is approximately at 12?W.The occupation area is only 120?m~2which is beneficial for increasing the fill-factor(FF).Therefore,the small area,the low power consumption and the good linearity can be achieved simultaneously,effectively solving the problem of the mutual limitation of pixel area and measurement accuracy.In addition,a TOF measurement which is suitable for long-distance and wide dynamic range is proposed.The measurement combines precise counting by TAC and digital coarse counting together,in which a digital counter is used to expand the full-scale time and the TAC circuit guarantees the minimum resolution,solving the problem of mutual limitation between detection accuracy and detection distance.(3)Simulation of photon arrival time modeling and TOF measurement.Based on the above designed circuit,a behavioral model of photon arrival time was proposed using Cadence and Verilog-A tools.Multiple simulations which is based on the TOF measurement are conducted to predict the time resolution and detection error combining the circuit characteristics.The detection distance in simulations is set to be 7.5 m under 10klux ambient light condition.The simulated distance of 7.44m with an error of 6 cm and a time resolution of 390 ps are acquired via extracting the photon arrival time through more than 300 times of simulations.The simulation results show that the establishment of the model combined with circuit simulation evaluates the performance of the circuit and verifies the feasibility of the TOF measurement method,providing a reference for the application of the SPAD pixel based on TOF in the field of Li DAR imaging.(4)Tape-out and verification of functional circuit,realization of wide dynamic range pixel circuit.The layout of each pixel circuit is designed and verified through tape-out.Experimental results exhibit that the quenching time of the active quenching gated circuit is measured at 8.0 ns with a load capacitance of 30 p F.Moreover,the large voltage swing of 1 V,the hold time of 1 ms and the minimum resolution of 3.9 m V are achieved for the high-impedance switching TAC.In addition,a good linearity with a small DNL of 0.1 LSB and INL of 0.15 LSB are obtained simultaneously.Owing to its good performance,the proposed TAC is suitable to be applied in the long distance and high precision imaging.Finally,the pixel circuit is further implemented based on the experimental results.The system clock period is set at 8 ns,so that the full scanning range reaches 2048 ns while ensuring a time resolution of 32 ps,through which not only the detection distance is expanded,but also the detection accuracy is guaranteed,facilitating the integrated application of large arrays and laying the foundation for high-precision 3D imaging at long distances. |
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