结合高分辨率TDC的单光子探测系统设计

Design of single photon detection system with high resolution TDC

针对时间数字转换器(time-to-digital converter,TDC)的时间分辨率和测量误差相互制约,单光子探测系统工作频率低、测量死时间长等问题,设计了一款用于荧光寿命成像的高速单光子探测系统.该系统集成了一个6×6单光子雪崩二极管(single photon avalanche diode,SPAD)阵列和一个两级结构的TDC.其中,SPAD之间相互并联以增大感光面积;淬灭电路自动控制两条放电支路,减小测量死时间的同时降低了后脉冲效应;TDC采用两级结构同时实现了高分辨率和大动态范围,其中第2级TDC采用三通道游标结构有效降低了测量误差;存储器将时间测量结果暂存在对应的地址中,测量结束后由串口电路按地址顺序读出到上位机中处理.该系统基于TSMC 0.18μm CMOS工艺仿真验证,芯片整体面积为2800μm×1800μm.仿真结果表明:SPAD的击穿电压约为11.3 V,雪崩电流约为10-3 A,淬灭电路的死时间约为40 ns;TDC的时间分辨率为30 ps,动态范围为241 ns;整个系统在526 MHz时钟频率下对两个荧光信号进行检测,测量误差均小于10 ps.

To solve the problems that the time resolution and measurement error of time-to-digital converter(TDC)are mutually restricted,and the single photon detection system has low working frequency and long dead time,a high-speed single photon detection system for fluorescence lifetime imaging was designed.The system integrates a 6×6 single photon avalanche diode(SPAD)array and a two-stage TDC.In this system,the SPADs are connected in parallel with each other to increase the photosensitive area.The quenching circuit could effectively reduce the detection dead time and afterpulse effect by automatically controlling the two discharge branches.Both high resolution and large dynamic range were achieved by the two-stage TDC structure,where a novel three-channel vernier structure was adopted to reduce the measurement uncertainty.The memory temporarily stored the measurement time interval in the corresponding address,and after the measurement,the UART circuit read data into the computer by the order of address.The system was simulated on the basis of TSMC 0.18μm CMOS technology,and the overall layout area was 2800μm×1800μm.Simulation results show that the breakdown voltage of SPAD was about 11.3 V,the avalanche current was about 10-3 A,and the dead time of quenching circuit was about 40 ns.TDC could achieve a time resolution of 30 ps and a total conversion dynamic range of 241 ns.The whole system detected two fluorescent signals at a clock frequency of 526 MHz,and the quantization error was less than 10 ps.