| In recent years,with the continuous development of photoelectric detection technology and the gradual extremes of the device working environment,the requirements for signal detection sensitivity have become higher and higher,and ordinary devices and circuit designs have become powerless.Avalanche photon diode(APD)is a new type of highly sensitive photoelectric detection and sensor device,which has the characteristics of high sensitivity,fast detection speed,high integration and convenient use.It has been widely used in astronomical observation,national defense measurement and control,aerial exploration and other fields.APD working in linear mode has the advantages of high photon detection rate,low power consumption,continuous detection,etc.It is widely used in various weak light signal detection imaging systems.During detection,the APD receives the light signal and converts it into a current output.Because the output pulse current signal has a low amplitude and short pulse width,it is necessary to design a readout circuit to process the output current to obtain a recognized voltage pulse signal.Based on the actual needs of optical signal detection and readout applications,and according to the characteristics of linear mode APD devices,a highly integrated pixel-level readout circuit suitable for array applications has been designed,including transimpedance amplifiers,sampling circuits,and output-buffers circuit and other modules.The device characteristics and working principles of the APD was been studied,and obtains the equivalent model of the APD after parameter analysis and equivalent simulation.For the problem of APD output current being too small in linear mode,the structure and principle of various input stages of the readout circuit are studied and analyzed,combined with the application background,a capacitive feedback transimpedance amplifier(CTIA)is selected and designed as the input of the readout circuit stage,amplify the signal to improve the gain of the linear APD,this structure has the advantages of large gain,high linearity and high injection efficiency;a correlated double sampling circuit and a small area unity gain output buffer circuit is designed to reduce the noise in the circuit and transmit the signal losslessly to achieve the purpose of reading weak light signals;at the same time aiming at the non-uniformity problem of the readout circuit array that needs to be solved urgently,and different from the current mainstream algorithm solution or external FPGA,digital and other methods,a new type of adaptive analog correction circuit that can be integrated on the chip is designed,the correction circuit mainly includes modules such as transimpedance amplifier,sampling circuit,voltage comparator,flip detection,shift register,switch drive register,DAC and output-buffer stage,which can effectively solve the problem of the array non-uniformity that caused by the difference in the offset of the detection element.When the circuit is working,one of the detection elements in the array is used as a reference,by correcting the rest of the detection elements and adjusting the result to be the same as the reference unit,the problem of array non-uniformity is solved.Based on the Global Foundry 0.35μm process and cadence simulation platform,simulation verification shows that the readout circuit designed in this article can accurately read out the photogenerated current signal and convert it into a voltage signal to be amplified and read out.When the photocurrent is 6p A,the output voltage of the readout circuit is 1.660 V,when the photocurrent is 10 p A,the output voltage is 1.205 V.The non-uniformity correction circuit can correct the output error voltage of 483.5m V to about 4.7m V when the applied bias has a deviation of 1V,finally,a 2×2 APD array readout circuit with a system of non-uniformity correction module is completed,and submitted it for tape-out. |
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