An SPAD operating overvoltage calibration circuit and calibration method

Abstract

The present application provides a SPAD operating overvoltage calibration circuit and a calibration method. By comparing the detected maximum voltage with the breakdown voltage of the first MOS transistor and automatically determining whether the output voltage of the charge pump needs to be adjusted, the output voltage of the charge pump is adjusted only once when adjustment is required, achieving precise control of the operating voltage of the SPAD. This solution can monitor in real time the voltage at the first terminal of the SPAD during operation (i.e., the voltage at avalanche breakdown) and capture its maximum value. In the case of a shift in the breakdown voltage due to changes in operating conditions or device aging, it can also respond quickly and make adjustments, effectively avoiding potential damage to the SPAD and subsequent circuits caused by overvoltage. This solution reduces the need for manual intervention and lowers the maintenance cost and complexity of the system through its built-in real-time detection and automatic calibration functions.

Description

Technical Field

[0001] This application relates to the technical field of single - photon avalanche diodes. Specifically, it relates to a SPAD operating over - voltage calibration circuit and a calibration method. Background Art

[0002] A single - photon avalanche diode (SPAD) and its key quenching circuit work together in an environment supported by a charge pump (adjustable voltage pump). The voltage pump precisely regulates the necessary operating voltage for the SPAD. This voltage consists of the SPAD's inherent breakdown voltage (BV) plus a carefully designed over - voltage to ensure that the SPAD operates stably in Geiger mode. When the SPAD captures a photon and triggers the avalanche breakdown effect, the voltage at the Vex node will soar instantaneously. At this time, the quenching circuit quickly intervenes to adjust the voltage across the SPAD back to the BV level, effectively cutting off the avalanche current. Subsequently, the MN0 circuit starts and slowly releases the voltage at the Vex node to 0V, making full preparations for the next photon detection.

[0003] Currently, the industry mostly uses methods such as directly monitoring the SPAD breakdown voltage and adding a fixed value, or comparing a single / multiple reference voltages with Vex to avoid damage to the quenching circuit due to over - voltage. However, directly detecting BV not only has a complex process but also needs to be achieved by gradually adjusting the charge pump voltage and monitoring the SPAD response, which is time - consuming and affects the ranging operation efficiency. The multi - comparator scheme may lead to inaccurate detection of the Vex peak due to input offset, bandwidth limitation, and insufficient response speed, and also requires multiple voltage adjustments to trigger the comparator flip, increasing the time cost. Summary of the Invention

[0004] The purpose of the embodiments of this application is to provide a SPAD operating over - voltage calibration circuit and a calibration method to solve the technical problems of inaccurate over - voltage detection, complex process, and high time cost of existing SPADs.

[0005] A SPAD operating over - voltage calibration circuit provided by the embodiments of this application includes: a charge pump, a SPAD, a first MOS transistor, and a control module;

[0006] The output terminal of the charge pump is connected to the second terminal of the SPAD. The first terminal of the SPAD is connected to the drain of the first MOS transistor. The source of the first MOS transistor is grounded. The input terminal of the control module is connected to the first terminal of the SPAD, and the output terminal of the control module is connected to the input terminal of the charge pump;

[0007] The control module is used to detect the maximum value of the voltage at the first end of the SPAD when the SPAD undergoes avalanche breakdown, compare the maximum value with the breakdown voltage of the first MOS transistor, and if the maximum value is greater than the breakdown voltage of the first MOS transistor, output a control signal indicating a reduction in the output voltage of the charge pump to the charge pump.

[0008] In the above technical solution, by comparing the detected maximum voltage value with the breakdown voltage of the first MOS transistor and automatically determining whether the output voltage of the charge pump needs to be adjusted, the output voltage of the charge pump is adjusted only once when it is determined that adjustment is required, achieving precise control of the operating voltage of the SPAD. This solution can monitor the voltage at the first end of the SPAD during operation (i.e., the voltage during avalanche breakdown) in real time and capture its maximum value. In the case of a shift in the breakdown voltage due to changes in operating conditions or device aging, it can also respond quickly and make adjustments, effectively avoiding potential damage to the SPAD and subsequent circuits caused by overvoltage. This solution reduces the need for manual intervention through its built-in real-time detection and automatic calibration functions, lowering the maintenance cost and complexity of the system. Moreover, by precisely controlling the operating voltage of the SPAD, this solution helps to optimize the response speed and sensitivity of the SPAD, thereby improving its performance in applications such as optical detection and photon counting.

[0009] In some alternative embodiments, the control module includes: an ADC, a pulse shaping circuit, a delay unit, and an MCU;

[0010] The delay unit is used to output a start conversion signal to the ADC and the MCU after a delay time for the voltage signal at the first end of the SPAD;

[0011] The ADC is used to start working when receiving the start conversion signal, sample and convert the voltage signal at the first end of the SPAD into a digital signal, and output the digital signal to the MCU;

[0012] The MCU is used to output different delay times to the delay unit, receive the digital signals corresponding to different delay times sampled by the ADC; determine the maximum value of the digital signals based on the digital signals corresponding to all delay times, and use the maximum value of the digital signals as the maximum value of the voltage at the first end of the SPAD.

[0013] In the above technical solution, sampling is performed using an ADC, which can convert the analog voltage signal at the first end of the SPAD into a digital signal with high precision. This enables the MCU to process and analyze the voltage data more accurately, thereby more precisely determining the overvoltage situation and making corresponding adjustments. The delay unit allows the conversion of the voltage signal to start after a certain delay time after the SPAD undergoes avalanche breakdown. The MCU, as the control core, can receive and process the digital signals corresponding to different delay times from the ADC. By comparing these signals, the MCU can accurately determine the maximum value of the voltage at the first end of the SPAD, thereby more precisely controlling the output voltage of the charge pump.

[0014] In some alternative embodiments, the first end of the ADC is connected to the first end of the SPAD, the second end of the ADC is connected to the MCU, and the third end of the ADC is connected to the second end of the delay unit; the first end of the delay unit is connected to the first end of the SPAD, the second end of the delay unit is connected to the MCU, and the third end of the delay unit is connected to the MCU.

[0015] In some alternative embodiments, the control module further includes: a pulse shaping circuit;

[0016] The pulse shaping circuit is used to broaden the voltage signal at the first end of the SPAD;

[0017] The delay unit is used to output a start conversion signal to the ADC and the MCU after a delay time for the broadened signal.

[0018] In the above technical solution, the pulse shaping circuit performs a broadening process on the voltage signal at the first end of the SPAD, which helps to improve the waveform and quality of the signal. When the SPAD undergoes avalanche breakdown, the generated voltage signal is often very steep and rapidly changing. Directly sampling such a signal may cause the ADC to fail to accurately capture its peak value. By broadening the signal, the pulse shaping circuit makes the waveform of the signal smoother, which is beneficial for the ADC to perform more stable and accurate sampling.

[0019] In some alternative embodiments, the first end of the ADC is connected to the first end of the SPAD, the second end of the ADC is connected to the MCU, and the third end of the ADC is connected to the second end of the delay unit; the first end of the pulse shaping circuit is connected to the first end of the SPAD, and the second end of the pulse shaping circuit is connected to the first end of the delay unit; the second end of the delay unit is connected to the MCU, and the third end of the delay unit is connected to the MCU.

[0020] In some alternative embodiments, the control module further includes: a second MOS transistor and a current source;

[0021] The first end of the SPAD is connected to the gate of the second MOS transistor. The drain of the second MOS transistor is connected to the power supply VDD. The source of the second MOS transistor is connected to a current source and then grounded. The source of the second MOS transistor is connected to the ADC and the pulse shaping circuit.

[0022] In the above technical solution, by introducing a source follower, the source follower has the characteristics of low output impedance and high input impedance. It can transmit the SPAD anode voltage to the ADC with less distortion. This solution avoids introducing too much capacitance before the ADC sampling point and ensures that the dead time of the quenching circuit is not changed. Among them, the dead time is the time required for the quenching circuit to recover to a state where it can be triggered again after the SPAD avalanche breakdown, which is crucial for the stable operation and accurate measurement of the SPAD. Avoiding the influence of capacitance on the dead time helps to maintain the stable performance of the quenching circuit and ensures that the SPAD maintains high efficiency and accuracy in high-speed and continuous photon detection. Moreover, by introducing a source follower, this solution effectively isolates the high voltage between the ADC sampling circuit and the SPAD anode, reducing the anode voltage to a safe level that the ADC can withstand. This not only protects the ADC from high voltage damage but also ensures the safety and reliability of sampling.

[0023] In some alternative embodiments, the control module further includes: a second MOS transistor and a third MOS transistor;

[0024] The first end of the SPAD is connected to the gate of the second MOS transistor. The drain of the second MOS transistor is connected to the power supply VDD. The source of the second MOS transistor is connected to the drain of the third MOS transistor. The drain of the third MOS transistor is connected to the gate of the third MOS transistor. The source of the third MOS transistor is grounded. The source of the second MOS transistor is connected to the ADC and the pulse shaping circuit.

[0025] In the above technical solution, a voltage dividing circuit is formed by using the second MOS transistor and the third MOS transistor to reduce the voltage input to the ADC, protecting the ADC from high voltage damage and ensuring the safety and reliability of sampling.

[0026] A method for calibrating the overvoltage during the operation of an SPAD provided by an embodiment of the present application includes:

[0027] Using the MCU to send an initial control signal to the charge pump, where the initial control signal is used to instruct the charge pump to output a voltage VSPAD; where VSPAD is greater than the SPAD breakdown voltage;

[0028] When the SPAD undergoes avalanche breakdown, using the pulse shaping circuit to broaden the voltage VEX at the first end of the SPAD;

[0029] Using the delay unit to generate a start conversion signal after delaying the broadened signal by a delay time;

[0030] When the ADC receives the start conversion signal, the ADC samples and outputs a digital signal to the MCU;

[0031] Using the MCU, by adjusting the delay time, digital signals corresponding to different delay times are obtained;

[0032] Using the MCU, based on the digital signals corresponding to all delay times, the maximum value of the digital signals is determined, and the maximum value of the digital signals is used as the maximum value of the first terminal voltage of the SPAD;

[0033] Using the MCU, the maximum value of the first terminal voltage of the SPAD is compared with the breakdown voltage of the first MOS transistor. If the maximum value is greater than the breakdown voltage of the first MOS transistor, a control signal indicating a reduction in the output voltage of the charge pump is output to the charge pump; if the maximum value is less than or equal to the breakdown voltage of the first MOS transistor, a control signal indicating that the output voltage of the charge pump remains unchanged is output to the charge pump. Description of the Drawings

[0034] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required for use in the embodiments of the present application will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present application, and thus should not be regarded as limiting the scope. For those of ordinary skill in the art, other related drawings can be obtained based on these drawings without creative efforts.

[0035] Figure 1 A SPAD operating overvoltage calibration circuit provided for the first embodiment of the present application;

[0036] Figure 2 A SPAD operating overvoltage calibration circuit provided for the second embodiment of the present application;

[0037] Figure 3 A SPAD operating overvoltage calibration circuit provided for the third embodiment of the present application;

[0038] Figure 4 A SPAD operating overvoltage calibration circuit provided for the fourth embodiment of the present application;

[0039] Figure 5 A SPAD operating overvoltage calibration circuit provided for the fifth embodiment of the present application;

[0040] Figure 6 A flowchart of the steps of a SPAD operating overvoltage calibration method provided for the embodiments of the present application;

[0041] Figure 7 A schematic diagram of the waveform of the start conversion signal provided for the embodiments of the present application. Detailed implementation manners

[0042] The technical solutions in the embodiments of the present application will be described below with reference to the accompanying drawings in the embodiments of the present application.

[0043] Please refer to Figure 1 , Figure 1 A SPAD working overvoltage calibration circuit provided by the first embodiment of the present application includes: a charge pump, a SPAD, a first MOS transistor M0, and a control module.

[0044] Among them, the output end of the charge pump is connected to the second end of the SPAD, the first end of the SPAD is connected to the drain of the first MOS transistor M0, the source of the first MOS transistor M0 is grounded, the input end of the control module is connected to the first end of the SPAD, and the output end of the control module is connected to the input end of the charge pump; the control module is used to detect the maximum value of the voltage at the first end of the SPAD when the SPAD undergoes avalanche breakdown, and compare the maximum value with the breakdown voltage of the first MOS transistor M0. If the maximum value is greater than the breakdown voltage of the first MOS transistor M0, a control signal for instructing to reduce the output voltage of the charge pump is output to the charge pump.

[0045] In the embodiments of the present application, by comparing the detected maximum voltage value with the breakdown voltage of the first MOS transistor M0 and automatically determining whether to adjust the output voltage of the charge pump, and only adjusting the output voltage of the charge pump once when it is determined that adjustment is needed, the working voltage of the SPAD can be accurately controlled. This solution can monitor the voltage at the first end of the SPAD during operation (i.e., the voltage during avalanche breakdown) in real time and capture its maximum value. In the case of a shift in the breakdown voltage due to changes in operating conditions or device aging, it can also respond quickly and make adjustments, effectively avoiding potential damage to the SPAD and subsequent circuits caused by overvoltage. This solution reduces the need for manual intervention through its built-in real-time detection and automatic calibration functions, reducing the maintenance cost and complexity of the system. Moreover, by accurately controlling the working voltage of the SPAD, this solution helps to optimize the response speed and sensitivity of the SPAD, thereby improving its performance in applications such as optical detection and photon counting.

[0046] Please refer to Figure 2 , Figure 2 The structural diagram of the SPAD working overvoltage calibration circuit provided by the second embodiment of the present application, the control module of this embodiment includes: an ADC, a pulse shaping circuit, a delay unit, and an MCU. The first end of the ADC is connected to the first end of the SPAD, the second end of the ADC is connected to the MCU, and the third end of the ADC is connected to the second end of the delay unit; the first end of the delay unit is connected to the first end of the SPAD, the second end of the delay unit is connected to the MCU, and the third end of the delay unit is connected to the MCU.

[0047] Among them, the delay unit is used to output a start conversion signal to the ADC and the MCU after a delay time for the first terminal voltage signal of the SPAD; the ADC is used to start working when receiving the start conversion signal, sample and convert the first terminal voltage signal of the SPAD into a digital signal, and output the digital signal to the MCU; the MCU is used to output different delay times to the delay unit, receive the digital signals corresponding to different delay times sampled by the ADC; and determine the maximum value of the digital signals according to the digital signals corresponding to all the delay times, and use the maximum value of the digital signals as the maximum value of the first terminal voltage of the SPAD.

[0048] In the embodiment of the present application, sampling is performed using the ADC, and the analog voltage signal at the first terminal of the SPAD can be converted into a digital signal with high precision. This enables the MCU to process and analyze the voltage data more accurately, thereby more precisely determining the overvoltage situation and making corresponding adjustments. The delay unit allows the conversion of the voltage signal to start after a certain delay time after the SPAD undergoes avalanche breakdown. The MCU, as the control core, can receive and process the digital signals corresponding to different delay times from the ADC. By comparing these signals, the MCU can accurately determine the maximum value of the first terminal voltage of the SPAD, thereby more precisely controlling the output voltage of the charge pump.

[0049] Please refer to Figure 3 , Figure 3 which is the structural diagram of the SPAD operating overvoltage calibration circuit provided in the third embodiment of the present application. The control module of this embodiment includes an ADC, a pulse shaping circuit, a delay unit, an MCU, and a pulse shaping circuit. The first terminal of the ADC is connected to the first terminal of the SPAD, the second terminal of the ADC is connected to the MCU, and the third terminal of the ADC is connected to the second terminal of the delay unit; the first terminal of the pulse shaping circuit is connected to the first terminal of the SPAD, and the second terminal of the pulse shaping circuit is connected to the first terminal of the delay unit; the second terminal of the delay unit is connected to the MCU, and the third terminal of the delay unit is connected to the MCU.

[0050] Among them, the pulse shaping circuit is used to broaden the first terminal voltage signal of the SPAD; the delay unit is used to output a start conversion signal to the ADC and the MCU after a delay time for the broadened signal. The ADC is used to start working when receiving the start conversion signal, sample and convert the first terminal voltage signal of the SPAD into a digital signal, and output the digital signal to the MCU; the MCU is used to output different delay times to the delay unit, receive the digital signals corresponding to different delay times sampled by the ADC; and determine the maximum value of the digital signals according to the digital signals corresponding to all the delay times, and use the maximum value of the digital signals as the maximum value of the first terminal voltage of the SPAD.

[0051] In the embodiments of the present application, the pulse shaping circuit broadens the voltage signal at the first end of the SPAD, which helps to improve the waveform and quality of the signal. When the SPAD undergoes avalanche breakdown, the generated voltage signal is often very steep and changes rapidly. Directly sampling such a signal may cause the ADC to fail to accurately capture its peak value. By broadening the signal, the pulse shaping circuit makes the waveform of the signal smoother, which is beneficial for the ADC to perform more stable and accurate sampling.

[0052] Please refer to Figure 4 , Figure 4 FIG. is the structural diagram of the SPAD operating overvoltage calibration circuit provided by the fourth embodiment of the present application. The difference from the third embodiment is that: the control module of this embodiment further includes a second MOS transistor M1 and a current source; the first end of the SPAD is connected to the gate of the second MOS transistor M1, the drain of the second MOS transistor M1 is connected to the power supply VDD, the source of the second MOS transistor M1 is connected to the current source and then grounded, and the source of the second MOS transistor M1 is connected to the ADC and the pulse shaping circuit.

[0053] In the embodiments of the present application, by introducing a source follower, the source follower has the characteristics of low output impedance and high input impedance. It can transmit the SPAD anode voltage to the ADC with less distortion. This solution avoids introducing too much capacitance before the ADC sampling point, ensuring that the dead time of the quenching circuit is not changed. Among them, the dead time is the time required for the quenching circuit to recover to a state where it can be triggered again after the SPAD avalanche breakdown, which is crucial for the stable operation and accurate measurement of the SPAD. Avoiding the influence of capacitance on the dead time helps to maintain the stable performance of the quenching circuit and ensure that the SPAD maintains high efficiency and accuracy in high-speed and continuous photon detection. And, by introducing a source follower, this solution effectively isolates the high voltage between the ADC sampling circuit and the SPAD anode, reducing the anode voltage to a safe level that the ADC can withstand. This not only protects the ADC from high voltage damage but also ensures the safety and reliability of sampling.

[0054] Please refer to Figure 5 , Figure 5 FIG. is the structural diagram of the SPAD operating overvoltage calibration circuit provided by the fifth embodiment of the present application. The difference from the third embodiment is that: the control module of this embodiment further includes a second MOS transistor M1 and a third MOS transistor M2; the first end of the SPAD is connected to the gate of the second MOS transistor M1, the drain of the second MOS transistor M1 is connected to the power supply VDD, the source of the second MOS transistor M1 is connected to the drain of the third MOS transistor M2, the drain of the third MOS transistor M2 is connected to the gate of the third MOS transistor M2, the source of the third MOS transistor M2 is grounded, and the source of the second MOS transistor M1 is connected to the ADC and the pulse shaping circuit.

[0055] In the embodiments of the present application, a voltage dividing circuit is formed by using the second MOS transistor M1 and the third MOS transistor M2 to reduce the voltage input to the ADC, protecting the ADC from high voltage damage and ensuring the safety and reliability of sampling.

[0056] Please refer to Figure 6 , Figure 6 which is a flowchart of the steps of a SPAD working overvoltage calibration method provided by the embodiments of the present application, specifically including:

[0057] Step S1: Use the MCU to send an initial control signal to the charge pump. The initial control signal is used to indicate the output voltage VSPAD of the charge pump. At this time, the SPAD can work in the Geiger mode; wherein, VSPAD is greater than the breakdown voltage of the SPAD.

[0058] Among them, the Geiger mode, also known as the avalanche mode, is a working mode of the SPAD. In this mode, the working voltage of the SPAD is higher than its breakdown voltage (Vbreak). When a photon is absorbed by the depletion region of the SPAD, an electron-hole pair will be generated. Under the action of a high electric field, this electron-hole pair will quickly split and accelerate, continuously collide and ionize, generating more electron-hole pairs, thus triggering an avalanche-like multiplication effect. This multiplication effect enables the SPAD to detect the weak signal of a single photon. The high sensitivity and time resolution of the SPAD in the Geiger mode make it a key component in lidar. By measuring the time of flight (ToF) of the light pulse, the distance and speed of the target object can be accurately calculated.

[0059] Step S2: When the SPAD undergoes avalanche breakdown, use the pulse shaping circuit to broaden the voltage VEX at the first end of the SPAD.

[0060] Among them, the pulse shaping circuit broadens the voltage signal at the first end of the SPAD, which helps to improve the waveform and quality of the signal. When the SPAD undergoes avalanche breakdown, the generated voltage signal is often very steep and changes rapidly. Directly sampling such a signal may cause the ADC to be unable to accurately capture its peak value. By broadening the signal, the pulse shaping circuit makes the waveform of the signal smoother, which is beneficial for the ADC to perform more stable and accurate sampling.

[0061] Step S3: Use the delay unit to generate a start conversion signal after delaying the broadened signal by a delay time.

[0062] Step S4: When the ADC receives the start conversion signal, the ADC samples and outputs a digital signal Dout to the MCU.

[0063] Step S5: Use the MCU to obtain digital signals corresponding to different delay times by adjusting the delay time;

[0064] Specifically, as Figure 7 shown, different start conversion signals are generated at different delay times, and the ADC is controlled to sample according to the start conversion signals, so that different VEX values can be obtained.

[0065] Step S6: Use the MCU to determine the maximum value of the digital signals according to the digital signals corresponding to all the delay times, and use the maximum value of the digital signals as the maximum value of the first terminal voltage of the SPAD.

[0066] Step S7: Use the MCU to compare the maximum value of the first terminal voltage of the SPAD with the breakdown voltage of the first MOS transistor M0. If the maximum value is greater than the breakdown voltage of the first MOS transistor M0, output a control signal indicating to reduce the output voltage of the charge pump to the charge pump; if the maximum value is less than or equal to the breakdown voltage of the first MOS transistor M0, output a control signal indicating that the output voltage of the charge pump remains unchanged to the charge pump.

[0067] In the embodiments provided in the present application, it should be understood that the disclosed device and method can be implemented in other ways. The device embodiments described above are only illustrative. For example, the division of the units is only a logical function division, and there may be other division methods in actual implementation. For another example, multiple units or components can be combined or integrated into another system, or some features can be ignored or not executed. Another point is that the displayed or discussed couplings or direct couplings or communication connections to each other can be through some communication interfaces. The indirect couplings or communication connections of the devices or units can be in electrical, mechanical or other forms.

[0068] In addition, the units described as separate components may or may not be physically separated, and the components displayed as units may or may not be physical units, that is, they may be located in one place, or may be distributed to multiple network units. Some or all of the units can be selected according to actual needs to achieve the purpose of the solution of this embodiment.

[0069] Furthermore, in each embodiment of the present application, the functional modules can be integrated together to form an independent part, or each module can exist alone, or two or more modules can be integrated to form an independent part.

[0070] In this article, relational terms such as first and second are only used to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations.

[0071] The above are only the embodiments of the present application and are not intended to limit the protection scope of the present application. For those skilled in the art, the present application may have various modifications and changes. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A SPAD working overvoltage calibration circuit, characterized in that: include: Charge pump, SPAD, first MOS tube and control module; The output end of the charge pump is connected to the second end of the SPAD, the first end of the SPAD is connected to the drain of the first MOS tube, the source of the first MOS tube is grounded, the input end of the control module is connected to the first end of the SPAD, and the output end of the control module is connected to the input end of the charge pump; The control module is used to detect the maximum value of the voltage at the first terminal of the SPAD when the SPAD undergoes avalanche breakdown, and compare the maximum value with the withstand voltage of the first MOS tube, and output a control signal instructing to reduce the output voltage of the charge pump to the charge pump if the maximum value is greater than the withstand voltage of the first MOS tube; The control module includes: ADC, pulse shaping circuit, delay unit and MCU; The pulse shaping circuit is used to widen the voltage signal of the first terminal of the SPAD; The delay unit is used to output a start conversion signal to the ADC and the MCU after a delay time for the stretched signal; The ADC is used to start working upon receiving a start conversion signal, sample the voltage level of the first terminal of the SPAD and convert it into a digital signal, and output the digital signal to the MCU; The MCU is used to output different delay times to the delay unit, and receive digital signals corresponding to different delay times obtained by sampling the ADC; determine the maximum value of the digital signal according to the digital signals corresponding to all the delay times, and use the maximum value of the digital signal as the maximum value of the first terminal voltage of the SPAD; The control module further includes: a second MOS tube and a current source; The first end of the SPAD is connected to the gate of the second MOS tube, the drain of the second MOS tube is connected to the power supply VDD, the source of the second MOS tube is connected to the current source and then grounded, and the source of the second MOS tube is connected to the ADC and the pulse shaping circuit.

2. The circuit according to claim 1, characterized in that The control module further includes: a third MOS tube; The source of the second MOS tube is connected to the drain of the third MOS tube, the drain of the third MOS tube is connected to the gate of the third MOS tube, and the source of the third MOS tube is grounded.

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