Transimpedance Preamplifier Input Impedance Adaptive Adjustment Circuit Structure and Control Method

Abstract

The present invention provides a circuit structure and a control method for adaptively adjusting the input impedance of a transimpedance preamplifier. It mainly consists of one current-mode preamplifier, two current discriminators, two counters, and one output impedance adaptive control module. The present invention can overcome the problem that the current-mode front-end readout circuit in the traditional solution can only perform impedance matching for a specified type of detector, and can be applicable to different types or models of detectors, ensuring the stable operation of the detector signal and improving the versatility of the current-mode transimpedance amplifier.

Description

Technical Field

[0001] The present invention belongs to the field of nuclear electronics, and particularly relates to a circuit structure and control method for adaptively adjusting the input impedance of a transimpedance preamplifier. It can be applied to dedicated integrated circuits for current-mode front-end readout of different types of radiation detectors, gas detectors, and photodetectors. Background Art

[0002] Nuclear radiation applications such as X- and γ-ray imaging, high-energy physics experiments, and nuclear physics experiments require detector front-end readout electronics systems with large dynamic range, high count rate, and high time resolution. In these readout electronics, dedicated integrated circuits (ASICs) are generally used to implement the front-end readout circuit system. When implementing the ASIC, a "charge integration type" front-end readout circuit based on a nano-CMOS process or a deep submicron "current mode" front-end readout circuit is generally used to obtain a large dynamic range, high count rate, and high time resolution. However, the solution of implementing the circuit using the nano-CMOS process not only has problems such as reduced input-output range and decreased signal-to-noise ratio caused by the decrease in power supply voltage, but also increases the manufacturing cost. Using a current-mode front-end readout circuit can overcome the above problems and is the main solution to simultaneously obtain a large dynamic range, high count rate, and high time resolution. However, the current-mode front-end readout ASIC will also encounter a problem: the detector and the front-end readout circuit need to be impedance-matched to maintain system stability.

[0003] To understand the problem of impedance matching between the detector and the front-end readout circuit, an electrical model of the detector and the front-end readout circuit is first established. As Figure 1 shown, on the left is the simplified model of the detector, which consists of a detector current source and a detector capacitor C det ; on the right is the simplified model of the preamplifier, including an input resistor R in , an input capacitor C in , an output resistor R out , and an output capacitor C out . The connection between the detector and the readout circuit is made by wire bonding, and a parasitic inductance L wb is used for modeling. By using Kirchhoff's law analysis, it can be obtained that the small-signal current I in flowing into the preamplifier and the output voltage V out can be expressed as:

[0004]

[0005] where Q 0$Q$ is the charge of the pulse current generated by the detector, and $A$ represents the current gain of the preamplifier in the current mode. In the above model, the detector charge is generated instantaneously, and the rise time of the input current pulse it generates is determined by the input impedance of the preamplifier, the detector capacitance, and the input capacitance of the preamplifier. Theoretically, as long as the input impedance of the preamplifier is small enough, the shorter the rise time of the input pulse current, the faster the speed.

[0006] Since there are series or parallel RLC circuits in the equivalent model, it is necessary to select a sufficiently high input impedance to avoid series resonance and a sufficiently low input impedance to avoid parallel resonance. Therefore, by calculating the quality factors under the two resonances, the value range of the input impedance under the stable conditions as shown in the following formula can be determined:

[0007]

[0008] The transient simulation results of the input current of the preamplifier changing with time under different input impedance matching conditions are as Figure 2 shown. From Figure 2 it can be seen that when the input impedance is large, parallel resonance will occur. At this time, the rise time of the input current signal increases, and a ringing effect occurs at the peak current. Eventually, the parallel resonance leads to an extension of the discharge time, affecting the output count rate. In addition, a large input impedance will reduce the current conversion efficiency of the front-end readout circuit, affecting the dynamic range and resolution. As the input impedance decreases, the above effects are alleviated. However, when the input impedance is small, series resonance will occur, resulting in a damping effect. Therefore, when designing the current-mode preamplifier, the input impedance cannot be too high or too low.

[0009] The literature "J.M.Fernández-Telado,Optimal design of single-photon sensor front-end electronics for fast-timing applications,IEEE,NSS / MIC 2020." gives a scheme for impedance matching of radiation detectors, that is, to design a fully customized front-end readout ASIC according to the performance parameters of the selected detector to ensure system stability. However, this fully customized method will also encounter the following problems: If another radiation detector of different specifications or types is replaced, the input impedance of the ASIC will be mismatched with the parameters of the new detector, resulting in unstable output signals. To solve this problem, a new circuit technology needs to be proposed so that the input impedance of the current-mode front-end readout ASIC can be adaptively adjusted according to the detector parameters, thereby solving the system stability problem when the detector and the ASIC are interconnected. Summary of the Invention

[0010] In order to overcome the deficiencies of the prior art, the present invention provides a transimpedance preamplifier input impedance adaptive adjustment circuit structure and control method. It mainly consists of 1 current-mode preamplifier, 2 current discriminators, 2 counters, and 1 output impedance adaptive control module. The present invention can overcome the problem that the current-mode front-end readout circuit in the traditional scheme can only perform impedance matching for a specified specification of the detector, and can be applicable to different types or models of detectors, ensuring the stable operation of the detector signal and improving the versatility of the current-mode transimpedance amplifier.

[0011] A transimpedance preamplifier input impedance adaptive adjustment circuit structure, characterized in that it consists of 1 current-mode preamplifier, 2 current discriminators, 2 counters, and 1 output impedance adaptive control module. Among them, the detector output current pulse signal is read out by the current-mode preamplifier and replicated into two currents, and then flows into the two current discriminators to generate voltage pulse signals TRIG1 and TRIG2. The voltage pulse signals become the count signals Count1 and Count2 of the adaptive impedance control module through the counters. When Count1 and Count2 are valid, the adaptive impedance control module starts to work, judges whether the resonance effect occurs according to the signals TRIG1 and TRIG2, and controls the input impedance of the current-mode preamplifier to change so as to match the detector impedance.

[0012] The input stage of the described current-mode preamplifier adds two transistors MN on the basis of the RCG amplifier 5 , MN 6 , and realizes the conduction and cut-off of the transistors MN 5 , MN 6 by turning on and off the output control signals D1 and D2 of the adaptive impedance control module; the output stage replicates two currents through a current mirror, amplifies them by N times, subtracts them from the threshold currents I th1 and I th2 , and then flows into the two current discriminators.

[0013] The present invention also provides a control method for a transimpedance preamplifier input impedance adaptive adjustment circuit structure as described above, characterized in that the steps are as follows:

[0014] Step 1, Detect the output current signal: When the radiation signal arrives, the detector generates a pulsed current and is in State 1. The current-mode preamplifier reads out the current and duplicates it into two paths and sends them to the current discriminator. The low-threshold current discriminator generates a trigger signal TRIG1, and the high-threshold current discriminator generates a trigger signal TRIG2. When the detection signal D_C of the adaptive control module is at a high level, two counters respectively record the number of discriminator triggers, denoted as Count1 and Count2. When D_C is at a low level, the counting stops. Meanwhile, judge the value of Count2. If it is 0, enter Step 2 and enter State 2. If it is 1, enter Step 3 and enter State 3. If it is greater than 1, return to the reset state;

[0015] Step 2, Detect series resonance: Enter State 2. If D_C is 0 and the value of Count1 is 0, series resonance occurs. Enter Step 4 and enter State 4. If the value of Count1 is not 1, the current state is invalid, and return to Step 1 to start detecting again;

[0016] Step 3, Detect parallel resonance: Enter State 3. Judge the magnitude of Count1. If the value of Count1 is 1, the system impedance matching is normal. Enter Step 4 and enter State 4. If it is greater than 2, parallel resonance occurs. Enter Step 4 and enter State 4;

[0017] Step 4, Generate a control signal: Enter State 4. Determine whether to continue the detection according to the level of D_C. If it is 1, continue the detection, and the state machine returns to State 1. If it is 0, do not detect, and the state machine stays in State 4 all the time, keeping the state of the input impedance adaptive control module unchanged until the next detection control signal D_C is at a high level and then return to Step 1 for the next round of detection.

[0018] The beneficial effects of the present invention are as follows: By adopting the method of input impedance adaptive control to control the current-mode preamplifier, the function of automatically adapting and adjusting the multi-stage input impedance can be realized, ensuring that the readout circuit using this structure can be matched with a variety of radiation detectors with different indexes and different types, overcoming the limitation that the fully customized readout circuit is only applicable to a certain specified detector, and enhancing the versatility of the readout circuit. Description of the Drawings

[0019] Figure 1 is a schematic diagram of the equivalent electrical model of the detector readout system based on the current-mode preamplifier;

[0020] In the figure, Q 0 - The charge quantity of the pulsed current generated by the detector, C det - The detector capacitance, L wb - The parasitic inductance, I in - The small-signal current flowing into the preamplifier, Rin - Input resistance, C in - Input capacitance, A - Current gain of the current - mode pre - amplifier, R out - Output resistance, C out - Output capacitance, V out - Output voltage;

[0021] Figure 2 is the current output pulse diagram of the silicon detector under different input impedances;

[0022] Figure 3 is the structural diagram of the input - impedance adaptive - regulation circuit of the trans - impedance pre - amplifier of the present invention;

[0023] In the figure, IN - Detector output current pulse signal, I out - Output current of the current - mode pre - amplifier, I th1 、I th2 - Threshold current, TRIG1 - Trigger signal of counter 1, TRIG2 - Trigger signal of counter 2, Counter1 - Counter 1, Counter2 - Counter 2, Count1 - Count value of counter 1, Count2 - Count value of counter 2, CLK - Clock signal, RST - Reset signal, D_C - Detection signal of the adaptive control module, D1, D2 - Output signals of the input - impedance adaptive - control module;

[0024] Figure 4 is the simulation result diagram of using two current discriminators to distinguish resonant signals from normal signals;

[0025] In the figure, (a) - Parallel resonance and its discriminator trigger result diagram, (b) - Series resonance and its discriminator trigger result diagram, (c) - Normal signal and its discriminator trigger result diagram;

[0026] Figure 5 is the state - transition diagram of the control method of the input - impedance adaptive - regulation circuit structure of the trans - impedance pre - amplifier of the present invention;

[0027] In the figure, IDLE - Reset state, State1 - State 1, State2 - State 2, State3 - State 3, State4 - State 4, Count1 - Count value of counter 1, Count2 - Count value of counter 2, D1, D2 - Output signals of the input - impedance adaptive - control module, RST - Reset signal, D_C - Detection signal of the adaptive control module;

[0028] Figure 6 is the comparison diagram of transient simulation results of the output current with and without the adaptive impedance - regulation module;

[0029] Figure 7It is a diagram of the system phase margin varying with the detector capacitance;

[0030] Figure 8 It is a structural diagram of an input impedance programmable current - mode pre - amplifier;

[0031] In the figure, I IN - Input current of the current - mode pre - amplifier, D1, D2 - Output signals of the input impedance adaptive control module, MN 1 ~MN 9 - NMOS transistors, MP 5 、MP 9 ~MP 12 - PMOS transistors, R 1 - Resistor, I th1 、I th2 - Threshold current, I out - Output current of the current - mode pre - amplifier, N - Constant;

[0032] Figure 9 It is a structural diagram of a current discriminator;

[0033] In the figure, I in - Input current of the current discriminator, MN 0 、MN 4 、MN 7 、MN 9 、MN 12 、MN 13 、MN 15 、MN 17 - NMOS transistors, MP 5 、MP 6 、MP 8 、MP 10 、MP 14 、MP 16 - PMOS transistors, VSSA - Analog negative power supply, VDDA - Analog positive power supply, V 1 ~V 6 - Node voltages, V out - Output voltage of the current discriminator;

[0034] Figure 10 It is a schematic diagram of the RTL level of a two - bit counter;

[0035] In the figure, TRIG_A - Counter trigger signal, Reset - Counter reset signal, >Clk - Counter trigger port, D - Counter input data port, Q - Counter normal output data port, - Counter output inverted data port, - Counter output signal;

[0036] Figure 11 It is a timing diagram controlled by an adaptive control method.

[0037] In the figure, TRIG1 is the trigger signal of counter 1, TRIG2 is the trigger signal of counter 2, Count1 is the count value of counter 1, Count2 is the count value of counter 2, CLK is the clock signal, RST is the reset signal, D_C is the detection signal of the adaptive control module, and D1 and D2 are the output signals of the input impedance adaptive control module. Specific implementation manner

[0038] The present invention will be further described below in conjunction with the accompanying drawings and embodiments. The present invention includes but is not limited to the following embodiments.

[0039] As Figure 3 shown, a transimpedance preamplifier input impedance adaptive adjustment circuit structure is characterized in that it mainly consists of 1 current-mode preamplifier, 2 current discriminators, 2 counters and 1 output impedance adaptive control module. Among them, the detector output current pulse signal is read out and copied into two currents by the current-mode preamplifier, and then flows into two current discriminators to generate voltage pulse signals TRIG1 and TRIG2. The voltage pulse signals become the counting signals Count1 and Count2 of the adaptive impedance control module through the counters. When Count1 and Count2 are valid, the adaptive impedance control module starts to work and controls the change of the two output signals. If the input signal generates a resonance effect due to the mismatch between the detector and the input impedance of the preamplifier, the adaptive impedance control module will judge whether a resonance effect occurs according to the discriminator output signals TRIG1 and TRIG2. After the resonance occurs, the adaptive impedance control module controls the input impedance of the current-mode preamplifier to change so as to match it with the detector impedance.

[0040] To achieve adaptive adjustment, it is first necessary to distinguish the types of input current signal responses caused by impedance matching and impedance mismatch. The principles for distinguishing series resonance, parallel resonance and normal signals are as follows: The charge Q generated after the detector is triggered by a single radiation signal is certain. However, due to the input impedance mismatch, the instantaneous magnitude of its current i(t) is not constant, but the integral value of its current signal from the generation to the end of the current signal within the time period t 0 must be the charge Q, that is:

[0041]

[0042] For example, in Figure 2Among the four current input signals shown, regardless of whether the current signal is caused by impedance matching or mismatch, the integral value of the current signal from the rising to the end time is equal. When the signal input impedance is too large and series resonance occurs in the signal, the discharge time of the signal is greatly extended, and the current peak value of the signal is greatly reduced, as shown in Figure 2 the R in in = 75 Ω and the signal of R in = 300 Ω; when parallel resonance occurs due to too low input impedance of the signal, the signal current peak value is very high, and at the same time, secondary oscillation ripples are generated, as shown in Figure 2 the R in in = 6 Ω. When the signal does not oscillate, its pulse width and current peak value are within a reasonable range.

[0043] Based on the principle that the charge generated by the above single event remains unchanged, the current flowing into the preamplifier readout is copied into two paths and amplified by N times and then sent to two current discriminators. By setting appropriate current thresholds for the two current discriminators, various resonance signals can be distinguished. Figure 4 is the simulation result of using two current discriminators to distinguish resonance signals from normal signals. It can be seen from this that after setting appropriate threshold currents I th1 and I th2 for the two current discriminators respectively, it is very easy to distinguish resonance signals from non-resonance signals according to the number of pulses triggered by the discriminators. When parallel resonance occurs, due to the generation of second harmonics, the low-threshold current discriminator will be triggered more than twice, and the high-threshold current discriminator will be triggered once; when series resonance occurs, the low-threshold current discriminator will be triggered once, and the high-threshold current discriminator will not be triggered; when no resonance occurs, both the low-threshold current discriminator and the high-threshold current discriminator will be triggered once. Finally, different impedance-matched signals are distinguished through the trigger results of the two discriminators.

[0044] After distinguishing resonance signals from normal signals, it is also necessary to implement an input impedance adaptive control method, as shown in Figure 5 shown, which mainly includes the following four steps:

[0045] Step 1, Detect the output current signal: When the radiation signal arrives, the detector generates a pulsed current and is in State 1. The current-mode preamplifier reads out the current and duplicates it into two paths and sends them to the current discriminators. The low-threshold current discriminator generates a trigger signal TRIG1, and the high-threshold current discriminator generates a trigger signal TRIG2. When the detection signal D_C of the adaptive control module is at a high level, two counters respectively record the number of times the discriminator is triggered, denoted as Count1 and Count2. When D_C is at a low level, the counting stops. At the same time, the value of Count2 is judged. If it is 0, go to Step 2 and enter State 2. If it is 1, go to Step 3 and enter State 3. If it is greater than 1, return to the reset state. The reset state will control the input impedance of the preamplifier to be a moderate value. At this time, the output of the adaptive control module is 10;

[0046] Step 2, Detect series resonance: Enter State 2. If D_C is 0 and the value of Count1 is 0, series resonance occurs, go to Step 4 and enter State 4. If the value of Count1 is not 1, the state is invalid at this time, and return to Step 1 to start detecting again;

[0047] Step 3, Detect parallel resonance: Enter State 3. Judge the magnitude of Count1. If the value of Count1 is 1, the system impedance matching is normal, go to Step 4 and enter State 4. If it is greater than 2, parallel resonance occurs, go to Step 4 and enter State 4;

[0048] Step 4, Generate a control signal: Enter State 4. Judge whether to continue the detection according to the level of D_C. If it is 1, continue the detection, and the state machine returns to State 1. If it is 0, do not detect, and the state machine stays in State 4 all the time, so that the state of the input impedance adaptive control module remains unchanged until the next detection control signal D_C is at a high level and then returns to Step 1 for the next round of detection.

[0049] Figure 6 The transient simulation comparison results of the output current of the signal with and without the adaptive impedance adjustment circuit are given. It can be seen that whether it is a series resonance or a parallel resonance signal, the output current can remain stable in the case of an adaptive impedance adjustment circuit, and the signal has no oscillation and no long tail. Figure 7 The variation of the system phase margin with the detector capacitance is given. The phase margin is an important index indicating the stability of the system. For an electronic system, the higher the phase margin, the more stable the system. From Figure 7 it can be seen that when there is no adaptive impedance control circuit, the system phase margin is very small. Although it slowly increases with the input capacitance, it is still small. While after adding the adaptive impedance control circuit, the phase margin always remains at a large stable value, and the system stability is good.

[0050] Figure 8 A specific circuit structure diagram of a digitally controlled current-mode preamplifier with input impedance is given. The input stage is an adjustable preamplifier (RCG) circuit structure, which consists of transistors MN 1 , MN 2 , MN 4 , MP 5 , MP 9 . The input impedance of this structure is very small. In order to make the input impedance of the RCG amplifier digitally controllable, transistors MN 5 , MN 6 are added on the basis of the RCG amplifier. The conduction and cutoff of transistors MN 5 , MN 6 are realized by turning on and off digital switches D1 and D2, so as to realize digitally programmable input impedance. The output stage is to copy two paths of current through a current mirror, amplify them by N times, and then subtract the threshold currents I th1 and I th2 , and then flow into two current discriminators.

[0051] Figure 9 A specific circuit structure diagram of a current discriminator is given. This circuit has an input port of a positive feedback stage and is realized by connecting the sources of PMOS and NMOS. Therefore, the input impedance is:

[0052]

[0053] where g m4 represents the transconductance of transistor MN 4 , and g m5 represents the transconductance of transistor MP 5 . The value of this input impedance can be very small, so this circuit is very sensitive to low currents. When the input current is greater than the threshold current and flows into the input of the positive feedback circuit, if V 1 remains constant, at this time V 2 decreases and V 3 increases. With positive feedback, transistor MP 8 conducts quickly, while transistor MN 9 turns off. Therefore, V 5 is set to high level and V 6 drops to low level. This structure can distinguish currents from several hundred nA to several mA and complete the conversion from weak current to large voltage pulse signal.

[0054] The counter adopts an asynchronous event-driven architecture, that is, when a trigger signal arrives, the counter counts, and if no trigger signal arrives, the counter does not work. This can reduce power consumption as much as possible and reduce the interference of digital clocks on analog signals at the same time. The RTL schematic diagram of the counter is as Figure 10As shown, an asynchronous two-bit counter structure is adopted.

[0055] The digital timing of the output impedance adaptive control method can be written according to Figure 5 to finally obtain Figure 11 the control timing of the adaptive control method. Combining Figure 5 and Figure 11 for Verilog digital circuit implementation, then using EDA tools to synthesize and generate a gate-level netlist, and finally using Cadence encounter to generate a specific layout to complete the digital method to circuit implementation.

Claims

1. A transimpedance preamplifier input impedance adaptive adjustment circuit structure, characterized in that: It is composed of 1 current-mode preamplifier, 2 current discriminators, 2 counters and 1 output impedance adaptive control module. Among them, the detector output current pulse signal is read out and copied into two currents by the current-mode preamplifier, and then flows into the two current discriminators to generate voltage pulse signals TRIG1 and TRIG2. The voltage pulse signals become the counting signals Count1 and Count2 of the adaptive impedance control module through the counters. When Count1 and Count2 are valid, the adaptive impedance control module starts to work, judges whether the resonance effect occurs according to the signals TRIG1 and TRIG2, and controls the input impedance of the current-mode preamplifier to change so as to match the detector impedance; The input stage of the described current-mode preamplifier adds two transistors MN on the basis of the RCG amplifier. 5 and MN 6 , and realizes the conduction and cut-off of transistors MN 5 and MN 6 by turning on and off the output control signals D1 and D2 of the adaptive impedance control module; the output stage copies two paths of current through a current mirror, amplifies them by N times, and then subtracts them from the threshold currents I th1 and I th2 and then flows into two current discriminators.

2. A control method for the transimpedance preamplifier input impedance adaptive adjustment circuit structure according to claim 1, characterized in that the steps are as follows: Step 1, detect the output current signal: when the radiation signal arrives, the detector generates a pulse current and is in state 1. The current-mode preamplifier reads out the current and copies it into two paths and sends them to the current discriminators. The low-threshold current discriminator generates a trigger signal TRIG1, and the high-threshold current discriminator generates a trigger signal TRIG2. When the detection signal D_C of the adaptive control module is at a high level, the two counters respectively record the trigger times of the discriminators, which are respectively recorded as Count1 and Count2. When D_C is at a low level, the counting stops. At the same time, judge the value of Count2. If it is 0, enter Step 2 and enter state 2. If it is 1, enter Step 3 and enter state 3. If it is greater than 1, return to the reset state; Step 2, detect series resonance: enter state 2. If D_C is 0 and the value of Count1 is 0, series resonance occurs, enter Step 4 and enter state 4. If the value of Count1 is not 1, the state is invalid at this time, and return to Step 1 to start detecting again; Step 3, detect parallel resonance: enter state 3, judge the magnitude of Count1. If the value of Count1 is 1, the system impedance matching is normal, enter Step 4 and enter state 4. If it is greater than 2, parallel resonance occurs, enter Step 4 and enter state 4; Step 4, generate a control signal: enter state 4, judge whether to continue detection according to the level of D_C. If it is 1, continue detection, and the state machine returns to state 1. If it is 0, do not detect, and the state machine stays in state 4 all the time, so that the state of the input impedance adaptive control module remains unchanged until the next detection control signal D_C is at a high level and then returns to Step 1 for the next round of detection.

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