CN104124947B - baseline voltage holding structure and pulse shaper - Google Patents

Abstract

The invention discloses a pulse shaper with a baseline voltage maintaining structure and a DC coupling structure. The invention realizes the suppression of the baseline voltage drift of the readout circuit, and the baseline drift value, that is, the difference between the baseline voltage and the reference voltage is converted into a current value and amplified, and the amplified current is fed back to the input terminal of the previous stage to form a high gain , Narrow bandwidth negative feedback loop, so that the baseline voltage is fixed at the reference value. The method of the present invention enables the baseline voltage value to be determined by the feedback loop without being affected by non-ideal factors such as parameter adaptation of circuit components; the switch of the feedback loop is controlled by the effective signal in the main signal path, ensuring the baseline maintenance module Has no effect on valid signals. The invention can meet the requirements of low power consumption and low noise radiation detection system, and will be widely used.

Description

Baseline voltage hold structure and pulse shaper

technical field

The invention belongs to the technical field of a readout structure and a readout method of a readout circuit of a radiation detection system.

Background technique

Radiation detection systems are widely used in high-energy physics, military affairs, industry and agriculture, medicine, astronomy and other fields. The core components of a radiation detection system include radiation detectors and readout circuits. The function of the radiation detector is to realize the conversion of the radiation quantity to the charge quantity, and the function of the readout circuit is to complete the processing and readout of the charge pulse signal. The readout circuit has a significant impact on the performance of the radiation detection system. In recent years, with the development of CMOS technology, large-scale CMOS radiation detector readout circuits have become mainstream.

A typical radiation detector readout circuit includes a Charge Sensitive Amplifier (CSA for short), a Pulse Shaper (Pulse Shaper), a discriminator, and an output buffer. The circuit structure is shown in Figure 1. The charge sensitive amplifier is the interface between the radiation detector and the readout circuit, and completes the functions of receiving and integrating the charge pulse signal. The pulse shaper is generally a band-pass filter, which completes the noise reduction and shaping of the output pulse signal of the previous stage. The discriminator can use a hysteresis comparator to reduce the influence of noise interference, complete the comparison between the pulse signal and the threshold voltage Vth, and send the comparison result to the counter for statistics and output.

The charge sensitive amplifier generally adopts an integrator in the form of capacitive negative feedback, which can avoid the influence on the amplification gain of the charge signal due to the difference in the capacitance of the detector. At the same time, in order to avoid the saturation of the output terminal of the charge-sensitive amplifier, the circuit uses a negative feedback resistor to realize continuous-time charge discharge. The transfer function of the charge-sensing amplifier can be expressed as:

Among them, C _f is the negative feedback capacitance, R _f is the negative feedback resistance, and _Qin inputs the total amount of charge.

The pulse shaper adopts the CR-RC ² structure, which realizes the further amplification of the signal while completing the noise reduction and shaping of the output signal of the previous stage. Since the amount of the charge pulse signal generated by the radiation detector may be small, the readout circuit needs to achieve a large gain, and this part of the gain is mainly realized by the pulse shaper. The shaper that directly adopts the form of AC coupling will produce overshoot at the output end, and the output overshoot requires a long recovery time, which seriously affects the counting rate of the detection system. The shaper with pole-zero cancellation structure can avoid output overshoot by eliminating poles and improve the counting rate of the detection system. The pole-zero cancellation circuit structure is shown in Figure 2. Set R _f C _f = R _pz C _dif , so that the zero point generated by the pulse shaper can cancel the pole point of the charge-sensing amplifier, where R _pz and C _dif are pole-zero The resistance and capacitance of the cancellation module.

The existence of the resistor R _pz makes the entire main signal path have a DC path, and the mismatch existing in the circuit is amplified step by step with the effective signal, which eventually causes a large drift in the baseline voltage of the output stage. The mismatch in the circuit is a random value, and the main contributors to the baseline voltage drift value are the input mismatch of the charge sense amplifier and the mismatch of the pole-zero canceling circuit. At the same time, the mismatch existing in the circuit also changes with the ambient temperature. Therefore, the baseline voltage drift value is a random voltage value that varies with the ambient temperature, and it is difficult to compensate the circuit with a fixed voltage or current to reduce the baseline voltage drift value.

Contents of the invention

The present invention provides a novel baseline voltage holding (baseline holder) structure, which forms a local negative feedback loop inside the pulse shaper, with the purpose of reducing the baseline voltage without increasing power consumption and without sacrificing signal-to-noise ratio. stable at a fixed reference voltage value.

The technical solution of the present invention is to convert the difference between the baseline voltage of the output signal and the reference voltage into a current value, and further amplify it and feed it back to the previous stage circuit to form a negative feedback loop, because the negative feedback loop achieves a larger gain , the baseline voltage of the output signal is finally fixed at the reference voltage value. The negative feedback loop is used as an auxiliary loop to suppress the drift of the baseline voltage. It should be ensured that it does not affect the effective signal. Therefore, a switch is added in the feedback loop, and the feedback loop is disconnected when the main signal path responds to the effective input signal. road. For the main signal path, the shaper achieves a large gain, so the charge-sensitive amplifier is more sensitive to noise. From the perspective of noise, the current negative feedback is not fed back to the input terminal of the charge-sensitive amplifier, but is fed back to the middle node of the shaper. The circuit structure of the baseline voltage holding module is shown in Figure 3. The baseline holding module forms a local negative feedback loop in the shaper, so that the DC-coupled pulse shaper has the characteristic of baseline holding. The structure mainly includes a transconductance structure and a bidirectional low-pass current mirror with a switch structure, the structure is very simple, and it is easy to realize the design requirements of low power consumption and low noise.

The _{transconductance} structure composed of operational amplifier OTA and resistor RT realizes the conversion from voltage to current. The positive input terminal of the operational amplifier OTA is the reference voltage V _ref , so the voltage value of the negative input terminal of the OTA is also clamped at the reference voltage V _ref , that is, the voltage values at both ends of the resistor R _T are the shaper output voltage V _out and the reference voltage V _ref , the voltage difference is proportional to the current flowing through the resistor RT _.

The switch in the feedback loop is controlled by the effective signal in the main signal path. When the effective signal passes through, the discriminator responds and generates a narrow pulse. The jump edge of the narrow pulse is used to trigger a pulse generator, which generates a A pulse with a fixed width τ is used to control the switch in the feedback loop, and the pulse width τ is determined by the effective signal time constant.

When there is no effective signal passing through the main signal path, the output voltage value V _out of the shaper is the baseline voltage. At this time, the switch is in the closed state, and one end of the bidirectional current mirror is in the conductive state. When the output baseline voltage is lower than the reference voltage, the low-pass current mirror composed of PMOS transistors Mp1, Mp2 and capacitor C1 is in the on state, and the NMOS transistor current mirror is in the off state. The feedback current fed back to the previous stage can be expressed as:

Where ω _p ≈ _{g ml} /Cl, g _ml is the transconductance value of the MOS tube Mp1, and N is the magnification of the current mirror.

The current value flowing through Mp1 is equal to the current flowing through the resistor RT, _Mp2 realizes the N times amplification of the current, and the capacitor C1 limits the bandwidth of the current mirror. When ω _p is much smaller than the time constant of the shaper, the bandwidth of the current mirror is much smaller than the operating frequency of the main signal path. On the one hand, it ensures that the noise introduced by the feedback loop is minimal, and on the other hand, it ensures the stability of the feedback loop. When the output baseline voltage is higher than the reference voltage, the PMOS transistor current mirror is in the off state, and the low-pass current mirror composed of the NMOS transistors Mn1, Mn2 and the capacitor C2 is in the on state.

When there is no baseline hold module, the baseline voltage drift value is ΔV _out , after adding the baseline hold module, the baseline voltage drift value is ΔV _{out, closed} , the relationship between the two can be expressed as:

Among them, A _open is the open-loop gain of the main signal path, β is the feedback coefficient, V _os is the input mismatch voltage of the operational amplifier OTA, and the loop gain β·A _open can be expressed as:

Where τ ₀ =R _sh C _sh is the time constant of the pulse shaper, R _sh and C _sh are the feedback resistance and capacitance respectively. According to the above expression, the following conclusions can be drawn: the greater the low-frequency gain of the loop, the smaller the baseline voltage drift; the smaller the ω _p , the narrower the loop bandwidth. When ω _p is much smaller than τ ₀ , the loop can be regarded as a single The pole structure can ensure the stability of the loop, and at the same time, the noise fed back to the front stage is smaller.

When a valid signal passes through the main signal path, the output voltage value V _out of the shaper contains the response to the input signal, and the switch is in an off state at this time. The current value flowing through Mp1 (or Mn1) is still equal to the current flowing through the resistor RT; the switch is _turned off, the capacitors Cl and C2 have no charge and discharge path, and the voltage maintains the voltage value at the moment before the switch is turned off, so the current flowing through Mp2 and Mn2 The feedback current value is also the current value flowing through the MOS tube immediately before the switch is turned off. Since the baseline drift value is usually not transient, the feedback current can still be used to suppress the drift of the baseline. The switch control signal of the baseline holding module may be delayed, and some effective signals enter the feedback loop. Due to the extremely narrow bandwidth of the feedback loop, the effective signals are filtered out in the loop, which further ensures the stability of the baseline voltage.

Description of drawings

Fig. 1 is a structural block diagram of a typical radiation detector readout circuit;

Fig. 2 is a structural block diagram of a radiation detector readout circuit including a pole-zero cancellation structure;

Fig. 3 is a novel baseline maintenance module structure diagram proposed in the present invention;

Fig. 4 is a graph showing the relationship between the baseline voltage value and the temperature change of the radiation detector readout circuit when there is no baseline maintenance module;

Fig. 5 is a comparison diagram of analog output and switch control signal of the radiation detector readout circuit with or without the baseline maintenance module.

detailed description

The present invention will be described in detail below through embodiments.

The amount of charge received by the radiation detector readout circuit is often very weak, which requires the pulse shaper to achieve a large multiple of gain. The readout circuit in the design verification achieves a magnification of about 40 times. The input mismatch of the charge-sensitive amplifier is usually on the order of several millivolts. Considering the mismatch of the resistors and capacitors in the circuit and the mismatch of the pulse shaper, the final baseline voltage drift can reach more than 100 millivolts. The tape-out test of the circuit structure without the baseline holding module is carried out. The test results of multiple channels show that the baseline drift value is random, and the maximum drift value can reach ±150mV. A channel is randomly selected for temperature testing. The relationship between the baseline voltage value (baseline) and the ambient temperature is shown in Figure 4, and the reference voltage is 1.6V.

In order to suppress baseline drift, a new baseline holding structure shown in Figure 3 is added to the circuit structure. Adding this module must not sacrifice the original performance of the circuit while effectively suppressing the baseline drift, mainly including the power consumption and signal-to-noise ratio of the circuit. In terms of power consumption, the main consumption module of the baseline maintenance module is the operational amplifier OTA. OTA only requires high gain and no bandwidth, and can realize low power consumption design. In this embodiment, the current consumption of OTA is lower than 1 μA. In terms of noise, capacitors C1 and C2 greatly limit the loop bandwidth, which requires the loop bandwidth to be much smaller than the operating frequency of the main signal path. The noise caused by the feedback loop is negligible compared with the main signal path. In this implementation Capacitors used to limit bandwidth are on the order of nanofarads.

The loop stability simulation is carried out on the negative feedback loop formed by the baseline holding module. The low-frequency gain of the loop is 40dB, and the unity gain bandwidth is 240Hz. The low-frequency gain of 40dB reduces the baseline drift value caused by mismatch by 99%. Carry out time domain simulation and noise performance simulation on the circuit. The simulation results are shown in Figure 5 below. The figure compares the output response of the readout circuit with or without the baseline hold module. The reference voltage is set to 1.6V, --- is the output response when there is no baseline hold module, due to mismatch, the output baseline voltage drift value is about 125mV, --- is the output response when the baseline hold module is added, the output baseline voltage restores the reference The voltage effectively suppresses the baseline drift, and the pulse width of the switch control signal is 7.25 μS, which is about the effective signal width, which ensures that the effective signal will not enter the feedback loop, and at the same time does not affect the counting rate of the readout circuit. The simulation results in Figure 5 include the influence of noise, and the results show that the output noise after adding the baseline hold module is almost the same as that without the baseline hold module.

It can be seen from the above results that the baseline holding structure proposed by the present invention can effectively suppress the drift of the baseline voltage of the readout circuit, and has the characteristics of low power consumption and low noise without affecting the performance of the circuit itself.

Finally, it should be noted that the purpose of the disclosed embodiments is to help further understand the present invention, but those skilled in the art can understand that various replacements and modifications can be made without departing from the spirit and scope of the present invention and the appended claims. It is possible. Therefore, the present invention should not be limited to the content disclosed in the embodiments, and the protection scope of the present invention is subject to the scope defined in the claims.

Claims (1)

1. a kind of dc-couple, with baseline voltage keep function pulse shaper, it is characterised in that put by three-stage operational Big device, discriminator, impulse generator and baseline voltage holding structure composition, the wherein series connection of first order opamp input terminal by Resistance R_pzWith electric capacity C_difAmplify respectively with the first order at the reinforced concrete structure that parallel connection is constituted, the two ends of another group of resistance capacitance parallel-connection structure Input, the output end connection of device；Second, third grade of operational amplifier configuration is identical, is input series resistance, input, it is defeated Go out the two ends that end connects resistance capacitance parallel-connection structure respectively；Three-stage operational amplifier is connected in series；The input connection of discriminator To the output end of second level operational amplifier, the output end connection impulse generator input of discriminator, impulse generator output End produces switch controlling signal；Wherein, baseline voltage holding structure includes one by operational amplifier OTA and resistance R_TComposition Transconductance modulator and a two-way low pass current mirror module being made up of PMOS current mirrors and NMOS current mirrors, wherein operational amplifier OTA positive input terminal is connected on baseline voltage Vref, resistance R_TOne end is connected to the output end of third level operational amplifier, separately A pair of NMOS tubes and the source of PMOS that the negative input end and source electrode that one end is connected to operational amplifier OTA are connected, it is described The grid of a pair of NMOS tubes and PMOS is connected to operational amplifier OTA output end, the pair of NMOS tube and PMOS Draining end of NMOS tube and PMOS drain terminal are connected respectively to PMOS current mirrors input pipe Mp1 drain terminals and NMOS current mirror input pipes Mn1 Drain terminal；PMOS current mirror efferent ducts Mp2 grid connection electric capacity C1 one end, electric capacity C1 other end ground connection, NMOS current mirrors Efferent duct Mn2 grid connection electric capacity C2 one end, electric capacity C2 other end ground connection；Mp2 drain terminal is connected with Mn2 drain terminal, And it is connected to the input of second level operational amplifier；Two-way low pass current mirror module is controlled by two switches, PMOS current mirrors One of switch of the efferent duct Mp2 grids through described two switches be connected to input pipe Mp1 grids, NMOS current mirrors it is defeated Another switch of outlet pipe Mn2 grids through described two switches is connected to input pipe Mn1 grid, and two switches, which are received, comes from arteries and veins Rush the switch controlling signal of generator generation.