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

Abstract

The invention discloses the pulse shaper of a kind of baseline voltage holding structure and dc-couple structure.The present invention realizes the suppression drifted about to reading circuit baseline voltage, baseline drift value, that is the difference of baseline voltage and reference voltage is converted into current value and is amplified, current feedback after amplification returns prime input, high-gain, a feedback loop for narrow bandwidth are formed, so that baseline voltage is fixed on reference value.The method of the present invention causes baseline voltage value to be determined by feedback control loop, without being influenceed by non-ideal factors such as circuit components parameter adaptations；The switch of feedback control loop is controlled by useful signal in main signal path, it is ensured that baseline keeps module not produce influence to useful signal.The present invention can meet low-power consumption, the requirement of low noise radiation detecting system, it will be widely applied.

Description

Baseline voltage holding structure and pulse shaper

Technical field

The invention belongs to the reading out structure of radiation detecting system reading circuit and reading method technical field.

Background technology

Radiation detecting system has a wide range of applications in fields such as high-energy physics, military affairs, industrial or agricultural, medical science, astronomy.Radiation The core component of detection system includes radiation detector and reading circuit.The effect of radiation detector is to realize amount of radiation to electric charge The conversion of amount, the effect of reading circuit is the processing and reading that complete charge pulse signal.Reading circuit is to radiation detecting system Performance have a major impact.In recent years, with the development of CMOS technology, extensive CMOS radiation detectors reading circuit into For main flow.

One typical radiation detector reading circuit includes charge amplifier (Charge Sensitive Amplifier, abbreviation CSA), pulse shaper (Pulse Shaper), several parts, circuit knot such as discriminator and output buffer Structure is as shown in Figure 1.Charge amplifier is the interface of radiation detector and reading circuit, completes the reception of charge pulse signal With integration etc. function.Pulse shaper is generally a bandpass filter, completes the noise reduction to prime output pulse signal and shaping. Discriminator can use hysteresis loop comparator, to reduce the influence of noise jamming, complete between pulse signals and threshold voltage vt h Compare, comparative result feeding counter is counted, exported.

Charge amplifier typically uses the integrator of capacitance negative feedback type, the integrator of the form can avoid because Detector electric capacity is different and influence that produced to charge signal gain amplifier.It is to avoid charge amplifier output end from satisfying simultaneously With circuit realizes the charge discharging resisting of continuous time using negative feedback resistor.The transfer function of charge amplifier can be represented For：

Wherein C_fFor negative-feedback electric capacity, R_fFor negative feedback resistor, Q_inInput charge total amount.

Pulse shaper employs CR-RC²Structure, is realized while noise reduction and shaping is completed to prime output signal The further amplification of signal.Due to the possible very little of charge pulse semaphore that radiation detector is produced, reading circuit needs to realize Larger gain, this fractionated gain is mainly realized by pulse shaper.Directly using the reshaper of AC coupled form defeated Overshoot can be produced by going out end, and output overshoot needs longer turnaround time, has a strong impact on the counting rate of detection system.Using the phase of pole zero The reshaper for the structure that disappears then can avoid output from overshooting by eliminating limit, improve the counting rate of detection system.The phase of pole zero Electric eliminating line structure is as shown in Fig. 2 set R_fC_f=R_pzC_dif, allow zero point and Charge sensitive amplifier that pulse shaper is produced The limit cancellation of device, wherein R_pzAnd C_difThe respectively resistance capacitance of pole-zero cancellation module.

Resistance R_pzPresence make it that whole main signal path has a DC channel, mismatch and useful signal present in circuit Amplified step by step together, it is final to cause output stage baseline voltage to produce larger drift.Mismatch in circuit is a random value, What baseline voltage drift value was played a major role is the input mismatch of charge amplifier and the mismatch of pole-zero cancellation circuit.Together When, mismatch present in circuit changes also as the change of environment temperature.Therefore, baseline voltage drift value be one it is random and With the magnitude of voltage of variation of ambient temperature, it is difficult to compensate to reduce base to circuit by fixed a voltage or electric current Line voltage drift value.

The content of the invention

(baseline holder) structure is kept the invention provides a kind of new baseline voltage, the structure is in pulse Reshaper is internally formed local negative feedback loop, it is therefore an objective to by baseline electricity on the premise of not increasing power consumption and not sacrificing signal to noise ratio Pressure is stable in a fixed reference potential value.

The technical scheme is that the difference of the baseline voltage of output signal and reference voltage is converted into current value, and Further amplified and feed back to front stage circuits formation feedback loop, it is defeated because feedback loop realizes larger gain The baseline voltage for going out signal is subsequently fixed at reference voltage level.Feedback loop is as secondary ring, to suppress baseline electricity The drift of pressure, should ensure that it does not produce influence, therefore increase switch in the feedback loop to useful signal, main signal path is to having Effect input signal disconnects feedback control loop when responding.For main signal path, reshaper realizes larger gain, therefore electric charge Sense amplifier is more sensitive for noise, considers from noise point, and Current Negative Three-Point Capacitance is not fed back to charge amplifier Input, but feed back to reshaper intermediate node.Baseline voltage keeps the circuit structure of module as shown in figure 3, baseline is kept Module forms a local negative feedback loop in reshaper so that have base using the pulse shaper of dc-couple structure The characteristic that line is kept.The structure mainly includes the two-way low pass current mirror of a transconductance structure and a belt switch structure, structure Very simple, it is easy to accomplish low-power consumption, the design requirement of low noise.

Operational amplifier OTA and resistance R_TThe transconductance structure of composition realizes voltage to the conversion of electric current.Operational amplifier OTA positive input terminal is reference voltage V_ref, therefore OTA negative input end magnitude of voltage is also clamped at reference voltage V_ref, i.e., it is electric Hinder R_TBoth end voltage value is respectively reshaper output voltage V_outWith reference voltage V_ref, voltage difference is with flowing through resistance R_TElectric current It is directly proportional.

Switch and controlled by useful signal in main signal path in feedback control loop, useful signal by when, discriminator makes phase Should, a burst pulse is produced, the hopping edge of the burst pulse produces one to trigger an impulse generator, the impulse generator Width is fixed as τ pulse to control the switch in feedback control loop, and pulse width τ is determined by useful signal time constant.

In main signal path without useful signal by when, reshaper output voltage values V_outFor baseline voltage, now at switch In closure state, one end of bidirectional current mirror is in the conduction state.When exporting baseline voltage less than reference voltage, PMOS The low pass current mirror of Mp1, Mp2 and electric capacity C1 compositions is in the conduction state, and NMOS tube current mirror is closed, before feeding back to The feedback current of level is represented by：

Wherein ω_p≈g_ml/ Cl, g_mlFor metal-oxide-semiconductor Mp1 transconductance value, N is current mirror multiplication factor.

Flow through Mp1 current value be equal to flow through resistance R_TElectric current, Mp2 realize electric current N times amplify, and electric capacity C1 is then limited The bandwidth of current mirror is made.Work as ω_pMuch smaller than reshaper time constant when, the bandwidth of current mirror is much smaller than main signal path Working frequency, on the one hand ensure that the noise that feedback control loop is introduced is minimum, on the other hand ensure that the stability of feedback control loop.When When exporting baseline voltage higher than reference voltage, pmos current mirror is closed, NMOS tube Mn1, Mn2 and electric capacity C2 compositions Low pass current mirror it is in the conduction state.

When keeping module without baseline, baseline voltage drift value is Δ V_out, increase baseline and keep after module, baseline voltage drift Shifting value is Δ V_{Out, closed}, both sides relation is represented by：

Wherein A_openFor main signal path open-loop gain, β is feedback factor, V_osFor operational amplifier OTA input mismatch electricity Pressure, loop gain β A_openIt is represented by：

Wherein τ₀=R_shC_shFor pulse shaper time constant, R_shAnd C_shRespectively feedback resistance electric capacity.According to above-mentioned table It can be drawn the following conclusions up to formula：Loop low-frequency gain is bigger, and baseline voltage drift value is smaller；ω_pSmaller, loop bandwidth is narrower, Work as ω_pMuch smaller than τ₀When, loop can regard first order pole structure as, it is ensured that loop stability, while the noise for feeding back to prime is got over It is small.

When main signal path have useful signal by when, reshaper output voltage values V_outContain the sound to input signal Should, now switch is off.Mp1 (or Mn1) current values are flowed through still equal to flowing through resistance R_TElectric current；Switch off, Electric capacity Cl and C2 is without charge and discharge electric pathway, and voltage maintained switch disconnects previous transient voltage value, therefore flows through Mp2 and Mn2 feedback Current value is all to switch off the current value that previous moment flows through metal-oxide-semiconductor.Because baseline drift value is generally not in transition, instead Supply current can still be used for suppresses the drift of baseline.Baseline keeps the switch controlling signal of module to there may be delay, and part is effectively Signal enters feedback control loop, and because feedback control loop very bandwidth is narrow, useful signal is cut in the loop, further ensures baseline The stability of voltage.

Brief description of the drawings

Fig. 1 is typical radiation detector reading circuit structure block diagram；

Fig. 2 is the radiation detector reading circuit structure block diagram for including pole-zero cancellation structure；

Fig. 3 is that the new baseline proposed in the present invention keeps function structure chart；

The baseline voltage value and temperature change graph of a relation of radiation detector reading circuit when Fig. 4 is no baseline holding module；

Fig. 5 is to whether there is baseline to keep the simulation output comparison diagram of radiation detector reading circuit and switch control during module to believe Number.

Embodiment

The present invention is described in detail below by embodiment.

The quantity of electric charge that radiation detector reading circuit is received is usually very faint, and this requires pulse shaper is realized larger The multiplication factor that the reading circuit being directed in the gain of multiple, design verification is realized is at 40 times or so.Charge amplifier Mismatch is inputted generally in several millivolts of magnitudes, while in view of the mismatch of the mismatch of resistance capacitance in circuit, and pulse shaper, Final baseline voltage drift value is reachable more than 100 millivolts.Flow survey has been carried out to the circuit structure that module is kept without baseline Examination, multiple lane testing results show that baseline drift value is random quantity, and maximum drift value can reach ± 150mV.Randomly select one Individual passage carries out temperature test, and the relation of baseline voltage value (baseline) and environment temperature is as shown in figure 4, reference voltage is 1.6V。

In order to suppress baseline drift, increase the new baseline holding structure shown in Fig. 3 in circuit structure.Increase the module The original performance of sacrifice circuit must be unable to while baseline drift is effectively suppressed, mainly includes the power consumption and noise of circuit Than.In power consumption, it is operational amplifier OTA that baseline, which keeps module main consumable module, OTA only require high-gain should not bandwidth, Low power dissipation design can be realized, OTA current drain is less than 1 μ A in the present embodiment.On noise, electric capacity C1 and C2 is very big Limit loop bandwidth, it is desirable to loop bandwidth must be much smaller than the working frequency of main signal path, make an uproar caused by feedback control loop Sound is negligible compared to main signal path, and the electric capacity for being used for limiting bandwidth in the present embodiment is nanofarad magnitude.

The feedback loop for keeping module composition to baseline has carried out loop stability emulation, and loop low-frequency gain is 40dB, unity gain bandwidth is 240Hz, and 40dB low-frequency gain causes the baseline drift value that mismatch is caused to reduce 99%.It is right Circuit carries out time-domain-simulation and noiseproof feature emulation, and simulation result is illustrated in fig. 5 shown below, and compared for reading circuit whether there is baseline in figure Keep the output response of module.Reference voltage is set to 1.6V, --- it is output response when module is kept without baseline, due to losing Match somebody with somebody, output baseline voltage drift value is about 125mV, --- output response during module, output baseline electricity are kept for increase baseline Pressure recovers reference voltage, effectively inhibits baseline drift, and switch controlling signal pulse width is 7.25 μ S, and about useful signal is wide Degree, that is, ensure that useful signal will not enter feedback control loop, while not influenceing the counting rate of reading circuit again.Fig. 5 simulation results Influence comprising noise, as a result shows that increase baseline keeps almost identical when output noise is with without baseline holding module after module.

As can be seen that baseline holding structure proposed by the present invention can effectively inhibit reading circuit base in from the above The drift of line voltage, while having low-power consumption, the characteristic of low noise, does not produce influence to circuit performance itself.

It is finally noted that, the purpose for publicizing and implementing example is that help further understands the present invention, but this area Technical staff be appreciated that：Without departing from the spirit and scope of the invention and the appended claims, it is various to replace and repair It is all possible for changing.Therefore, the present invention should not be limited to embodiment disclosure of that, and the scope of protection of present invention is to weigh The scope that sharp claim is defined is defined.

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.