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(1)Faculty of Computer Science, Electronics and Telecommunications Department of Electronics. Doctoral Thesis. Chemiluminescence Detection Using Silicon Photomultiplier in Stationary and Microfluidic Systems. Author: M.Sc. Mateusz Baszczyk. Thesis supervisor: Professor Wojciech Kucewicz. Kraków 2017.

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(3) I would like to thank my supervisor professor Wojciech Kucewicz for the encouragement and support in preparation of this thesis. I would like to thank my auxiliary supervisor Ph.D. Witold Reczyński for help and assistance in the chemical analysis. I would like to express my gratitude to M.Sc. Piotr Dorosz, M.Sc. Sebastian Głąb and Ph.D. Łukasz Mik for their support and inspiring discussions..

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(5) 1. Introduction .............................................................................................................. 1 1.1. The Purpose of the Research ............................................................................ 3. 2. Silicon Photomultiplier ............................................................................................ 4. 3. A Phenomenon of the Chemiluminescence ........................................................... 10. 4. Detection Methods of the Signals from the Silicon Photomultipliers .................... 14 4.1. Problems in the Chemiluminescence Measurements...................................... 14. 4.2. A Front-end Circuit ......................................................................................... 15. 4.2.1 SiPM Model ................................................................................................ 17 4.2.2 Fast Shaper .................................................................................................. 19 4.2.2.1 Prototype .............................................................................................. 22 4.2.2.2 Measurements of the Light ................................................................... 27 4.2.3 Comparator .................................................................................................. 29 4.2.4 Charge to Time Converter ........................................................................... 30 4.2.4.1 Prototype .............................................................................................. 32 4.2.4.2 Measurements of the Light ................................................................... 34. 5. 4.3. Gain Compensation Method ........................................................................... 36. 4.4. The Reduction of the Silicon Photomultipliers Thermal Generation ............. 42. 4.5. Conclusions ..................................................................................................... 44. Application Specific Integrated Circuit.................................................................. 46 5.1. Methodology ................................................................................................... 47. 5.1.1 Top Down Design Procedures .................................................................... 47 5.1.2 Aspects of the Mixed-Signal Integrated Circuit Design ............................. 50 5.1.3 Circuit Modelling Oriented to the Analog Design ...................................... 53 5.1.4 Technology Parameters ............................................................................... 56 5.1.5 Transistor Sizing ......................................................................................... 63 5.2. Analog Circuit................................................................................................. 68.

(6) 5.2.1 Circuit Modeling with Verilog-A ............................................................... 68 5.2.2 Transconductance Amplifier ...................................................................... 76 5.2.3 Output Range of the Front-end Circuit ....................................................... 81 5.2.4 Pole Zero Cancellation Circuit ................................................................... 85 5.2.5 Timings ....................................................................................................... 87 5.2.6 Layout and Parasitic Extraction .................................................................. 88 5.2.7 Stability Analysis ........................................................................................ 90 5.3. Digital Circuit ................................................................................................. 91. 5.3.1 Design Synthesis ......................................................................................... 92 5.3.2 Layout Generation ...................................................................................... 94. 6. 5.4. Measurements of the ASIC ............................................................................ 97. 5.5. Conclusion .................................................................................................... 101. Analysis of the Chemiluminescence Measurements ........................................... 102 6.1. Chemiluminescence Measurements in the Stationary System ..................... 102. 6.1.1 Fast Shaper................................................................................................ 105 6.1.1.1 Amplitude Detection .......................................................................... 105 6.1.1.2 Numerical Integration ........................................................................ 108 6.1.2 Comparator ............................................................................................... 110 6.1.2.1 Number of Events .............................................................................. 111 6.1.2.2 Width of the Pulse.............................................................................. 113 6.1.3 Charge to Time Converter ........................................................................ 114 6.2. Comparison of the Stationary Detection Methods ....................................... 117. 6.3. Kinetics of the Chemiluminescence Reaction .............................................. 124. 6.4. Comparison with the Photomultiplier Tube ................................................. 126. 6.5. Chemiluminescence Measurements in the Microfluidic System ................. 130. 6.5.1 Microfluidic System ................................................................................. 130 6.5.2 Bubble Trap Kit ........................................................................................ 137.

(7) 6.5.3 Comparison of the Microfluidic Detection Methods ................................ 142 7. Summary .............................................................................................................. 144. 8. Acknowledgement ................................................................................................ 146. 9. References ............................................................................................................ 147.

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(9) 1 Introduction The chemiluminescence is one of the luminescence phenomena. It is triggered by a chemical reaction and is used in a vast range of applications in medicine, chemistry, biology, biotechnology,. environmental. protection.. The. basic. methods. to. compare. the. chemiluminescence systems and the exemplary applications are presented in chapter 3. The devices used to measure the chemiluminescence are chemiluminometers. These devices are intended primarily for laboratory use. They use Photomultiplier Tubes as light detectors. They are not portable and need several conditions to be fulfilled to perform good quality measurement. The Photomultiplier Tube requires high bias voltage in the range from several hundred volts up to several thousand volts and it is sensitive to the influence of external magnetic field. Due to its structure, it is less mechanically resistant. The aim of the research is to prove that the silicon photomultiplier (SiPM) is capable of measuring the chemiluminescence phenomenon on the level of single photons. The SiPM is described in chapter 2. The use of the SiPMs in the chemiluminescence measurements can reduce the construction costs of research equipment and can allow to design a portable Labon-a-chip systems more resistant to mechanical damage. Comparing with other popular sensing techniques, a fluorescence and the chemiluminescence are two the most common used luminescence phenomena. Fluorescence sensing technique requires to use an optical filters in order to remove the background light generated by the stimulating light source. The chemiluminescence sensing contains negligible background light. Hence, there is no need to use intermediary optical filters and the construction of the system is simpler and cheaper. The thesis consists of three essential sections. The first one describes difficulties in the chemiluminescence detection, proposes the detection methods and presents the front-end electronics (chapter 4). The second one describes the Application Specific Integrated Circuit (ASIC) design (chapter 5). The third one discusses the results of the chemiluminescence in the stationary and microfluidic system (chapter 6). Chapter 4 characterizes the detection methods. Apart from measuring single photons the readout electronics should be sufficiently fast in order to distinguish overlapping signals generated by the chemiluminescence light. A dedicated font-end electronics was constructed to measure the chemiluminescence and to choose the most optimal solution. The front-end 1.

(10) circuit consists of a preamplifier, a fast shaper and charge to time converter (QTC). Five detection methods are compared: the fast shaper allows to measure the amplitude and charge, the QTC allows to measure the width of the pulse, whereas the comparator is used to measure the number of events and the width of the pulse (time over threshold). The chapter presents the characteristic of the prototype, design problems and the results of the light measurements. The variability of the gain of the silicon photomultiplier is one of the problems discussed in the thesis. The gain compensation method brings the opportunity to add the gain compensation in the front-end circuit with a little effort (chapter 4.3). The influence of the thermal generation on the chemiluminescence measurements is presented and the method how to reduce its consequences is explained (chapter 4.4). In the chapter 5 the ASIC is described. The unit explains the methodology and presents the design of the analog and digital part. The methodology section consists of the description of the top-down design procedure, the aspects of the mixed-signal integrated circuit and circuit modeling oriented to the analog design. The procedure of parameters extraction of the AMS 350 nm technology is presented. The transistor sizing method utilizes the information of the inversion level of the transistor. Section 5.2 demonstrates the analog part of the ASIC which is designed with the top-down procedure. The Verilog-A language is used for the analog circuit modeling. The considerations of the transconductance amplifier are discussed. The unit presents the important specifications of the analog part: an output range, a pole zero cancellation circuit, timings, a layout, a stability analysis. Section 5.3 describes the synthesis and the automatic layout generation of the digital part which cooperate with the analog circuit. Chapter 5.4 presents the results of the measurements of the manufactured ASIC. The results of the chemiluminescence detection in the stationary and microfluidic system are presented in the chapter 6. The performance of the five detection methods is verified with measurements of the chemiluminescence of the luminol which is used e.g. in the criminology to detect traces of blood. The relationship between the light intensity and the concentration of the luminol is analyzed (section 6.1). The comparison of the efficiency of chemiluminescence detection methods is carried out (section 6.2). The measurements of the kinetics of the chemical reaction are discussed in section 6.3. The results of the chemiluminescence detection with the Photomultiplier Tube and the comparison with the SiPM are presented in section 6.4. Unit 6.5 describes the microfluidic measurement system.. 2.

(11) The main problems of the chemiluminescence measurements in the microfluidic system are presented and solved.. 1.1 The Purpose of the Research The chemiluminescence detection methods using the Silicon Photomultipliers has not been developed so far. The SiPMs have got several advantages over the PMTs and allow to design a portable, more mechanically resistant measurement systems which can be integrated in the Lab-on-a-chip or Micro Total Analysis Systems. The purpose of the research is to prove that the Silicon Photomultiplier can be used in the detection of the chemiluminescence in the stationary and microfluidic system. The author has developed and compared the different methods for detection of the signals of the chemiluminescence. The data acquisition system has been designed and constructed for the chemiluminescence measurements in the stationary and microfluidic system. The author has confirmed the functionality of the chemiluminescence detection methods using the SiPMs and has characterized the optimal methods adopted to the specific applications and circuit demands.. 3.

(12) 2 Silicon Photomultiplier A Silicon Photomultiplier (SiPM) is a novel type of photodetector with Geiger mode of operation for a low intensity light detection. For several decades Photomultiplier Tubes (PMT) were used for low intensity photon flux measurements practically without an alternative [1]. A silicon structure of the avalanche breakdown mode micro cells with a quenching resistor and common electrodes was the first concept of the SiPM, which appeared in the late nineties [2,3,4,5]. The Silicon Photomultipliers are devices which can register single photons. They have high gain on the level of one million electrons per photon. The SiPM introduces the new alternative in the measurement of light. It can be used for small, sensitive, mobile light detectors. The examples of the SiPMs are presented in Figure 2.1.. 1 mm. Figure 2.1 Silicon photomultipliers in various packages. With respect to the latest state of the art the elements used the most often as visible light detectors in luminescence applications are avalanche photodiodes, photomultiplier tubes [6], light-sensitive CCD or CMOS [7]. The Photomultiplier Tube dominates in a study of very small light intensities. When a photon falls on a photocathode of the PMT then an electron is emitted, which is reproduced by the system of dynodes. In order to have the gain of the PMT large enough the signal from one photon must be amplified by several dynodes. Therefore the bias voltage of the detector must be in the range from several hundreds volts up to several thousands volts. The PMT has lower noises than the SiPM, however, it has a larger size. The PMT is sensitive to the influence of external magnetic field. Due to its structure, it is less. 4.

(13) mechanically resistant. The creation of an avalanche multiplication will be impossible when any dynode shifts itself. The Silicon Photomultiplier is an array of avalanche photodiodes connected in parallel. Pixel (avalanche photodiode) consists of an active area and quenching resistor. Each pixel is separated from another by an isolating ring (Figure 2.2). Active Area. Quenching Resistor. Isolating ring. Figure 2.2 Schematic of the SiPM. When a photon falls on the pixel avalanche, an effect occurs in the whole volume of pixel. The charge of the entire area of the pixel is generated. When two photons fall on two different pixels at the same time then generated charge is twice higher. Such construction makes it possible to distinguish exactly how many photons fall on the array (Figure 2.3) [4] [2]. Figure 2.4 presents the current voltage characteristic of the SiPM produced by the Hamamatsu. When the polarization voltage is smaller than the breakdown voltage, the SiPM works as a photodiode or an avalanche photodiode and the gain is low. When the polarization voltage is higher than the breakdown voltage, the SiPM works in Geiger mode. Each time the photon enters the junction and generates first electron-hole pair every free carrier inside the cell participates in the process of creating the avalanche. The total charge of every avalanche is similar. In the Geiger operating mode it is possible to achieve the amplification on the level of one million electron-hole pairs per photon.. 5.

(14) Figure 2.3 The pulses from the SiPM resulting from one and two photons gathered with the RAPSODI chip [8]. S10362-11-100C. Current [A]. Operating point Breakdown voltage. Bias voltage [V] Figure 2.4 The current voltage characteristic of Hamamatsu MPPC S10362-11-100C. Thermally generated avalanches can appear in the SiPM. The charge can be created, even when no photon felt on the surface of the detector. The thermal generation produces the background in the light detection and it cannot be easily removed from the measurements of the chemiluminescence. The parameter which describes this feature is called Dark Count Rate 6.

(15) (DCR). The frequency of the thermal generation at the room temperature is on the level of hundreds of kHz. The DCR for S10362-11-100C is equal to 755 000 counts per second [9]. The improved Hamamatsu detectors, starting from S12571 series, have a better dark count rate on the level of 75 300 counts per second. They have also got less afterpulses. The afterpulses appear mainly when the electron tunnels through the isolating ring to another pixel and generates there an avalanche. Figure 2.5 shows the signals from the previous and the new generation of the SiPMs. The waveforms present the difference in the number of afterpulses. a). b) Afterpulses. Figure 2.5 Waveforms for the previous series of detectors S10362-11-100C (a) and improved S12571-100C (b) [10]. The Photon Detection Efficiency (PDE) describes the sensitivity of the SiPM. It is calculated by the product of the quantum efficiency, fill factor and the avalanche probability. The avalanche probability depends on the operating voltage. The quantum efficiency depends on the wavelength of the light. The fill factor is determined by the surface of the active and dead (isolating rings, quenching resistors) area. The PDE is presented in the Figure 2.6. The peak sensitivity of the SiPM is at the wavelength between 400 nm and 500 nm. The SiPMs have got dimensions of the order of fractions of square millimeters to several square millimeters (Table 2.1). the Silicon Photomultipliers need a smaller bias voltage on the level of several dozen volts in comparison to the PMT. They are insensitive to the magnetic field, and have more durable design than the PMTs [11] [12]. S12571-100C (Table 2.1) has got the lowest DCR. S10362-11 has got the worst DCR however, it has got the highest PDE. S10984-050P and ArraySL-4-30035 are the arrays of the SiPMs.. 7.

(16) Figure 2.6 Photon Detection Efficiency of S12571-100C [13]. SiPM. Channel. Area. Pixel. Number. Bias. DCR. [mm2]. size. of pixels. [V]. [kcps]. Gain. PDE [%]. [µm] S10362-11-100C. 1. 1. 100. 100. 70.8. 755. 2.4e6. 65. S12571-100C. 1. 1. 100. 100. 65.5. 75.3. 2.8e6. 35. S10984-050P. 4. 4x1. 50. 400. 71.7. 400. 7.5e5. 45. ArraySL-4-30035. 16. 16x9.3. 45. 4774. 31.5. 5000. 2.4e6. 14. Table 2.1 Parameters of the SiPMs used in the research produced by Hamamatsu and SensL. Among the standard silicon detectors, photodiode has got a much smaller gain than the silicon photomultiplier, it generates only one electron-hole pair per photon while the avalanche photodiode generates up to several hundreds of electron-hole pairs per photon [5]. CCD and CMOS sensors have got a sensitivity several orders smaller than the SiPMs. The arrays optimized in terms of sensitivity, for example Electron Multiplying CCD, have got less sensitivity of about two orders [14]. SiPMs can be used to build the multichannel detector. However, the resolution of CCD and CMOS is better than the resolution of the SiPMs currently available on the market. The described characteristics of the detectors show a gap, which justifies the reason why the research problem should be taken. On the one hand, the PMT can measure very low light intensities. On the other hand, the detectors such as CCD or CMOS are not able to measure a 8.

(17) single photons. The SiPM combines the positive features of the both groups. Against the background of the presented sensors, the SiPM could be, in the future, the most optimal for the construction of sensitive, small, electro-mechanically resistant and portable devices.. 9.

(18) 3 A Phenomenon of the Chemiluminescence The luminescence is an emission of the light which can be produced by chemical reactions, electrical energy, absorption of the electromagnetic radiation, mechanical action, ionizing radiation. The sensing of the luminescence is popular technique in many research fields. The chemiluminescence is one of them. The chemiluminescence is a physicochemical process, during which some exothermic chemical reactions appear. As a result of an exothermic reaction energy is released. The particles during their transition from the excited state to the ground state emit an electromagnetic radiation [7]. The chemiluminescence appears when there are molecules which can accumulate the energy of the reaction in a form of the exited states and can achieve high efficiency of the excitation. To analyze the low light intensity of the chemiluminescence sensitive detectors have to be used. To study these phenomena chemiluminometers are used. These devices are intended primarily for laboratory use. They are large in size and use photomultiplier tubes. Therefore, they are not portable and need several condition to be fulfilled to perform good quality measurement. The use of the SiPMs in chemiluminescence measurements can reduce the construction costs of research equipment and can allow to design a portable systems more resistant to mechanical damage. Analysis of the reaction which generate the chemiluminescence is complicated due to its complexity and high dynamics of changes. The basic methods to compare the chemiluminescence systems are: •. Intensity of the light emission – the number of emitted photons is proportional to the analyte and can be used to estimate its concentration. •. Kinetics of the reaction – chemiluminescence decay time. •. Light spectrum. When a chemical reaction occurs in living organisms it is called bioluminescence. There. are. many. chemical. compounds. manifesting. the. phenomenon. of. chemiluminescence and bioluminescence, which are used in many applications in medicine, chemistry, biology, biotechnology, environmental protection. One of the popular ones is a. 10.

(19) luminol. It is an organic chemical compound, in which chemiluminescence is associated with the oxidation of the luminol in the presence of specific activators. The molecular formula of the luminol is C8H7N3O2. Its structure and the chemiluminescence reaction are presented in the Figure 3.1. In the chemical reaction the luminol needs the oxidizer and alkaline environment. As a result the light is generated.. COO-. + O2 + 2OH-. + N2 + 2H2O + light COO-. luminol Figure 3.1 Structure of the luminol and the phenomena of the chemiluminescence reaction of the luminol [15]. The wavelength of the emitted light depends on the oxidation conditions and activators. It usually takes a value in a range of between 400 nm and 500 nm where the SiPM has the highest detection efficiency PDE. The Luminol is used to find traces of blood. The Luminol with an addition of oxidizing reagents located at the place where previously blood was, will generate electromagnetic radiation. Iron in hemoglobin will act as an activator. The research has got and interdisciplinary nature, has got an impact on areas such as electronics, medicine, chemistry, biology, biotechnology, environmental protection [16] [7] [17]. Examples of application of the chemiluminescence are: •. Quantitative determination of DNA immobilized on nylon membranes (possible reaction with the luminol). •. Detection of nitric oxide production in cell cultures by luciferine – luciferase chemiluminescence. •. Determination of very small samples of DNA from the hair, spots of blood, tissue sections (possible reaction with the luminol). •. Finding traces of blood - the iron in hemoglobin acts as an activator (possible reaction with the luminol) 11.

(20) •. The study of naturally occurring substances in plants. •. Determination of enzymes acting as activators in the chemiluminescence reaction. •. The interaction of substances in cells - study receptor dimerization (i.e. insulin receptor), luciferase (found naturally in jellyfish), a mutant of GFP (Green Fluorescent Protein), etc.. •. Monitoring active nuclear proteins in living cells. •. The in-vivo - gene expression testing of animals. •. Infectious diseases - using a variety of bioluminescent markers. •. The immune system - determination of antigens, antibodies or molecules by enzymes. •. Biotechnology - monitoring the fermentation process, nucleic acid testing, testing food samples (estimation of storage time). •. Protection of the environment - testing drinking water, mineral, marine activators are metal ions. •. Portable systems for the study of water bottom - underwater probes (i.e. search of the underwater hydrothermal craters by looking for Manganese ions) [6]. •. human saliva test - detecting the presence of drugs in the body, research cortisol, mRNA, HIV, hepatitis, certain hormones, enzymes, bacteria (i.e. progesterone, testosterone, steroids). Determination of oxidation of the body.. The fluorescence and the chemiluminescence are two the most common used luminescence sensing techniques. The fluorescence light is emitted as a result of the absorption of the electromagnetic radiation. To generate the fluorescence light a stimulating light source (i.e. laser) has to be used. Therefore, the light detected by the sensor includes a background which can be removed by utilizing an optical filter [18] [19]. To increase the sensitivity and dynamic range high performance and narrowband filters should be used. It complicates the optical system. In the chemiluminescence there is negligible background light because the phenomenon is generated as a result of a chemical reaction. The chemiluminescence does not require additional optics between analyte and detector. It improves the efficiency and sensitivity of the detection and decrease the costs of the measurement system [20].. 12.

(21) The chemiluminescence can be also detected in a microfluidic systems [21]. It consists of microchannels, reaction and emission spirals, pumps and light detectors [22] [23]. Integrated microfluidic systems allow to build a Lab-on-chip or Micro Total Analysis System (µTAS), which are multi-purpose analytical systems. The Silicon Photomultiplier can be integrated with the microchannel, improving measurement performance and efficiency. The advantages of microfluidic systems are miniaturization, reducing the amount of reagents, faster response of the system, higher efficiency of the light detection.. 13.

(22) 4 Detection. Methods. of. the. Signals. from. the. Silicon. Photomultipliers 4.1 Problems in the Chemiluminescence Measurements The first measurement of the chemiluminescence (CL) is presented in the Figure 4.1. It was taken using the SiPM from Hamamatsu S10362-11-100C. The measurement system consisted of a simple amplifier and an oscilloscope. The signal was sampled with the rate of 100 kHz. 120. no CL. CL. Amplitude [mV]. 100. 80. 60. 40. 20. 0. 0. 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. Time [ms]. Figure 4.1 The first measurement of the chemiluminescence with Hamamatsu S10362-11-100C. The SiPM generates a pulse when photon falls on its surface. When the bunch of photons fall on the surface of the detector it produces a charge proportional to the number of the photons and generates an electrical pulse. The electronic circuit should be fast enough to distinguish photons generated by the chemiluminescence. The first 5 ms of the measurement (Figure 4.1) show the situation where there was no chemiluminescence light. The signals gathered during that period represent mainly the thermal generation and background light.. 14.

(23) The time interval of 5 ms to 10 ms presents the light from the chemiluminescence. The mean amplitude of pulses is higher (higher charge is generated) than in the previous period. There is a problem with a base line restoration. There are many pile up signals. The pulses from the light are overlapping and it is difficult to measure the correct value of the amplitude, the charge or the time duration of the pulse. The signals form the light and from the thermal generation cannot be easy distinguished. The thermal generation introduces the background to the measurement. The improved Hamamatsu detectors, starting from S12571 series, have got a lower dark count rate and less afterpulses than the S10362 series (Figure 2.5). In the measurement of the chemiluminescence the newest series of the SiPMs were used. The chemiluminescence phenomenon generates the light randomly. During the measurement there is no trigger which could be used to distinguish the light and the thermal generation like in fluorescence measurements [8]. Taking into account the features of SiPMs and chemiluminescence phenomenon the measurement system should be fast. The sampling rate of 100 kHz is too small. The front-end electronics should shape signal in time to decrease the number of pile up signals. The algorithm should be optimal and simple enough to allow to build multichannel systems in the future.. 4.2 A Front-end Circuit The front-end electronics allow to check several algorithms to find the most appropriate ones for the chemiluminescence detection. The front-end includes a circuit which shapes the signal in time to separate the overlapping pulses. The system allows to measure the amplitude, the charge and the width of pulse from SiPM. Figure 4.2 presents the front-end circuit dedicated to the chemiluminescence detection. It consists of a preamplifier, a fast shaper with a comparator and the charge to time converter (QTC) [24]. It is designed as a fully differential circuit.. 15.

(24) SiPM. PreAmp. Shaper. Comp. QTC Figure 4.2 Block diagram of the front-end circuit. The shaper produces a fast signal. It generates accurate information of the input photons. The system permits to measure the amplitude and the charge of the pulse. The measurement chain for the shaper requires using a very fast ADC to sample the whole signal from the shaper or the fast ADC with some additional circuit, i.e. peak and hold detector to measure and sample only the amplitude. The comparator after the shaper can be used to measure the width of the pulse (which includes information about the input photons) or just to count the events. In the second option the trigger signal is created when one or more photons fall at the same time on the detector and the system counts only such events. The measurement chain does not require any ADC. It needs the counter (i.e. implemented in FPGA or microcontroller) or time to digital converter (TDC) implemented i.e. in FPGA. The QTC produces a pulse whose width includes the information of the number of input photons. The QTC allows to measure signals in wide dynamic range. The measurement chain for the QTC requires to use the TDC. There is no one optimal solution for the problems in the chemiluminescence measurements. The presented detection methods have got their advantages and disadvantages which are presented in Table 4.1. The further research will present the characteristics of each method.. Circuit. Attribute. Readout - how to. Advantages. Disadvantages. measure Shaper. Amplitude. Very high speed ADC or Fast and linear high speed ADC with peak and hold detector 16. response. Complicated readout.

(25) Charge. Very high speed ADC or Fast and linear high speed ADC with. Complicated readout. response. integrator Comparator Width of after shaper. TDC. Simpler readout, Non-linear response. the pulse Number of. fast response Counter. The simplest. events. Non-linear response. readout, fast response. QTC. Width of. TDC. Wide range,. the pulse. Non-linear response. simpler readout. Table 4.1 Characteristic of the front-end circuit and detection methods. 4.2.1 SiPM Model The electrical characterization of the SiPMs is a topic widely described in publications [25] [26] [27]. The Proper model of the SiPM is an important component to analyze the presented algorithms. In the first phase the circuit was developed in the simulations in the PSpice. In the next phase the prototype was built and analyzed. Both phases demand developing suitable SiPM model which should be simple and should allow to easily generate the charge corresponding to different number of photons. The presented model is based on the measured signals form the SiPMs. In the simulations with the PSpice environment the SiPM equivalent circuit is built as an exponential current source (Figure 4.3). The current source I1 generates the charge corresponding to a single photon. Then the current control current source I2 is used to generate the charge corresponding to any number of photons. Figure 4.4 presents the voltage generated by the model on the 50 Ω resistor connected to the SIPM_out. The amplitude corresponding to the single photon is equal to 1 mV. The pulse has got a short rising time and a long, exponential decay. The rising edge of the pulse was designed to be several ns and the falling edge to be about 50 ns [11] (baseline restores after about 100ns). The rising edge for the PSpice model is equal to 1.57 ns and the falling edge 44 ns.. 17.

(26) HV. R2. I1=iSIPM. R1. I2=PhotonsNum· iSiPM SiPM_out. Figure 4.3 SiPM model in the PSpice environment. The current source I1is an exponential current source. I2 is a current controlled current source. 1.2. Voltage [mV]. 1 0.8 0.6 0.4 0.2 0 -0.2. 0. 10. 20. 30. 40. 50. 60. 70. 80. 90. 100. time [ns] Figure 4.4 Voltage signal of the PSpice SiPM model gathered when 50 Ω resistor is connected to the output of the model to ground. In the measurement circuit the model of the SiPM was implemented as an RC circuit. Figure 4.5 presents the model. R1 and C1 are a discrete elements on the PCB and R2, C2 represent the input impedance of the amplifier. The values of the R1 and C1 were founded empirically to have similar timings like in the simulation in the PSpice.. 18.

(27) PCB. R1 Falling edge. C1 C2. R2. Amplifier. PCB. Figure 4.5 SiPM model in the measurement circuit. The pulse is generated on the falling edge of the square wave using the generator. Figure 4.6 shows the wave from oscilloscope on the output of the preamplifier. The rising edge is equal to 4.4 ns and falling edge 49.6 ns. The amplitude is higher than in the PSpice model (Figure 4.4) because the amplifier gains the signal.. Figure 4.6 Waveform of the RC model of SiPM with rise and fall time extraction. 4.2.2 Fast Shaper A fast shaper is one of the components of the front-end circuit (Figure 4.2). Its main function is to shape the SiPM signal in time and to produce short signal. The fast shaper is based on the pole-zero cancellation circuit and is realized as a single stage pass-band active filter built on fully differential amplifier (Figure 4.7). It produces the most optimal signal in terms of the pulse width, without overshoots and undershoots. The fast shaper generates unipolar pulse. 19.

(28) C1. C3. R1. R3. High Voltage C0. C2 R2. In+ OutSiPM. In+ OutVOCM In- Out+. VOCM In- Out+. Figure 4.7 Schematic diagram of the fast shaper with the preamplifier and the SiPM. Figure 4.8 presents the simulations outcome of the fast shaper’s output. The FWHM is equal to 3.5 ns and peaking time is equal to 2.5 ns. The amplitude of the signal resulting from the single photon is equal to 35 mV. The simulation was taken using the ADA4927-1 fully differential amplifier. The parameter of the amplifier are: • 3 dB bandwidth is equal to 2.3 GHz • 0.1 dB gain flatness: 150 MHz • Slew rate: 5000 V/µs, 25% to 75% • Fast 0.1% settling time: 10 ns 40. Voltage [mV]. 30. 20. 10. 0. 0. 5. 10. 15. 20. 25. 30. 35. time [ns] Figure 4.8 Output signal from the shaper simulated with the PSpice.. 20. 40. 45. 50.

(29) Figure 4.9 explains why the fast shaper is important in the chemiluminescence detection to properly measure overlapping pulses. Figure 4.9 shows an exemplary signal generated with the SiPM model. The first three pulses represent single photons. They are generated with 10 ns delay and they are overlapping. The maximum value of the pulses is increasing although they represent the same number of photons. The fast shaper properly distinguish the signals because the maximum value of each pulse is the same. The response of the fast shaper is short enough (lower than 10 ns) to distinguished pile up signals.. a). 500. 20. SiPM Current [ µA]. Number of photons. b). 400. 1 300. 10. 200. 1. 100. 10. 10 1. 1. 111. 0. 0 700. 50. 100. 150. 200. 250. 300. 50. 100. 150. 200. 250. 300. Shaper output [mV]. 600 500 400 300 200 100 0 0. time [ns] Figure 4.9 The simulated SiPM signal with overlapping signals with various number of photons (a) and the fast shaper’s response (b). Then a signal of 10 photons is generated with one photon signal shortly after. It is hard to find a smaller signal after a higher one in the waveform from the SiPM. The shaper correctly resolves and finds the correct pulses: one pulse form 10 photons, another one from a single photon. The situation is similar for the next pulses (different amplitudes and delays). 21.

(30) Three signals resulting form 10 photons have the same amplitude. The amplitude of the shaper’s output is proportional to the number of photons. The signal from 10 photons is 10 times higher than the signal from a single photon. The signal from 20 photons is 20 times higher than the signal from a single photon. The simulations show that it is possible to build the fast shaper with an output signal faster than 10 ns. It is also possible to resolve the pile up signals with presented amplitudes and delays. 4.2.2.1 Prototype The prototype of the fast shaper was built. During the research four different version of the PCBs were constructed. Figure 4.10 presents the shaper’s output of the first version. The system was oscillating.. Figure 4.10 The shaper’s output waveform with oscillation of the first version of the measurement system. 22.

(31) b). a). SiPM. preAmp shaper. QTC. comparator. DAC. Figure 4.11 The PCB layout of the channel with AD8137 (a) and with ADA4930 (b). A lot of effort was inserted into the proper project of the PCB (Figure 4.11). The main problems which were resolved during the research and the main changes into the PCB are [28,29,30,31,32]: •. the PCB with four layers – the top and the bottom layer are for the signals, two internal layers are for the power supply distribution and the ground. •. The amplifiers – should have high quiescent current and slew rate because of capacitive load, the gain bandwidth should not be so high (as low as possible to have proper shaping) 23.

(32) •. Footprints – the surface mounting, for the capacitors and the resistors the 0604 footprint was used. •. Decoupling – the capacitance ladder with the low inductance capacitor 100 nF 0508 footprint, 100 nF 0402 footprint. •. Wires – as much symmetrical as possible, short. •. Placement of elements – as much symmetrical as possible. •. Ground – internal layer, one plane. •. Power – internal layer, the plane divided on the analog and the digital supply with the independent stabilizers. •. Additional separating filters between inverting and non-inverting outputs of the amplifiers. In the Figure 4.11 there are presented two layouts of the final version of the PCB. One is for the slower channel with AD8137 and the other one is for the faster ADA4930. The comparator is ADCMP604 with a low glitch LVDS compatible output stage and 1.6 ns propagation delay. The amplifiers are in different packages (Figure 4.12). The outputs and inputs are placed in a way to have short connection in the feedback network. The LFCSP package has got an additional feedback output which is useful to connect the feedback resistor or the capacitor between the output and the input.. a). b). Figure 4.12 The 8-Lead Standard Small Outline Package (SOIC) of the AD8137 (a) and the 16Lead Frame Chip Scale Package (LFCSP) of the ADA4930 (b).. 24.

(33) The parameters of fully differential amplifiers are presented in Table 4.2 [33] [34]. The ADA4930 has got a ten times better slew rate, quiescent current and larger signal bandwidth. The settling time is about twenty times lower. AD8137. ADA4930. Small Signal Bandwidth. 75. 1350. MHz. Large Signal Bandwidth. 107. 937. MHz. Quiescent Current. 2.6. 34. mA. Slew Rate. 375. 3400. V/µs. Settling Time to 0.1%. 110. 6. ns. Open-Loop Gain. 89. 64. dB. Input. Differential. 800. 150. kΩ. Resistance. Common-mode. 400. 3000. kΩ. 1.8. 1. pF. Input Capacitance. Table 4.2 Parameters of the amplifiers AD8137 and ADA4930. Table 4.3 presents the values of capacitances and resistances in the front-end circuit. The values were achieved during the optimizations of the instability problem and to reduce the duration time of the pulse for each amplifier. AD8137. ADA4930. C0. 68. 68. pF. R1. 1. 2. kΩ. C1. 2. 2.2. pF. R2. 200. 300. Ω. C2. 47. 47. pF. R3. 430. 681. Ω. C3. 4.7. 6.9. pF. Table 4.3 The values of the resistors and capacitors in the front-end circuit (Figure 4.7). The measurements of the prototype were performed with Hamamatsu S12571-100C MPPC. The Figure 4.13 shows the waveform from the oscilloscope of the fast shaper’s output using the amplifier ADA4930 and the signals were gathered using the thermal generation. The Figure 4.14 show the waveform from the oscilloscope of the fast shaper’s output using the amplifier AD8137 and the signals come from the light measurements. The persistence mode 25.

(34) in the oscilloscope was turned on. In each waveform there are specific levels of the photons entering the SiPM at the moment of measurement. There are distinguishable waves from one photon, two photons, three photons in Figure 4.13 and Figure 4.14. The peaking time for the single photon in the Figure 4.13 is equal to 3.6 ns and the FWHM is 3.8 ns. For the slower amplifier AD8137 in Figure 4.14 the peaking time for the single photon is equal to 7.5 ns and the FWHM is 9.8 ns.. Figure 4.13 The waveform of the shaper’s output. The circuit consists of the amplifiers ADA4930 and SiPM MPPC S12571-100C (the oscilloscope with persistence). Figure 4.14 The waveform of the shaper’s output. The circuit consists of the amplifiers AD8137 and SiPM MPPC S12571-100C (the oscilloscope with persistence). 26.

(35) The waveforms form Figure 4.13 and Figure 4.14 were gathered using the differential output of the amplifier. Figure 4.15 presents the outputs of the amplifier when they are measured single-ended. Channel 1 presents out- and channel 2 out+ (Figure 4.7). When the signals are measured in the single-ended way the distortions appear. The amplitudes of the pulses are changed by distortions. Channel A represents the fully-differential method when the difference between out+ and out- is measured. There are no distortions and the pulses have got a proper unipolar shape.. Figure 4.15 The differential measurement of the signal from the shaper. Channel 1 and 2 represent the out- and out+ of the amplifier. Channel A is the difference between channel 2 and 1. 4.2.2.2 Measurements of the Light The measurements of the light was performed to study the relationship between the light intensity and the shaper’s response. The electrical signal from the fast shaper is investigated. The system with the pulse laser and the nanosecond generator was built to measure the light intensity (Figure 4.16). The laser is connected by the optical fiber directly to the SiPM. The data were gathered with the trigger from the generator. Therefore, the thermal generation has got a minimal impact on the results.. 27.

(36) Figure 4.16 The system for the light intensity measurements. To extract the information of the input photons the amplitude of the shaper’s output signal was extracted. The histograms of the amplitudes in the Figure 4.17 show the results of the light measurements with the Hamamatsu MPPC S12571-100C. Figure 4.14 presents the accumulated electrical signals in time for the ‘intensity 1’. The peaks in each histogram represent a specific photon number starting from one photon. One photon was falling the most often on the surface of the detector for the ‘intensity 1’. For the ‘intensity 2’ five photons were appearing the most often. For the ‘intensity 3’ it was fifteen photons. The system works properly, the photon’s peaks are distinguishable. The quantity of the detected photons changes with the light intensity.. Number of Counts. 1000. intensity 1 intensity 2 intensity 3. 800. 600. 400. 200. 0. 0. 0.5. 1. 1.5. 2. Amplitude [V] Figure 4.17 Histogram of the fast shaper’s amplitudes for three light intensities. 28.

(37) The shaper can generate fast pulses with the FWHM up to 3.8 ns. It can separate the pile up signals with the high resolution. The measurement system properly distinguishes photon levels. The system is optimal enough to measure the chemiluminescence.. 4.2.3 Comparator The comparator after the shaper can be used to measure the width of a pulse or to count the events number. Figure 4.18 shows the comparator output signal. The comparator generates the signal whenever the bunch of the photons fall on the surface of the SiPM. It can be easily used to count the number of events. The width of pulses is correlated with the number of photons. The relationship is non-linear. The efficiency of the measurement of the chemiluminescence using the comparator is analyzed in the chapter 6.1.2.. a). 500. 20. SiPM Current [ µA]. Number of photons. 1 300. 10. 200 100 0 40. Comparator afte shaper [V]. b). 400. 1. 10. 10 1. 1. 111 50. 100. 50. 100. 150. 200. 250. 300. 150. 200. 250. 300. 3. 2. 1. 0. 0. time [ns] Figure 4.18 The simulated overlapping SiPM signal with various numbers of photons (a) and the comparator’s response (b). 29.

(38) 4.2.4 Charge to Time Converter The QTC contains a fully differential integrating amplifier and a comparator (Figure 4.19) [35] [36] [37]. The pulse width of the QTC depends on the number of photons entering the SiPM at the moment of measurement. The discharge resistor placed in the amplifier’s feedback was tuned so as to obtain short pulses. The diode allows to work with a wide range of input signals. It prevents the amplifier from saturation and causes the nonlinear relationship between the input and output signal.. D5 C5 R5 R4 In+ Out-. In+. Q. VOCM Out+ In-. In-. nQ. Figure 4.19 Schematic diagram of the QTC. Figure 4.20 presents the simulated signal in the PSpice with the SiPM model and the response of the QTC. For the single photon the QTC generates the short pulse. The signals for 10 and 20 photons have got longer duration time. The signals for one photon generated on the falling edge of 10 photons (with delay 10 ns and 20 ns) are not detected by the QTC as a separated signals. However they have got slightly different width which could be used to extract the proper photon number. The relationship between the pulse width and the number of photons is not linear. The advantage of the QTC is that it allows to work with signals in the wide dynamic range.. 30.

(39) a). 500. 20. SiPM Current [ µA]. Number of photons 400. 1 300. 10. 200. 1. 100. 0 4. QTC output [V]. 1. 111. 0. b). 10. 10 1. 50. 100. 50. 100. 150. 200. 250. 300. 150. 200. 250. 300. 3. 2. 1. 0. 0. time [ns] Figure 4.20 The simulated overlapping SiPM signal with various number of photons (a) and the QTC’s response (b). The diode type in the feedback of the amplifier (Figure 4.19) has got the influence on the output characteristic [38]. Figure 4.21 presents the output characteristic of the QTC simulated for various diodes. In the wide range the response of the QTC is nonlinear nevertheless, the curves can be linearized in the interval from 8 to 60 photons. Table 4.4 presents the values of the following parameters: the parameters of diodes, r-squared and time per photon. The values were extracted from the simulations. The fastest switching diode (plot number 1) has the worst linearity and resolution (time per photon). The slower switching diode (plot number 2) has got a better resolution. The highest linearity and resolution is achieved with varicap diodes. Plots number 3,4,5 show varicap diodes with different capacitance. The linearity does not change much in those three cases. The higher capacitance results in larger pulse width for the same number of photons.. 31.

(40) 120. Pulse Width [ns]. 100 80 60 40 20 0. 0. 10. 20. 30. 1. switching, trr=4 ns 2. switching, trr=50 ns 3. varicap, 5 pF 4. varicap, 9 pF 5. varicap, 12 pF 40 50 60. Number of Photons Figure 4.21 Relationship between the output pulse width and the number of photons for different types of diodes (simulations). No.. Diode. Diode. Series. R-squared. Time per photon. Type. capacitance. resistance. [pF]. [Ω]. In range from 8 to 60 photons. [ns/photon]. 1. switching. 4. 3,5. 0,810. 0,17. 2. switching. 5. 7. 0,966. 0,76. 3. varicap. 5. 1,2. 0,995. 1,06. 4. varicap. 9. 1,8. 0,996. 0,98. 5. varicap. 12. 0,5. 0,998. 0,90. Table 4.4 Parameters of diodes and r-squared value with time per photon in range from 8 to 60 photons extracted from the simulation. 4.2.4.1 Prototype The measurement system for the QTC was built. The PCB design is presented in the Figure 4.11. The amplifiers are described in Table 4.2 and Figure 4.12. The main problem in the QTC design caused the oscillations which were occurring randomly. The exemplary waveform is presented in the Figure 4.22. The comparator was switching to the metastability state. Its output stayed at the level between low and high signal. 32.

(41) Figure 4.22 The exemplary waveform when the QTC starts to oscillate. Channel 4 presents the signal form the QTS’s output. Channel A is the preamplifier’s output. The solution was to add the RC filter on the differential output of the fully differential amplifier (Figure 4.23). The filter was placed on between the amplifier and the comparator (Figure 4.19). Feedback. In+. Out50 Ω 10 pF. In-. Out+ 50 Ω. Feedback Figure 4.23 The additional filter on the output of the fully differential Amplifier in the QTC. In the Figure 4.24 there is presented the QTS’s output signal for the thermal generation. The data was gathered with Hamamatsu S12571-100C SiPM. The waveform presents the signal from the comparator with the distinguishable falling edges. Each pulse’s width corresponds to the different number of photons.. 33.

(42) 3 ph 2 ph 1 ph. Distinguishable falling edges. Figure 4.24 The QTS’s output gathered with the oscilloscope in the persistence mode. 4.2.4.2 Measurements of the Light The histograms in the Figure 4.25 show the results of the light measurements with the Hamamatsu MPPC S12571-100C. The measurement system is similar to the one used for the shaper in the Figure 4.16. The plots present the histograms of the pulse width of the QTC. Each peak represents different number of photons falling on the surface of the detector at the moment of measurement. There are separated peaks up to over a dozen photons. The maximal value of the number of the counts in histogram is increasing when the light intensity increases. The light intensities correspond to those measured with the fast shaper presented in the Figure 4.17.. 250. intensity 1 intensity 2 intensity 3. Number of Counts. Number of photons 200. 1. 2. 3 4 5 6 7 8 9…. 150. 100. 50. 0. 0. 20. 40. 60. 80. 100. Pulse Width [ns] Figure 4.25 Histogram of the QTC pulse width for three different intensities.. 34. 120.

(43) The characteristic of the QTC in the wide range was measured. Figure 4.26 presents the measurement system. The computer gathers data from the oscilloscope and controls the generator. The SiPM model presented in the Figure 4.5 was used. Figure 4.27 presents the relationship between the output pulse width and the number of photons. Plot 1 was measured with the switching diode LL4148, plot number 2 with the varicap diode BB833 and number 3 with the varicap SMV1236 diode. Similarly like in the simulations slower diodes have got a better resolution. In the range up to several hundreds of photons good approximation of the QTC’s response is the logarithmic curve. The r-squared value for the switching diodes is equal to 0.986, for the varicap 9 pF 0.984, for the varicap 12 pF 0.956.. Figure 4.26 The system for the measurement of characteristic of the QTC. 400 350. 1. switching, trr=4 ns 2. varicap, 12 pF 3. varicap, 9 pF. Pulse width [ns]. 300 250 200 150 100 50 0 0 10. 10. 1. 10. 2. Number of photoelectrons Figure 4.27 Relationship between the QTC’s output width of the pulse and the number of photons (measurements). 35.

(44) 4.3 Gain Compensation Method Measurements using the Silicon Photomultiplier as a photon detector have required the stable gain specially for a sensitivity on the level of a single photon [39]. The gain of the SiPM changes with the temperature [40] [41] [42]. Moreover, each manufactured detector has got its own gain. In multichannel systems it is necessary to moderate the amplification in each channel separately to have similar gain among channels [43]. The gain compensation method exploits the fully differential readout circuit. The gain is adjusted by moderating the bias voltage of the SiPM. The polarization voltage is adjusted indirectly by tuning the output common mode voltage (VOCM) of the fully differential amplifier by the DAC (Figure 4.28). Changes of VOCM introduce the same change to the amplifier’s input (+in, -in) DC voltage. Thereby the bias voltage of the SiPM connected to the amplifier is also modified [44].. High Voltage. SiPM. In+ Out-. In+ Out-. VOCM Out+ In-. VOCM In-. Out+. DAC Figure 4.28 Block diagram of the gain compensation method. The advantage of the algorithm is the possibility to set the bias of each SiPM in the array independently so they all could operate in very similar conditions (have similar gain and dark count rate). The measurement system requires only one high voltage power supply when the multichannel system is built. Figure 4.29 presents the interconnection for the system with more than one channel. The SiPMs are connected to the same high voltage (HV) and the gain is compensated in each channel individually by the DACs.. 36.

(45) High Voltage Power Supply HV In+. In+. OutVOCM Out+ In-. OutVOCM Out+ In-. Channel 1. DAC 1. HV In+. In+. OutVOCM Out+ In-. OutVOCM Out+ In-. Channel n. DAC n Figure 4.29 The schematic of the multichannel system with gain compensation. The compensation is also possible for the detectors with the common cathode output, i.e. S10984-050P is a four channel SiPM with the common cathode (Figure 4.30). When the high voltage connected to the cathode is changing the bias voltage for all channels change in the same way. It is impossible to adjust the bias of each channel separately. The presented compensation method allows to adjust the gain for the common cathode detectors.. Cathode. Ch1. Ch2. Ch3. Ch4. Figure 4.30 The schematic of the four channel S10984-050P with the common cathode output. The gain of the SiPM is calculated by measuring the mean amplitude of the signal resulting from the detection of the single photon. The measurements were performed using thermal generation. The histograms of the amplitudes of the single photon for different values of the VOCM are presented in the Figure 4.31.. 37.

(46) 350. ∆VOCM = 510mV ∆VOCM = 110mV ∆VOCM = -290mV. 300. Number of Counts. 250. 200. 150. 100. 50. 0 0.02. 0.04. 0.06. 0.08. 0.1. 0.12. 0.14. 0.16. Amplitude of the Signal from the Single Photon [V]. Figure 4.31 Histograms of the amplitudes of the single photon for different VOCM using Hamamatsu MPPC S12571-100C. The relationship between the VOCM and the gain per one photon is linear. The measurement was carried out with Hamamatsu MPPC S12571-100C (Figure 4.32) and SensL ArraySL-4-30035 [45] (Figure 4.33). The error bars represent standard deviation of the amplitude of the one photon. The R-square parameter for both detectors is higher than 0.99. The S12571-100C is more sensitive to the bias variations. For ∆VOCM = 1 V the gain changes in the range of 60 mV for Hamamatsu and 13 mV for SensL.. 38.

(47) 110. Gph=-0,06*VOCM+76. R2=0.9997. Gain per Photon [mV]. 100 90 80 70 60 50 40 -600. -400. -200. 0. 200. 400. 600. ∆VOCM [mV]. Figure 4.32 Relationship between the gain per one photon and the VOCM for Hamamatsu MPPC S12571-100C 54. Gph=-0.0128*VOCM+43.1. 52. R2=0.9923. Gain per Photon [mV]. 50 48 46 44 42 40 38 36 34 -600. -400. -200. 0. 200. 400. 600. ∆VOCM [mV] Figure 4.33 Relationship between the gain per one photon and the VOCM for SensL ArraySL-430035. 39.

(48) The algorithm of the compensation consists of the steps: 1) Define the relation between the VOCM and the gain per photon (Figure 4.32, Figure 4.33) 2) Calculate the value of the VOCM for the given gain using the linear equation from the previous point 3) Set the VOCM with DAC To verify the algorithm of compensation the measurements with Hamamatsu and SensL SiPMs were carried out. In each measurement the same bias voltage was set on the high voltage power supply for all the channels. Table 4.5 and Figure 4.34 present the results of the four instances of MPPC S12571-100C. The value of the gain of the SiPMs before the compensation when VOCM is equal to 0V is presented. The gain differs among different instances of the SiPMs. The compensation were performed to achieve the gain 80 mV per photon. For all models and all measurements the same bias voltage was set to 65.50 V on the power supply. MPPC parameters Serial. Vop. Magnitude. no.. [V]. 805. 65.55. 2.81E+06. 807. 65.55. 2.80E+06. Bias [V]. Before compensation Gain/ph. σ [mV]. [mV]. After compensation Gain/ph. σ. ∆Vocm. [mV]. [mV]. [mV]. 72.9. 3.89. 79.5. 3.46. -164. 77.8. 3.68. 80.1. 3.65. -13.4. 65.5 808. 65.54. 2.81E+06. 84.4. 4.00. 80.0. 4.08. 40. 809. 65.5. 2.80E+06. 81.1. 3.69. 79.8. 3.91. 6. Table 4.5 Parameters of S12571-100C and gain compensation results.. 40.

(49) 90. 85. 85. Gain per Photon [mV]. Gain per Photon [mV]. 90. 80. 75. 75. 70. 70. 65. 80. 805. 807. 808. 65. 809. 805. 807. 808. 809. Serial Number. Serial Number. Figure 4.34 MPPC S12571-100C before (left) and after (right) compensation. Table 4.6 and Figure 4.35 present the compensation results for four channel S10984050P. The gain was compensated to achieve the 60 mV per photon. MPPC parameters Channel. Vop. Bias. Magnitude. [V]. [V]. Before compensation σ [mV]. Gain/ph [mV]. 1. 71.72. 7.48E+05. 2. 71.72. 7.47E+05. After compensation Gain/ph. σ. ∆Vocm. [mV]. [mV]. [mV]. 61.9. 4.79. 60.0. 4.69. 20.7. 62.6. 4.39. 59.6. 4.85. 55. 71.7 3. 71.76. 7.52E+05. 61.8. 4.69. 59.9. 4.29. 36.6. 4. 71.83. 7.52E+05. 57.8. 4.08. 60.0. 4.21. -36.6. 70. 70. 68. 68. 66. 66. Gain per Photon [mV]. Gain per Photon [mV]. Table 4.6 Parameters of S10984-050P and gain compensation results. 64 62 60 58 56. 64 62 60 58 56. 54. 54. 52. 52. 50. 1. 2. 3. 4. 50. 1. Channel. 2. 3. Channel. Figure 4.35 MPPC S10984-050P before (left) and after (right) compensation. 41. 4.

(50) Table 4.7 and Figure 4.36 present the compensation results of the first four channels A, B, C, D of 16 channel SensL ArraySL-4-30035. The gain was compensated to achieve the 45 mV per photon. MPPC parameters Channel. Vop. Bias. Magnitude. Before compensation. [V]. Gain/ph. [V]. σ [mV]. [mV]. A. After compensation Gain/ph. σ. ∆Vocm. [mV]. [mV]. [mV]. 47.1. 2.3. 45.0. 2.5. 210. 46.2. 2.0. 45.0. 2.0. 197. C. 45.9. 2.1. 44.9. 2.1. 175. D. 45.3. 2.0. 44.83. 2.1. 80. B 31.5. 2.4E+06. 31.5. 50. 50. 49. 49. 48. 48. Gain per Photon [mV]. Gain per Photon [mV]. Table 4.7 Parameters of SensL ArraySL-4-30035 and gain compensation results. 47 46 45 44 43 42. 46 45 44 43 42 41. 41 40. 47. A. B. C. D. 40. A. B. C. D. Channel. Channel. Figure 4.36 SensL ArraySL-4-30035 before (left) and after (right) compensation. 4.4 The Reduction of the Silicon Photomultipliers Thermal Generation The thermal generation has got an impact on the results of the light measurements. The higher is the number of the thermally generated pulses the higher is the background and the signal of the buffer. The intensity of the thermal generation depends on temperature. When the temperature is lower there is less thermal pulses. However, this solution complicates the measurement system. The other option is to set the threshold of the detection above several photons to avoid thermal generation. This solution is also inappropriate, the thesis assumes to detect light on the level of single photons. The measurements with the trigger (i.e. like in the. 42.

(51) fluorescence) are not possible in the chemiluminescence. The solution which is applicable in the chemiluminescence detection is the measurement in coincidence. The process of thermally generated avalanches is stochastic and can be eliminated by simultaneous measurement of signals in two or more SiPMs. If signals are generated by different SiPMs in the same time (with low delay) it is high probability that they are generated by the light. In other case when only one detector out of several generates signal it is high probability that it comes from the thermal generation [46]. The idea of the coincidence block is presented in the Figure 4.37.. Channel 1 Channel 2. Reset Channel 1 Channel 2 Channel 1 Channel 2. OR. AND. Start. Out. Time window Coincidence discriminator. 0. 100. 200 t [ns] Coincidence. Stop. Counter. 0. 100. 200 t [ns]. Figure 4.37 Schematic of coincidence idea and exemplary coincidence signal for 100 ns time window. When one channel generates pulse then the counter starts. If during the fixed time window the other channel generates the signal then coincidence occurs. If the signal is not generated the coincidence system resets the counter, ignores coming data and waits for the next signal [47]. The influence of the coincidence system on the thermal generation was measured using two instances of S10362-11-100C. The SiPMs were in the black box and the thermal generation was measured. The Figure 4.38 presents the dependencies between the DCR and the time of the coincidence window. The longer is the coincidence window, the higher value of the dark count rate appears. When the coincidence signal lasts 12ns then measured DCR has got the value of 8 kcps. The influence of the thermal generation on the measurement is reduced by two orders of magnitude.. 43.

(52) Dark count rate [cps]. 10. 10. 10. 10. 6. 5. 4. 3. 0. 20. 40. 60. 80. 100. 120. 140. Time of coincidence window [ns] Figure 4.38 DCR in relation with time of coincidence window. 4.5 Conclusions The readout system was designed to measure the light intensity of the chemiluminescence phenomenon. The front-end electronic allows to investigate five methods (Table 4.1) and to find the most optimal solution for the chemiluminescence detection. The system consists of the fast shaper with the comparator and the charge to time converter. The shaper generates fast pulses which the amplitude and the integral can be measured. The comparator allows to measure the width of the shaper’s pulse (time over threshold) and to count the events. The QTC produces pulses whose width provides the information of the number of photons. Figure 4.39 presents the outputs of the shaper, the QTC and the comparator. The measurement for all the devices was taken at the same time. Therefore, the same light intensity was measured. The Hamamatsu MPPC S12571-100C was used. Each waveform shows distinguishable levels of the single photons.. 44.

(53) Comparator. 1 photon. QTC 1 photon. Shaper. 1 photon. Figure 4.39 Comparison of the outputs of fast shaper (Channel A), QTS (Channel 4) and comparator (Channel 3). The waveforms were measured simultaneously. The gain compensation method for the SiPM brings the opportunity to add the gain compensation in the front-end circuit with a little effort, by adding only the DAC to the output common mode voltage. The relation between the gain per photon and the output common mode voltage is linear. The proposed readout methods are effective in measurements of the random light signals where frequent events tend to pile-up. The method enables to distinguish those overlapping signals and get the reliable information of the number of detected photons. In the next chapters detection methods are investigated in the chemiluminescence measurements. The methods are analyzed in terms of accuracy of the light intensity measurements.. 45.

(54) 5 Application Specific Integrated Circuit The Application Specific Integrated Circuit (ASIC) was designed. The circuit is made in AMS 350 nm technology. It consists of the analog and the digital part. The circuit is presented in the Figure 5.1. Its dimensions are 2100 µm to 1880 µm.. Figure 5.1 The ASIC designed in the AMS 350 nm technology. The structure of ASIC is presented in the Figure 5.2. The analog part includes the preamplifier, the shaper and the QTC amplifier. The digital part consists of the serial in parallel out shift register and three DACs. The digital part controls parameters of the analog circuit such as gain, output range, timings and shape of the output signals. The ASIC has got an analog input from the SiPM and the digital inputs for the shift register. The outputs are the analog signals from the preamplifier, the shaper and the QTC. Chapter 5.1 presents the design procedure. It describes the top down technique. The aspects of the mixed-signal integrated circuit shows the interaction between the analog and the digital part. The analog design methodology and general analog design approach is. 46.

(55) presented in chapters 5.1.3, 5.1.4, 5.1.5. The section 5.2 presents the analog circuit and results of simulations. Chapter 5.3 illustrate the synthesis of the digital part. ASIC Analog PreAmp. Shaper. SiPM input. Analog Outputs QTC. Digital Shift Register. DACs. Digital Inputs Figure 5.2 The structure of ASIC. The circuit was designed in Cadence environment. The analog part was prepared using Virtuoso. For the digital part the RTL Compiler and Encounter were used. The author has experience gained in the LHCb experiment in the design of the CLARO8 ASIC for the RICH detector [48,49].. 5.1 Methodology 5.1.1 Top Down Design Procedures The analog and the digital part were designed with the Top Down procedure. The Figure 5.3 presents the idea of the Top Down method. The design flow starts from an overall description of the system then in the next steps it is developed in details. After the behavioral modeling the description of the analog and the digital part with Hardware Description Language (HDL) is prepared.. 47.

(56) Figure 5.3 Top Down System design. Figure 5.4 shows in detail steps in the design flow with the information in which the tool operations were performed. The digital part was synthesized in the RTL Compiler and the layout was generated automatically with the Encounter. The analog part was prepared in the Virtuoso. The chip was assembled using the Encounter and Virtuoso. The final checking were performed in the Virtuoso: Design Rule Check (DRC), Layout Versus Schematic (LVS), Design for Manufacturability (DFM).. 48.

(57) INIT. INIT Virtuoso. HDL Description. RTL Compiler. Schematic and VerilogA description. Attributes, Constraints. Basic Blocks. Synthesis, mapping, optimization. Calculations, transistor sizing. Functional Simulation. Simulation Layout. OK? Post layout simulation. Encounter Attributes, Constraints Floorplan Placement clock tree synthesis. Encounter/ Virtuoso Chip Assembly Power Routing OK?. Timing opt Route Virtuoso. Timing opt Flow HDL Timing Simulation. Design Check: DRC, DFM, LVS. OK?. OK?. END Figure 5.4 The design flow. 49.

(58) 5.1.2 Aspects of the Mixed-Signal Integrated Circuit Design Mixed signal integrated circuits are devices where there is the coexistence of the analog and the digital circuits. The demands on mixed circuits are continuously increasing. There are many aspects which designers must consider. They must take into account cost, accuracy, performance, power of the integrated circuit. Integration of different circuits and functionalities in one chip is becoming more and more important. Evolution of the integrated circuits manufacturing technologies leads to smaller length of transistor gate. There is more transistor in the same area. From the point of view of the analog designer the analog circuits become more difficult to design. When the size decreases, the precision of the components is degraded and the problem with matching appears. Not every topology can be used in low power supply voltage. In a new technologies there are no on-off switches, there are only almost on and almost off switches. Under these conditions the analog design is a challenge for a designer [50] [51] [52]. There are many challenges in the mixed signal design. Some aspects should be considered to build high performance circuits [53] [54] such as: •. Trade-off between analog and digital. •. Digitally assisted analog circuits. •. Noise coupling in mixed mode ICs. •. Interference effects CMRR/PSRR. •. Design for ESD. •. Matching impairments. In mixed signal circuits the analog and the digital circuits are in the one chip together. Thanks to the reduction of the technology size, even large and complicated digital circuits are becoming relatively small. High performance analog circuits can be realized by building complicated, precise topologies. Alternative to the precise circuits is the digital calibration [50] [55] [56]. The digital calibration is the solution to build high performance analog circuits with the assistance of digital. It enables to use even digital processes to build a mixed signal design [57]. The digital calibration block could be synthesized automatically which consume less time and effort to design a circuit.. 50.

(59) The digital compensation can be used in the analog circuit to improve the device imperfections caused by technology variation. The same transistors from the design can have different parameters in reality. It leads to mismatch, non-linearity, offset. Variations in the factors of the environment where the chip is working influence the device performance. For example temperature variations, humidity, ageing of the device. The digital calibration could have place [50]: •. One time after production. For example offsets in the circuit could be calibrated one time. •. Each power on. •. Continuously during the chip operations. For example temperature could be compensated in this way. Finally the digital calibration can operate in two ways: •. On the system level, when the calibration firstly is injected to the input and after the observation of the output some changes are made. •. On the block level, when the imperfection is detected firstly then the calibration is made. To implement the digital compensation two nodes in the circuit must be defined: detection node (the signal which is compensated) and compensation node (the signal which is introduced to the circuit to perform compensation). Relying on the detection node some device imperfection can be corrected. To set appropriate value of some parameter the detection node is read. This observed value allows to determine if there is need to decrease or increase the value in the compensation node. As an example the differential pair from the Figure 5.5 can be analyzed. If the circuit were ideal, the offset and the mismatch problem would not appear. In that case currents I1 and I2 would be the same. Due to technology variation I1 and I2 are different. Using the digital calibration the output of differential pair could be the detection node. It is easier to add currents so there are two compensation node, which introduce to the differential pair currents Icomp1 and Icomp2. Based on the value gathered from the output compensation is made. The compensation goal is to have the output value close enough to some expected value [58] [59].. 51.

(60) Icomp1. Icomp2. I1. I2. Figure 5.5 Compensation idea in a differential pair. The calibration of the offset of operational amplifier can be performed using two topologies: 1) Closed-loop: • Low detection level – gain of the imperfection is determined by close-loop gain of the amplifier. • Calibration during operation is possible. 2) Open-loop: • High detection level – gain of the imperfections determined by the high openloop gain of the amplifier. • Calibration during operation is impossible. In the open-loop topology the signal level at the detection node is higher than in the closed loop, because it is multiplied by an open loop gain of the amplifier. The open loop topology is more appropriate to use during the offset calibration due to the higher detection level. On the other side in this topology the calibration cannot be performed during the operation, it needs some additional circuit. The close loop topology doesn’t have this problem. The choice depends on the applications demand. In the designed ASIC the compensation of gain, timings and signal shape is introduced. The gain should be compensated continuously during the chip operations. The timings and the 52.

(61) shape of the output signal should be compensated each time when the new SiPM is connected to the AISC. Therefore, the closed-loop topology with the continuous compensation is chosen.. 5.1.3 Circuit Modelling Oriented to the Analog Design The analog design still plays an important role in many systems on chip nowadays. The development of the submicron technologies lead to the improvement of the digital systems. The digital circuit becomes more precise, faster and occupies a smaller area. Hence the design of the analog circuits becomes more sophisticated task in terms of gain, speed and acceptable surface. The modern submicron technologies cause some difficulties. There are larger parasitic capacitances and smaller output resistances. There is no general analog design methodology or general design approach. A designer must deal with a large degree of freedom. They must consider circuit level parameters such as: 1) Gain 2) Speed 3) Power consumption 4) Dynamic 5) Noise 6) Linearity 7) Area The parameters are correlated to themselves. It is impossible to have high demands to all parameters together. The designers must choose what is more important. The circuit specifications are defined by the transistor level parameters: gds, gm, ft, vdsat, vnoise, ID, W, L, Cgs, Cds. The understanding of the device physics and preparing hand calculations can be very useful and can decrease the time of the design process. There are several models of the MOS transistors. In the design process there were used two of them: BSIM and EKV. Both are dedicated to the submicron technology. The BSIM is threshold voltage based empirical model. It has good accuracy and continuous derivatives. Version BSIM3v3 became the industrial standard and is the most widely used model for the. 53.