Strategies towards high performance (high-resolution/linearity) time-to-digital converters on field-programmable gate arrays

Time-correlated single-photon counting (TCSPC) technology has become popular in scientific research and industrial applications, such as high-energy physics, bio-sensing, non-invasion health monitoring, and 3D imaging. Because of the increasing demand for high-precision time measurements, time-to-digital converters (TDCs) have attracted attention since the 1970s. As a fully digital solution, TDCs are portable and have great potential for multichannel applications compared to bulky and expensive time-to-amplitude converters (TACs). A TDC can be implemented in ASIC and FPGA devices. Due to the low cost, flexibility, and short development cycle, FPGA-TDCs have become promising. Starting with a literature review, three original FPGA-TDCs with outstanding performance are introduced. The first design is the first efficient wave union (WU) based TDC implemented in Xilinx UltraScale (20 nm) FPGAs with a bubble-free sub-TDL structure. Combining with other existing methods, the resolution is further enhanced to 1.23 ps. The second TDC has been designed for LiDAR applications, especially in driver-less vehicles. Using the proposed new calibration method, the resolution is adjustable (50, 80, and 100 ps), and the linearity is exceptionally high (INL pk-pk and INL pk-pk are lower than 0.05 LSB). Meanwhile, a software tool has been open-sourced with a graphic user interface (GUI) to predict TDCs’ performance. In the third TDC, an onboard automatic calibration (AC) function has been realized by exploiting Xilinx ZYNQ SoC architectures. The test results show the robustness of the proposed method. Without the manual calibration, the AC function enables FPGA-TDCs to be applied in commercial products where mass production is required.Time-correlated single-photon counting (TCSPC) technology has become popular in scientific research and industrial applications, such as high-energy physics, bio-sensing, non-invasion health monitoring, and 3D imaging. Because of the increasing demand for high-precision time measurements, time-to-digital converters (TDCs) have attracted attention since the 1970s. As a fully digital solution, TDCs are portable and have great potential for multichannel applications compared to bulky and expensive time-to-amplitude converters (TACs). A TDC can be implemented in ASIC and FPGA devices. Due to the low cost, flexibility, and short development cycle, FPGA-TDCs have become promising. Starting with a literature review, three original FPGA-TDCs with outstanding performance are introduced. The first design is the first efficient wave union (WU) based TDC implemented in Xilinx UltraScale (20 nm) FPGAs with a bubble-free sub-TDL structure. Combining with other existing methods, the resolution is further enhanced to 1.23 ps. The second TDC has been designed for LiDAR applications, especially in driver-less vehicles. Using the proposed new calibration method, the resolution is adjustable (50, 80, and 100 ps), and the linearity is exceptionally high (INL pk-pk and INL pk-pk are lower than 0.05 LSB). Meanwhile, a software tool has been open-sourced with a graphic user interface (GUI) to predict TDCs’ performance. In the third TDC, an onboard automatic calibration (AC) function has been realized by exploiting Xilinx ZYNQ SoC architectures. The test results show the robustness of the proposed method. Without the manual calibration, the AC function enables FPGA-TDCs to be applied in commercial products where mass production is required

A 10 nV/rt Hz noise level 32-channel neural impedance sensing ASIC for local activation imaging on nerve section

A 10 nV/rt Hz noise level 32-channel neural impedance sensing ASIC is presented for the application of local activation imaging in nerve section. It is increasingly known that the monitoring and control of nerve signals can improve physical and mental health. Major nerves, such as the vagus nerve and the sciatic nerve, consist of a bundle of fascicles. Therefore, to accurately control a particular application without any side effects, we need to know exactly which fascicle was activated. The only way to find locally activated fascicle is to use electrical impedance tomography (EIT). The ASIC to be introduced is designed for neural EIT applications. A neural impedance sensing ASIC was implemented using CMOS 180-nm process technology. The integrated input referred noise was calculated to be 0.46 μVrms (noise floor 10.3 nVrms/rt Hz) in the measured noise spectrum. At an input of 80 mV, the squared correlation coefficient for linear regression was 0.99998. The amplification gain uniformity of 32 channels was in the range of + 0.23% and - 0.29%. Using the resistor phantom, the simplest model of nerve, it was verified that a single readout channel could detect a signal-to- noise ratio of 75.6 dB or more. Through the reservoir phantom, real-time EIT images were reconstructed at a rate of 8.3 frames per second. The developed ASIC has been applied to in vivo experiments with rat sciatic nerves, and signal processing is currently underway to obtain activated nerve cross-sectional images. The developed ASIC was also applied to in-vivo experiments with rat sciatic nerves, and signal processing is currently underway to obtain locally activated nerve cross-sectional images

Bidirectional Neural Interface Circuits with On-Chip Stimulation Artifact Reduction Schemes

Bidirectional neural interfaces are tools designed to “communicate” with the brain via recording and modulation of neuronal activity. The bidirectional interface systems have been adopted for many applications. Neuroscientists employ them to map neuronal circuits through precise stimulation and recording. Medical doctors deploy them as adaptable medical devices which control therapeutic stimulation parameters based on monitoring real-time neural activity. Brain-machine-interface (BMI) researchers use neural interfaces to bypass the nervous system and directly control neuroprosthetics or brain-computer-interface (BCI) spellers. In bidirectional interfaces, the implantable transducers as well as the corresponding electronic circuits and systems face several challenges. A high channel count, low power consumption, and reduced system size are desirable for potential chronic deployment and wider applicability. Moreover, a neural interface designed for robust closed-loop operation requires the mitigation of stimulation artifacts which corrupt the recorded signals. This dissertation introduces several techniques targeting low power consumption, small size, and reduction of stimulation artifacts. These techniques are implemented for extracellular electrophysiological recording and two stimulation modalities: direct current stimulation for closed-loop control of seizure detection/quench and optical stimulation for optogenetic studies. While the two modalities differ in their mechanisms, hardware implementation, and applications, they share many crucial system-level challenges. The first method aims at solving the critical issue of stimulation artifacts saturating the preamplifier in the recording front-end. To prevent saturation, a novel mixed-signal stimulation artifact cancellation circuit is devised to subtract the artifact before amplification and maintain the standard input range of a power-hungry preamplifier. Additional novel techniques have been also implemented to lower the noise and power consumption. A common average referencing (CAR) front-end circuit eliminates the cross-channel common mode noise by averaging and subtracting it in analog domain. A range-adapting SAR ADC saves additional power by eliminating unnecessary conversion cycles when the input signal is small. Measurements of an integrated circuit (IC) prototype demonstrate the attenuation of stimulation artifacts by up to 42 dB and cross-channel noise suppression by up to 39.8 dB. The power consumption per channel is maintained at 330 nW, while the area per channel is only 0.17 mm2. The second system implements a compact headstage for closed-loop optogenetic stimulation and electrophysiological recording. This design targets a miniaturized form factor, high channel count, and high-precision stimulation control suitable for rodent in-vivo optogenetic studies. Monolithically integrated optoelectrodes (which include 12 µLEDs for optical stimulation and 12 electrical recording sites) are combined with an off-the-shelf recording IC and a custom-designed high-precision LED driver. 32 recording and 12 stimulation channels can be individually accessed and controlled on a small headstage with dimensions of 2.16 x 2.38 x 0.35 cm and mass of 1.9 g. A third system prototype improves the optogenetic headstage prototype by furthering system integration and improving power efficiency facilitating wireless operation. The custom application-specific integrated circuit (ASIC) combines recording and stimulation channels with a power management unit, allowing the system to be powered by an ultra-light Li-ion battery. Additionally, the µLED drivers include a high-resolution arbitrary waveform generation mode for shaping of µLED current pulses to preemptively reduce artifacts. A prototype IC occupies 7.66 mm2, consumes 3.04 mW under typical operating conditions, and the optical pulse shaping scheme can attenuate stimulation artifacts by up to 3x with a Gaussian-rise pulse rise time under 1 ms.PHDElectrical EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/147674/1/mendrela_1.pd

Energy Efficient Computing with Time-Based Digital Circuits

University of Minnesota Ph.D. dissertation. May 2019. Major: Electrical Engineering. Advisor: Chris Kim. 1 computer file (PDF); xv, 150 pages.Advancements in semiconductor technology have given the world economical, abundant, and reliable computing resources which have enabled countless breakthroughs in science, medicine, and agriculture which have improved the lives of many. Due to physics, the rate of these advancements is slowing, while the demand for the increasing computing horsepower ever grows. Novel computer architectures that leverage the foundation of conventional systems must become mainstream to continue providing the improved hardware required by engineers, scientists, and governments to innovate. This thesis provides a path forward by introducing multiple time-based computing architectures for a diverse range of applications. Simply put, time-based computing encodes the output of the computation in the time it takes to generate the result. Conventional systems encode this information in voltages across multiple signals; the performance of these systems is tightly coupled to improvements in semiconductor technology. Time-based computing elegantly uses the simplest of components from conventional systems to efficiently compute complex results. Two time-based neuromorphic computing platforms, based on a ring oscillator and a digital delay line, are described. An analog-to-digital converter is designed in the time domain using a beat frequency circuit which is used to record brain activity. A novel path planning architecture, with designs for 2D and 3D routes, is implemented in the time domain. Finally, a machine learning application using time domain inputs enables improved performance of heart rate prediction, biometric identification, and introduces a new method for using machine learning to predict temporal signal sequences. As these innovative architectures are presented, it will become clear the way forward will be increasingly enabled with time-based designs

Atténuation des interactions électromagnétiques entre le module de détection LabPET II et l’IRM

Les scanners TEP/IRM simultanés offrent une occassion unique d'examiner en même temps les propriétés anatomiques et fonctionnelles des tissus malins, tout en évitant l'incertitude des systèmes séquentiels de TEP/IRM. Cependant, le couplage électromagnétique entre les deux modalités constitue un défi important à relever. Ces interférences électromagnétiques entravent les performances du scanner et altèrent la qualité d'image de chaque modalité. Bien que les métaux possèdent d'excellentes propriétés de blindage contre les fréquences radioélectriques, ils ne constituent pas nécessairement une option de blindage appropriée pour modifier les champs magnétiques induisant des courants de Foucault dans les couches métalliques. En conséquence, il existe une demande considérable pour un nouveau matériau de protection et une approche originale pour retirer les pièces métalliques du champ de vision IRM. L’objectif de ce projet était d’initier les études en vue de la réalisation d’un scanner TEP/IRM simultané basé sur des modules de détection LabPET II hautement pixélisés afin d’obtenir une résolution spatiale millimétrique pour le cerveau humain et le chien. L'électronique LabPET II comprend des circuits intégrés à application spécifique dans lesquels le signal est numérisé à proximité de la photodiode à avalanche et offre un environnement moins sensible aux interférences électromagnétiques. Pour atteindre l'objectif principal, premièrement, l'effet du matériau métallique des modules de détection LabPET II sur les performances de la TEP et de l'IRM est examiné théoriquement. Les résultats confirment que les composants métalliques du module de détection LabPET II altèrent le champ magnétique, génèrent des courants de Foucault ce qui augmente leur température. Ensuite, les performances électroniques des modules de détection LabPET II sous l’influence de bobines d’IRM faites sur mesure sont examinées. La résolution en énergie et la résolution temporelle se détériorent en présence de bobines RF et de bobines à gradient en raison des perturbations électromagnétiques. Subséquemment, un module de détection LabPET II blindé par une fine couche de composite cuivre-argent est étudié, prouvant que le blindage contre les interférences électromagnétiques avec le composite rétablit les performances en TEP, fournissant moins d'induction par courants de Foucault. En outre, une nouvelle configuration de blindage basée sur un composite de couche flexible de nanotubes de carbone a été fabriquée pour limiter les interférences électromagnétiques. Les composites de nanotubes de carbone créent une couche hautement conductrice avec des chemins conducteurs minimaux, ce qui permet de réduire les courants de Foucault. Le principal résultat scientifique de ce projet est que le blindage composite empêche les interférences de basses et hautes fréquences et réduit l'induction de courants de Foucault, offrant ainsi la flexibilité nécessaire pour acquérir une séquence rapide de commutation de gradients. D'un point de vue technique, le module de détection LabPET II ainsi blindé présente une excellente performance dans un environnement de type IRM, ce qui permet de concevoir un insert TEP basé sur la technologie LabPET II.Abstract: Simultaneous PET/ MRI scanners provide a unique opportunity to investigate anatomical and functional properties of malignant tissues at the same time while avoiding the uncertainty of a sequential PET/MRI systems. However, electromagnetic coupling between the two modalities is a significant challenge that needs to be addressed. These electromagnetic interferences (EMI) hinder the performance of both scanners and distort the image quality of each modality. Although metals have excellent radio-frequency shielding properties, they are not necessarily an appropriate shielding option for altering magnetic fields that induce eddy currents in any metallic layer. Thus, there is a considerable demand for a new shielding material and an original approach to remove metallic parts from the MRI field of view. The objective of this project was to initiate the realization of a simultaneous PET/MRI scanner based on highly pixelated LabPET II detection modules to achieve millimeter spatial resolution for the human brain and dogs. The LabPET II electronics include application specific integrated circuits where the signal is digitized near the avalanche photodiode and offers an environment less susceptible to EMI. To fulfill the main aim, for the first time, the effect of the metallic material of LabPET II on PET and MRI performance was theoretically examined. Results confirm that metallic components of the LabPET II detection modules distort the magnetic field, generate eddy currents, and increase temperature. Then, the LabPET II electronics performance under the influence of custom-made MRI coils was investigated. Its energy and timing resolutions deteriorate in the presence of both RF and gradient signals because of EMIs. Thus, a LabPET II detection module shielded by a thin layer of the copper-silver composite was investigated, proving that shielding EMIs with the composite restores the PET performance, with less eddy current induction. Besides, a new shielding configuration based on a flexible layer of carbon nanotube (CNT) composite was fabricated to limit the EMIs. The CNT composite creates a highly conductive layer with minimal conductive paths that allows eddy currents to be decreased. The primary scientific outcome of this project is that the novel composite shielding rejects both low and high-frequency interferences and reduces eddy current induction, offering the flexibility to acquire a fast gradient switching sequence. From a technical point of view, the shielded LabPET II detection module demonstrates an excellent performance in an MRI-like environment supporting the feasibility of designing a PET-insert based on LabPET II technology

A Wireless, High-Voltage Compliant, and Energy-Efficient Visual Intracortical Microstimulator

RÉSUMÉ L’objectif général de ce projet de recherche est la conception, la mise en oeuvre et la validation d’une interface sans fil intracorticale implantable en technologie CMOS avancée pour aider les personnes ayant une déficience visuelle. Les défis majeurs de cette recherche sont de répondre à la conformité à haute tension nécessaire à travers l’interface d’électrode-tissu (IET), augmenter la flexibilité dans la microstimulation et la surveillance multicanale, minimiser le budget de puissance pour un dispositif biomédical implantable, réduire la taille de l’implant et améliorer le taux de transmission sans fil des données. Par conséquent, nous présentons dans cette thèse un système de microstimulation intracorticale multi-puce basée sur une nouvelle architecture pour la transmission des données sans fil et le transfert de l’énergie se servant de couplages inductifs et capacitifs. Une première puce, un générateur de stimuli (SG) éconergétique, et une autre qui est un amplificateur de haute impédance se connectant au réseau de microélectrodes de l’étage de sortie. Les 4 canaux de générateurs de stimuli produisent des impulsions rectangulaires, demi-sinus (DS), plateau-sinus (PS) et autres types d’impulsions de courant à haut rendement énergétique. Le SG comporte un contrôleur de faible puissance, des convertisseurs numérique-analogiques (DAC) opérant en mode courant, générateurs multi-forme d’ondes et miroirs de courants alimentés sous 1.2 et 3.3V se servant pour l’interface entre les deux technologies utilisées. Le courant de stimulation du SG varie entre 2.32 et 220μA pour chaque canal. La deuxième puce (pilote de microélectrodes (MED)), une interface entre le SG et de l’arrangement de microélectrodes (MEA), fournit quatre niveaux différents de courant avec la valeur maximale de 400μA par entrée et 100μA par canal de sortie simultanément pour 8 à 16 sites de stimulation à travers les microélectrodes, connectés soit en configuration bipolaire ou monopolaire. Cette étage de sortie est hautement configurable et capable de délivrer une tension élevée pour satisfaire les conditions de l’interface à travers l’impédance de IET par rapport aux systèmes précédemment rapportés. Les valeurs nominales de plus grandes tensions d’alimentation sont de ±10V. La sortie de tension mesurée est conformément 10V/phase (anodique ou cathodique) pour les tensions d’alimentation spécifiées. L’incrémentation de tensions d’alimentation à ±13V permet de produire un courant de stimulation de 220μA par canal de sortie permettant d’élever la tension de sortie jusqu’au 20V par phase. Cet étage de sortie regroupe un commutateur haute tension pour interfacer une matrice des miroirs de courant (3.3V /20V), un registre à décalage de 32-bits à entrée sérielle, sortie parallèle, et un circuit dédié pour bloquer des états interdits.----------ABSTRACT The general objective of this research project is the design, implementation and validation of an implantable wireless intracortical interface in advanced CMOS technology to aid the visually impaired people. The major challenges in this research are to meet the required highvoltage compliance across electrode-tissue interface (ETI), increase lexibility in multichannel microstimulation and monitoring, minimize power budget for an implantable biomedical device, reduce the implant size, and enhance the data rate in wireless transmission. Therefore, we present in this thesis a multi-chip intracortical microstimulation system based on a novel architecture for wireless data and power transmission comprising inductive and capacitive couplings. The first chip is an energy-efficient stimuli generator (SG) and the second one is a highimpedance microelectrode array driver output-stage. The 4-channel stimuli-generator produces rectangular, half-sine (HS), plateau-sine (PS), and other types of energy-efficient current pulse. The SG is featured with low-power controller, current mode source- and sinkdigital- to-analog converters (DACs), multi-waveform generators, and 1.2V/3.3V interface current mirrors. The stimulation current per channel of the SG ranges from 2.32 to 220μA per channel. The second chip (microelectrode driver (MED)), an interface between the SG and the microelectrode array (MEA), supplies four different current levels with the maximum value of 400μA per input and 100μA per output channel. These currents can be delivered simultaneously to 8 to 16 stimulation sites through microelectrodes, connected either in bipolar or monopolar configuration. This output stage is highly-configurable and able to deliver higher compliance voltage across ETI impedance compared to previously reported designs. The nominal values of largest supply voltages are ±10V. The measured output compliance voltage is 10V/phase (anodic or cathodic) for the specified supply voltages. Increment of supply voltages to ±13V allows 220μA stimulation current per output channel enhancing the output compliance voltage up to 20V per phase. This output-stage is featured with a high-voltage switch-matrix, 3.3V/20V current mirrors, an on-chip 32-bit serial-in parallel-out shift register, and the forbidden state logic building blocks. The SG and MED chips have been designed and fabricated in IBM 0.13μm CMOS and Teledyne DALSA 0.8μm 5V/20V CMOS/DMOS technologies with silicon areas occupied by them 1.75 x 1.75mm2 and 4 x 4mm2 respectively. The measured DC power budgets consumed by low-and mid-voltage microchips are 2.56 and 2.1mW consecutively

Design and validation of key components for the readout electronics of future PET scanners

This thesis work discusses the design and validation of two circuit components used in the electronic readout of positron emission tomography (PET) scanners for biomedical applications: a constant fraction discriminator (CFD) and an integrated CMOS time to digital converter (TDC). The former is used in the read out of a double-head PET scanner already developed by the group of medical physics at INFN Pisa for non-invasive dose delivery monitoring in hadrontherapy. The goal of the work has been the optimization of the front-end PCB in terms of timing performances so as to reduce the dead time and resolution at system level. A new CFD board has been implemented and experimental results have shown a significant enhancement of the timing characteristics which have enabled performing in-beam PET data acquisition which is fundamental in hadrontherapy treatment. The design of an integrated CMOS TDC to be used for the time of flight measurement in a magnetic field-compatible PET block detector is the second topic of the thesis. The required time resolutions, linear behaviour as well as the communication with other readout elements have been taken into account in the definition of the circuit topology. Cadence and Verilog simulations have shown that a bin size of 100 ps can be obtained with the combination of a submicron technology (UMC 65 nm LLLVT) and a pipeline approach where a 10 bit systolic counter coupled to a 4 stage delay locked loop (DLL) are exploited. This translates into a nominal resolution of 29 ps. In addition, the use of a short DLL leads to a high linearity which is an issue in PET measurements. Despite lower resolutions are obtained in literature with different TDC topologies, achieving good performances in terms of both time resolution and linearity is not straightforward. The converter also features a real-time validation algorithm which is capable to reject noise inputs generated by the photodetector without impairing the acquisition capability of the system. A standard-cell unit has been also designed which is in charge of data buffering and serial communication with external readout boards. A 47 bit output word is provided by the semi-custom stage at a measurement rate which is selectable between 31.25 MHz and 62.5 MHz with a double hit resolution of 170 ns. An 8 channel prototype of 1.875 x 1.875 mm2 has been submitted in March 2013 in order to validate simulated data with experimental results

Hardware improvements for detecting modulated near-infrared light in diffuse optical measurements of the human brain

Abstract. Functional near-infrared spectroscopy (fNIRS) is a non-invasive in vivo brain imaging method. fNIRS systems can be used to detect diseases that alter the hemodynamics of the brain, but they are also applicable to the study of hemodynamics in healthy brains to investigate, for example, how brain hemodynamics change in response to external stimuli. The work carried out in this thesis involved improving parts of the light detection hardware that forms the core of a frequency-domain/spatially-resolved fNIRS system. Different narrow band-pass filter configurations were analyzed to determine the best option for a system of that type. Among the various alternatives, multifeedback filters and resonator filters were simulated and compared. Finally, two different resonator filter sets were physically implemented using printed circuit technology. The second part of the thesis describes the manufacture of six low-noise, high-sensitivity NIR light detectors designed to extend the light detection capabilities of the system. All these detectors were implemented on printed circuit boards. After implementation, the circuits were tested separately and in combination with other parts of the system, achieving good results in both cases. The most significant result was the detection of blood flow pulsations from the finger and forehead of a subject using the designed light detectors in combination with the designed filters and a lock-in amplifier. This result shows that the circuits are fully functional and can be used to expand the capabilities of the fNIRS device.Laitteistoparannuksia moduloidun lähi-infrapunavalon detektoimiseksi ihmisaivojen diffuusi optisessa mittausmenetelmässä. Tiivistelmä. Toiminnallisella lähi-infrapunaspektroskopialla tarkoitetaan usein ei-invasiivista aivojen optista kuvantamista. Menetelmää voidaan hyödyntää aivoperäisten sairauksien tutkimisessa, mutta myös terveiden aivojen toimintojen tutkimisessa, esimerkiksi tutkimalla kuinka ulkoinen ärsyke aiheuttaa aktivaation aivoissa, näkyen menetelmällä mitattavissa olevina aivojen happitasojen muutoksina. Tämän diplomityön aiheena oli olemassa olevan laitteiston vastaanotintekniikan kehittäminen, jota käytetään lähi-infrapunavalon FD-modulointiin perustuvassa tekniikassa. Useanlaisia kapeakaistaisia suodatinkonfiguraatioita analysoitiin parhaan suodatintyypin valitsemiseksi. Eri vaihtoehdoista valittiin ns. multifeedback- ja resonaattorisuodattimet, joita simuloitiin ja verrattiin keskenään. Lopuksi suunniteltiin kaksi resonaattorisuodatinsarjaa toteuttaen PCB-piirilevyllä. Diplomityön toisena osana suunniteltiin kuusi pienikohinaista ja herkkää lähi-infrapunavalovastaanotinta käytettäväksi olemassa olevassa laitteistossa. Kaikki vastaanottimet rakennettiin PCB-piirilevylle. Suunnitellut piirilevyt testattiin erikseen ja yhdistettynä laitekokonaisuuteen, saaden siinä hyviä tuloksia sekä suodattimilla että vastaanottimilla. Testeissä veren virtauksen pulsaatioita pystyttiin mittaamaan sormesta ja aivoista otsalohkon alueelta hyödyntäen olemassa olevaa ns. lock-in tekniikkaa. Testitulokset osoittivat, että suunnitellut piirilevyt toimivat hyvin ja paransivat mittalaitteen vastaanottimen suoritustasoa