L'ochratoxine A (OTA) est un métabolite secondaire produit par des espèces appartenant aux genres Aspergillus et Penicillium qui a été liée à certaines conditions avec des effets néphrotoxiques, immunotoxiques, tératogènes et cancérogènes. La présence d'OTA dans le café vert a été détectée depuis 1974 et sa transmission à la boisson a été mise en évidence en 1989. La torréfaction du café est un procédé thermique qui peut avoir un effet sur la teneur en OTA, avant 1988, on pensait que l'OTA était détruite pendant la torréfaction, mais après plusieurs chercher sont des résultats contradictoires publiés dans % de réduction (de 0 à 100%). Plusieurs auteurs émis les hypothèses suivantes pour expliquer cette réduction : Isomérisation de la toxine dans la position C3 formant un diastéréoisomère moins toxique (Studer-Rohr et al, 1995 et Bruinink et al, 1997), protection de la dégradation d’OTA par l'humidité du grain (Boudra et al 1995; Blanc et al, 1998 et Stegen et al, 2001.), existence de réactions avec le café toxine parent ou réarrangements de la molécule OTA à températures de torréfaction (Suarez-Quiroz et al, 2005). Une autre étude sur la dégradation thermique de OTA pure a montré la formation de deux composés moins toxiques, 14-(R)-ochratoxine A et de la 14-descarboxi-ochratoxine A (Cramer et al, 2008). Parce qu'il n'y a pas de données concluantes sur l'effet de la torréfaction sur l'OTA dans le grain et le besoin de bases scientifiques pour établir des règles pour l'exportation de café vert, l'objectif de ce travail était d'étudier l'impact des différents types de torréfaction sur la stabilité thermique de l'OTA dans le café et l'élucidation chimique des produits de transformation. Deux niveaux de contamination ont été obtenus à partir de café contaminés artificiellement par Aspergillus westerdijkiae (5,3 et 57,2 ppb d'OTA). Ces lots sont grillés à 230° en utilisant deux méthodes : La torréfaction à tambour (TR) et à lit fluidisé (LF). Les échantillons ont été prélevés toutes les 3 min pour TR et chaque min 0,9 pour LF pour quantifier la valeur résiduelle d¡¯OTA. Les résultats ont montré que le procédé de torréfaction par TR (plus lent) était plus efficace que la FL dans l'élimination de l'OTA (67% et 36%, respectivement, pour une torréfaction moyenne). Nous avons déterminé le taux de dégradation thermique de OTA pure et de l'OTA mélangée avec les composants du café (5 sucres, 3 acides aminés, la caféine et les acides chlorogéniques), montrant que les interactions se déroulent en fonction des conditions de pH et de pKa des composants testés, dans ce cas, en influant sur la réactivité et la vitesse de dégradation de l'OTA. Un produit de transformation (PT) a été observé sur les chromatogrammes obtenus à partir de l'interaction de l'OTA avec les composants du café. Des tests d'alcalinisation et de chauffage de OTA pure ont confirmé que le PT provient de la modification structurale de la molécule d'OTA et n’est pas un produit de l'interaction avec les composants naturels du café. L'effet du pH et de la température sur l'extraction de l'OTA dans le café contaminé a été testé dans ce travail, les résultats ont montré une plus grande extraction de la toxine à un pH de 8,5 et 6°. Au même pH à 20° il y avait une plus importante formation d'un produit de transformation. Le PT a été purifié pour mener à bien sa caractérisation chimique. La nature chimique du produit de transformation, les données spectroscopiques telles que celles obtenus sous UV-Vis (max: 237nm), l'affinité avec la phase mobile de l'OTA, l'analyse de l'alcalinisation (phénomène de régénération de l'OTA et PT), l'analyse d’isotopes stables (SIDA’s) et le spectre de masse (ion moléculaire M+1: 420 m/z), suggèrent que la structure de le PT d'OTA durant le processus de torréfaction correspond à un analogue de l'OTA qui conserve son groupe carboxyle acide et conformément à la fragmentation correspond à la Hydroxy-ochratoxine A (OH-OTA), avec des quantités mineures d’ OTA et de ses isomères. / Ochratoxin A (OTA) is a secondary metabolite produced by species belonging to the genera Aspergillus and Penicillium and this toxin has been associated with certain illnesses within nephrotoxic effects, immunotoxic, teratogenic and carcinogenic. The presence of OTA in green coffee was detected in 1974 and its transmission into the beverage was made evident in 1989. Roasting coffee is a thermal process that have an effect on the OTA content. Before 1988 it was thought that the OTA was destroyed during roasting, but after several investigations, results published are contradictory results published in the % reduction (from 0 to 100%). Several authors have established hypothesis that try explain this reduction: Isomerization of the toxin in the C3 position forming a less toxic diastereomers (Studer-Rohr et al., 1995 and Bruinink et al., 1997), Protection of grain moisture degradation OTA (Boudra et al., 1995; Blanc et al., 1998 and Stegen et al., 2001), Existence of reactions with the parent toxin coffee or rearrangements of the OTA molecule roasting temperatures (200¡ã and 250¡ãC) (Su¨¢rez-Quiroz et al., 2005). Another study on thermal degradation of pure OTA showed the formation of two less toxic compounds 14 - (R)-ochratoxin A and the 14-descarboxi-Ochratoxin A (Cramer et al., 2008).Because there are no conclusive data regarding the effect of roasting on OTA in grain and the need for scientific bases for establishing regulations for export of green coffee, the objective of this work was to study the impact of different types of roasting on the thermal stability of OTA in coffee and chemical elucidation of the transformation products.Two levels of contamination were obtained by the contamination of coffee with a strain of A. westerdijkiae (5.3 and 57.2 ppb of OTA). These lots were roasted at 230 ¡ã C using two methods: Drum rotation (TR) and fluidized bed (LF). Samples were taken every 3 min from TR and every 0.9 min for LF to quantify the residual OTA. The results showed that in roasting process by TR (slower), it was more effective than with LF in the elimination of OTA (67% and 36%, respectively, for a medium roast). The thermal degradation rate of pure OTA and of OTA mixed with the components of coffee (5 sugars, 3 amino acids, caffeine and chlorogenic acids), were determined, showing that interactions took place dependent themselves on the conditions of pH and pKa values of the components tested, in this case by influencing by the reactivity and the rate of degradation of OTA. A transformation product (TP) was observed in the chromatograms obtained from the interaction of OTA with the components of coffee. A test of alkalinization and warming of pure OTA confirmed that the TP comes from the structural modification of the OTA molecule and is not a product of interaction with the natural components of coffee. The pH and temperature showed an effect in extraction of OTA in contaminated coffee, the results show better extraction of the toxin at pH 8.5 at 60 ¡ãC. At the same pH at 20 ¡ãC, it was shown a greater formation of the transformation product.The TP was purified to carry out its chemical characterization. The chemical nature of compound transformation and spectroscopic data such as UV-Vis (¦Ëmax: 237nm), the affinity with the mobile phase of the OTA, the analysis of alkalinization (OTA regeneration phenomenon and TP) analysis of stable isotopes (SIDA's) and the mass spectrum (molecular ion M +: 420 m / z), suggest that structurally the TP of OTA during the roasting process corresponds to an analogue of OTA which retains its acidic carboxyl group and in accordance to fragmentation corresponds to the Hydroxi- Ochratoxin A (OH-OTA), as well as minor amounts of OTA and its isomers.

No description available.

A general framework for performance optimization of continuous-time OTA-C (Operational Transconductance Amplifier-Capacitor) filters is proposed. Efficient procedures for evaluating nonlinear distortion and noise valid for any filter of arbitrary order are developed based on the matrix description of a general OTA-C filter model . Since these procedures use OTA macromodels, they can be used to obtain the results significantly faster than transistor-level simulation. In the case of transient analysis, the speed-up may be as much as three orders of magnitude without almost no loss of accuracy. This makes it possible to carry out direct numerical optimization of OTA-C filters with respect to important characteristics such as noise performance, THD, IM3, DR or SNR. On the other hand, the general OTA-C filter model allows us to apply matrix transforms that manipulate (rescale) filter element values and/or change topology without changing its transfer function. The above features are a basis to build automated optimization procedures for OTA-C filters. In particular, a systematic optimization procedure using equivalence transformations is proposed. The research also proposes suitable software implementations of the optimization process. The first part of the research proposes a general performance optimization procedure and to verify the process two application type examples are mentioned. An application example of the proposed approach to optimal block sequencing and gain distribution of 8th order cascade Butterworth filter (for two variants of OTA topologies) is given. Secondly the modeling tool is used to select the best suitable topology for a 5th order Bessel Low Pass Filter. Theoretical results are verified by comparing to transistor-level simulation withCADENCE. For the purpose of verification, the filters have also been fabricated in standard 0.5mm CMOS process. The second part of the research proposes a new linearization technique to improve the linearity of an OTA using an Active Error Feedforward technique. Most present day applications require very high linear circuits combined with low noise and low power consumption. An OTA based biquad filter has also been fabricated in 0.35mm CMOS process. The measurement results for the filter and the stand alone OTA have been discussed. The research focuses on these issues.

The focus of the present thesis is the circuit-level implementation of an excess loop delay compensation scheme which optimizes excess loop delay in Analog-to-Digital Converter(ADC) by using a programmable delay block and synchronizes the signal passing through Dynamic Element Matching block, used to mitigate mismatch effects of multi-bit Digital-to-Analog Converter(DAC). The proposed delay block has tuning range of T/10 to T/2 seconds, with a step size of T/30 seconds, where T is the time period (1.25 nanoseconds) of sampling signal (800 MHz) in high IF (200 MHz) Bandpass [sigma delta] ADC. The implementation details of the element rotation scheme used to calibrate the multi-bit DAC static error mismatch are also presented. Also presented is the design of high frequency highly linear Operational Transconductance Amplifier(OTA) targeted for continuous-time filters in a high resolution High Intermediate Frequency (200 MHz) Bandpass [sigma delta] ADC for Software Radio applications. Proposed OTA uses super source follower input stage to enhance its voltage-to-current conversion linearity. The design has been simulated using TSMC 0.18 μm CMOS process. The OTA has small signal transconductance of 0.9 mA/V, IM3 below -79 dB (for 0.3 Vpp input), Signal-to-Noise Ratio of 82 dB and power consumption of 6.8 mW, when tested in unity gain configuration.

Hochsch. für Wirtschafts-, Rechts- und Sozialwiss., Diss.--St. Gallen, 1991.

Hochs., Diss., 1991--St. Gallen.

In this thesis, a second-order low pass square root domain filter (SRD filter) based on operational transconductors amplifiers (OTAs) is presented. The SRD filter consists of two translinear filters and four OTAs. Because the OTA has small voltage swings, which may violate the large signal natural of the SRD filter. We investigate the dynamic range of this compact SRD filter with different quality factor(Q). The circuit has fewer numbers of transistors and operate in low voltage, therefore, it has less power consumption and less chip area. The circuit has been fabricated with 0.35£gm CMOS technology. It operates with a supply voltage 1.5V and the biasing current varies from 10uA to 80uA. Measurement results lts show that Im/I0≥40% when the external capacitance C is 3.5pF¡B7pF and Im/I0≥53% when the external capacitance C is 3pF¡B8.5pF. The cutoff frequency of the filter can be tuned from 1.24MHz to 5.53MHz when the external capacitance C is 3.5pF¡B7pF and the cutoff frequency can be tuned from 900KHz to 4.46MHz when the external capacitance C is 3pF¡B8.5pF. The total harmonic distortion is 0.908% and the power consumption is 506£gW.

Operational Transconductance Amplifiers (OTAs) with high linearity and wide input range characteristics have become a focus of interest for analog continuous-time circuits. In this research, we intend to develop a low distortion, wide swing CMOS OTA. The supply voltage is 5V. Simulation results show that the OTA is linear, tunable and wide swing. The transconductance can be tuned from 23£gA/V to 37£gA/V. When a 3VPP input signal with 1MHz is applied, THD of the OTA is 51dB. The OTA is used to realize a 1-MHz 7th-order Butterworth lowpass filter. When a 1VPP input signal with 100KHz is applied, THD of the lowpass filter is 50dB. All the circuits are designed based on the UMC 0.5£gm 2p2m CMOS process technology.

No description available.

This thesis is an exploration into the design of configurable analog block (CAB) for field programmable analog arrays (FPAAs) designed in modern complementary metal-oxide-semiconductor (CMOS) technologies. Specifically, this thesis develops a single configurable analog block (CAB) using an operational transconductance amplifier (OTA). A fully differential OTA is selected for its flexibility and the reliability of differential signals with respect to noise when compared to single-ended signals. The OTA is combined with a set of switches, controlled by a serial shift register, to allow for reconfiguration of the internal wiring, and two capacitor arrays used to fine-tune the frequency response of the circuit. Simulation results are provided for an OTA, the OTA operating in situ, and a band-pass filter, thus demonstrating the use of the CAB. A single CAB is constructed on a 0.13 µm CMOS chip. / Integrated Circuits and Systems