Dissertations / Theses on the topic 'Digital converters'

As the rapid development of the wireless communication system and mobile video devices, the integrated chip with low power consuming and high conversion efficiency is widely needed. ADC and DAC are playing an important role in these applications.

The aim of this thesis is to verify a post-correction method which is used for improving the performance of ADC. First of all, this report introduces the development and present status of ADC, and expatiate its important parameters from two different classes (static performance and dynamic performance). Based on the fundamental principle, the report then focuses on the dynamic integral non-linearity modeling of ADC. Refer to this model, one post-correction method is described and verified.

Upon the face of post-correction, this method is to modify the output signals which have been converted from analog to digital format by adding a correction term. Improvement made by the post-correction needs to be checked out. Thus the performance analysis mainly relay on the measures of total harmonic distortion and signal to noise and distortion ratio is also included in this thesis.

This thesis deals with digital post-correction ofanalog-to-digital converters (ADCs). The performance ofpractical ADCs is deteriorated by nonidealities and flaws inthe converter. Methods for mitigating these errors by applyingdigital signal processing to the output of the converter havebeen proposed in the past. These methods are often referred toas postcorrection methods. This work is mainly concerned withpost-correction methods based on look-up tables.

Practical ADCs often exhibit dynamical error behavior,meaning that the error is dependent on the dynamics of theinput signal. In this thesis an extension of previouslyproposed post-correction methods is proposed. The method usesthe present sample in conjunction with a number of past samplesto form the table index. In order to reduce the number of indexbits, and thereby the size of the table, the method comprises a‘bit mask’, which selects a subset of the availablebits to be used in the index. Evaluations using experimentalADC data show that the proposed method improves the converterperformance, but also that the choice of index bits has asignificant impact on the outcome of the correction. Theincorporation of a bit mask enables an analysis of the effectof different bit masks. The analysis results in a framework forcomparing different correction tables.

The framework is then applied in an optimization problem.The goal is to find the best allocation of a fixed number ofindex bits. Two different criterions are applied: minimizationof the total harmonic distortion and maximization of thesignal-to-noise and distortion ratio. The results of theoptimization, performed with experimental data, show that theoptimal bit allocation is different depending on whichcriterion is used. Moreover, the performance of a correctionscheme deteriorates only slowly with decreasing table size, ifappropriate index bits are selected.

There is a need for a foundation of a research study aimed at investigations on near real-time reliability awareness of Gallium Nitride devices in high-frequency power converters for which we need advanced hardware and algorithms. This dissertation is moving beyond traditional reliability analysis and looking to more applicable and accurate analytical tools by introducing deep learning techniques and advanced sensing solutions. The computational structures will be applied at the edge of the power converter through online sensing and data processing units as well as on a remote server. They will provide an iterative ability to predict the time until the device may fail or reach a pre-defined degradation threshold.

With the availability of the most granular information deduced from advanced devices, a new data-driven scheme is proposed for system monitoring and possible lifetime extension Gallium Nitride power converters. The approach relies on the real-time on-resistance data extraction from the power converter, and calibration of an adaptive model using multi-physics co-simulations under power cycling. More specifically, the focus is on deploying machine learning algorithms to exploit for the parameter estimation in power electronics engineering reliability. The proposed techniques in this work are quite new and have not yet been developed and analyzed for high-frequency power converters specifically with Gallium Nitride power semiconductor devices.

Denna avhandling behandlar analog-digitalomvandling. I synnerhet behandlas postkorrektion av analog-digitalomvandlare (A/D-omvandlare). A/D-omvandlare är i praktiken behäftade med vissa fel som i sin tur ger upphov till distorsion i omvandlarens utsignal. Om felen har ett systematiskt samband med utsignalen kan de avhjälpas genom att korrigera utsignalen i efterhand. Detta verk behandlar den form av postkorrektion som implementeras med hjälp av en tabell ur vilken korrektionsvärden hämtas.

Innan en A/D-omvandlare kan korrigeras måste felen i den mätas upp. Detta görs genom att estimera omvandlarens överföringsfunktion. I detta arbete behandlas speciellt problemet att skatta kvantiseringsintervallens mittpunkter. Det antas härvid att en referenssignal finns tillgänglig som grund för skattningen. En skattare som baseras på sorterade data visas vara bättre än den vanligtvis använda skattaren baserad på sampelmedelvärde.

Nästa huvudbidrag visar hur resultatet efter korrigering av en A/D-omvandlare kan predikteras. Omvandlaren antas här ha en viss differentiell olinjäritet och insignalen antas påverkad av ett slumpmässigt brus. Ett postkorrektionssystem, implementerat med begränsad precision, korrigerar utsignalen från A/D-omvandlaren. Ett utryck härleds som beskriver signal-brusförhållandet efter postkorrektion. Förhållandet visar sig bero på den differentiella olinjäritetens varians, det slumpmässiga brusets varians, omvandlarens upplösning samt precisionen med vilken korrektionstermerna beskrivs.

Till sist behandlas indexering av korrektionstabeller. Valet av metod för att indexera en korrektionstabell påverkar såväl tabellens storlek som förmågan att beskriva och korrigera dynamiska fel. I avhandlingen behandlas i synnerhet tillståndsmodellbaserade metoder, det vill säga metoder där tabellindex bildas som en funktion utav flera på varandra följande sampel. Allmänt gäller att ju fler sampel som används för att bilda ett tabellindex, desto större blir tabellen, samtidigt som förmågan att beskriva dynamiska fel ökar. En indexeringsmetod som endast använder en delmängd av bitarna i varje sampel föreslås här. Vidare så påvisas hur valet av indexeringsbitar kan göras optimalt, och experimentella utvärderingar åskådliggör att tabellstorleken kan reduceras avsevärt utan att fördenskull minska prestanda mer än marginellt.

De teorier och resultat som framförs här har utvärderats med experimentella A/D-omvandlardata eller genom datorsimuleringar.

Analog-to-digital conversion and quantization constitute the topic of this thesis. Post-correction of analog-to-digital converters (ADCs) is considered in particular. ADCs usually exhibit non-ideal behavior in practice. These non-idealities spawn distortions in the converters output. Whenever the errors are systematic, it is possible to mitigate them by mapping the output into a corrected value. The work herein is focused on problems associated with post-correction using look-up tables. All results presented are supported by experiments or simulations.

The first problem considered is characterization of the ADC. This is in fact an estimation problem, where the transfer function of the converter should be determined. This thesis deals with estimation of quantization region midpoints, aided by a reference signal. A novel estimator based on order statistics is proposed, and is shown to have superior performance compared with the sample mean traditionally used.

The second major area deals with predicting the performance of an ADC after post-correction. A converter with static differential nonlinearities and random input noise is considered. A post-correction is applied, but with limited (fixed-point) resolution in the corrected values. An expression for the signal-to-noise and distortion ratio after post-correction is provided. It is shown that the performance is dependent on the variance of the differential nonlinearity, the variance of the random noise, the resolution of the converter and the precision of the correction values.

Finally, the problem of addressing, or indexing, the correction look-up table is dealt with. The indexing method determines both the memory requirements of the table and the ability to describe and correct dynamically dependent error effects. The work here is devoted to state-space--type indexing schemes, which determine the index from a number of consecutive samples. There is a tradeoff between table size and dynamics: more samples used for indexing gives a higher dependence on dynamic, but also a larger table. An indexing scheme that uses only a subset of the bits in each sample is proposed. It is shown how the selection of bits can be optimized, and the exemplary results show that a substantial reduction in memory size is possible with only marginal reduction of performance.

Pulse width modulated converters are becoming increasingly popular as their cost decreases and power rating increases. The new trend of smallscale power producers, often using renewable energy sources, has created new demands for delivery of energy to the grid.

A major advantage of the pulse width modulated converter is the ability to control the output voltage at any point in the voltage period. This enables rapid response to load changes and non-linear loads. In addition it can shape the voltage in response to the output current to create an outward appearance of a source impedance. This is called a virtual impedance.

This thesis presents a controller for a voltage controlled three phase pulse width modulated converter. This controller enables operation in standalone mode, in parallel with other converters in a microgrid, and in parallel with a strong main grid.

A time varying virtual impedance is presented which mainly attenuates reactive currents. A method of investigating the overall impedance including the virtual impedance is presented.

New net standards have been introduced, requiring the converter to operate even during severe dips in the grid voltage. Experiments are presented verifying the operation of the controller during voltage dips.