Analysis, Estimation and Compensation of Mismatch Effects in A/D Converters

University dissertation from Linköping : Linköping University

Abstract: The trend in modern communication systems is to replace as much analog circuits as possible with digital ones, to decrease size, energy consumption and cost. An analog to digital converter (ADC) is the interface between the analog and digital parts. Replacing analog parts, such as mixers, with digital ones requires higher sampling rates. The bottleneck in a digital communication system is often the ADC. Requirements on low power consumption, small chip area and high sample rates are often contradictory to requirements on high accuracy in the manufacturing process.The traditional way to improve the accuracy is to calibrate the ADC before use. However, calibration is time consuming and costly. Furthermore, the errors usually change during the lifetime of the ADC due to, for instance, temperature variation and aging. This means that the ADC must be recalibrated at regular intervals.In this thesis, we investigate how the errors in an ADC can be estimated and compensated for while the ADC is used. The estimation must then be done without any special calibration signal. Two different types of errors are discussed in this thesis. The first type of error is static errors in the reference levels, caused by resistor mismatch. Two methods are proposed for estimation and correction of these errors. The most general method requires only that the amplitude distribution is smooth, while the other one requires knowledge of the amplitude distribution of the input signal butgives a little better performance.The second type of error occurs in time interleaved ADCs, where several ADCs are used in parallel. Due to component mismatch, three different static errors appear: Time errors (static jitter), amplitude offset errors and gain errors. A method for estimation and compensation of these errors is proposed. The method requires basically only that the input signal is band limited to the Nyquist frequency for the system.Another way to decrease the impact of the mismatch errors in a time interleaved ADC is to randomize the selection of which order the ADCs should be used. This randomization spreads the distortion to a more noise like shape. How the mismatcherrors aect the spectrum of a randomly interleaved ADC is also analyzed in this thesis.To confirm that the analysis and estimation methods work in practise the methods are evaluated on both simulated data and data from real ADCs.

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