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Chapter 3:
DACs Used in ADC Architectures and Read-in ICs
Digital-to-analog converters make up a significant portion of the components within many ADC algorithms and circuits. DACs are often used as feedback for subtracting an ideal analog signal from the real analog signal entering the device. The quantization error difference between the input analog signal and a subtracted ideal analog signal in this type of feedback is called a residue. ADC architectures utilizing some form of DAC feedback include pipeline, sub-ranging, two-step, successive approximation, and delta-sigma modulation. DACs are also widely used to perform calibration tasks and provide supporting static or dynamic bias functions for almost all medium- and highresolution architectures. To fully understand a broad range of ADC architectures, you must first understand some basic DAC architectures. This chapter will focus on this basic understanding in preparation for the subsequent chapters on ADCs and will ignore more advanced DAC architectures and algorithms. If you already have a basic understanding of DAC architectures, you may wish to move directly to Chapter 4. An ideal DAC will accept a digital word and then translate that digital word into the analog domain using the DAC references to set the analog range and the ideal quantization to set the point in the range where the output settles. Throughout the following two chapters, we will call the highest and lowest references VRefHigh and VRefLow, respectively. The ideal DAC output will range between these two references.
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