Theory and Practical Implementation of a 5th-Order Sigma-Delta A/D Converter
One-bit noise-shaping converters with high-order loop filters have long been considered difficult or impossible to stabilize. In this paper we discuss a new approach that simplifies the analysis of high-order loops, and present several topologies that can be used for the implementation of such loops. An 18-bit A/D converter IC has been designed using this approach that uses a 5th-order loop filter with an unusual loop topology. Implementation of the 5th-order loop filter using differential switched-capacitor techniques is described. A one-step decimation filter with 115 dB stopband attenuation is used to remove out-of-band shaped quantization noise. Preliminary measurements indicate a dynamic range of 105 dB, a number which is compatible with the requirements of professional audio.
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