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