USE OF FEEDFORWARD AND FEEDBACK IN OVERSAMPLED ANALOG TO DIGITAL CONVERTERS TO BYPASS PERFORKANCE CONSTRAINTS RELATED TO ANALOG CIRCUIT COMPONENTS (original) (raw)

Abstract

system performance of an oversampled Analog to Digital Converter (ADC) with feedback noise shaping is limited by the precision of the Digital to Analog Converter (DAC) in the feedback path as well as by the number of integrators in the loop(s). The integrators in the feedback loop of the quantizer are the source of the transmission zeros which suppress the in-band quantizing noise. The design bandwidth, defined as the fraction of the sample rate for which the noise level is sufficiently suppressed, increases with the order of the loop. Standard designs avoid the DAC precision problem by restricting the ADC and DAC to a single bit while stability and matching considerations limit systems to three loops. This in turn defines the oversample ratio for a given effective tandwidth and noise performance. We present simple modifications to the oversampled ADC which avoids these limitations via local feedback in the feedback path and local feedforward in the feedforward path of the original delta sigma loop structure. These modifications result in wider conversion bandwidth and greater dynamic range than is available from standard configurations.

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