Comparative study of sigma delta and nonuniform sampling A/D converters

This paper compares the performance of the sigma delta analog-to-digital converter (ΣΔADC) and the nonuniform sampling analog-to-digital converter (NUSADC) in terms of their achieved signal-to-quantization noise ratio (SQNR) and their computational complexity. Relaxation of the performance requirements of an anti-aliasing filter preceding an ADC can be achieved with oversampling and/or nonuniform sampling of the input signal. Oversampling is performed in the ΣΔADC, and nonuniform sampling is implemented in the NUSADC. For the NUSADC, 4-bit and 7-bit voltage crossing levels are used and for the ΣΔADC, first- and second-order modulators with a single-bit quantizer are used. The NUSADC uses an additive “dither signal” to force threshold crossing events and achieve a predictable average sampling rate. The average rate of nonuniform samples is calculated for both 4-bit and 7-bit NUSADC. The equivalent sampling frequency is used for the ΣΔADC, allowing us to compare two architectures using equivalent sampling frequencies. We found that the first-order ΣΔADC has inferior performance as compared to NUSADC, while the second-order ΣΔADC can achieve equivalent SQNR values. The results also show that the second-order ΣΔADC achieved an SQNR nearly equal to that of the NUSADC, with less computational cost.