Implementation considerations and limitations for dynamic range enhanced analog to digital converters

The authors review the operation of single and cascade oversampled noise-feedback converters operating with multilevel ADCs (analog/digital converters). Attention is given to techniques for improving the accuracy and the dynamic range of ADCs by operating a reduced resolution quantizer in a feedback loop. These techniques are seen to be special cases of noise-feedback coding in which the loop operates at sample rates far in excess of the signal bandwidth and uses spectral noise shaping to shift in-band quantizing noise power to out-of-band spectral positions. It is noted that implementing these techniques at very high sample rates (10⁸ samples per second) with combinations of analog and digital subsystems offers a number of challenging signal processing problems. It is shown that the mix of analog and digital segments and the ADC-DAC (digital/analog converter) delays have significant deleterious effects on the loop performance