15.3 A 115dB-DR audio DAC with -61dBFS out-of-band noise

Out-of-band noise (OBN) is troublesome in analog circuits that process the output of a noise-shaping audio DAC. It causes slewing in amplifiers and aliasing in sampling circuits like ADCs and class-D amplifiers. Non-linearity in these circuits also causes cross-modulation of the OBN into the audio band. These mechanisms lead to a higher noise level and more distortion in the audio band. OBN also leads to interference in the LF and MF band, compromising e.g. AM radio reception. To sufficiently avoid these problems, it is desired to reduce OBN power to below -60dBFS. An active low-pass filter after the DAC output can reduce the OBN power to acceptable low levels, but this solution is expensive in terms of power consumption and chip area. A FIR-DAC[1] approach implements a one-bit PWM modulator, followed by a semi-digital lowpass FIR reconstruction filter. It achieves high-end audio performance with sufficiently low OBN, but the FIR structure costs area, adds latency, and (like an analog low-pass filter) inherently limits the maximum output signal frequency. Multi-bit noise shapers employ smaller quantization steps and therefore output lower OBN. The cascaded modulator architecture[2,3] can directly be followed by an onchip amplifier without low-pass filtering. However, with only 330 quantization levels, it still cannot achieve the desired -60dBFS OBN without additional filtering. Moreover, this approach requires complex dynamic element matching (DEM) and inter-symbol interference (ISI) shaping mechanisms.