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. Nonlinearity 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 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 levels, but this solution is expensive in terms of power consumption and chip area. A FIR-DAC approach implements a 1b PWM modulator, followed by a semi-digital low-pass 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. A cascaded-modulator architecture can directly be followed by an on-chip 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. The paper present an approach that reduces OBN to below -60dBFS with minimal increase in power and area consumption. It consists of two paths . The main path is based on the work of van Tuijl et al. (2004), containing a 128× oversampled 5b 3rd-order noise shaper, thermometer decoder and real-time DEM algorithm followed by a current DAC. Since the digital noise shaper generates negligible in-band noise products, the error signal of the noise shaper is practically equal to the OBN. This error signal is integrated (as part of the loop filter), quantized and fed to a correction path with a differentiating DAC (DIFF-DAC). This DAC inverts the integration action, obtaining unity signal transfer. The output currents of both paths are subtracted, reducing OBN significantly. Quantization noise of the correction path is shaped because the error signal is differentiated after quantization. Depending on the shape of the noise transfer function of the main DAC, the DIFF-DAC needs an over-range in order to accommodate the increased signal swing caused by the integration action. Still, area and power cost is minimal because the range of the DIFF-DAC is still only a fraction of the main DAC range.