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

Abstract

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.We present an approach that reduces OBN to below -60dBFS with minimal increase in power and area consumption. It consists of two paths ( fig. 1). The main path is based on [4], containing a 128x oversampled 5-bit 3 rd 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 overrange 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.The DIFF-DAC is a semi-digital FIR-DAC with a 1-z A demonstrator chip was designed and manufactured in a 180nm CMOS process. The interpolation filters and the noise shaper were implemented in an FPGA for flexibility. The digital part of the chip contains the 5-bit thermometer decoder, the Real-Time DEM algorithm and the logic circuitry for the DIFF-DAC. To obtain ...

Studies on Selected Topics in Radio Frequency Digital-to-Analog Converters

“…Semi-digital FIR filters or sometimes called analog FIR filters, are used as frequency selective filters as well as converting from digital domain to analog domain since it uses analog multipliers as filter taps. The analog multipliers are implemented by means of current sources, in conventional SDFIR filters, if the output of the Σ∆ modulator quantizer is a single bit [116,122,18,34,87,105,124,46,109]. In fact each tap is one-bit DAC and they are weighted according to the filter coefficients.…”

“…6.2. Quantization error introduced by the limited coefficient precision will alter the FIR filter frequency response [116,18,87,124,46]. Secondly the SDFIR, in contrary to digital FIR, will have mismatch between the multipliers and this inaccuracy in FIR filter coefficients imposes a higher bound on achievable attenuation by the SDFIR filter [94,93].…”

“…5.1, to an interpolating stage, digital Σ∆ modulator and a semi-digital finite-impulse response (FIR) filter. Semi-digital FIR digital-to-analog converters (SDFIR DAC), used as filters and data converters, are implemented as switched-capacitor network or in current-steering architectures, and reported previously in [116,122,18,34,87,105,124,48,47,46]. In this configuration, N -bit baseband data is up-sampled and filtered through the interpolator and then applied to the digital Σ∆ modulator.…”

A Stereo Audio Delta-Sigma DAC with 40-kHz Bandwidth and 103-dB SNR

“…Recently, as the market for artificial intelligent speakers has expanded and become popular, high-quality audio coder-decoder (CODEC) is required. The deltasigma (ΔΣ) digital-to-analog converter (DAC), one of the key components of the CODEC, is essential for generating high-performance audio signals with a signalto-noise ratio (SNR) over 100 dB [1][2][3]. An anti-aliasing filter (AAF) is needed to reduce out-of-band noise (OBN).…”

“…And the ΔΣ modulator suppresses the quantization noise by noiseshaping. Multi-level modulation reduces the quantization noise more than 1-bit modulation [3,6]. But it requires a dynamic element matching (DEM) such as data-weighted averaging (DWA) to suppress the nonlinearities caused by the mismatch among elements [7,8].…”