This article describes the architecture of Wolfson Microelectronics' latest generation audio digital-to-analog converter (DAC), focusing on designing new device families that provide high-voltage line driver output in consumer electronics applications.
Fundamental
Incremental accumulation modulators are often described in complex terms, using mathematical formulas, state tables, and theoretical models. Although all of this is necessary to understand the complexity of incremental accumulation modulation, the key to the purpose of this article is to understand the benefits of the SDM architecture and their application in audio converter ICs.
The two basic principles of incremental accumulation modulation are:
â— Oversampling
The sampling process produces quantization errors; the difference between the sampling level at the output and the desired output level. The energy of the quantization noise depends on the resolution of the audio converter and is spread over the bandwidth of the sampling frequency.
The Nyquist sampling principle shows that in order to accurately convert a signal from the analog to the digital domain, the signal must be sampled at twice the frequency of the highest frequency component of the signal. The highest frequency component is also called the Nyquist frequency. For audio, the typical bandwidth is between 20Hz and 20KHz, and the sampling frequency tends to be 44.1KHz (for CD audio) to 192kHz (DVD audio).
A sampling frequency lower than twice the Nyquist frequency will cause aliasing, and the input signal will fold back to the audio frequency band with an image around the Nyquist frequency.
In SDM converters, data converters operate at frequencies much higher than twice the Nyquist frequency, usually 128 to 768 times the lowest sampling frequency.
The oversampling process diffuses the quantization noise over a wider bandwidth than other data conversion methods, so there is very little quantization noise in the audio frequency band.
â— Noise shaping
In addition to spreading quantization noise over a wide spectrum, SDM is also used as a low-pass filter to filter the input signal. A high-pass filter filters the quantization noise, pushing the quantization noise out of the audio band. For ADCs, this allows the converter to use fewer bits without reducing SNR.
The requirement of oversampling means that the incremental accumulation modulator design is best suited for low bandwidth applications, such as audio data conversion, such as audio data conversion.
Design considerations
SDM-based architectures are complex, and designers have many options to optimize their designs for specific applications. The key tradeoffs are order, resolution, and architectural topology.
The order of the incremental accumulation modulator:
The first-order and second-order SDMs are inherently stable and generate large in-band noise, but have very low out-of-band noise. Higher-order SDMs can be conditionally stable and generate greater out-of-band noise, so they are sensitive to clock jitter.
The latest DAC architecture of Wolfson Microelectronics is based on a second-order incremental accumulation modem, which drives the clock at a high speed to reduce in-band noise and is therefore insensitive to clock jitter.
â— DAC resolution
The increase in DAC resolution reduces the quantization error, thus improving the theoretical signal-to-noise ratio (SNR) of the DAC.
For each bit resolution, the theoretical maximum SNR is approximately 6xn, where n is the number of bits. Therefore, the theoretical maximum SNR of a 24-bit audio DAC is close to 144dB.
Wolfson ’s DAC design is based on a 5-bit or 6-bit converter, combined with the SDM architecture to provide a resolution of up to 24 bits.
For different noise sources, including analog and digital noise, the SNR cannot reach the theoretical maximum of -144dB. However, because of improved design methods, Wolfson's high-performance DAC of each generation strives to approach the theoretical maximum.
Performance, stability, size and cost are directly affected by the above design issues.
â— DAC architecture
It can be considered that a typical incremental accumulation DAC contains the following elements: insertion filter—increasing the effective bit rate, allowing the DAC to oversample the input signal.
Wolfson uses a three-stage cascade integral comb filter (CIC) to attenuate the image from 8fs to 128fs. This method greatly attenuates frequency components that are several times the input sampling rate, improving the DAC's tolerance to clock jitter.
Incremental accumulation modulator-has the advantages of oversampling and noise shaping, which is critical for the high-performance audio data conversion described above.
Digital-to-analog converter—Converts SDM output to analog output. The switched capacitor method is used to precisely control the output voltage, and the noise introduced by the noise shaper is filtered to further improve immunity to clock jitter.
The patented method adopted by Wolfson includes a unique dynamic cell matching (DEM) scheme, which can minimize the capacitance mismatch error, compared with other optional schemes, greatly improving the DAC linearity.
Low-pass filter—removes any remaining high-frequency components to achieve the most accurate reproduction of audio signals.
In fact, these four units are not completely isolated modules, and some functions are handled between these modules.
â— Output level requirements
The audio DAC usually outputs a full-scale signal. Under the condition of 5V power supply, the level is between 1.0 Vrms and 1.1 Vrms. When the power supply voltage is 3.3 V, it is 0.66 Vrms and 0.72 Vrms. In mainstream applications, the output of the DAC is fed into an active circuit, which has two purposes:
Low-pass filter-it can remove the high frequency noise inherent in the conversion process.
Amplifier-The output level is usually increased to 2Vrms, which requires a high-voltage power rail (usually between 9V ~ 12V) to power the active devices of external circuits. There are several reasons for its implementation, including meeting industry standards, providing noise tolerance, and meeting de facto standards for interfacing with audio equipment.
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