This paper introduces the architecture of Wolfson Microelectronics' latest generation of audio digital-to-analog converters (DACs), focusing on the design of new device families for high voltage line driver outputs in consumer electronics applications.
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Fundamental
Incremental accumulation modulators are often described in complex terms, using mathematical formulas, state tables, and theoretical models. While all of this is necessary to understand the complexity of incremental accumulation modulation, the key to the purpose of this paper is to understand the benefits of SDM architecture and their application in audio converter ICs.
The two basic principles of incremental accumulation modulation are:
â— Oversampling
The sampling process produces a quantization error; the difference between the sample 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 analog to 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 known as the Nyquist frequency. For audio, the typical bandwidth is between 20 Hz and 20 kHz, and the sampling frequency tends to be 44.1 kHz (for CD audio) to 192 kHz (DVD audio).
The sampling frequency is less than twice the Nyquist frequency, which causes aliasing and the input signal is folded back into the audio band with a mirror near the Nyquist frequency.
In SDM converters, the data converter operates at frequencies much higher than twice the Nyquist frequency, typically 128 to 768 times the lowest sampling frequency.
The oversampling process spreads the quantization noise over a wider bandwidth than other data conversion methods, so there is very little quantization noise in the audio 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, and 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 the SNR.
The requirement for 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
The SDM-based architecture is complex, and designers have many options to optimize their designs for specific applications. The key trade-offs are the order, resolution, and architectural topology.
Incrementally accumulate the order of the modulator:
The first and second order SDMs themselves are very stable, producing large in-band noise, but with very low out-of-band noise. Higher-order SDMs are conditionally stable and produce more out-of-band noise and are therefore sensitive to clock jitter.
Wolfson Microelectronics' recent DAC architecture is based on a second-order incremental accumulation modem that drives clock speeds high 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 maximum SNR of the 24-bit audio DAC theory is close to 144 dB.
Wolfson's DAC design is based on a 5- or 6-bit converter that combines SDM architecture to provide up to 24-bit resolution.
Different noise sources, including analog and digital noise, cannot achieve a theoretical maximum of -144dB. However, because of the improved design methodology, Wolfson's high-performance DACs strive to approach theoretical maximums.
Performance, stability, size and cost are directly affected by the design issues above.
â— DAC architecture
A typical incremental accumulation DAC can be thought of as having the following elements: Insert Filter—Increases the effective bit rate, allowing the DAC to oversample the input signal.
Wolfson uses a three-stage integrated comb filter (CIC) to attenuate the image from 8fs to 128fs. This method attenuates the frequency components of the input sample rate several times, improving the DAC's resistance to clock jitter.
Incremental Accumulator Modulators - with the advantages of oversampling and noise shaping, which is critical for the high performance audio data conversion described above.
Digital to Analog Converter—Converts the SDM output to an analog output. The switched capacitor method is used to accurately control the output voltage, and the introduced noise is filtered by a noise shaper to further improve the immunity to clock jitter.
The patented method used by Wolfson includes a unique Dynamic Cell Matching (DEM) solution that minimizes capacitance mismatch errors and greatly improves DAC linearity compared to alternatives.
Low Pass Filter—Remove any remaining high frequency components for the most accurate reproduction of the audio signal.
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 typically outputs a full-scale signal that is between 1.0 Vrms and 1.1Vrms at 5V supply and 0.66Vrms to 0.72Vrms at 3.3V supply. In mainstream applications, the output of the DAC is fed into the active circuit for two purposes:
Low pass filter - it removes high frequency noise inherent in the conversion process.
Amplifier—The output level is typically increased to 2Vrms, which requires a high-voltage supply rail (typically between 9V and 12V) to power active devices in external circuitry. It is implemented for several reasons, including meeting industry standards, providing resistance to noise, and meeting the de facto standards for interfacing with audio equipment. Why is 2Vrms?
Various formal and de facto industry standards have been developed that require line levels of 2Vrms between consumer audio equipment (such as DVD recorders) and televisions. However, there is a common misconception that the signal level must be 2Vrms plus or minus a specified tolerance. In fact, in most standard industry tests, audio equipment, such as DVD recorders, must be able to accept signal levels up to 2Vrms plus a tolerance. A device such as a DVD player must have an output level of no more than 2Vrms plus tolerance. There is no minimum specification for signal levels, although to meet certain industry standards, the output signal level must be around 1Vrms.
In summary, the audio transmitter must output a signal level between 1Vrms and 2Vrms, and the audio receiver must be able to receive input signals up to 2Vrms. Because most consumer audio ICs have a 5V or 3.3V supply and it is not possible to generate 2Vrms from a 5V supply, designers must use external active devices to generate the required output levels from the DAC circuit.
WM8501 and WM8522
In the design, Wolfson Microelectronics considered these line level requirements and designed a new series of audio devices. The output analog audio signals of these devices are 1.7Vrms line level and the power supply is 5V. This meets industry standards and eliminates the need for high voltage rails that boost the DAC output. This high voltage rail is still needed in some applications, such as SCART signal processing. But this is not required in all applications, eliminating 12V power rails and associated power traces, active components of the DAC output filter, and associated PCB space, enabling significant cost savings and reduced circuit area.
The same high quality audio output can be obtained with a passive filter at the DAC output. The complexity and cost are greatly reduced compared to active filters that are required to provide the necessary gain to amplify 1.0Vrms to 2.0Vrms.
The WM8501 is a stereo audio DAC with a line driver output of 1.7Vrms. The WM8522 is a 6-channel version of the same product designed for multi-channel audio systems. Both chips produce audio signals with dynamic ranges greater than 100 dB, which greatly exceeds any industry standard requirements and therefore meet the de facto standards set by consumer audio device critics.
Conclusion
The facts show that the design of incremental accumulators is constantly evolving to meet market demand for performance and cost. In addition to improving the core architecture of their ICs, Wolfson Microelectronics continues to discover ways to provide customers with new ways to maximize performance, cost savings and simplify end product design.
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