Here are some pitfalls to avoid when evaluating power consumption in audio DACs or codecs
BY ERIC HABER
Wolfson Microelectronics
Edinburgh, U.K.
http://www.wolfsonmicro.com
Consumers are looking for more and better functionality on their portable multimedia players. System designers have responded by creating increasingly complex systems, yet they must continue to balance cool features with small size and low power consumption. The selection of the audio DAC not only has a big impact on the audio performance of the PMP (portable media player) but also in extending battery life of the player.
PMP designers may spend hours scrutinizing the claims made by different silicon vendors in order to select an audio DAC. Their task is complicated because vendors spec parts differently. Indeed, audio input and output subsystems are especially tricky, since they include both analog and digital circuitry, typically with several separate supply voltages.
Reading into datasheets
A closer inspection of the circuitry in audio subsystems helps in understanding the true meaning of power dissipation figures in manufacturers’ datasheets. Figure 1 illustrates the main functional blocks used in creating audio output for a portable system. The audio DAC typically includes digital signal enhancement, digital-to-analog conversion, and analog mixing and amplification.
Where datasheets specify DAC power consumption, or DAC supply current, it’s important to be sure whether this figure includes the power requirements of amplifiers and other subcircuits associated with the DAC. If not, these need to be accounted for separately.
Fig. 1. The audio DAC typically includes digital signal enhancement, D/A conversion and analog mixing and amplification.
Likewise, quoted power consumption on datasheets for playback to headphones may not include on-chip enhancements such as limiting, 3D signal enhancement and equalization, making power consumption numbers look much lower than they would be under real-world operation. Some manufacturers exclude the digital audio interface when specifying playback power consumption, which bears no resemblance to reality, as the interface must be powered up to receive audio data for playback.
The real world
Other features of datasheets often do not correspond to a real-world scenario. For instance, the power dissipated inside loudspeakers and headphones during playback accounts for a large percentage of overall power consumption, but these figures are not usually included in datasheets.
More commonly, power consumption is specified in the quiescent (that is, playing absolute silence) state, which is represented in the digital domain as a long series of zeros. In this state, voltage across the load is zero and no current flows through it. While in a quiescent state, the audio IC itself consumes less power. On some datasheets, power consumption is measured without connecting a load.
To get meaningful data, a load must be connected to the system. Typically in consumer electronics, the load’s impedance is 8 Ω for small loudspeakers and 16 or 32 Ω for headphones. A realistic test signal must also be driven through all the relevant parts of the circuit and into the load.
Of course, the question of what constitutes a realistic test signal immediately arises. A 1-kHz sine wave is easy to generate, which means that it is often used as a test signal. But this kind of signal does not reflect the mix of frequencies or the variations of amplitude over time that usually characterize music or speech.
Perhaps the most useful kind of signal is provided by the IEC 60268-5 (formerly IEC 268-5) standard for loudspeakers. This standard uses so-called “pink” noise, which is a weighted mix of frequencies running right across the entire audio band. The “crest factor” — the difference between peak and long-term rms amplitude — is well defined in “pink” noise, reflecting how real-world signals vary between louder and quieter passages.
Fig. 2. The amplifier’s Class G mode saves around two mW compared to a traditional Class AB circuit at low amplitudes, whereas louder signals offer no savings.
Since the efficiency of any given amplifier varies with signal amplitude, it’s worth considering the power consumption of amplifiers across the signal’s entire dynamic range, as Fig. 2 shows. For instance, Class G amplifiers use different supply voltages depending on the signal amplitude, and usually have a discontinuity around the switch-over points.
In Fig. 2, the amplifier’s Class G mode saves around 2 mW compared to a traditional Class AB circuit at low amplitudes, whereas louder signals offer no savings. But a new Wolfson development used in the WM8903 ultra-low-power audio codec, dubbed Class W, enables further savings compared to Class G, and again the savings vary with amplitude, peaking at a signal level of 0.3 mW into 30 Ω. Very similar considerations apply for loudspeaker amplifiers, where Class D technology is now considered the industry standard.
Quiescent states
Apart from amplifiers, there are other circuits in which power consumption is lower in the quiescent state than in real life. This situation also applies to analog circuits such as mixers and programmable-gain amplifiers as well as digital CMOS circuitry.
In CMOS logic, power consumption is largely a function of how frequently bits toggle between the 0 and 1 states, so that a signal consisting only of zeros (that is, quiescent) leads to an unrealistically low supply current. To deliver meaningful power data, all components should be processing a real, nonzero signal.
Master vs. slave modes
Audio ICs such as DACs or ADCs can be configured as either master or slave devices. This is important, because in “master” mode, the audio IC drives the digital audio interface and therefore requires more current than in slave mode.
Of course, this shouldn’t be taken to imply that slave mode is always preferable. After all, if the audio IC isn’t driving the interface, then the component on the other side has to do it, so that the power requirement is simply shifted around the system, rather than being eliminated.
A further tip: Look for the load capacitance specification, as that determines how much extra current will be required. Vendors may use unrealistically low load capacitances to bring the headline power consumption figures down.
Some audio components have special clocking modes that eliminate the need for a power-hungry low-jitter phase-locked loop (PLL). Many audio DACs and codecs, for instance, have a “USB mode” in which audio clocks are generated directly from a 12-MHz USB clock. In this case, the power saved by integrating the clocking normally far exceeds the power consumed in the audio interface.
Low-voltage power supply
Apart from the most basic ICs, all of these circuits need more than one supply rail. Typically, there will be at least one analog supply, a digital I/O supply for the audio and control interfaces and a separate digital core supply. The overall power consumption for any IC is calculated by adding the power needed in each supply rail — which means the most obvious way to save power is to use the lowest possible voltage for each supply.
In the case of digital I/O voltage, this may be given by the other system components with which the audio IC needs to interface. It’s possible to reduce the digital core voltage right down to its lower limit, which can normally be found under “Recommended Operating Conditions” on datasheets.
Ideally, datasheets would provide graphs of each supply current versus voltage in every possible scenario. Where such data are missing or incomplete, it’s possible to make some educated guesses.
For instance, current scales proportionally to voltage in CMOS logic. This means that a voltage reduction is doubly beneficial — with a 50% reduction in supply voltage resulting in a reduction of 75% in the power used on that rail.
Analog circuits are somewhat more complex, since they often contain constant-current sources. Typically, though, after halving an analog supply voltage, the power consumed by that part of the IC (excluding any load) is somewhere between half and a quarter of its original value. ■
For more on audio DACs, visit http://www2.electronicproducts.com/AnalogMixICs.aspx.
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