As audio quality becomes increasingly important in electronic systems, many more designers must become skilled in the "art" of analog and mixed-signal design. Some designers find this trend intimidating. In truth, though, an amplifier is still just an amplifier. For the amplifiers used in audio, a little more care is taken to ensure the device's performance over the audio band. Keep in mind that a digital device will largely perform to specification—regardless of the devices around it. In an audio-converter chip, however, the system designer must carefully consider the environment that immediately surrounds the chip.

To begin, a designer must clearly identify the audio requirements of the design: Is it an integral feature or a "nice-to-have" add-on? If it's an add-on, absolute performance might not be so critical. Be realistic about the design's performance requirements. It may be nice "specmanship" to have a digital-to-analog converter (DAC) with a -120-dB noise floor in a PDA. But will the rest of the system ever be able to reflect that performance level? Is the battery likely to keep up? Although this example is obviously a contrived one, improperly specifying audio requirements may lead to lower system performance in more areas than just audio.

Many choices must be made when choosing an audio device. Although some choices will be dictated by the end application, others will not. Just a few of this myriad of options are the number of channels, audio sample rate, data-interface type, control-interface type, power supply, package type, and performance.

In general, a PDA will require a DAC to either generate sounds or for MP3 playback. It also may offer voice-recording capabilities, which mandate the choice of a codec (record and playback). The selection of a codec depends on making an analog-to-digital converter (ADC) capable of hi-fi audio performance (at least 44.1 kHz and 16 b). This quality is much higher than the level that's required for voice recording (8/16 b and 8/16 kHz). As a result, the designer must ensure that the chosen codec will support sample and bit rates that are low enough to record voice without creating huge files.

After choosing an audio device that meets his or her functional requirements, the designer must get this device to perform to specification in the system. Generally, any queries that relate to devices not meeting their specified performance can be traced to four sources: failure to read the documentation, clocking problems, or issues with the printed-circuit-board (PCB) layout, external components, or signal timing.

Undoubtedly, the biggest of these problems is the datasheet. Perhaps as many as 50% of the application inquiries that we receive are answered by our own documentation. The consequences of not understanding a device may not be immediately apparent. One day, however, chances are that a designer will encounter a problem that could have been easily avoided. Problems at this stage tend to relate to having incorrect register writes or signal-timing and clocking-related issues. Any one of these problems is enough to stop a system from running or greatly degrade its performance compared to what was expected.

In addition, the designer must ensure that the signal levels being passed into the audio chip are correct. Say the audio chip has a maximum input swing of 1 V p-p, but the digital signal processor (DSP) outputs a 2-V p-p signal. A divider network is needed to pad down the incoming signal, thereby ensuring that the converter chip will not clip its outputs.

Obviously, the single biggest factor that will affect any system's audio quality is noise. Noise can be introduced to the system from many places, such as poorly designed power supplies, badly routed signals, ill-chosen external components, bad clocks, or poor signal timing. All of these elements can lead to drastic reductions in expected performance.

Problems also arise from the pressure to design all products to a cost. Designers must recognize the point at which cost cutting makes it impossible to maintain performance. With a careful printed-circuit-board layout and sensible component choices, incorporating an audio device shouldn't be too traumatic.

Good PCB design and layout is one of the simplest ways to help reduce system noise ingress. Digital-to-analog converters, analog-to-digital converters, and codecs all have both digital and analog interfaces. Noise must be minimized in order to get the maximum converter performance. To minimize interference between digital and analog signals, try to view the PCB in analog and digital blocks. Keep these blocks as far apart physically as is practical.

Analog and digital tracks should never be run side by side. If digital signal tracks must be run through analog parts of the board, spacing should be as generous as possible to help avoid interference. Keep the analog output tracks as short as possible and far enough apart to allow some grounding between them (unless they are differential audio tracks).

When using differential audio in a design, keep both the positive and negative track lengths as close to the same as possible. Power and analog tracks should always be made larger than the other tracks on the board. Digital tracks should never be any smaller than 8thou on the PCB with 10thou being recommended. Analog and power tracks should be 10 to 50thou. For analog and power, the recommendations are 20thou and 30thou, respectively. Where vias are used in tracks, it's a good idea to keep the track width the same as the via's diameter.

Now, consider the grounding of the integrated circuit (IC). Ideally, the circuit board will have a single continuous groundplane with all of the ground pins connected to it. The components will be located in such a way that high-speed digital devices are kept away from analog devices. They also will be placed so that noise currents don't stray where they aren't wanted.