Audio converter subsystem design in the 21st century – Part 1: Overview

The habitat of audio converters is ever-changing. Early converters were built into stand-alone digital audio equipment. Later, they were built as stand-alone conversion devices with dedicated digital audio connectivity. Nowadays they are increasingly offered as peripherals for PCs or computer networks. All the while performance targets have been rising.

In some ways these changes have impacted audio converter design; in other ways it's business as usual. Herein are the random musings of an old warhorse, struggling to focus on a few key strategies in the eternal battle for optimum audio converter performance.

INTRODUCTION
It seems to me that the plan of campaign for most engineers setting out to design an audio converter subsystem (by which I mean a stand-alone converter or that part of an equipment which deals with audio conversion) is as follows: First, choose a data converter device (by which I mean an ADC, DAC or CODEC chip) which will meet the project requirements - performance, channel count, cost, features etc. Then implement a design around it based on the manufacturer's application note plus whatever additional features, interfaces etc. are required.

Yet the majority of converter subsystems perform somewhat below the potential of their chosen data converter – either all the time, or in certain situations. The random topics which follow are the result of my personal prejudices about why that usually is.

If you're wondering whether to read on, I can summarise the battle plan under these headings:

• Sort out your clocking;
• Learn to tame the switch-mode power supply;
• Strive for a top quality analogue signal path;
• Don't overlook the voltage reference;
• Make sure the digital parts don't ambush you.

But first, I should explain.

WHY DATA CONVERTERS ARE SORTED
When digital audio was new, the data converter itself was almost the only consideration – since it was impossible to build one with as much dynamic range and linearity as the professional or high-end consumer user was used to in analogue equipment. All we used to ask was 'what chip is in it?'

Nowadays, where workmanlike data converters can exceed a dynamic range of 130dB and THD+N of 110dB (audio band, rms, unweighted), the weak link in a conversion system is most often elsewhere. Surely there is no case for applying design effort/budget to the data converter itself, except in the most exacting of applications, and where it has already been applied in great measure to the rest of the converter subsystem?

OK, maybe the decimation and interpolation filters aren't sorted. Maybe there isn't enough silicon on the planet, I don't know, so I'm moving on. From here on in it's 'How to get the best out of your chosen data converter.' The mantra is 'Painstaking design and relentless assessment.'

DESIGN

Typical ADC and DAC subsystems
For clarification, typical ADC and DAC subsystems are shown in Figures 1 and 2. I've marked the parts which you have to take special care of (the analogue bits) and also the parts which will try to make that difficult (the rest). For spendthrifts, I've indicated a cut line where you might consider isolating the analogue and digital parts, but there are lots of ways you could do it, or not at all.

So, with the problem defined, here, in no particular order, here are my 'converter design' musings…