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WAN-0129

Decoupling and Layout Methodology for Wolfson DACs, ADCs and CODECs

Wolfson Microelectronics plc 

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WAN-0129

Decoupling and Layout Methodology

for Wolfson DACs, ADCs and CODECs

INTRODUCTION

APPLICATION

GROUNDING

DIGITAL SUPPLY

This application note looks at the general decoupling and layout practice required for Wolfson

converters to achieve maximum performance in a typical high-speed mixed-signal circuit.

Wolfson DACs, ADCs and CODECs have both digital and analogue interfaces. It is important that

noise is minimised to get the maximum converter performance. In our datasheets there are a

number of supply pins on each chip. They are labelled according to their internal connection, not

necessarily how they should be connected externally. The datasheets show typical supply

connections and decoupling arrangements and this report goes into further details.

First consider the grounding of the IC. Ideally the circuit board will have a single continuous ground

plane, with all ground pins connected to it. The components will be located so that high-speed digital

devices are kept away from analogue devices, so that noise currents do not stray where they are not

wanted.

In some cases a cut between analogue and digital planes is required, because the component

positions cannot be optimised. In this case the analogue and digital grounds must be connected

under the converter with a wide copper track. Just using a zero-ohm link may not provide a good

enough connection in some circumstances. Ensure any tracks going from the board’s analogue

section to the digital section (e.g. data lines and clocks) are tracked over the ground connection, not

the cut, to minimise loop area for the return currents. Wolfson converters are designed with a pin-out

which ensures tracks need only go over the appropriate ground plane. If you have other high-speed

currents (including ESD currents) flowing between analogue and digital grounds, make sure this is

not the only connection between them, or you will make the converter performance worse.

Digital supply pins can be connected to the main digital supply rail if the noise levels are not too high.

If there is no nearby digital supply, or it is too noisy, then one can be made from the analogue supply

using an LC filter like below: this will make sure the noise does not go back to the analogue supply.

AVdd

10µ H

DVdd

0.1µ F

Figure 1 Converter Digital Supply from Analogue Supply

We name the digital pins like this: DVDD, DCVDD. Be sure to place a 0.1µF multi-layer ceramic

capacitor close to each of these pins (within 3mm) to decouple it properly to DGND. If there is no

large-value capacitor within 50mm, then also add one 10µF low-ESR capacitor per rail. This can be

multi-layer ceramic, low-ESR tantalum or low-ESR organic semiconductor electrolytic type. Check

the datasheet shows the part has less than 200mΩ ESR for best performance.

WOLFSON MICROELECTRONICS plc

www.wolfsonmicro.com

January 2003, Rev 1.0

Copyright 2003 Wolfson Microelectronics plc

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