WAN-0129 - Wolfson Microelectronics Co., Ltd.

w WAN-0129

Decoupling and Layout Methodology

for Wolfson DACs, ADCs and CODECs

WOLFSON MICROELECTRONICS plc

www.wolfsonmicro.com

January 2003, Rev 1.0

INTRODUCTION

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.

APPLICATION

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.

GROUNDING

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

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.

0.1

AVdd DVdd

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.

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ANALOGUE SUPPLY

Analogue supply pins must be connected to a low-noise analogue supply rail, which ideally comes

from a low-noise linear regulator. We name these pins like this: AVDD, HPVDD, SPKVDD. Be sure

to place a 0.1µF multi-layer ceramic capacitor close to each of these pins (within 3mm) to decouple it

properly to AGND. 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.

On some of our devices there are internal supplies or references which require external decoupling.

We names these pins like: VMID, VREF, VREFOUT, CAP2. The pins connect just to a capacitor,

not to an external supply. Be sure to place a 0.1µF multi-layer ceramic capacitor close to each of

these pins (within 3mm) to decouple it properly to AGND. Also add one 10µF low-ESR capacitor per

pin. 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.

SUPPLY LAYOUT

Ideally the supply connections will be made with planes on a multi-layer circuit board. If these planes

are on adjacent layers, the parasitic capacitance of these layers will further improve high-frequency

decoupling. Where this is not possible, use tracks much wider than the current rating requires.

SOLUTION

Figure 2 Decoupling Example

CONCLUSION

With careful consideration of layout and provision of low-noise supplies, the performance specified in

our datasheets should be achieved.

APPLICATION SUPPORT

If you require more information or require technical support please contact Wolfson Microelectronics

Applications group through the following channels:

Email: apps@wolfsonmicro.com

Telephone: +44 (0)131 272 7070

Fax: +44 (0)131 272 7001

Mail: Applications at the address on last page.

or contact your local Wolfson representative.

Additional information may be made available from time to time on our web site at

http://www.wolfsonmicro.com

DVDD

DGND

AGND

AVDD

Notes: 1. AGND and DGND should be connected as close to the IC as possible.

2. C

, C

and C

should be 0.1µF multi-layer ceramics, positioned as close to the IC as possible.

3. Capacitor types for C1, C5, C9 should be carefully chosen. 10µF capacitors with low ESR are

recommended for optimum performance.

VREFP

VREFN

3 C

4 C

DVDD

AVDD

DGND

AGND

VMI

AGND

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IMPORTANT NOTICE

Wolfson Microelectronics plc (WM) reserve the right to make changes to their products or to discontinue any product or service

without notice, and advise customers to obtain the latest version of relevant information to verify, before placing orders, that

information being relied on is current. All products are sold subject to the WM terms and conditions of sale supplied at the time of

order acknowledgement, including those pertaining to warranty, patent infringement, and limitation of liability.

WM warrants performance of its products to the specifications applicable at the time of sale in accordance with WM’s standard

warranty. Testing and other quality control techniques are utilised to the extent WM deems necessary to support this warranty.

Specific testing of all parameters of each device is not necessarily performed, except those mandated by government

requirements.

In order to minimise risks associated with customer applications, adequate design and operating safeguards must be used by the

customer to minimise inherent or procedural hazards. Wolfson products are not authorised for use as critical components in life

support devices or systems without the express written approval of an officer of the company. Life support devices or systems are

devices or systems that are intended for surgical implant into the body, or support or sustain life, and whose failure to perform when

properly used in accordance with instructions for use provided, can be reasonably expected to result in a significant injury to the

user. A critical component is any component of a life support device or system whose failure to perform can be reasonably

expected to cause the failure of the life support device or system, or to affect its safety or effectiveness.

WM assumes no liability for applications assistance or customer product design. WM does not warrant or represent that any

license, either express or implied, is granted under any patent right, copyright, mask work right, or other intellectual property right of

WM covering or relating to any combination, machine, or process in which such products or services might be or are used. WM’s

publication of information regarding any third party’s products or services does not constitute WM’s approval, license, warranty or

endorsement thereof.

Reproduction of information from the WM web site or datasheets is permissible only if reproduction is without alteration and is

accompanied by all associated warranties, conditions, limitations and notices. Representation or reproduction of this information

with alteration voids all warranties provided for an associated WM product or service, is an unfair and deceptive business practice,

and WM is not responsible nor liable for any such use.

Resale of WM’s products or services with statements different from or beyond the parameters stated by WM for that product or

service voids all express and any implied warranties for the associated WM product or service, is an unfair and deceptive business

practice, and WM is not responsible nor liable for any such use.

ADDRESS:

Wolfson Microelectronics plc

Westfield House

26 Westfield Road

Edinburgh

EH11 2QW

United Kingdom

Tel :: +44 (0)131 272 7000

Fax :: +44 (0)131 272 7001

Email :: apps@wolfsonmicro.com