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INTRODUCTION
WAN-0129
Decoupling and Layout Methodology
for Wolfson DACs, ADCs and CODECs
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鈥檚 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.
10碌 H
AVdd
0.1碌 F
DVdd
Figure 1 Converter Digital Supply from Analogue Supply
We name the digital pins like this: DVDD, DCVDD. Be sure to place a 0.1碌F 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 10碌F 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 200m鈩?ESR for best performance.
WOLFSON MICROELECTRONICS plc
January 2003, Rev 1.0
Copyright
餂?003
Wolfson Microelectronics plc
www.wolfsonmicro.com
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