WT32i REFERENCE DESIGN - silabs.com · Bluegiga Technologies Oy Page 6 of 19 ... capacitors with 5% tolerance were chosen for the external audio PA to minimize the pop sound when

WT32i REFERENCE DESIGN

APPLICATION NOTE

Monday, 26 May 2014

Version 1.1

Bluegiga Technologies Oy

Copyright © 2000-2014 Bluegiga Technologies

All rights reserved.

Bluegiga Technologies assumes no responsibility for any errors which may appear in this manual. Furthermore, Bluegiga Technologies reserves the right to alter the hardware, software, and/or specifications detailed here at any time without notice and does not make any commitment to update the information contained here. Bluegiga’s products are not authorized for use as critical components in life support devices or systems.

The WRAP is a registered trademark of Bluegiga Technologies

The Bluetooth trademark is owned by the Bluetooth SIG Inc., USA and is licensed to Bluegiga Technologies. All other trademarks listed herein are owned by their respective owners.


VERSION HISTORY

Version Comment

1.0 First version

1.1 Schematic 4/5 corrected


TABLE OF CONTENTS

1 Introduction ....................................................................................................................................................5

2 Hardware Design of DKWT32i v2 .................................................................................................................6

2.1 Choosing the Capacitors and Resistors Used in the Audio Path .........................................................6

2.2 How to Avoid Pops and Cracks When Powering the Board or Establishing A2DP connection ...........7

2.3 Schematic and Assembly of DKWT32i v2 ............................................................................................9

2.4 Layout of DKWT32i ............................................................................................................................ 15

2.5 RF, EMC and Audio Layout Considerations ...................................................................................... 17

3 Contact Information .................................................................................................................................... 19


Page 5 of 19

1 Introduction

This documents describes detailed the design of WT32i development kit.


Page 6 of 19

2 Hardware Design of DKWT32i v2

2.1 Choosing the Capacitors and Resistors Used in the Audio Path

Metal film resistors have lower noise than carbon resistors which makes them more suitable for high quality audio.

Non-linearity of capacitors within the audio path will have an impact on the audio quality at the frequencies where the impedance of the capacitors become dominant. At higher frequencies the amplitude is not determined by the value of the capacitors, but at the lower frequencies the impact of the capacitors will be seen.

Ceramic capacitors should be X5R or X7R type capacitors with relative high voltage rating. The higher the capacitance value, the lower is the frequency where the non-linearity will start to have an impact. Thus it is not a bad idea to select the capacitors value bigger than necessary from the frequency response point of view.

For optimal audio quality the best selection is to use film capacitors. Film capacitors have excellent linearity and they are non-polarized which makes them perfect choice for using in audio path. The drawback of film capacitors is bigger physical size and higher cost.

Figure 1 shows a modulation distortion measurement when using different type of capacitors in the audio paths. Modulation distortion measures the amount of distortion between two closely located sine waves. The difference between the different capacitors is obvious at low frequencies where the impedance of the capacitor is dominant.

Figure 1: Intermodulation distortion with different type of capacitors


Page 7 of 19

2.2 How to Avoid Pops and Cracks When Powering the Board or Establishing A2DP connection

Figure 2: Extenal audio PA connection

To match the impedances Z1 and Z2 it is important that the capacitors at the input of the amplifier are well matched together. The tolerance of a ceramic chip capacitor is typically in the range of 5% - 20% and generally ceramic capacitors are more accurate than film capacitors. Thus in this case ceramic X7R capacitors with 5% tolerance were chosen for the external audio PA to minimize the pop sound when the A2DP link is established or switched off.


Page 8 of 19

Figure 3: Equivalent circuit for simulating the transient caused by the impedance mismatch


Page 9 of 19

2.3 Schematic and Assembly of DKWT32i v2

Figure 4: DKWT32i v2 Schematic (1/5)


Page 10 of 19

U16 and U17 disconnec t UART converter when the cable is not plugged

2

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10DCD

11CTS

12SHTD

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14EE_DATA

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V B U S



Page 11 of 19



Page 12 of 19



Page 13 of 19

Software I2C interface to configure the I2S codec

15MIC B IA S

10MIC 1L/LIN E 1L

12MIC 2L/LIN E 2L

13MIC 2R/LINE 2R

14MIC 3L/LIN E 3L

16MIC 3R/LINE 3R

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Page 14 of 19

Figure 9: WT32i Development Board Assembly Drawing


Page 15 of 19

2.4 Layout of DKWT32i

Figure 10: Top layer layout of DKWT32i contains signal and audio tracing

Figure 11: 2nd layer layout of DKWT32i contains a solid GND plane


Page 16 of 19

Figure 12: 3rd layer layout of DKWT32i contains supply voltages

Figure 13: Bottom layer layout of DKWT32i contains GND and signal tracing


Page 17 of 19

2.5 RF, EMC and Audio Layout Considerations

GND planes are stitched with vias at the edgesOf the PCB to prevent emissions

GND vias are placed right next to any GND pad to connectGND pads directly to the solid GND plane. This is to makesure that the return current path is low impedance withoutany loops.

Figure 14: GND vias in DKWT32i

Differential audio signals are traced parallelto make sure they have perfect common mode rejection . All the audio traces are routed on asolid GND plane.

Figure 15: Routing of differential audio signals


Page 18 of 19

Metal clearance area under the antenna. Antenna placed at the edge of the PCB.

GND plane is required on both sides of theAntenna. The antenna uses the GND plane as a part of radiator and the lack of GND planewill signifficantly reduce the performance.

Figure 16: Clearance area under the antenna


Page 19 of 19

3 Contact Information

Sales: [email protected]

Technical Support: www.bluegiga.com/support

Orders: [email protected]

WWW: www.bluegiga.com

www.bluegiga.hk

Head Office / Finland:

Phone: +358-9-4355 060

Fax: +358-9-4355 0660

Sinikalliontie 5A

02630 ESPOO

FINLAND

Postal address / Finland:

P.O. BOX 120

02631 ESPOO

FINLAND

Sales Office / USA:

Phone: +1 770 291 2181

Fax: +1 770 291 2183

Bluegiga Technologies, Inc.

3235 Satellite Boulevard, Building 400, Suite 300

Duluth, GA, 30096, USA

Sales Office / Hong-Kong:

Phone: +852 3972 2186

Bluegiga Technologies Ltd.

mailto:[email protected]

http://www.bluegiga.com/support

mailto:[email protected]

http://www.bluegiga.com/

http://www.bluegiga.hk/

Documents

WT32i REFERENCE DESIGN - silabs.com · Bluegiga Technologies Oy Page 6 of 19 ... capacitors with 5% tolerance were chosen for the external audio PA to minimize the pop sound when