The introduction above provides a nearlycomplete character outline for the circuitunder consideration. In brief, it is a 16-bitstereo version of a digital to analogue con-verter with USB interface. The converter maybe used for a range of audio applications. Butthe first thing that comes to mind is an exter-nal sound card for PCs and laptops without

such a card, or as an expansion cardin the event a second one is desired.

The circuit is conspicuously com-pact and simple by design, becauseit consists mainly of an integratedD/A-converter supplemented by twoopamps. This simplicity however,does not imply that the quality of

this ‘USB audio-DAC’ has been com-promised. On the contrary, we wouldlike to say. A short profile:

The DAC possesses an integratedUSB interface that complies withversion 1.0 of the standard. Itaccepts 16-bit stereo and monauralUSB data streams and is equippedwith an 8× oversampling digital fil-ter. The circuit contains an‘Enhanced multi-level delta-sigmamodulator’ and is compatible withsampling rates of 32, 44.1 and48 kHz. In addition, it has a digitalattenuator, a soft-mute function aswell as optical Suspend- and Play-back-indicators. And finally, there isno need for additional drivers whenused with Windows 98 (and later).

The description above sounds allvery nice, but it is the end result thatthis all leads to, that is more impor-tant. Well then, in a separate table,all the measuring results are sum-marised. Examining the results withmore than a cursory glance, you willhave to reach the conclusion that theperformance is very reasonable.

PCM2702

The heart of the circuit consists ofa PCM2702, a two-channel single-chip D/A-converter with integratedUSB interface controller. Figure 1shows the block diagram detailingthe internals of the IC. For moreinformation you may refer to the(condensed) datasheet printed else-

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44 Elektor Electronics 12/2000

USB Audio-DACA mini PC Sound CardDesign by T. Giesberts

This circuit is pre-eminently suitable for PCs and laptops that are notequipped with a built-in sound card but do have a USB interface.However, the ‘mini PC sound card’ can also be useful for otherapplications and various experiments.

nals, and the, so-called Feature Unit.The input terminal is defined as ‘USB

stream’. This input accepts two channelaudio data streams. The output terminal isdefined as a ‘speaker’. The Feature Unit sup-ports volume and mute control.

The built-in digital volume control can beadjusted from 0.0 dB to –64.0 dB in steps of1 dB. Each channel can be set independently,but master control of both channels simulta-neously is also possible. Mute is only avail-able in master control.

Interface #1 has three alternative settings.Setting #0 is the ‘Zero Bandwidth’ setting (akind of mute function). Setting #1 selects16-bit stereo and #2 is the 16-bit monauralsetting.

The PCM2702 requires a 12 MHz clock forUSB and audio functions. The on-chip oscillator with an external crys-tal (which is how it is used here) may gener-ate this clock. Or, if desired, an external clocksignal may be applied to pin XTI instead.

The IC includes an internal power-on resetcircuit, which automatically initialises thedigital logic when the power supply isswitched on. The PCM2702 is ready toreceive audio data after completion of thereset sequence and a connection to the USBbus.

The Circuit

The circuit (Figure 2) has largely been copiedfrom the evaluation board DEM-PCM2702from Burr-Brown. As can be observed,besides the PCM2702, the active componentsare limited to a dual opamp, a couple of gen-eral-purpose transistors and a voltage regu-lator. A handful of standard parts completesthe circuit.

We refrained from fitting a power supplyconnector and mini-jack on the PCB. As a con-sequence, either a mains power adapter or asmall PCB transformer (voltage >8 V, power>0.5 VA) may be used. A mains poweradapter with regulated output is preferred,because the ripple suppression ratio of the 5 Vregulator 78L05 (IC3) on the PCB is only 50 dB.

To connect active PC speakers, for exam-ple, a mini-jack or cinch connectors may beused. Cinch connectors permit the connectionof a normal power amplifier without the needfor a special adapter cable.

Because of the relatively large supply cur-rent (approximately 60 mA) it was not con-sidered appropriate to power the circuit fromthe USB bus (since the maximum current per-mitted is only 100 mA, a second device wouldincrease the current consumption to unac-ceptable levels). The PCM2702 has goodpower supply rejection which makes addi-

where in this issue.Control and audio data are both

transferred to the PCM2702 via pinsD+ and D–. All data to and from theIC are transferred at full speed. VBUS(pin 8) and DGNDU (pin 9) are alsoconnected to the USB bus. VBUS doesnot consume any power and is usedonly to detect the connection to the

USB bus.The PCM2702 has two interfaces.

Each interface is constructed bysome specific setting. Interface #0has only one setting. This settingdescribes the standard audio inter-face

The most important three connec-tions are the input and output termi-

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4512/2000 Elektor Electronics

Measurement Results(measured at 0 dB, 44.1 kHz unless otherwise indicated)

input signal USB audio datanominal output voltage 62 % Vcc (1.096 Veff bij5.000 V)bandwidth (10 kΩ load) 5 Hz to fs/2 (fs =32/44.1/48 kHz)amplitude at 20 kHz –0.25 dBanalogue filter bandwidth 30 kHz (2nd order 0.25dB-Chebyshev)output impedance 100 Ωsignal to noise ratio > 101 dBATHD+N (1 kHz, B = 80 kHz) < 0.0035 %

(20 kHz, B = 80 kHz) < 0.025 %IMD (60 Hz/7 kHz = 4:1) < 0.006 %channel separation > 116 dBstopband attenuation digital filter > 82 dBcurrent consumption < 60 mA

The distortion figures are, without exception, very good. The numbers for signalto noise and channel separation are no worse. The frequency response (mea-sured using a test CD) of the sound card is remarkably flat. For the benefit of thepurists, this graph has been reproduced here. The only irregularity consists of asmall amplitude increase that is caused by the Chebyshev output filter. It appearslike a large ‘bump’, but when closely examining the scale of the graph it is realisedthat the ripple is less than 0.25 dB!

Figure 1. Block diagram of the single-chip D/A-converter PCM2702.

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46 Elektor Electronics 12/2000

Figure 2. The complete schematic of the USB audio-DAC is remarkably simple.

Figure 3. The PCB is very compact, in part because IC1 and IC2 are SMD devices.

R5

1M

X1

12MHz

C3

18p

C4

18p

R8

10k0

R13

10k0

R10

8k25

R15

8k25

R11

100Ω

R16

100Ω

R2

22Ω

R3

22Ω

R4

22Ω

R1

1k

5

R9

10

k0

R14

10

k0

R12

22

0k

R17

22

0k

R6

47

0Ω R7

68

0Ω

C12

1n5

C15

1n5

C13

270p

C16

270p

C2

100n

C5

100n

C7

100n

C8

100n

C9

100n

6

5

7IC2b

2

3

1IC2a

C11 100n

C6 10µ63V

C14

3µ3

C17

3µ3

PCM2702

PLY

BC

K

VOUTL

VOUTR

DG

ND

C

AG

ND

R

AG

ND

L

AG

ND

P

TEST0

TEST1

TEST2

TEST3

SS

PN

D

DG

ND

U

VCOMIC1VBUS

VD

DC

DG

ND

VC

CP

VC

CL

VC

CR

AG

ND

ZERO

VD

D

XTI XTO

VC

C

D+

28

21D-

23

19

272517

18 24 26 20

22

16

15

14

13

12

10 11

1

4

6

7

8

2

35 9

T2BC557BT1

D4

SUSPEND

D3

PLAYBACK

C1

10µ63V

C10

10µ63V

D1

D21N4148

K1

1

2

4

3

USB

L

R

IC2

8

4

2x

5V

IC2 = OPA 2353UA2x

B1

B80C1500

C26

22n

C23

22n

C25

22n

C24

22n C22

470µ25V

C20

10µ63V

C21

100n

C19

100n

78L05

IC3

>9VDC

>8VAC

5V

C18

470n

000169 - 12

B1

C1C

2

C3

C4

C5

C6

C7 C

8

C9

C10

C11

C12

C13

C14

C15

C16

C17

C18

C19

C20

C21

C22

C23C24

C25C26

D1

D2

D3

D4

H1 H2

H3H4

IC1

IC2

IC3

K1

OUT1OUT2

R1

R2

R3

R4

R5

R6

R7

R8

R9

R10

R11R12

R13

R14

R15

R16R17

T1

T2

X1

000169-1

R L

T T

USB

>8VAC

>9VDC

~~

(C) E

LEK

TOR

000169-1

The amplifier selected for the output filteris an OPA2353UA. This is a high speed, sin-gle supply, rail to rail, low noise CMOS opampwith a maximum output current of 40 mA. Asa result of using this amplifier, an additionalpower supply is avoided and the output filtercan simply be powered from the same 5 Vthat is already present.

The outputs have to be DC decoupled, ofcourse. This is achieved with C14/C17 (3.3µFMKT; check that the pitch is either 5 or7.5 mm!), R11/R16 ensure that capacitiveloads will never be able to cause trouble.

R12/R17 make sure that C14/C17 arealways charged. This prevents a loud ‘pop’ ifthe loudspeakers are connected to the circuitafter this has already been powered up.

Two LEDs indicate the status of the circuit.

tional SMD capacitors unnecessary.RF decoupling is provided by stan-dard 100 nF ceramic capacitors (with5 mm lead pitch).

With the crystal oscillator, keep inmind that C3 and C4 act as Cload forthe attached crystal (here Cload =C3/2 + Cparasitic (when C3=C4); ifnecessary, the frequency may bemeasured at the XTO pin and isallowed to deviate by at most500 ppm).

This audio DAC, for a change,

does not use Sallen-Key filters, butuses Multiple Feedback filters (MFB)instead. The output filters are DCcoupled. This is possible becausethe Vcom output biases the opampsto exactly half of the power supplyrail. An additional advantage of theMFB filters is the relative insensitiv-ity to component tolerances. C11provides RF decoupling of the biasvoltage. Because these are only 2nd

order filters, the Chebyshev typewith 0.25 dB ripple was selected.This is a compromise between thesmaller bandwidth of the analogueoutput filter (better suppression ofthe mixing products in the audioband) and quality.

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4712/2000 Elektor Electronics

COMPONENTS LIST

Resistors:R1 = 1kΩ5R2,R3,R4 = 22ΩR5 = 1MΩR6 = 470ΩR7 = 680ΩR8,R9,R13,R14 = 10kΩ0 1%R10,R15 = 8kΩ25 1%R11,R16 = 100ΩR12,R17 = 220kΩ

Capacitors:C1,C6,C10,C20 = 10µF 63V

radialC2,C5,C7,C8,C9,C11,C19,C21 =

100nF ceramic, lead pitch 5mmC3,C4 = 18pFC12,C15 = 1nF5 1%

polystyrene/polypropyleneC13,C16 = 270pF 1%

polystyrene/polypropylene C14,C17 = 3µF3 MKT (Siemens),

lead pitch 5 or 7.5mmC18 = 470nF, lead pitch 5mmC22 = 470µF 25V radialC23-C26 = 22nF ceramic, lead

pitch 5mm

Semiconductors:D1,D2 = 1N4148D3 = green high-efficiency LEDD4 = red high-efficiency LEDB1 = B80C1500 (80V piv, 1.5A

peak), round caseT1,T2 = BC557BIC1 = PCM2702E (SSOP-28 case)

(Burr-Brown)IC2 = OPA2353UA (SO-8 case)

(Burr-Brown)IC3 = 78L05

Miscellaneous:K1 = USB connector, receptacle

style B (PCB mount)X1 = 12MHz quartz crystal PCB, order code 000169-1 (see

Readers Services)

Filter softwareOn the Internet, at the Burr-Brown website (http://www.burrbrown.com/applications) theprogram FilterPro may be found. The program that may be downloaded is called filter.zip,which contains, among other things, filter2.exe. This is a DOS program that enables the cal-culation of the well-known Sallen-Key filters as well as the MFB filters that are used here.Several parameters may be adjusted and the exact values of the E-96 series resistors can becalculated. The capacitors are automatically adjusted for the selected scaling resistors (E-12values), but it is also possible to choose your own values, this is particularly useful whenusing previously measured capacitors. Those who disagree with the choice of the Cheby-shev filter for this circuit, may modify the filter themselves to, for example, a Butterworthor a Bessel type.

D3 is lit up when the PCM2702 isplaying back audio data (Playback).D4 is lit up when the USB interruptsthe audio data stream to thePCM2702 (Suspend).

Construction

The PCB layout for the USB audioDAC is shown in Figure 3. It isremarkably compact, having dimen-sions of only 46 × 74 mm.

Both the PCM2702 and the dualopamp OPA2353VA are surfacemount devices. A considerableamount of soldering skill is required.The problems are only minor whensoldering IC2, but for IC1 a solderingiron with very small tip is absolutelynecessary (the pins have only a0.65 mm pitch!). Use desolderingwick to remove any excess solder. Toprevent overheating of the IC, allow itto cool down from time to time. Thisalso applies to the soldering opera-tion.

Once both SMD ICs have beencarefully soldered to the PCB, theremainder of the parts may be fitted.These are, without exception, stan-dard components of which there areonly a very small number, so noproblems are anticipated here.

Installation

We installed the circuit on a PC withWindows98SE. Windows98 (andlater) contains drivers for USB audioplayback as standard.

For testing purposes it is possibleto power the circuit using a 9 V bat-tery. The red LED (D4) should light.Once the connection to the USBcable is made (cable type A to B),the red LED should extinguishimmediately and a window shouldpop up with the message ‘Burr-Brown Japan PCM2702’. This is fol-lowed by the window ‘Add NewHardware Wizard’ which will wantto install the drivers for a ‘USB Com-posite Device’ (refer to Figure 4a).Once the drivers are installed, theWizard window will reappear whichnow will want to install a ‘USB audioDevice’ (Figure 4b). Having com-

pleted all of this, there should be, inControl Panel, under System Proper-ties in the Device Manager under‘Sound, video and game controllers’a ‘USB audio device’ and under ‘Uni-versal Serial Bus controllers’ a ‘USBcomposite device’ (Figure 4c). Ifanother sound card is alreadyinstalled, then using Control Panelunder Multimedia Properties in theAudio tab it is now also possible toselect ‘USB Audio Device’ as the Pre-ferred device for Playback.

We assume that every PC thesedays is equipped with a CD-ROMdrive. With a regular sound card, theanalogue output from the CD playeris normally directly connected to thiscard. This permits the listening to, orthe processing of, audio-CDs usingthe PC. If you would like to listen toaudio CDs using the USB audio DAC,then, in Multimedia Properties in the‘CD Music’ tab tick ‘Enable digitalCD audio for this CD ROM device’(refer Figure 4d). There are now fourcontrols in the ‘Volume Controls’window: one for ‘speaker’ (master),one for ‘CD Player’, one for ‘Wave’and one for ‘SW Synth’ (for MIDI, aSoftware Wavetable Synthesiser).

Practical Hints

There are older generations of PCmotherboard (not ATX) that do pos-sess a USB controller but do not havethe appropriate connectors fitted.Separate USB-dual-brackets areavailable (standard with two USBconnectors type A) with an 8 or 10way socket that may replace anunused expansion slot cover. If noexpansion slots are available, then itmay be possible to remove thebracket with the 25 way printer portand 9 way serial port and movethese to the break-out openings inthe computer case instead. The USBbracket may then be fitted in thenewly created space.

There are three variations (therecould be more) of the socket for thisbracket, so pay close attention to theheader pinout. The connections inthe socket are usually easilyswapped around. By carefully liftingup the plastic locking tabs andsimultaneously pulling on the wires,the individual connectors can beremoved from the socket.

(000169-1)

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48 Elektor Electronics 12/2000

Figure 4. The installation on the PC isnot particularly difficult. The textprovides a number of useful hints.

a

b

c

d