Analog Devices AN-404 Application Notes

AN-404

a

ONE TECHNOLOGY WAY • P.O. BOX 9106

Considerations for Mixed Signal Circuit Board Design

(How to Design a PCB Layout/Assembly Compatible with the AD1845

INTRODUCTION

Analog Devices’ AD1845

Port®, Stereo Codec

CS4231
“pin-for-pin” compatible. Many customers have had dif­ficulties using these codecs interchangeably because
they have

ments and the parts’ documentation recommend differ­ing power supply circuitry

contribute to the challenges of PC OEMs trying to design
a compatible “socket” (a PCB layout compatible with
both codecs) for these highly popular audio codecs.

This application note shows designers how to put a
“socket” in their PC motherboard or plug-in card design
that is compatible with both parts (with some minor as­sembly differences). In addition to the recommended
design (provides the highest performance, but requires
the most assembly differences), this note describes
cost and performance tradeoffs that are available with
“compromise” components (reducing the number of as­sembly differences with compatible external circuitry).
Table I lists the assembly differences between an
AD1845 and CS4231 system for the recommended
codec “socket,” shown in Figure 1.

USING A CODEC ON YOUR PCB

This application note was inspired by the difficulties sev­eral Analog Devices customers reported when putting
an AD1845 into PC boards laid out using the specifica­tion in the Crystal Semiconductor CS4231 (AD1845 pin­compatible codec) data sheet. Reported problems
varied from reduced performance to complete part
breakdown.

This application note explains the design issues in­volved in designing a codec “socket” that provides the
highest performance from both parts. For simplicity, all
figures in this application note use AD1845/CS4231

Parallel Interface, Multimedia Audio Codec

slightly different external interface require-

Parallel-Port, 16-Bit, Sound-

and Crystal Semiconductor’s

. These design details

•

NORWOOD, MASSACHUSETTS 02062-9106

and CS4231 Codecs)

are

APPLICATION NOTE

617/329-4700

•

PLCC package pin numbers, but the design principles
covered apply as well to other Analog Devices package
types.

Including a codec in your PC motherboard or plug-in
card design (and getting reasonable performance
from the part) requires some effort. For the AD1845 and
CS4231, a small group of design considerations have a
profound influence on the performance of your final
design. The design considerations that relate to creating a
compatible codec “socket” (a PCB layout compatible
with both codecs) for these codecs include the following:

• Input Circuit Design
This section describes input circuit design and assem­bly differences between the two codecs for the highest
performance, compatible “socket” (shown in Figure 1).

• Power Supply Design
This section describes compatible codec “socket”
power supply design (including the two recom­mended power supply layouts) and explains what
makes some codec vendors’ recommended power
supply design incompatible with the AD1845.

• Layout Design
This section describes layout principles (component
placement priorities and grounding) for the highest
performance compatible codec “socket.”

• Cost/Performance Tradeoffs
This section describes a compatible codec socket that
does not require any assembly differences for the two
codecs (at the expense of lower performance).

The application circuits shown in this note are sugges­tions only. You should choose component values that
it the needs of your own design and fall with the specifi­cations of the AD1845 and CS4231 data sheets.

SoundPort is a registered trademark of Analog Devices, Inc.

+12V

FERRITE

BEAD

+5V

REGULATOR

V

V

OUT

IN

1µF

0.1µF10µF

0.1µF
FERRITE

BEAD

10µF

0.1µF

0.1µF0.1µF

0.1µF

0.1µF0.1µF

0.1µF

+5V

CS4231
ONLY

33pF

33pF

33pF

33pF

16.9344MHz

24.576MHz

1kΩ

1000pF

AD1845
ONLY

(SEE L_MIC CIRCUIT)

(1000pF)

(1000pF)

10µF

1µF

1µF

10µF

0.1µF

1µF

36 35

V

CC

(VA2, VA1)

21

XTAL21

22

XTAL20

17

XTAL11

18

XTAL10

L_MIC (LMIC)

29

R_MIC (RMIC)

28
30

L_LINE (LLINE)

27

R_LINE (RLINE)

39

L_AUX1 (LAUX1)

42

R_AUX1 (RAUX1)

38

L_AUX2 (LAUX2)

43

R_AUX2 (RAUX2)

46

M_IN (MIN)

R_FILT (RFILT)

26

L_FILT (LFILT)

31

V

(VREF)

32

REF

V

33

REF_F

GNDA

(AGND1, 2)

34

37

(VREFI)

1519

(VD4, VD3, VD1, VD2, NC, NC)

AD1845 PLCC

(CS4231 PLCC)

NC

(TEST)

(DGND3, 4, 7, 8, 1, 2, NC, NC)

55

16

7

1

V

DD

PWRDWN (PDWN)

GNDD

5320

45

RESET (NC)

M_OUT (MOUT)

R_OUT (ROUT)

L_OUT (LOUT)

CS (CS)

ADR1 (A1)
ADR0 (A0)

WR (WR)

RD (RD)

XCLL0 (XCTL0)
XCTL1 (XCTL1)

DATA7 (D7)
DATA6 (D6)
DATA5 (D5)
DATA4 (D4)
DATA3 (D3)
DATA2 (D2)
DATA1 (D1)
DATA0 (D0)

DBDIR (BDIR)

DBEN (DBEN)

PDRQ (PDRQ)

CDRQ (CDRQ)

PDAK (PDAK)
CDAK (CDAK)

INT (IRQ)

8264 2544

54

23
24

1µF

47

41

40

59

9
10
61
60

56
58
65
66
67
68

3

4

5

6
62
63

14
12
13
11
57

47kΩ

1µF

47kΩ

1µF

47kΩ

ANALOG AND DIGITALGND

CONNECTED AT ONE POINT

ANALOG

GND

+5V

100kΩ

1µF

ADDRESS

DECODE

74_245

DATA

DIR
G

BENEATH CODEC

18

DIGITAL
GND

ISA BUS

SA 19:2
AEN
SAI
SAO
IOWC
IORC

D7

D6

D5

D4

D3

D2

D1

D0

DRQx
DRQy
DAKx
DAKy
IRQz

Figure 1. Highest Performance AD1845/CS4231 Codec System Diagram

Table I. Assembly Differences Between AD1845 And CS4231 For Codec “Socket” (In Figure 1)

Component Function For AD1845 Install . . . For CS4231 Install . . .

Crystal oscillator and capacitors on XTAL2 input Not required 16.9334 MHz crystal

and 33 pF (2)

Antialiasing filter on L_MIC, R_MIC, L_LINE, R_LINE, L_AUX1, 1 kΩ and 1000 pF Not required, (but can
R_AUX1, L_AUX2, R_AUX2, and M_IN inputs be left installed)

External filtering capacitors for L_FILT and R_FILT inputs 1 µF (2) 1000 pF (2)

–2–

Input Circuit Design

AD1845

(CS4231)

2kΩ

1µF

1000pF

L_MIC (LMIC)
R_MIC (RMIC)
L_LINE (LLINE)
R_LINE (RLINE)
L_AUX1 (LAUX1)
R_AUX1 (RAUX1)
L_AUX2 (LAUX2)
R_AUX2 (RAUX2)
M_IN (MIN)

2kΩ

THE 2kΩ RESISTORS
ACT AS A VOLTAGE
DIVIDER

Figure 2 shows a portion of the AD1845/CS4231 system
and highlights the differences between input circuit de­signs for an optimum performance codec “socket.” This
section describes the following input structure differ­ences between the codecs.

• Crystal oscillators

• Analog input filtering

Crystal Oscillator

As shown in Figure 2, the CS4231 requires two crystal
inputs, 24.575 MHz (XTAL1) and 16.9344 MHz (XTAL2).
The AD1845 defaults to one crystal input (24.576 MHz),
but also can use other frequency sources including the

14.31818 MHz PC bus clock. The AD1845 uses its Vari­able Sample Frequency Generator to generate any of
50,000 selectable sample rates from the single crystal
input.

CS4231
ONLY

Input Filtering

As shown in Figure 2, each of the AD1845’s ADC analog
inputs (MIC, LINE, AUX1, AUX2, & M_IN) require an ex­ternal low pass antialiasing filter (1 kΩ and 1000 pF), and
the AD1845 uses 1 µF capacitors on the external filter
pins to apply a 2.6 Hz high pass filter to the ADC.

33pF

33pF

33pF

33pF

AD1845
ONLY

10µF

1kΩ

1000pF

16.9344MHz

24.576MHz

1µF

(1000pF)

1µF

(1000pF)

10µF

0.1µF

21

22

17

18

1µF

26

31

32

33

XTAL21

XTAL20

XTAL11

XTAL10

L_MIC (LMIC)
R_MIC (RMIC)
L_LINE (LLINE)
R_LINE (RLINE)
L_AUX1 (LAUX1)
R_AUX1 (RAUX1)
L_AUX2 (LAUX2)
R_AUX2 (RAUX2)
M_IN (MIN)

R_FILT (RFILT)

L_FILT (LFILT)

(VREF)

V

REF

V

(VREFI)

REF_F

Figure 2. AD1845/CS4231 Codec Input
Structures Diagram

CRYSTAL
OSCILLATOR
INPUT
DIFFERENCES

ANALOG
INPUT
FILTER
DIFFERENCES

AD1845 PLCC

(CS4231 PLCC)

The CS4231 applies its internal low pass antialiasing fil­tering after the input multiplexer stage and uses the ex­ternal filter pins to attach 1000 pF capacitors for the low
pass filtering.

Note that for a compatible codec “socket” the external
low pass antialiasing filter required for the AD1845 is
completely compatible with the CS4231 inputs, but the
capacitors on the external filter pins MUST change for
best performance.

If you are (for example) replacing a
CS4231 with an AD1845 in your system, the two 1000 pF
capacitors must be replaced with 1 µF caps. If the
1000 pF caps are left in, the AD1845’s high pass filter
break point moves from 2.6 Hz to 2.4 kHz seriously re­ducing the audio band frequency performance. This per­formance reduction includes a nonlinear gain vs.
frequency response and an overall reduction in gain.
The gain loss can be as much as –30 dB at 20 Hz.

Optional Input Level Scaler

The two codecs have slightly different input impedances
and their data sheets provide differing designs for scal­ing 2 V rms line level inputs. (The AD1845 and CS4231
codecs can handle 1 V rms signals.) Figure 3 shows an
example voltage divider circuit for use with 2 V rms line
level inputs that is compatible with both codecs. For
other application related circuits, see the AD1845 and
CS4231 Data Sheets.

Figure 3. AD1845/CS4231 Codec Input Structures with
Voltage Dividers Diagram

Power Supply Design

Your power supply distribution strategy must account
for the mixed signal (analog & digital) nature of the
AD1845 and CS4231 codecs. For power supply design
considerations, think of these codecs as having a digital
section (digital portions of ADC, DAC, and ISA bus driv­ers) and analog section (analog portions of ADC, DAC,
multiplexer, and output mixer stages).

This section presents two successful strategies for com­patible power supply design and explains what makes
the power supply strategy described by other codec
vendors’ documentation incompatible with the AD1845.

–3–