AD1847 datasheet(23/28 Pages) AD

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AD1847

REV. B

–23–

DATA FORMAT DEFINITIONS

There are four data formats supported by the AD1847: 16-bit

signed, 8-bit unsigned, 8-bit companded

µ-law, and 8-bit com-

panded A-law. The AD1847 supports these four formats because

each of them have found wide use in important applications.

16-Bit Signed Format

The 16-bit signed format (also called 16-bit twos-complement)

is the standard method of representing 16-bit digital audio. This

format yields 96 dB of dynamic range and is common in con-

sumer compact disk audio players. This format uses the value

– 32768 (8000h) to represent minimum analog amplitude while

32767 (7FFFh) represents maximum analog amplitude. Inter-

mediate values are a linear interpolation between minimum and

maximum amplitude values.

MAX

MIN

8000h

DIGITAL VALUE

0000h

7FFFh

Figure 11. 16-Bit Signed Format

8-Bit Unsigned Format

The 8-bit unsigned format is commonly used in the personal

computer industry. This format delivers 48 dB of dynamic

range. The value 0 (00h) is used to represent minimum analog

amplitude while 255 (FFh) is used to represent maximum ana-

log amplitude. Intermediate values are a linear interpolation be-

tween minimum and maximum amplitude values. The least

significant byte of the 16-bit internal data is truncated to create

the 8-bit output samples.

MAX

MIN

00h

DIGITAL VALUE

7Fh

FFh

Figure 12. 8-Bit Unsigned Format

8-Bit Companded Formats

The 8-bit companded formats (

µ-law and A-law) are used in the

telecommunications industry. Both of these formats are used in

ISDN communications and workstations;

µ-law is the standard

for the United States and Japan while A-law is used in Europe.

Companded audio allows either 64 dB or 72 dB of dynamic

range using only 8-bits per sample. This is accomplished using a

nonlinear formula which assigns more digital codes to lower am-

plitude analog signals at the expense of resolution of higher am-

plitude signals. The

µ-law format of the AD1847 conforms to

the Bell System

µ = 255 companding law while the A-law format

conforms to CCITT “A” law models. Figure 13 shows approxi-

mately how both the

µ-law and A-law companding schemes be-

have. Refer to the standards mentioned above for an exact

definition.

MAX

MIN

00h

FFh

80h

AAh

µ-law

A-law

D5h

2Ah

DIGITAL VALUE

Figure 13. 8-Bit Companded Format

APPLICATIONS CIRCUITS

The AD1847 Stereo Codec has been designed to require a mini-

mum of external circuitry. The recommended circuits are shown

in Figures 14 through 22. Analog Devices estimates that the to-

tal cost of all the components shown in these Figures, including

crystals, to be less than $3 in 10,000 quantities.

Industry-standard compact disc “line-levels” are 2 Vrms centered

around analog ground. (For other audio equipment, “line level”

is much more loosely defined.) The AD1847 SoundPort is a

+5 V only powered device. Line level voltage swings for the

AD1847 are defined to be 1 Vrms for a sine wave ADC input and

0.707 Vrms for a sine wave DAC output. Thus, 2 Vrms input ana-

log signals must be attenuated and either centered around the

reference voltage intermediate between 0 V and +5 V or

ac-coupled. The VREF pin will be at this intermediate voltage,

nominally 2.25 V. It has limited drive but can be used as a volt-

age datum to an op amp input. Note, however, that dc-coupled

inputs are not recommended, as they provide no performance

benefits with the AD1847 architecture. Furthermore, dc offset

differences between multiple dc-coupled inputs create the po-

tential for “clicks” when changing the input mux selection.

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