AD1847
D ATA FO RMAT D EFINITIO NS
8-Bit Com panded For m ats
T here 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.
T he 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. T his 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. T he µ-law format of the AD1847 conforms to
the Bell System µ = 255 companding law while the A-law format
conforms to CCIT T “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.
16-Bit Signed For m at
T he 16-bit signed format (also called 16-bit twos-complement)
is the standard method of representing 16-bit digital audio. T his
format yields 96 dB of dynamic range and is common in con-
sumer compact disk audio players. T his 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
MAX
MIN
MIN
00h
2Ah
8000h
0000h
7FFFh
FFh
D5h
80h µ-law
AAh A-law
DIGITAL VALUE
DIGITAL VALUE
Figure 11. 16-Bit Signed Form at
8-Bit Unsigned For m at
Figure 13. 8-Bit Com panded Form at
T he 8-bit unsigned format is commonly used in the personal
computer industry. T his format delivers 48 dB of dynamic
range. T he 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. T he least
significant byte of the 16-bit internal data is truncated to create
the 8-bit output samples.
AP P LICATIO NS CIRCUITS
T he AD1847 Stereo Codec has been designed to require a mini-
mum of external circuitry. T he 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.) T he 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. T hus, 2 Vrms input ana-
log signals must be attenuated and either centered around the
reference voltage intermediate between 0 V and +5 V or
MAX
ac-coupled. T he 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.
MIN
00h
7Fh
FFh
DIGITAL VALUE
Figure 12. 8-Bit Unsigned Form at
REV. B
–23–
ADI [ ADI ]
