AD1847
Changes in DAC output attenuation take effect only on zero
crossings of the digital signal, thereby eliminating “zipper” noise
on playback. Each channel has its own independent zero-crossing
detector and attenuator change control circuitry. A timer guar-
antees that requested volume changes will occur even in the ab-
sence of an input signal that changes sign. The time-out period
is 8 milliseconds at a 48 kHz sampling rate and 48 milliseconds
at an 8 kHz sampling rate. (Time-out [ms]
≈
384/F
S
[kHz]).
Digital Mixing
specified in the Codec’s internal registers. Note that when
µ-law
compressed data is expanded to a linear format, it requires 14
bits. A-law data expanded requires 13 bits.
15
COMPRESSED
INPUT DATA
MSB
8
LSB
7
0
15
EXPANSION
MSB
3/2
LSB
2/1
0
Stereo digital output from the ADCs can be mixed digitally with
the input to the DACs. Digital output from the ADCs going out
of the serial data port is unaffected by the digital mix. Along the
digital mix datapath, the 16-bit linear output from the ADCs is
attenuated by an amount specified with control bits. Both chan-
nels of the monitor data are attenuated by the same amount.
(Note that internally the AD1847 always works with 16-bit
PCM linear data, digital mixing included; format conversions
take place at the input and output.)
Sixty-four steps of –1.5 dB attenuation are supported to
–94.5 dB. The digital mix datapath can also be completely
muted, preventing any mixing of the digital input with the digi-
tal output. Note that the level of the mixed signal is also a func-
tion of the input PGA settings, since they affect the ADCs’
output.
The attenuated digital mix data is digitally summed with the
DAC input data prior to the DACs’ datapath attenuators. The
digital sum of digital mix data and DAC input data is clipped at
plus or minus full scale and does not wrap around. Because
both stereo signals are mixed before the output attenuators, mix
data is attenuated a second time by the DACs’ datapath
attenuators.
Analog Outputs
15
DAC INPUT
MSB
3/2
LSB
2/1
000 / 00
0
Figure 2. A-Law or
µ
-Law Expansion
When 8-bit companding is specified, the ADCs’ linear output is
compressed to the format specified.
15
ADC OUTPUT
MSB
LSB
0
15
TRUNCATION
MSB
3/2
LSB
2/1
0
15
COMPRESSION
MSB
8
LSB
7
00000000
0
Figure 3. A-Law or
µ
-Law Compression
Note that all format conversions take place at input or output.
Internally, the AD1847 always uses 16-bit linear PCM represen-
tations to maintain maximum precision.
Power Supplies and Voltage Reference
A stereo line-level output is available at external pins. Other
output types such as headphone and speaker must be imple-
mented in external circuitry. The stereo line-level outputs
should be capacitively coupled (ac-coupled) to the external cir-
cuitry. Each channel of this output can be independently
muted. When muted, the outputs will settle to a dc value near
V
REF
, the midscale reference voltage.
Digital Data Types
The AD1847 supports four global data types: 16-bit twos-
complement linear PCM, 8-bit unsigned linear PCM,
companded
µ-law,
and 8-bit companded A-law, as specified by
control register bits. Eight-bit data is always left-justified in 16-
bit fields; in other words, the MSBs of all data types are always
aligned; in yet other words, full-scale representations in all four
formats correspond to equivalent full-scale signals. The eight
least significant bit positions of 8-bit data in 16-bit fields are ig-
nored on digital input and zoned on digital output (i.e., truncated).
The 16-bit PCM data format is capable of representing 96 dB of
dynamic range. Eight-bit PCM can represent 48 dB of dynamic
range. Companded
µ-law
and A-law data formats use nonlinear
coding with less precision for large-amplitude signals. The loss
of precision is compensated for by an increase in dynamic range
to 64 dB and 72 dB, respectively.
On input, 8-bit companded data is expanded to an internal lin-
ear representation, according to whether
µ-law
or A-law was
The AD1847 operates from +5 V power supplies. Independent
analog and digital supplies are recommended for optimal perfor-
mance though excellent results can be obtained in single-supply
systems. A voltage reference is included on the Codec and its
2.25 V buffered output is available on an external pin (V
REF
).
The reference output can be used for biasing op amps used in
dc coupling. The internal reference must be externally bypassed
to analog ground at the V
REFI
pin, and must not be used to bias
external circuitry.
Clocks and Sample Rates
The AD1847 operates from two external crystals, XTAL1 and
XTAL2. The two crystal inputs are provided to generate a wide
range of sample rates. The oscillators for these crystals are on
the AD1847, as is a multiplexer for selecting between them.
They can be overdriven with external clocks by the user, if so
desired. At a minimum, XTAL1 must be provided since it is se-
lected as the reset default. If XTAL2 is not used, the XTAL2
input pin should be connected to ground. The recommended
crystal frequencies are 16.9344 MHz and 24.576 MHz. From
them, the following sample rates can be selected: 5.5125, 6.615,
8, 9.6, 11.025, 16, 18.9, 22.05, 27.42857, 32, 33.075, 37.8,
44.1, 48 kHz.
–8–
REV. B
AD [ ANALOG DEVICES ]
