AD1846
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 analog input with the digi-
tal input. 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.
In case the AD1846 is capturing data but ADC output data is
not removed in time (“ADC overrun”), then the last sample
captured before overrun will be used for the digital mix. In case
the AD1846 is playing back data hut input digital DAC data
fails to arrive in time (“DAC underrun”), then a midscale zero
will be added to the digital mix data.
Analog Outputs
15
COMPRESSED
INPUT DATA
MSB
8 7
LSB
0
15
EXPANSION
MSB
3/2
LSB
2/1
0
15
DAC INPUT
MSB
3/2
LSB
2/1
0
000/00
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
0
LSB
15
TRUNCATION
MSB
3/2
LSB
2/1
0
A stereo line level output is available at external pins. 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
15
COMPRESSION
MSB
8 7
LSB
00000000
0
Figure 3. A-Law or
µ
-Law Compression
The AD1846 supports four data types: 16-bit twos-complement
linear PCM, eight-bit unsigned linear PCM, companded
µ-law,
and 8-bit companded A-law, as specified by control register bits.
Data in all four formats is always transferred MSB first. Stereo
data is always transferred in the left-right order. All data formats
that are less than 16 bits are properly aligned to insure the utili-
zation of full system resolution.
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
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.
Note that all format conversions take place at input or output.
Internally, the AD1846 always uses 16-bit linear PCM represen-
tations to maintain maximum precision.
Power Supplies and Voltage Reference
The AD1846 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
REF
_F pin.
Clocks and Sample Rates
The AD1846 operates from external crystals. Two crystal inputs
are provided to generate a wide range of sample rates. The oscil-
lators for these crystals are on the AD1846, as is a multiplexer
for selecting between them. They can be overdriven with exter-
nal clocks by the user, if so desired. The recommended crystal
frequencies are 16.9344 MHz and 24.576 MHz. From them the
following sample rates are divided down: 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.
–10–
REV. A
AD [ ANALOG DEVICES ]
