AD1848 Ver la hoja de datos (PDF) - Analog Devices
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AD1848 Datasheet PDF : 28 Pages
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AD1848K
Digital Mixing
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 data port is unaffected by the digital mix. Along the digi-
tal 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 AD1848K 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 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 AD1848K 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 AD1848K is playing back data but 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
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 VREF, the mid-
scale reference voltage.
Digital Data Types
The AD1848K supports four global 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 trans-
ferred MSB first. Eight-bit data is always left justified in 16-bit
fields; said 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
ignored on input and zeroed on output.
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.
15
87
0
COMPRESSED
INPUT DATA MSB
LSB
15
EXPANSION MSB
3/2 2/1
0
LSB
15
DAC INPUT MSB
3/2 2/1
0
LSB 0 0 0 / 0 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
0
LSB
15
TRUNCATION MSB
3/2 2/1
0
LSB
15
COMPRESSION MSB
87
LSB
0
00000000
Figure 3. A-Law or µ-Law Compression
Note that all format conversions take place at input or output.
Internally, the AD1848K always uses 16-bit linear PCM repre-
sentations to maintain maximum precision.
Power Supplies and Voltage Reference
The AD1848K operates from +5 V power supplies. Indepen-
dent analog and digital supplies are recommended for optimal
performance 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
(VREF). 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 VREF_F pin.
Clocks and Sample Rates
The AD1848K operates from external crystals. Two crystal in-
puts are provided to generate a wide range of sample rates. The
oscillators for these crystals are on the AD1848K, as is a multi-
plexer for selecting between them. They can be overdriven with
external 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–
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