AD1848 datasheet(10/28 Pages) AD

10 / 28 page

AD1848K

REV. 0

–10–

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.

3/2

2/1

MSB

0 0 0 / 0 0

MSB

DAC INPUT

EXPANSION

COMPRESSED

INPUT DATA

3/2

2/1

LSB

8 7

LSB

Figure 2. A-Law or

µ-Law Expansion

When 8-bit companding is specified, the ADCs’ linear output is

compressed to the format specified.

LSB

3/2

2/1

MSB

0 0 0 0 0 0 0 0

MSB

ADC OUTPUT

TRUNCATION

COMPRESSION

LSB

8 7

LSB

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.

AD1848 Datasheet(PDF) 10 Page - Analog Devices

AD1848 Datasheet(HTML) 10 Page - Analog Devices

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28

Datasheet Download

Link URL