AD1847 datasheet(8/28 Pages) AD

8 / 28 page

AD1847

REV. B

–8–

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

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

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

VREF, 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

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.

EXPANSION

DAC INPUT

MSB

LSB

MSB

COMPRESSED

INPUT DATA

LSB

3/2

2/1

LSB

3/2

2/1

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.

TRUNCATION

COMPRESSION

MSB

LSB

MSB

LSB

MSB

LSB

3/2

2/1

ADC OUTPUT

0 0 0 0 0 0 0 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

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 (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 VREFI 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.

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