PCM1738ESLASH2K datasheet(40/47 Pages) TI2

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THEORY OF OPERATION

ADVANCED SEGMENT DAC

0–66

Digital Input

Analog

Output

Current

Segment

DAC

Advanced

DWA

0–62

Level

0–4

Level

Upper

6Bits

MSBand

Lower18Bits

ICOB

Decoder

3rd-Order,

5-Level,

Delta-Sigma

24-Bit8f

B0204-01

CONSIDERATIONS FOR APPLICATIONS CIRCUITS

PCB LAYOUT GUIDELINES

PCM1738

SBAS174C – FEBRUARY 2002 – REVISED FEBRUARY 2007

Figure 48. Architecture of Advanced Segment DAC

The PCM1738 uses the newly developed advanced segment DAC architecture to achieve excellent dynamic

performance and improved tolerance to clock jitter. The PCM1738 provides balanced current outputs, allowing

the user to optimize analog performance externally. The structure of the advanced segment DAC architecture is

shown in Figure 48.

Digital input data from the digital interpolation filter is split into six upper bits and 18 lower bits. The upper six bits

are converted to inverted complementary offset binary (ICOB) code. The lower 18 bits associated with the MSB

are processed by five-level, third-order, delta-sigma modulators operated at 64 fS by default conditions. The level

of the modulator is equivalent to one LSB of the ICOB code converter (decoder). The data groups processed in

the ICOB converter and the third-order delta-sigma modulator are summed together to create up to 66 levels of

digital code that is then processed by data-weighted averaging (DWA) to reduce noise produced by element

mismatch. The output data from the DWA block is then converted to an analog output using a differential current

segment DAC.

A typical PCB floor plan for the PCM1738 is shown in Figure 49. A ground plane is recommended, with the

analog and digital sections being isolated from one another using a split or cut in the circuit board. The

PCM1738 should be oriented with the digital I/O pins facing the ground plane split/cut to allow for short, direct

connections to the digital audio interface and control signals originating from the digital section of the board.

Separate power supplies are recommended for the digital and analog sections of the board. This prevents the

switching noise present on the digital supply from contaminating the analog power supply and degrading the

dynamic performance of the DACs. In cases where a common 5-V supply must be used for the analog and

digital sections, an inductance (RF choke, ferrite bead) should be placed between the analog and digital 5-V

supply connections to avoid coupling of the digital switching noise into the analog circuitry. Figure 50 shows the

recommended approach for single supply applications.

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