Datasheet

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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
6Bits
MSBand
Lower18Bits
ICOB
Decoder
3rd-Order,
5-Level,
Delta-Sigma
24-Bit8f
S
+
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 f
S
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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