Datasheet

PCM1741
18
SBAS175
FIGURE 14. 8-Level Delta-Sigma Modulator.
+
Z
–1
8-Level Quantizer
+
Z
–1
+
Z
–1
–
+
Z
–1
+
+
8f
S
64f
S
PCM1741
V
CC
V
DD
DGND
Output
Circuits
RF Choke or Ferrite Bead
Common
Ground
AGND
DIGITAL SECTION ANALOG SECTION
V
DD
Power Supplies
+3.3V +V
S
AGND
REG
–V
S
FIGURE 13. Single-Supply PCB Layout.
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 perfor-
mance of the PCM1741. In cases where a common +3.3V
supply must be used for the analog and digital sections, an
inductance (RF choke, ferrite bead) should be placed be-
tween the analog and digital +3.3V supply connections to
avoid coupling of the digital switching noise into the analog
circuitry. Figure 13 shows the recommended approach for
single-supply applications.
THEORY OF OPERATION
The delta-sigma section of the PCM1741 is based on an
8-level amplitude quantizer and a fourth-order noise shaper. This
section converts the oversampled input data to 8-level delta-sigma
format. A block diagram of the 8-level delta-sigma modulator is
shown in Figure 14. This 8-level delta-sigma modulator has the
advantage of stability and clock jitter sensitivity over the typical
one-bit (2-level) delta-sigma modulator. The combined
oversampling rate of the delta-sigma modulator and the interpo-
lation filter is 64f
S
.