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THEORY OF OPERATION
ADC SECTION
The PCM3002/3003 ADC consists of two reference circuits, a stereo single-to-differential converter, a fully
differential fifth-order delta-sigma modulator, a decimation filter (including digital high pass), and a serial interface
circuit. The block diagram in this data sheet illustrates the architecture of the ADC section, Figure 40 shows the
single-to-differential converter, and Figure 52 illustrates the architecture of the fifth-order delta-sigma modulator
and transfer functions.
DAC SECTION
The delta-sigma DAC section of the PCM3002/3003 is based on a 5-level amplitude quantizer and a third-order
noise shaper. This section converts the oversampled input data to a 5-level delta-sigma format. A block diagram
of the 5-level delta-sigma modulator is shown in Figure 53. This 5-level delta-sigma modulator has the advantage
of improved stability and reduced clock-jitter sensitivity over the typical one-bit (2-level) delta-sigma modulator.
The combined oversampling rate of the delta-sigma modulator and the internal 8
×
interpolation filter is 64 f
for a
256-f
S
system clock. The theoretical quantization noise performance of the 5-level delta-sigma modulator is
shown in Figure 54.
1
st
SW-CAP
Integrator
Analog
In
X(z)
+
+
2
nd
SW-CAP
Integrator
3
rd
SW-CAP
Integrator
+
4
th
SW-CAP
Integrator
+
+
+
+
+
+
+
+
5
th
SW-CAP
Integrator
Digital
Out
Y(z)
Comparator
Qn(z)
H(z)
1-Bit
DAC
STF(z) = H(z) / [1 + H(z)]
NTF(z) = 1 / [1 + H(z)]
Y(z) = STF(z) * X(z) + NTF(z) * Qn(z)
Signal Transfer Function
Noise Transfer Function
B0005-01
PCM3002
PCM3003
SBAS079A–OCTOBER 2000–REVISED OCTOBER 2004
An internal reference circuit with three external capacitors provides all reference voltages required by the ADC,
which defines the full-scale range for the converter. The internal single-to-differential voltage converter saves the
space and extra parts needed for the external circuitry required by many delta-sigma converters. The internal
full-differential signal processing architecture provides a wide dynamic range and excellent power supply
rejection performance. The input signal is sampled at a 64
×
oversampling rate, eliminating the need for a
sample-and-hold circuit, and simplifying antialias filtering requirements. The fifth-order delta-sigma noise shaper
consists of five integrators which use a switched-capacitor topology, a comparator, and a feedback loop
consisting of a one-bit DAC. The delta-sigma modulator shapes the quantization noise, shifting it out of the audio
band in the frequency domain. The high order of the modulator enables it to randomize the modulator outputs,
reducing idle tone levels.
The 64-f
S
, one-bit data stream from the modulator is converted to 1-f
S
, 16/20-bit data words by the decimation
filter, which also acts as a low-pass filter to remove the shaped quantization noise. The dc components are
removed by a high-pass filter function contained within the decimation filter.
Figure 52. Simplified Fifth-Order Delta-Sigma Modulator
34