CDB4328 International Power Sources, Inc., CDB4328 Datasheet - Page 13

• Manufacturer Part Number: CDB4328
• Description: 18-bit, Stereo D/A Converter For Digital Audio
• Manufacturer: International Power Sources, Inc.

THEORY OF OPERATION
The CS4328 architecture can be considered in
five blocks: Interpolation, sample/hold, delta-
sigma modulation, D/A conversion, and analog
filtering.
Audio data is input to the CS4328 digital inter-
polation filter which removes images of the input
signal that are present at multiples of the input
sample frequency, Fs (Figure 21). Following the
interpolation stage, the resulting frequency spec-
trum has images of the input signal at multiples
of eight times the input sample frequency, 8 Fs
(Figure 22). Eliminating the images between Fs
and 8 Fs greatly relaxes the requirements of the
analog filtering, allowing the suppression of im-
ages while leaving the audio band of interest
unaltered.
The CS4328 interpolation stage is followed by a
sample-and-hold function where the data points
from the interpolator are held for eight (64 Fs)
clock cycles. The resulting frequency response
is a sinx/x characteristic with zeros at 8 Fs mul-
DS62F3
(dB)
(dB)
Audio
Data
Figure 22. 8X Interpolated Data Spectrum
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24
Figure 21. Input Data Spectrum
Interpolator
Fs
8x
2Fs
8Fs
8 X S/H
Digital
16Fs
Figure 20. CS4328 Architecture
f (kHz)
f (kHz)
Modulator
Sigma
Delta
tiples. The sinx/x zeros completely attenuate
signals at 8 Fs and largely suppress the remain-
ing energy of the images (Figure 23). The 8
interpolation followed by the 8 sample-and-
hold results in data at a rate of 64 Fs.
The delta-sigma modulator takes in the 64 Fs
data (3.072 MHz for 48kHz sampled systems)
and performs fifth-order noise shaping. In the
digital modulator of the CS4328, 18-bit audio
data is modulated to a 1-bit, 64 Fs signal. The
5th-order noise shaper allows 1-bit quantization
to support 18-bit audio processing by suppress-
ing quantization noise in the bandwidth of
(dB)
interest. Figure 24 shows the frequency spec-
trum of the modulator output.
The CS4328’s digital modulator is followed by a
D-to-A converter that translates the 1-bit signal
into a series of charge packets. The magnitude
of the charge in each packet is determined by
sampling of a voltage reference onto a switched
(dB)
DAC
Figure 24. Modulator Output Spectrum
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24
Figure 23. Spectrum After S/H
Switched
LPF
Cap
8Fs
Analog Filter
Continuous
Time
Filter
16Fs
CS4328
f (kHz)
Analog
Output
f(kHz)
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