|
CS4328 Datasheet, PDF (13/31 Pages) Cirrus Logic – 18-Bit, Stereo D/A Converter for Digital Audio | |||
|
◁ |
CS4328
Audio
Data
8x
Interpolator
Digital
8 X S/H
Delta
Sigma
Modulator
DAC
Figure 20. CS4328 Architecture
Switched
Continuous
Cap
Time
LPF
Filter
Analog Filter
Analog
Output
THEORY OF OPERATION
The CS4328 architecture can be considered in
five blocks: Interpolation, sample/hold, delta-
sigma modulation, D/A conversion, and analog
filtering.
tiples. The sinx/x zeros completely attenuate
signals at 8Ã Fs and largely suppress the remain-
ing energy of the images (Figure 23). The 8Ã
interpolation followed by the 8Ã sample-and-
(dB)
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, 8Ã Fs
(Figure 22). Eliminating the images between Fs
and 8Ã Fs greatly relaxes the requirements of the
analog filtering, allowing the suppression of im-
ages while leaving the audio band of interest
unaltered.
(dB)
24
8Fs
16Fs f (kHz)
Figure 23. Spectrum After S/H
hold results in data at a rate of 64Ã Fs.
The delta-sigma modulator takes in the 64Ã 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, 64Ã 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
24 Fs 2Fs
f (kHz)
Figure 21. Input Data Spectrum
(dB)
(dB)
24
8Fs
16Fs f (kHz)
Figure 22. 8X Interpolated Data Spectrum
The CS4328 interpolation stage is followed by a
sample-and-hold function where the data points
from the interpolator are held for eight (64Ã Fs)
clock cycles. The resulting frequency response
is a sinx/x characteristic with zeros at 8Ã Fs mul-
DS62F3
24
f(kHz)
Figure 24. Modulator Output Spectrum
interest. Figure 24 shows the frequency spec-
trum of the modulator output.
The CS4328âs 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
13
|
▷ |