CS4328 datasheet(13/31 Pages) CIRRUS

13 / 31 page

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-

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

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

Audio

Data

Analog

Output

Continuous

Time

Filter

DAC

8 X S/H

Interpolator

Delta

Sigma

Modulator

Digital

Switched

Cap

LPF

Analog Filter

Figure 20. CS4328 Architecture

f (kHz)

8Fs

16Fs

(dB)

Figure 23. Spectrum After S/H

f (kHz)

8Fs

16Fs

(dB)

Figure 22. 8X Interpolated Data Spectrum

f(kHz)

(dB)

Figure 24. Modulator Output Spectrum

f (kHz)

2Fs

(dB)

Figure 21. Input Data Spectrum

CS4328

DS62F3

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