STA321 datasheet(52/157 Pages) STMICROELECTRONICS

52 / 157 page

FFX

STA321

52/157

Doc ID 15351 Rev 3

FFX

7.1

Functional description

Figure 29.

FFX processing schematic

The FFX modulator is a digital low-distortion low-noise PCM-to-PWM converter, based on a

pseudo-natural sampling technique, which converts the 4 by 24-bit digital inputs into

differential pulse-width modulated outputs at a frequency of either 384 or 768 kHz (selected

by register FFXCFG2, bit PWM_FREQ) and with a time resolution of 98.304 MHz. This

gives a dynamic range that is approaching 100 dB.

The signal is compared with two different carrier signals (rising and falling sawtooth

waveforms at the PWM frequency), to get a double edge modulation and to have the

possibility to drive a differential (full bridge) power stage.

The order of the noise shaper can be modified by the user, via register bit

FFXCFG2.NS_ORD, depending on the acceptable amount of noise out of the audio band,

that is, noise above 20 kHz. The higher the noise shaper order, the better is the SNR but the

higher is the out of band noise.

The PWM generator block converts the amplitude quantization into time quantization to

generate a PWM signal.

7.2

Modulation schemes

It is possible to use each of the two intersections with up-carrier and down-carrier to force a

rising or a falling edge on each of the two PWM outputs (A and B). This flexibility is achieved

through programming registers PWMOnCFG1-2 (where n is the number, 1 to 4, of the

output) beginning on page 91.

PWM output A can be modulated in one of, or a hybrid of, two basic ways via bits PM_nA:

with the wave starting from level 0 at the beginning of the period, and rising to level 1

when the audio signal intersects the down-carrier

with the wave starting from level 1 at the beginning of the period, and falling to level 0

when the audio signal intersects the up-carrier;

96 kHz

24 bits

rd, 4th, 5th -order

noise

shaper

1536 kHz

24 bits

PWM

generator

384/768 kHz

8/7 bits

pwm

Intersector

pwm

384/768 kHz

1 bits

16x

oversampling

stage

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