CS5317 datasheet(7/32 Pages) CIRRUS

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SYSTEM DESIGN WITH THE CS5317

Like a tracking ADC, the CS5317 continuously

samples and converts, always tracking the analog

input signal and updating its output register at a

20 kHz rate. The device can be read at any rate to

create any system-level sampling rate desired up

to 20kHz.

Clocking

Oversampling is a critical function in delta-sigma

A/D conversion. Although system-level output

sample rates typically remain between 7kHz and

20kHz in voiceband applications, the CS5317 ac-

tually samples and converts the analog input at

rates up to 2.56 MHz. This internal sampling rate

is typically set by a master clock which is on the

order of several megahertz. See Table1 for a com-

plete description of the clock relationships in the

various CS5317 operating modes.

Some systems such as echo-canceling modems,

though, require the output sampling rate to be

locked to a sampling signal which is 20 kHz or

below. For this reason the CS5317 includes an

on-chip phase-lock loop (PLL) which can gener-

ate its requisite 5.12 MHz master clock from a

20 kHz sampling signal.

The CS5317 features two modes of operation

which utilize the internal PLL. The first, termed

Clock Generation 1 (CLKG1), accepts a sam-

pling clock up to 20 kHz at the CLKIN pin and

internally generates the requisite 5.12 MHz clock.

The CS5317 then processes samples updating its

output register at the rate defined at CLKIN, typi-

cally 20 kHz. For a 20 kHz clock input the digital

filter’s 3 dB corner is set at 5.239 kHz, so CLKG1

provides a factor of 2X oversampling at the sys-

tem level (20 kHz is twice the minimum possible

sampling frequency needed to reconstruct a 5

kHz input). The CLKG1 mode is initiated by ty-

ing the MODE input to +5V.

+5V

Analog

Supply

0.1

µF

Ω

CS5317

AIN

Signal

Conditioning

VA-

AGND

DGND

VD-

0.1

REFBUF

VA+

VD+

Clock

Source

Signal

Analog

-5V

Analog

Supply

Serial

Data

Interface

uP or DSP

Control

(clock gen. mode / CLKG2)

VD- (clock override mode / CLKOR)

VD+ (clock gen. mode / CLKG1)

PHDT

µF

VCOIN

VA+

Ω

25 nF

25 k

Ω

± 2.75V

µF

0.1

µF

CLKIN

MODE

CLKOUT

DATA

DOE

RST

DOUT

0.1

µF 0.1 µF

Figure 1. System Connection Diagram with Example PLL Components

CS5317

DS27F4

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