PCM3500ESLASH2K datasheet(23/26 Pages) TI

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PCM3500

ANTI-ALIASING FILTER

STOPBAND CHARACTERISTICS

Frequency (Hz)

–5

–10

–15

–20

–25

–30

–35

–40

–45

–50

100

10k

100k

10M

ANTI-ALIASING FILTER

PASSBAND CHARACTERISTICS

Frequency (Hz)

–0.1

–0.2

–0.3

–0.4

–0.5

–0.6

–0.7

–0.8

–0.9

–1.0

100

10k

100k

The AFE circuit consists of a single-ended-to-differential

converter, with the first stage of the circuit doubling as a

low-pass, anti-alias filter. The frequency response for the

filter is shown in Figure 22. Since the delta-sigma modulator

oversamples the input at 64fS, the anti-alias filter require-

ments are relaxed, with only a single-pole filter being re-

quired. If an application requires further band limiting of the

input signal, a simple RC filter at the VIN input (pin 4) can

be used, as shown in Figure 23.

SOFTWARE MODEM

APPLICATIONS

The PCM3500 was designed to meet the requirements for

software-based analog modems, supporting up to 56kbps(1).

In a software modem application, the PCM3500 is paired

with a Data Access Arrangement (DAA) and a host CPU to

provide the complete modem function. Figure 24 shows a

simplified block diagram of a software modem using the

PCM3500.

The DAA provides the interface between the CODEC and

two-wire telephone line. The DAA provides numerous

functions, including two-to-four wire conversion, modem-

side to line-side isolation, ring detection, hook switch con-

trol, line current compensation, and overvoltage protection.

The host CPU provides the data pump and supervisory

functions for the software modem application. The host

executes modem software code, which includes the neces-

sary routines for transmit and receive functions, error detec-

tion and correction, echo cancellation, and CODEC/DAA

control and supervision.

NOTE: (1) Data transmission is limited to 53kbps over standard telephone

lines. Actual transmission rates vary depending upon the quality of the

lines and switching equipment for a given connection.

FIGURE 22. Anti-Alias Filter Frequency Response.

FIGURE 23. Optional External Low-Pass Filter for the

ADC.

FIGURE 24. Software Modem Block Diagram.

PCM3500

Analog

Input

–3dB =

π RC

Host

CPU

Controls (ring detect, off hook, etc.)

Modem

Software

PCM3500

CODEC

Data

Access

Arrangement

(DAA)

Data

Tip

Ring

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