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TS613 AS DRIVER FOR ADSL LINE INTERFACES - A SINGLE SUPPLY

IMPLEMENTATION WITH PASSIVE OR ACTIVE IMPEDANCE MATCHING

by C. PRUGNE

June 2000

ADSL CONCEPT

Asymmetric Digital Subscriber Line (ADSL), is a

new modem technology, which converts the exist-

ing twisted-pair telephone lines into access paths

for multimedia and high speed data communica-

tions.

ADSL transmits more than 8 Mbps to a subscriber,

and can reach 1Mbps from the subscriber to the

central office. ADSL can literally transform the ac-

tual public information network by bringing mov-

ies, television, video catalogs, remote CD-ROMs,

LANs, and the Internet into homes.

An ADSL modem is connected to a twisted-pair

telephone line, creating three information chan-

nels: a high speed downstream channel (up to

1.1MHz) depending on the implementation of the

ADSL architecture, a medium speed upstream

channel (up to 130kHz) and a POTS (Plain Old

Telephone Service), split off from the modem by

filters.

THE LINE INTERFACE - ADSL Remote

Terminal (RT):

The Figure1 shows a typical analog line interface

used for ADSL. The upstream and downstream

signals are separated from the telephone line by

using an hybrid circuit and a line transformer. On

this note, the accent will be made on the emission

path.

The TS613 is used as a dual line driver for the up-

stream signal.

For the remote terminal it is required to create an

ADSL modem easy to plug in a PC. In such an ap-

plication, the driver should be implemented with a

+12 volts single power supply. This +12V supply is

available on PCI connector of purchase.

The figure 2 shows a single +12V supply circuit

that uses the TS613 as a remote terminal trans-

mitter in differential mode.

The driver is biased with a mid supply (nominaly

+6V), in order to maintain the DC component of

the signal at +6V. This allows the maximum dy-

namic range between 0 and +12 V. Several op-

tions are possible to provide this bias supply (such

as a virtual ground using an operational amplifier),

such as a two-resistance divider which is the

cheapest solution. A high resistance value is re-

quired to limit the current consumption. On the

other hand, the current must be high enough to

bias the inverting input of the TS613. If we consid-

er this bias current (5

µA) as the 1% of the current

through the resistance divider (500

µA) to keep a

stable mid supply, two 47k

Ω resistances can be

used.

The input provides two high pass filters with a

break frequency of about 1.6kHz which is neces-

sary to remove the DC component of the input sig-

nal. To avoid DC current flowing in the primary of

the transformer, an output capacitor is used.

Figure 1 : Typical ADSL Line Interface

impedance

matching

twisted-pair

telephone

line

HYBRID

CIRCUIT

digital

treatment

LP filter

TS613

Line Driver

reception

(analog)

emission

(analog)

digital to

analog

analog to

digital

high output

current

reception

circuits

upstream

downstream

Figure 2 : TS613 as a differential line driver with

a +12V single supply

R1

R3

R2

_

+

_

+

Vi

Vi

Vo

Vo

3

2

6

5

1

7

4

GND

8

+12V

25

Ω

100Ω

1:2

Hybrid

&

Transformer

GND

+12V

47k

47k

10µ

100n

100n

100n

1k

1k

12.5

12.5

10n

1µ

AN1281

APPLICATION NOTE