AN1281 Datasheet, PDF (1/3 Pages) STMicroelectronics – TS613 AS DRIVER FOR ADSL LINE INTERFACES

AN1281
APPLICATION NOTE
TS613 AS DRIVER FOR ADSL LINE INTERFACES - A SINGLE SUPPLY
IMPLEMENTATION WITH PASSIVE OR ACTIVE IMPEDANCE MATCHING
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.
Figure 1 : Typical ADSL Line Interface
high output
current
digital to emission ^{LP filter}
analog (analog)
digital
treatment
analog to
digital
reception
(analog)
upstream
TS613
Line Driver
reception
circuits
impedance
matching
HYBRID
CIRCUIT
twisted-pair
telephone
line
downstream
by C. PRUGNE
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.
Figure 2 : TS613 as a differential line driver with
a +12V single supply
100n
+12V
1k
Vi
47k
3
+
8
+12V
2^_
1
R2
R1
Vi
10µ 47k 100n
1k
GND
100n
R3
6_
7
5₊
GND
4
1µ
12.5
10n
1:2
Vo
25Ω
Hybrid
&
100Ω
Transformer
Vo
12.5
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 (5µA) as the 1% of the current
through the resistance divider (500µA) to keep a
stable mid supply, two 47kΩ 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.
June 2000
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