ST AN1281 APPLICATION NOTE

AN1281

APPLICATION NOTE

TS613 AS DRIVER FOR ADSL LINE INTERFACES - A SINGLE SU PPLY

IMPLEMENTATION WITH PASSIVE OR ACTIVE IMPEDANCE MATCHING

by C. PRUGNE

ADSL CONCEPT

Asymmetric Digital Subscriber Line (ADSL), is a
new modem technology, which converts the exist­ing twisted-pair telephone lines into ac cess paths
for multimedia and high speed data communica­tions.

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 ac­tual public information network by bringing mov­ies, 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 chan­nels: 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 TS613 is used as a dual line driver for the up­stream signal.
For the remote terminal it is required to create an
ADSL modem easy to plug in a PC. In such an ap­plication, the driver should be imple men ted 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 trans­mitter in differential mode.

Figure 2 : TS613 as a differential line driver with

a +12V single supply

100n

+12V

1k

Vi

Vi Vo

100n

10µ 100n

1k

GND

47k

47k

8

3

+12V

+

_

1

2

R2

R1

R3

_

6

7

5

+

GND

4

12.5

12.5

1µ

10n

Vo

1:2

Hybrid
&
Transformer

100Ω

Ω

25

The Figure1 shows a typ ical analog lin e interface
used for ADSL. The upstream and downstream
signals are separat ed 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
analog

digital
treatment

analog to
digital

June 2000

emission
(analog)

reception
(analog)

LP filter

TS613
Line Driver

reception
circuits

upstream

impedance
matching

downstream

HYBRID
CIRCUIT

twisted-pair
telephone

line

The driver is biased with a m id supply (nominaly
+6V), in order to maintain the DC component of
the signal at +6V. Th is allows the maximum dy ­namic range between 0 and +12 V. Several op­tions 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 re­quired 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 consid­er this bias current (5µA) as the 1% of the current
through the resistance divi der (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 whi ch is neces­sary to remove the DC component of the input sig­nal. To avoid DC current flowing in the primary of
the transformer, an output capacitor is used.

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AN1281

The 1µF capacitance provides a path for low fre­quencies, the 10nF capacitance provides a path
for high end of the spectrum.

In differential mode the TS613 is able to deliver a
typical amplitude signal of 18V peak to peak.

The dynamic li ne i mpe danc e is 100Ω . The typical
value of the amplitude sign al required on the line
is up to 12.4V peak t o peak. B y us ing a 1 :2 trans­former ratio the reflected impedance back t o the
primary will be a quarter (25Ω ) and therefore the
amplitude of the signal required with this imped­ance will be the half (6.2 V peak to peak). Assum­ing the 25Ω series resistance (12.5Ω for both out­puts) necessary for impedance matching, the out­put signal amplitude required is 12.4 V peak to
peak. This value is acc eptable for the TS613. In
this case the load impedance is 25Ω for each driv­er.

For the ADSL up stream path, a lowpass filter is
absolutely necessary t o cuto ff the high er frequ en­cies from the DAC analog output. In this simple
non-inverting amplification configuration, it will be
easy to implement a Sallen-Key lowpass filter by
using the TS613. For AD SL over POTS, a maxi­mum frequency of 135kHz is reached. For ADSL
over ISDN, the maximum frequency will be
276kHz.

INCREASING THE LINE LEVEL BY USING AN
ACTIVE IMPEDANCE MATCHING

With passive matching, the output signal ampli­tude of the driver must be twice the amplitude on
the load. To go beyond this limitation an active
maching impedan ce can be used. With this tec h­nique it is possible to keep good impedance
matching with an amplitude on the load higher
than the half of the oup ut driver amplitude. This
concept is shown in figure3 for a differential line.

Figure 3 : TS613 as a differential line driver with

an active impedance matching

100n

+12V

1k

Vi

Vi Vo

100n

10µ 100n

1k

GND

47k

47k

8

3

+12V

+

_

2

R2

R3

R1

R5

R4

_

6

5

+

4

GND

12.5

1

Vo°

Vo°

12.5

7

1µ

10n

Vo

1:2

Hybrid
&
Trans for mer

100Ω

Ω

25

Compon ent calculat i on:

Let us consider the equivalent c ircuit for a single
ended configuration, figure4.

Figure 4 : Single ended equivalent circuit

+

Rs1

Vi

1/2

Let us consider the unloaded system . Assuming
the currents through R1, R2 and R3

as respectively:

As Vo° equals Vo without load, the gain in this
case becomes :

The gain, for the loaded system will be (1):

GL

As shown in figure5, this system is an ideal gener­ator with a synthesized impedance as the i nterna l
impedance of the system. From this, the output
voltage becomes:

with Ro the synthesized impedance and Iout the
output current. On the other hand Vo can be ex­pressed a s:

Vo

_

R2

R1

G

Vo withload()

------------------------------------

R3

2

Vi Vo°

()

Vi

--------------------------

---------

,

1

R

Vo noload()

-------------------------------

==

Vi

Vi

Vo ViG()RoIout()



1

Vi

++



---------------------------------------------- -

1

Vo°

-1

–

2

R

Vi Vo

()

----------------------- -

and

1

++

-----------------------------------

1

++

1

-----------------------------------

-- -

2

–= 2

2R2

---------- -

–

2

R

------ -

1

3

R

R

2

R

------ -

3

R

Vo

+

3

R

2R2

---------- -

1

R

2

R

-------

1

–

3

R

2R2

---------- -

1

R

2

R

------ -

1

–

3

R

,

()

Rs1Iout

---------------------

R

------ -

1

–

R

1/2

RL

2

R

------ -

3

R

2

R

-------

3

R

1(),==

3(),–=

2
3

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AN1281

By identification of both e quat ions (2) a nd (3), the
synthesized impedance is, with Rs1=Rs2=Rs:

Figure 5 : Equivalent schematic. Ro is the syn-

thesized impedance

Ro

Iout

Rs

---------------- -

R

------ -

1

–

R

RL

1/2

4(),=

2
3

Ro

Vi.Gi

Unlike the level Vo° required for a passive imped­ance, Vo° will be smaller than 2Vo in our case. Let
us write Vo°=kVo with k the matching factor vary­ing between 1 and 2. Assum ing that the current
through R3 i s negligeable, it comes the fo llowing
resistance divider:

Ro

RL2Rs

1+

kVoRL

---------------------------

=

After choosing the k factor, Rs will equal to
1/2RL(k-1).
A good impedance matching assume s:

1

-- -

Ro

,=

RL5()

2

From (4) and (5) it becomes:

R

------ -

R

2
3

2

Rs

--------- -

1

RL

6(),–=

By fixing an arbitrary value for R2, (6) gives:

R

1

2

Rs

--------- -

–

RL

2

R

-------------------

3

=

Finally, the values of R2 and R3 allow us to extract
R1 from (1), and it comes:

---------------------------------------------------------

1

R

21

2R2

2

R



------ -

–

R

3

GL



7(),=

2

R

------ -

1–

–

3

R

with GL the required gain.

GL (gain for the
loaded system)

R1 2R2/[2(1-R2/R3)GL-1-R2/R3]
R2 (=R4) Abrit ra ry fixed
R3 (=R5) R2/(1-Rs/0.5RL)

Rs 0.5RL(k-1)

GL is fixed for the application requirements
GL=Vo/Vi=0.5(1+2R2/R1+R2/R3)/(1-R2/R3)

CAPABILITIES

The table below shows the calculated compo­nents for different values of k. In this case
R2=1000Ω and the gain=16dB. The last column
displays the maximu m amplitude level on the line
regarding the TS613 maximum output capabilities
(18Vpp diff.) and a 1:2 line transformer ratio.

Active matching

TS613 Output

R1

k

(Ω)R3(Ω)Rs(Ω)

1.3 820 1500 3.9 8 27.5

1.4 490 1600 5.1 8.7 25.7

1.5 360 2200 6.2 9.3 25.3

1.6 270 2400 7.5 9.9 23.7

1.7 240 3300 9.1 10.5 22.3
Passive matching 12.4 18

Level to get

12.4Vpp on
the line

(Vpp diff)

Maximum
Line level

(Vpp diff)

MEASUREMENT OF THE POWER
CONSUMPTION IN THE ADSL APPLICATION

Conditions:

Passive impedance matching
Transformer turns ratio: 2
Maximun level required on the line: 12.4Vpp
Maximum output level of the driver: 12.4Vpp
Crest factor: 5.3 (Vp/Vrms)

The TS613 power consumption during em ission
on 900 and 4550 meter twisted pair telephone
lines: 360mW

Information furnished is bel ieved to be accurate and reliable. However, STMicroe lectronics assumes no responsibility for the
consequences of use of such information nor for any infringement of patents or other rights of third parties which may result from
its use. No li cense is granted by i mp lication or otherwise under any patent or pate nt rights of STMicroelectronics. Specifications
mentioned in this publication ar e subject to change without notice. This publication supersedes and replaces all information
previously supplied. S TMicroelectronics products are not authorized for use as critica l components in life suppo rt devices or
systems without express written approval of STMicroelectronics.

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