TS612

8/10

For the ADSL upstream path, a lowpass filter is

absolutely necessary to cutoff the higher frequen-

cies 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 TS612. For ADSL over POTS, a maxi-

mum 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 ampli-

tude of the driver must be twice the amplitude on

the load. To go beyond this limitation an active

maching impedance can be used. With this tech-

nique it is possible to keep good impedance

matching with an amplitude on the load higher

than the half of the ouput driver amplitude. This

concept is shown in figure3 for a differential line.

Component calculation:

Let us consider the equivalent circuit for a single

ended configuration, figure4.

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):

As shown in figure5, this system is an ideal gener-

ator with a synthesized impedance as the internal

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 ex-

pressed as:

By identification of both equations (2) and (3), the

synthesized impedance is, with Rs1=Rs2=Rs:

Figure 3 : TS612 as a differential line driver with

an active impedance matching

R4

R2

Vi

Vi

Vo

Vo

RL

100

Ω

1:n

Hybrid

&

Transformer

GND

Vcc+

10

µ

100n

100n

100n

1k

1k

Rs1

Rs2

10n

1

µ

R3

R5

Vo°

Vo°

GND

Vcc+

Vcc+

+

_

+

_

GND

1/2

R1

1/2

R1

Vcc/2

Figure 4 : Single ended equivalent circuit

R2

R3

+

_

Vi

Vo

Rs1

-1

Vo°

2Vi

R1

---------

Vi

Vo

°

–

()

R2

--------------------------and

Vi

Vo

+

()

R3

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

,

G

Vo noload

()

Vi

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

1

2R2

R1

-----------

R 2

R 3

-------

++

1

R2

R3

-------

–

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

==

GL

Vo withload

()

Vi

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

1

2

---

1

2R2

R1

-----------

R2

R3

-------

++

1

R2

R3

-------

–

----------------------------------- 1

()

,

==

Vo

ViG

()

RoIout

()

–

=

2

()

,

Vo

Vi 1

2R2

R1

-----------

R2

R3

-------

++





1

R2

R3

-------

–

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

Rs1Iout

1

R2

R3

-------

–

--------------------- 3

()

,

–

=

Ro

Rs

1

R2

R3

-------

–

----------------- 4

()

,

=