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TA0313

STMicroelectronics Solutions for ADSL Line Interfaces

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 matching impedance can be used. With this technique it is possible

to maintain good impedance matching with an amplitude on the load higher than half of the output driver

amplitude. This concept is shown in Figure 7 for a differential line.

Component calculation

Let us consider the equivalent circuit for a single-ended configuration, as shown in Figure 8.

For unloaded system, we can assume that currents through R1, R2 and R3 are respectively:

Figure 7: TS613 as a differential line driver with an active impedance matching

Figure 8: Single ended equivalent circuit

½R1

R4

R2

_

+

_

+

Vi

Vi

Vo

Vo

3

2

6

5

1

7

4

GND

8

+12V

25

100

Ω

1:2

Hybrid

&

Transformer

GND

+12V

3k9

3k9

10µ

100n

100n

100n

1k

1k

Rs1

10n

1µ

_

+

_

+

Vi

Vi

Vo

Vo

3

2

6

5

1

7

4

GND

8

+12V

25

Ω

1:2

Hybrid

&

Transformer

GND

+12V

3k9

3k9

10µ

100n

100n

100n

1k

1k

10n

1µ

Rs2

R5

R3

½R1

Vo°

Vo°

½R1

R4

R2

_

+

_

+

Vi

Vi

Vo

Vo

3

2

6

5

1

7

4

GND

8

+12V

25

100

Ω

1:2

Hybrid

&

Transformer

GND

+12V

3k9

3k9

10µ

100n

100n

100n

1k

1k

Rs1

10n

1µ

_

+

_

+

Vi

Vi

Vo

Vo

3

2

6

5

1

7

4

GND

8

+12V

25

Ω

1:2

Hybrid

&

Transformer

GND

+12V

3k9

3k9

10µ

100n

100n

100n

1k

1k

10n

1µ

Rs2

R5

R3

½R1

Vo°

Vo°

1/2

R1

R2

R3

+

_

Vi

Vo

Rs1

-1

Vo°

1/2

RL

2Vi

R1

---------

Vi

Vo

°

–

()

R 2

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

Vi

Vo

+

()

R 3

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

,