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TA0313

STMicroelectronics Solutions for ADSL Line Interfaces

The input provides two high-pass filters with a break frequency of about 1.6kHz which is necessary to

remove the DC component of the input signal. To avoid DC current flowing into the primary side of the

transformer, an output capacitor is used. The 1

µF capacitance provides a path for low frequencies, the

10nF capacitance provides a path for high end of the spectrum.

Filtering

As the hybrid circuit cannot perfectly separate the upstream signal and the downstream signal, any

distortion from the upstream signal could affect the downstream signal. For the upstream path, a low-

pass filter becomes absolutely necessary in order to cut off the higher frequencies from the DAC analog

output and the driver distortions. In this simple non-inverting amplification configuration, it is easy to

implement a Sallen-Key low-pass filter by using the TS613 or TS612.

A first solution is to use a LC cell before the driver to provide the low-pass filtering. Nevertheless, as

shown in Figure 6, using 2nd order active filtering is a good solution especially as regards cost and

space-saving considerations.

In the configuration shown in Figure 6, we assume R4=R6, R5=R7, C1=C3 and C2=C4.

Figure 5: Transmission path filtering

Figure 6: TS613 line driver with 2nd order active filtering

30 kH z

PO T S

U p stream

Dow n s t re a m

lo w pas s filter (trans m is s ion path)

D is tor tion

13 0kH z (o v e r P O T S )

27 6kH z (o v e r IS D N )

½R1

R3

R2

_

+

_

+

Vi

Vi

Vo

Vo

3

2

6

5

1

7

4

GND

8

+12V

25

GND

+12V

3k9

3k9

10µ

100n

100n

100n

1k

1k

12.5

Ω

10n

_

+

_

+

Vi

Vi

Vo

Vo

3

2

6

5

1

7

4

GND

8

+12V

25

Ω

GND

3k9

3k9

10µ

100n

100n

100n

1k

1k

10n

µ

½R1

12.5

Ω

R4

R5

C2

C4

R6

R7

C1

C3

½R1

R3

R2

_

+

_

+

Vi

Vi

Vo

Vo

3

2

6

5

1

7

4

GND

8

+12V

25

GND

+12V

3k9

3k9

10µ

100n

100n

100n

1k

1k

12.5

Ω

10n

_

+

_

+

Vi

Vi

Vo

Vo

3

2

6

5

1

7

4

GND

8

+12V

25

Ω

GND

3k9

3k9

10µ

100n

100n

100n

1k

1k

10n

µ

½R1

12.5

Ω

R4

R5

C2

C4

R6

R7

C1

C3