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STMicroelectronics Solutions for ADSL Line Interfaces

TA0313

The resistances R1, R2 and R3 allow us to calculate the gain of the structure as follows:

The damping factor can be derived from these resistances and the capacitances C1, C2, C3 and C4:

with:

The higher the gain, the more sensitive the damping factor is. When the gain is higher than 1 it is

preferable to use very stable resistance and capacitance values.

Moreover this expression shows that it is possible to shift the cut-off frequency by simply changing the

values of the resistances R4, R5 or R6, R7 — with neither a change of capacitance nor of the damping

factor.

The following table shows a calculations of components for a cut-off frequency around 130kHz for the

ADSL over POTS and 270kHz for the ADSL over ISDN. The final, accurate settings are made by

compromising between the attenuation of the highest frequencies of the upstream signal and the impact

of the distortion on the downstream signal. This is best done directly in the application. Nevertheless we

can start with the following initial values:

Transformation ratio

In differential mode, the TS613 and TS612 are able to deliver a typical amplitude signal of 18V peak to

peak.

The dynamic line impedance is 100

Ω. The typical value of the amplitude signal required on the line is up

to 12.4V peak to peak. By using a 1:2 transformer ratio the reflected impedance back to the primary will

be a quarter (25

Ω) and therefore the amplitude of the signal required with this impedance will be halved

(6.2 V peak to peak). Assuming a 25

Ω series resistance (12.5Ω for both outputs) is necessary for

impedance matching, the output signal amplitude required is 12.4V peak to peak. This value is

acceptable for both TS613 and TS612. In this case, the load impedance is 25

Ω for each driver in single

ended.

R1

(

Ω)

R2

R3

(

Ω)

R4

R6

(

Ω)

R5

R7

(

Ω)

C1

C3

(nF)

C2

C4

(nF)

Gain

(dB)

fc

(kHz)

ζ

180

536

261

261

10

2.2

16.8

130

0.73

180

536

127

127

10

2.2

16.8

270

0.73

Gain

1

2R2

R1

-----------

+1

2R 3

R 1

-----------

+

==

ζ

2 C1

αC2

–

2 C1C 2

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

2C 3

αC4

–

2 C3C 4

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

==

α

2R 2

R 1

-----------

2R3

R1

-----------

==

fc

1

2

π R4R5C1C2

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

1

2

π R6R7C3C4

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

==