TA0313 Datasheet, PDF (5/9 Pages) STMicroelectronics – STMicroelectronics Solutions for ADSL Line Interfaces

STMicroelectronics Solutions for ADSL Line Interfaces
TA0313
The resistances R1, R2 and R3 allow us to calculate the gain of the structure as follows:
Gain = 1 + ²----^R-----2- = 1 + ²----^R-----³-
R1
R1
The damping factor can be derived from these resistances and the capacitances C1, C2, C3 and C4:
ζ = ²----^C-----¹----^–-----^α----^C-----²- = ²----^C-----³----^–-----^α----^C-----⁴-
2 C1C2
2 C3C4
with:
α = ²--_R--^R---₁--²- = ²--_R--^R---₁--³-
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.
The value of the gain does not affect the cut-off frequency, f_c, which is derived as follows:
fc = ---------------------¹---------------------- = ---------------------¹----------------------
2π R4R5C1C2 2π R6R7C3C4
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:
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
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 100Ω. 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 (25Ω) and therefore the amplitude of the signal required with this impedance will be halved
(6.2 V peak to peak). Assuming a 25Ω 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 25Ω for each driver in single
ended.
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