TA0313 Datasheet, PDF (4/9 Pages) STMicroelectronics – STMicroelectronics Solutions for ADSL Line Interfaces

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 1µF 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.
Figure 5: Transmission path filtering
U pstream
D istortion
Dow nstream
POTS
30 kH z
130kH z (over PO TS)
27 6kH z (o v er IS D N )
lo w p a s s filte r (tra n s m is s io n p a th )
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.
Figure 6: TS613 line driver with 2nd order active filtering
100n
R4
R5
+12V
1k
Vi
C1
3k9
Vi
10µ _3k9 100n
C3
1k _GND
R6
R7
100n
C2
3 ₊ ⁸ _+12V
2^_
1
R2
½ R1
½ R1
6 ^{_ R3}
7
5+
GND
4
C4
12.5Ω
12.5Ω
µ
10n
Vo
25Ω
Vo
In the configuration shown in Figure 6, we assume R4=R6, R5=R7, C1=C3 and C2=C4.
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