TS612 ST Microelectronics, Inc., TS612 Datasheet - Page 8

• Manufacturer Part Number: TS612
• Description: Dual Wideband Op-amp With High Output Current
• Manufacturer: ST Microelectronics, Inc.

TS612
For the ADSL upstream path, a lowpass filter is
absolutely necessary to cutoff the higher frequen-
cies from the DAC analog output. In this simple
non-inverting amplification configuration, it will be
easy to implement a Sallen-Key lowpass filter by
using the TS612. For ADSL over POTS, a maxi-
mum frequency of 135kHz is reached. For ADSL
over ISDN, the maximum frequency will be
276kHz.
INCREASING THE LINE LEVEL BY USING AN
ACTIVE IMPEDANCE MATCHING
With passive matching, the output signal ampli-
tude of the driver must be twice the amplitude on
the load. To go beyond this limitation an active
maching impedance can be used. With this tech-
nique it is possible to keep good impedance
matching with an amplitude on the load higher
than the half of the ouput driver amplitude. This
concept is shown in figure3 for a differential line.
Figure 3 : TS612 as a differential line driver with
8/10
Vi
Vi
100n
100n
1k
1k
10
Vcc+
GND
an active impedance matching
100n
1/2
1/2
R1
R1
+
+
_
_
Vcc/2
R2
R3
R5
R4
Vcc+
GND
Vcc+
GND
Vo°
Vo°
Rs1
Rs2
Vo
Vo
10n
1
RL
Transformer
Hybrid
1:n
&
100
Component calculation:
Let us consider the equivalent circuit for a single
ended configuration, figure4.
Figure 4 : Single ended equivalent circuit
Let us consider the unloaded system. Assuming
the currents through R1, R2 and R3
as respectively:
As Vo° equals Vo without load, the gain in this
case becomes :
The gain, for the loaded system will be (1):
As shown in figure5, this system is an ideal gener-
ator with a synthesized impedance as the internal
impedance of the system. From this, the output
voltage becomes:
with Ro the synthesized impedance and Iout the
output current. On the other hand Vo can be ex-
pressed as:
By identification of both equations (2) and (3), the
synthesized impedance is, with Rs1=Rs2=Rs:
GL
Vo
=
G
=
1/2
Vo withload
----------------------------------- -
=
Vi
R1
Vo
Vi 1
---------------------------------------------- -
2Vi
-------- -
R1
Vo noload
------------------------------ -
=
Vi
Ro
R2
------------------------- - and
+
+
_
1
Vi
Vi Vo
ViG
2R2
---------- -
–
R1
R3
=
–
R2
R2
------ -
R3
---------------- - 4
1
–
+
–
Rs
=
R2
------ -
R3
=
RoIout
R2
------ -
R3
Vo°
1
-- -
2
1
---------------------------------- -
1
---------------------------------- - 1
Rs1
,
-1
–
+
+
----------------------- -
1
Rs1Iout
--------------------- 3
Vi
2R2
---------- -
1
2R2
---------- -
R1
1
R1
–
–
,
R3
–
+
Vo
R2
------ -
R3
2
R2
------ -
R3
R2
------ -
R3
Vo
+
+
R2
------ -
R3
R2
------ -
R3
1/2
,
RL
,