Datasheet

Active Termination
3-4
3.3 Active Termination
Active termination is a technique that allows the designer to use a small value
resistor for the series resistance (R17 or R18). The circuit then uses positive
feedback to make the impedance of this resistor appear much larger, when
looking from the line side. This accomplishes two things:
1) A very small resistance is evident when the line driver amplifier transmits
signals to the line. This lowers the driver stage output voltage swing range
requirement.
2) Proper matching impedance when looking from the line to the amplifier
Figure 3−3 shows the basic circuit for differential positive feedback.
Figure 3−3. Differential Positive Feedback
R1
R6
R16
R7
<<< Zin
<<< Zin
Z1
0
R5
Z2
0
= RF
R24
0
+V
Line =
R23
0
TP2
Vout+
= RF
J6
Vout+
= RP
= RPR11
= RS
TP1
Vo+
= RSR17
= 2 RG
C3
TP4
Vout −
J1
IN1
+
−
U1B
THS6043
12
11
13
R12
J2
IN2
R18
J7
Vout −
V Line
TP3
Vo −
1:n
+
−
U1A
THS6043
2
3
1
14
4
−V
49.9 W
210 W
1 mF
49.9 W
49.9 W
750 W
750 W
49.9 W
100 W
Active feedback creates larger impedance (Z) than what is actually placed
there by series resistors R
S
:
(2)
Z(W) +
R
S
1–
R
F
R
P
The important thing to consider is that regardless of the forward gain from
Vin to Vo, the active impedance (Z) value remains constant.