Datasheet

Active Termination
3-3
THS6182DWEVM Applications
3.2 Active Termination
Although this application is specifically for use as an ADSL line driver, the
principals shown can be applied to other applications.
Active termination is a technique that allows the designer to use a small value
resistor for the series resistance (R6 and, or R15). The circuit then utilizes pos-
itive feedback to make the impedance of this resistor appear much larger,
when looking from the line-side. This accomplishes two things:
- A very small resistance exists between the amplifier and the transformer.
This lowers the output voltage swing range required from the driver stage.
- Proper matching impedance appears when looking from the line to the
amplifier.
Figure 3−2 shows the basic circuit for differential positive feedback.
Figure 3−2. Differential Positive Feedback
+V
1
13
2
8
20
R2 = RF
R15 = Rs
C2
R3 = Rp
R6 = Rs
R12 = Rp
C1
+
−
U1B
THS6182
19
R1
R10
Z1 R9
J2
Vin1
−V
+
−
U1A
THS6182
18
3
R11 = RF
R19
Z2 R18
J4
Vin2
TP4
1:n
TP2
Vout+
Line =
V Line
Vout+
Vo1
TP3
Vo2
TP1
49.9 W
0 W 0 W
12.4 W
1.5 k W
2 k W
0.1 mF
750 W
R23 = 2 RG
2 k W
1.5 k W
12.4 W
49.9 W
0 W 0 W
100 W