Texas Instruments THS6132EVM, Руководство пользователя

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Active Termination
3-3 THS6132EVM 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
positive 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
14
3
2(5)
2
12
R2 = RF
R15 = Rs
C2
R3 = Rp
R6 = Rs
R12 = Rp
C1
+
-
U1B
THS6132
11
R1
R10
Z1 R9
J2
Vin1
+
-
U1A
THS6132
R11 = RF
R19
Z2 R18
J4
Vin2
TP4
1:n
TP2
Vout+
Line =
V Line
Vout+
Vo1
TP3
Vo2
TP1
49.9
0 0
12.4
1 k
2 k
0.1 F
500
R23 = 2 RG
2 k
12 k
12.4
49.9
0 0
100
Note: Component pin numbers shown are for THS6132RGW while pin numbers in parentheses are for THS6132VFP.