Datasheet

Snubber Circuit
3-6
3.4 Snubber Circuit
Figure 3−4. Addition of Snubber Circuit to Active Termination
49.9 W
= RP
R18
R5
Z2
0
R1
R6
J7
Vout−
TP3
Vo−
= RP
J6
Vout+
C5
+
−
U1A
THS6043
2
3
1
14
4
−V
TP4
Vout−
J1
IN1
= RF
R24
0
R23
0
1:n
= RF
TP2
Vout+
= RS
C3
R19
R7
<<< Zin
+
−
U1B
THS6043
12
11
13
<<< Zin
J2
IN2
R11
V Line
= 2 RG
R17
Z1
0
= RS
R12
Line =
TP1
Vo+
R16
+V
49.9 W
210 W
1 mF
750 W
750 W
49.9 W
49.9 W
100 W
R19 and C5 are located on the EVM so that a snubber circuit may be
implemented. Some transformers have a high resonance frequency (as low
as 25 MHz but as high as 150 MHz). When using traditional termination (just
R17 and R18—no active termination), there is typically no reason to use these
components. But, when active termination is used, the effective impedance of
these two resistor values drops substantially. Thus, there can be very small
resistor isolation between the amplifier—and a resonance problem. Couple
this with the feedback path of R6 and R11, and this can cause the amplifier to
oscillate. The snubber is utilized to eliminate this oscillation. As a rule of thumb,
use the following calculations to select the proper snubber values:
(5)
Then select C5:
C5 +
1
2 p R19 F
C
(6)
R19 + 2
R
LINE
n
2
where F
C
= at least 10 X the highest operating frequency (1.104MHz is the
highest ADSL operating frequency). 20X or even larger may be preferable.