Datasheet

Active Termination
3-4
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.
Solving equation 2 for Rp, the following equation is produced:
(3)
R
P
+
R
F
1 *
R
S
P
Using Z = 50 Ω and values from Figure 3−2 in equation 3, yields 1995 Ω for
R
P
. The closest E96(1%) value to 1995 Ω is 2 kΩ, as shown in Figure 3−2.
Now that the return impedance is corrected, forward voltage gain from input
to output is calculated. Equation 3 shows the simplified forward gain from Vin
to Vo.
(4)
A
V
+
V
O
"
V
in
"
+
1 )
ǒ
R
F
R
G
|| R
P
Ǔ
1 *
ǒ
R
F
R
P
Ǔǒ
R
L
R
L
)R
S
Ǔ
if R
L
tt R
P
where
R
L
+
R
LINE
2n
2
(5)
With a transformer ratio( n ) of 1 and a R
LINE
of 100 Ω, R
L
is 50 Ω.
When the value R
L
and the values in Figure 3−2 are used in equation 4, the
resulting voltage gain is 14.5. Because R
G
does not affect the value of the
apparent output impedance of the circuit, voltage gain can be adjusted by
changing R
G
.
The reader is cautioned that active termination is a very complex topic, with
many considerations. Please carefully read the Texas Instruments Application
Report Active Output Impedance for ADSL Line Drivers, (SLOA100) to gain
a more complete understanding of the topic and all the subtle implications of
active termination.
R1 and C1 are located on the EVM so that a snubber circuit may be
implemented. Some transformers have a high resonant frequency (as low as
25 MHz but as high as 150 MHz). When using traditional termination (just R6,
and R15—no active termination), there is typically not a 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