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Active Termination
Active feedback creates larger impedance (Z) than what is actually placed
there by series resistors R S :
S
1– F
P
Z(W) +
R
R
R
(2)
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:
R +
F
1 * S
P
R
R
P
(3)
Using Z = 50 ? and values from Figure 3?2 in equation 3, yields 1995 ? for
R P . The closest E96(1%) value to 1995 ? is 2 k ? , 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.
A + O
1 *
F
P
G
L
R ) R
V V
V
in
"
"
+
1 )
R
R
F
P
R
R || R
R
L
S
if R
L
tt R
P
(4)
where
R +
LINE
L
R
2 n 2
(5)
With a transformer ratio( n ) of 1 and a R LINE of 100 ? , R L is 50 ? .
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
3-4
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