Datasheet

AD8392A
Rev. 0 | Page 9 of 12
TYPICAL ADSL/ADSL2+ APPLICATION
In a typical ADSL/ADSL2+ application, a differential line driver
is used to take the signal from the analog front end (AFE) and
drive it onto the twisted pair telephone line. Referring to the
typical circuit representation in
Figure 18, the differential input
appears at V
IN+
and V
IN−
from the AFE, while the differential
output is transformer coupled to the telephone line at tip and
ring. The common-mode operating point, generally midway
between the supplies, is set through V
COM
.
R4
R4
R3
R3
R
m
R2
R1
R2
V
COM
1:N
TIP
RING
R
OUT
R
IN
R
m
V
OA
V
OA
V
P
V
P
R
BIAS
R
BIAS
V
IN–
V
IN+
06477-024
Figure 18. Typical ADSL/ADSL2+ Application Circuit
In ADSL/ADSL2+ applications, it is common practice to
conserve power by using positive feedback to synthesize the
output resistance, thereby lowering the required ohmic value
of the line matching resistors, R
m
. The circuit in Figure 18 is
somewhat unique in that the positive feedback introduced via
R3 has the effect of synthesizing the input resistance as well.
The following definitions and equations can be used to calculate
the resistor values necessary to obtain the desired gain, input
resistance, and output resistance for a given application. For
simplicity, the following calculations assume a lossless
transformer.
The following values are used in the design equations and are
assumed already known or chosen by the designer.
Value Definition
V
IN
Differential input voltage
R
IN
Desired differential input resistance
N
Transformer turns ratio
V
LINE
Differential output voltage at tip and ring
R
m
Each is typically 5% to 15% of the transformer reflected
line impedance
R2
Recommended in the amplifier data sheet
V
P
Voltage at the + inputs to the amplifier, approximately
½ V
IN
(must be less than V
IN
for positive input resistance)
R
L
Transformer reflected line impedance
Additional definitions for calculating resistor values include:
Value Definition
V
OA
Voltage at the amplifier outputs
k
Matching resistance reduction factor
A
V
Gain from V
IN
to transformer primary
β
Negative feedback factor
α
Positive feedback factor
Note: R1 must be calculated before β and α.
(
)
N
kV
V
LINE
OA
+
=
1
L
m
R
R
k
2
=
IN
LINE
V
VN
V
A =
R2R1
R1
2
β
+
=
(
) k
−
=
1βα
With the above known quantities and definitions, the remaining
resistors can readily be calculated.
P
OA
P
VV
RV
R1
−
=
22
(
)
IN
PININ
V2
VVR
R4
−
=
(
)
()
R2R1R
RR2RR1RR1RR1R4A
R3
L
LLL
m
V
2α
2αα2
+
−
−
+
=
() R4R3R4
R4R3
R
BIAS
+−
=
α
α
After building the circuit with the closest 1% resistor values,
the actual gain, input resistance, and output resistance can be
verified with the following equations.
()
()
R3
R4
R
R4
R3
R4
k
N
GAIN
BIAS
LINEtoV
IN
−
⎟
⎠
⎞
⎜
⎝
⎛
+++
=
11β
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
−
=
L
L
m
V
IN
RR4
RR
A
R4
R
2
β
1
2
()
⎟
⎟
⎟
⎟
⎠
⎞
⎜
⎜
⎜
⎜
⎝
⎛
+
+
+
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
+
−
=
BIAS
BIAS
BIAS
BIAS
m
OUT
RR4
RR4
R3
R2R1
RR4R1
RR4
NR
R
2
1
2
2