Data Sheet
AD8390A
Assuming low values for back termination resistor RM, R3 is
approximated as
ADSL AND ADSL2+ APPLICATIONS
In a typical ADSL/ADSL2+ application, a differential line driver
drives the signal from the analog front end (AFE) onto the
twisted pair telephone line. Referring to the typical circuit
representation in Figure 17, the differential input appears at
R3 ≅ R1×2×k ×AV
where AV is the voltage gain.
R2 is given by
V
IN+ and VIN− from the AFE. 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 VCOM.
R3
1− k
R2 =
With RM, R3, and R2 calculated, the closest 1% resistors are
chosen and the gain rechecked with the following equation:
In ADSL/ADSL2+ applications, it is common practice to
conserve power by using positive feedback (R3 in Figure 17) to
synthesize the output resistance, lowering the required value of
the line matching resistors, RM.
R2× R3
AV =
R1
[
RM + R2
(
k +1
)
− R3
]
Table 6 compares the results of the exact values, the simplified
approximation, and the closest 1% resistor value calculations. In
this example, R1 = 1.0 kΩ, AV = 10, and k = 0.1.
R2
R3
VCC
0.1µF
R1
10µF
Note that decreasing the value of the back termination resistors
attenuates the receive signal by approximately 1/k. Advances in
low noise receive amplifiers permit the use of k values as small
as 0.1.
0.1µF
+IN
R
R
M
1:N
–OUT
+
VCOM
R
V
L
OUT, DM
–
The line impedance, turns ratio, and k factor specify the output
voltage and current required from the AD8390A. To accom-
modate higher crest factors or lower supply rails, the turns ratio,
N, may need to be increased. Because higher turns ratios and
smaller k factors both attenuate the receive signal, a large
increase in N may require an increase in k to maintain the
desired noise performance. Any particular design process
requires that these trade-offs be addressed.
+OUT
R3
0.1µF
10µF
M
R1
–IN
R
ADJ
R2
VEE
0.1µF
Figure 17. ADSL/ADSL2+ Application Circuit
The differential input impedance to the circuit is 2 × R1.
R1 is chosen by the designer to match system requirements.
Table 6. Resistor Selection
The synthesized value of the back termination resistor is given
by the following equation.
Exact
Value
Approximate Standard 1%
Component
R1 (Ω)
R2 (Ω)
R3 (Ω)
RM (Ω)
Calculation
Resistor Value
1000
2246.95
2022.25
5
1000
1000
RL
RM = k×
2×N2
2222.22
2000
2210
2000
where RL is the line impedance, and N is the turns ratio of the
transformer.
5
4.99
Actual AV
Actual k
10.000
0.1
9.889
0.1
10.138
0.095
The factor k defines the relationship between the negative and
positive feedback resistors and is given by
LIGHTNING AND AC POWER FAULT
R3
R2
k =1 −
When the AD8390A is an ADSL/ADSL2+ line driver, it is
transformer-coupled to the twisted pair telephone line. In this
environment, the AD8390A is subject to large line transients
resulting from events such as lightning strikes or downed power
lines. Additional circuitry is required to protect the AD8390A
from damage due to these events.
Commonly used values for k are between 0.1 and 0.25. Values
less than 0.1 can lead to instability and are not recommended.
Rev. B | Page 11 of 12
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