Data Sheet

AD8390A

Rev. C | Page 11 of 12

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

transformer-coupled to the telephone line at tip and ring. The

common-mode operating point, generally midway between the

supplies, is set through VCOM.

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

+

–

–OUT

+OUT

R3

R2

R1

R1

R3

0.1µF

R2

1:N

10µF

0.1µF

VCC

+IN

VCOM

–IN

10µF

0.1µF

0.1µF

VEE

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.

The synthesized value of the back termination resistor is given

by the following equation.

2

2 N

R

k

R

L

M

×

×

=

transformer.

The factor k defines the relationship between the negative and

positive feedback resistors and is given by

2

1

R

R3

k

−

=

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.

approximated as

V

A

k

R1

R3

×

×

×

≅

2

R2 is given by

k

R3

R2

−

=

1

chosen and the gain rechecked with the following equation:

(

)

[

]

R3

k

R2

R

R1

R3

R2

A

M

V

−

+

+

×

=

1

Table 6 compares the results of the exact values, the simplified

approximation, and the closest 1% resistor value calculations. In

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.

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.

Table 6. Resistor Selection

Component

Exact

Value

Approximate

Calculation

Standard 1%

Resistor Value

R1 (Ω)

1000

1000

1000

R2 (Ω)

2246.95

2222.22

2210

R3 (Ω)

2022.25

2000

2000

5

5

4.99

10.000

9.889

10.138

Actual k

0.1

0.1

0.095

LIGHTNING AND AC POWER FAULT

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.