AD8390A 查看數據表（PDF） - Analog Devices

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AD8390A

Low Power, High Output Current Differential Amplifier

Analog Devices 

Data Sheet

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

VIN+ 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.

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.

VCC

R2

0.1µF 10µF

R3

R1

+IN

0.1µF

VCOM

R1

0.1µF

–IN

VEE

0.1µF

10µF

RADJ

–OUT RM

+OUT RM

R3

R2

1:N

+

RL VOUT, DM

–

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.

RM

=

k×

RL

2×N2

where RL is the line impedance, and N is the turns ratio of the

transformer.

The factor k defines the relationship between the negative and

positive feedback resistors and is given by

k = 1 − R3

R2

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.

AD8390A

Assuming low values for back termination resistor RM, R3 is

approximated as

R3 ≅ R1× 2× k × AV

where AV is the voltage gain.

R2 is given by

R2 = R3

1−k

With RM, R3, and R2 calculated, the closest 1% resistors are

chosen and the gain rechecked with the following equation:

AV

=

R1 [R M

R2 × R3

+ 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.

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

R1 (Ω)

1000

R2 (Ω)

2246.95

R3 (Ω)

2022.25

RM (Ω)

5

Actual AV

10.000

Actual k

0.1

Approximate

Calculation

1000

2222.22

2000

5

9.889

0.1

Standard 1%

Resistor Value

1000

2210

2000

4.99

10.138

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.

Rev. B | Page 11 of 12

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