AD8390

Rev. C | Page 11 of 16

+

–

–OUT

+OUT

R3

R2

R1

R1

R3

0.1

µF

R2

1:N

10

µF

0.1

µF

+IN

–IN

10

µF

0.1

µF

0.1

µF

Figure 24. ADSL/ADSL2+ Application Circuit

Referring to Figure 24, the following describes how to calculate

the resistor values necessary to obtain the desired input imped-

ance, gain, and output impedance.

The differential input impedance to the circuit is simply 2R1.

As such, R1 is chosen by the designer to yield the desired input

impedance.

When synthesizing the output impedance, a factor k is

introduced, which is used to express the ratio of the negative

feedback resistor to the positive feedback resistor by

R2

R3

k =

−

1

(5)

Along with the turns ratio N, k is also used to define the value of

are 0.1 to 0.25. A k value of 0.1 would result in back termination

resistors that are only 1/10 as large as those in the simplest case

described above. Lower values of k result in greater amounts of

positive feedback. Therefore, values much lower than 0.1 can

lead to instability and are generally not recommended.

2

2 N

R

k

R

L

M

×

×

=

(6)

then used to calculate the necessary values for R3 and R2.

(

)

M

M

V

V

V

R

k

R

k

R1

A

R1

A

k

R1

A

R3

×

−

+

×

×

×

×

+

×

×

=

2

(7)

for R3.

V

A

k

R1

R3

×

×

×

≅

2

(8)

k

R3

R2

−

=

1

(9)

be chosen and the gain rechecked with the following equation:

R3

R2

R2

k

R

R3

R2

A

M

V

×

−

+

×

+

×

=

(10)

Table 6 shows a comparison of the results using the exact values,

the simplified approximation, and the closest 1% resistor values.

and 0.1 kΩ, respectively.

It should be noted that decreasing the value of the back termi-

nation resistors attenuates the receive signal by approximately

1/k. However, advances in low noise receive amplifiers permit

k values as small as 0.1 to be commonly used.

The line impedance, turns ratio, and k factor specify the output

voltage and current requirements from the AD8390. To accom-

modate higher crest factors or lower supply rails, the turns ratio,

N, may have to be increased. Since 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 visited.

Table 6. Resistor Selection

Component

Exact

Values

Approximate

Calculation

Standard 1%

Resistor

Values

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 (Eq. 5)

0.1

0.1

0.095

MULTITONE POWER RATIO (MTPR)

Multitone power ratio is a commonly used figure of merit that

xDSL designers use to help describe system performance.

MTPR is the measured delta between the peak of a filled

frequency bin and the harmonic products that appear in an

intentionally empty frequency bin. Figure 25 illustrates this

principle. The plots in Figure 10 and Figure 13 show MTPR

performance in various power modes. All data were taken with

a circuit with a k factor of 0.1, a 1:1 turns ratio transformer, and

a waveform with a 5.4 peak-to-average ratio, also known as the

crest factor (CF).

–70dBc

10dB/DIV

CENTER 431.25kHz

SPAN 10kHz

1kHz/DIV

Figure 25. MTPR Measurement