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LT1969

APPLICATIO S I FOR ATIO

and assuming RP >> RL, we require

Va = Vo (1 – 1/n)

solving

RF/RP = 1 – 1/n

So to reduce the back-termination by a factor of 3 choose

RF/RP = 2/3. Note that the overall gain is increased to:

Vo/Vi = (1 + RF/RG + RF/RP)/[2(1 – RF/RP)]

ADSL Driver Requirements

The LT1969 is an ideal choice for ADSL upstream (CPE)

modems. The key advantages are:

±200mA output drive

with only 1.7V worst-case total supply voltage headroom,

high bandwidth, which helps achieve low distortion, low

quiescent supply current of 7mA per amplifier and a

space-saving, thermally enhanced MS10 package.

An ADSL remote terminal driver must deliver an average

power of 13dBm (20mW) into a 100

Ω line. This corre-

sponds to 1.41VRMS into the line. The DMT-ADSL peak-to-

average ratio of 5.33 implies voltage peaks of 7.53V into

the line. Using a differential drive configuration and trans-

former coupling with standard back-termination, a trans-

former ratio of 1:2 is well suited. This is shown on the front

page of this data sheet along with the distortion perfor-

mance vs line voltage at 200kHz, which is beyond ADSL

requirements. Note that the distortion is better than

–73dBc for all swings up to 16VP-P into the line. The gain

of this circuit from the differential inputs to the line voltage

is 10. Lower gains are easy to implement using the

compensation techniques of Figure 5. Table 2 shows the

drive requirements for this standard circuit.

The above design is an excellent choice for desktop

applications and draws typically 550mW of power. For

portable applications, power savings can be achieved by

reducing the back-termination resistor using positive feed-

back as shown in Figure 9. The overall gain of this circuit

Figure 9. Power Saving ADSL Modem Driver

–

8.45

Ω

523

Ω

1.21k

523

Ω

µF

–Vi

AV = 10

8.45

Ω

1969 F09

1:1

100

Ω

Table 2. ADSL Upstream Driver Designs

STANDARD

LOW POWER

Line Impedance

100

Ω

100

Ω

Line Power

13dBm

Peak-to-Average Ratio

5.33

Transformer Turns Ratio

Reflected Impedance

Ω

100

Ω

Back-Termination Resistors

12.5

Ω

8.35

Ω

Transformer Insertion Loss

1dB

0.5dB

Average Amplifier Swing

0.79VRMS

0.87VRMS

Average Amplifier Current

31.7mARMS

15mARMS

Peak Amplifier Swing

4.21V Peak

4.65V Peak

Peak Amplifier Current

169mA Peak

80mA Peak

Total Average Power Consumption

550mW

350mW

Supply Voltage

Single 12V

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