12

LT1886

APPLICATIO S I FOR ATIO

= 12.3.

Note that the overall gain is increased:

V

V

RR

R

nR

R

R

R

R

o

i

PP

P

FG

P

P

P

=

+

()

+

()

()

[]

22

1

12

1

11

1

/

//

/

/

A simpler method of using positive feedback to reduce the

back-termination is shown in Figure 8. In this case, the

drivers are driven differentially and provide complemen-

tary outputs. Grounding the inputs, we see there is invert-

solving

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

ADSL Driver Requirements

The LT1886 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 SO-8 package.

An ADSL remote terminal driver must deliver an average

power of 13dBm (20mW) into a 100

Ω line. This corre-

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

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

is also 10, but the power consumption has been reduced

to 350mW, a savings of 36% over the previous design.

Note that the reduction of the back-termination resistor

has allowed use of a 1:1 transformer ratio.

1886 F07

1 +

n

=

= 1 –

–

1

n

()

1 +

()

()

1 + 1/n

Figure 7. Back-Termination Using Positive Feedback

–

+

1886 F08

n

=

n =

1 –

2

+

1 +

1 –

1

()

Figure 8. Back-Termination Using Differential Positive Feedback