13

LT1886

APPLICATIO S I FOR ATIO

Table 2 compares the two approaches. It may seem that

the low power design is a clear choice, but there are further

system issues to consider. In addition to driving the line,

the amplifiers provide back-termination for signals that

are received simultaneously from the line. In order to

reject the drive signal, a receiver circuit is used such as

shown in Figure 10. Taking advantage of the differential

nature of the signals, the receiver can subtract out the

drive signal and amplify the received signal. This method

works well for standard back-termination. If the back-

termination resistors are reduced by positive feedback, a

portion of the received signal also appears at the amplifier

outputs. The result is that the received signal is attenuated

–

+

8.45

Ω

1k

523

Ω

1.21k

1.21k

523

Ω

1

µF

8.45

Ω

1886 F09

1k

1:1

100

Ω

1886 F10

1:n

+

–

+

–

LT1813

LT1813

= REFLECTED IMPEDANCE

=

SET

Figure 9. Power Saving ADSL Modem Driver

Figure 10. Receiver Configuration

Table 2. ADSL Upstream Driver Designs

STANDARD

LOW POWER

Line Impedance

100

Ω

100

Ω

Line Power

13dBm

13dBm

Peak-to-Average Ratio

5.33

5.33

Transformer Turns Ratio

2

1

Reflected Impedance

25

Ω

100

Ω

Back-Termination Resistors

12.5

Ω

8.35

Ω

Transformer Insertion Loss

1dB

0.5dB

Average Amplifier Swing

Average Amplifier Current

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

Single 12V

by the same amount as the reduction in the back-termina-

tion resistor. Taking into account the different transformer

turns ratios, the received signal of the low power design

will be one third of the standard design received signal.

The reduced signal has system implications for the sensi-

tivity of the receiver. The power reduction may, or may not,

be an acceptable system tradeoff for a given design.

Demo Board

Demo board DC304 has been created to provide a versatile

platform for a line driver/receiver design. (Figure 11 shows

a complete schematic.) The board is set up for either single

or dual supply designs with Jumpers 1–4. The LT1886 is

set up for differential, noninverting gain of 3. Each amp is

configured as in Figure 5 for maximum flexibility. The

amplifiers drive a 1:2 transformer through back-termina-

tion resistors that can be reduced with optional positive

feedback. The secondary of the transformer can be iso-

lated from the primary with Jumper 5.

A differential receiver is included using the LT1813, a dual

100MHz, 750V/

µs operational amplifier. The receiver gain

from the transformer secondary is 2, and the drive signals

are rejected by approximately a factor of 14dB. Other

optional components include filter capacitors and an RC

snubber network at the transformer primary.