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LT1969

Figure 10. Receiver Configuration

RBT

–Va

–VL

RBT

1969 F10

1:n

–

VBIAS

VRX

–

LT1813

= REFLECTED IMPEDANCE

2n2

= ATTENUATION OF Va

+ RBT

2n2

SET

+ RBT

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.

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

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.

Controlling the Quiescent Current

The quiescent current of the LT1969 is controlled via two

control pins, CTRL1 and CTRL2. The pins can be used to

either turn off the amplifiers, reducing the quiescent cur-

rent on

±2.5V supplies to less than 500µA per amplifier, or

to control the quiescent current in normal operation.

Figure 11 shows how the control pins are used in conjunc-

tion with external resistors to program the supply current.

In normal operation, each control pin is biased to approxi-

mately 1V above V – and by varying the resistor values, the

current from each control pin can be adjusted. It is this

current that sets the supply current of both amplifiers. If

one of the resistors is open, i.e. R2, the supply current of

the amplifiers will be set by CTRL1 and R1. Figure 12

shows supply current vs resistor value.

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