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LT1794

–12V

12V

10 (SHDN)

RSHDN

110

Ω

OUT (+)

OUT (–)

10k

EIN

0.01

µF

RL ≈ 50Ω

1:2*

10k

49.9

Ω

110

Ω

12.7

Ω

1794 TC

11 (SHDNREF)

–

0.1

µF

4.7

µF

0.1

µF

12V

–12V

–

–12V

12.7

Ω

VOUT(P-P)

100 LINE LOAD

SUPPLY BYPASSING

*COILCRAFT X8390-A OR EQUIVALENT

VOUTP-P AMPLITUDE SET AT EACH AMPLIFIER OUTPUT

DISTORTION MEASURED ACROSS LINE LOAD

SPLITTER

MINICIRCUITS

ZSC5-2-2

4.7

µF

0.1

µF

4.7

µF

TEST CIRCUIT

APPLICATIO S I FOR ATIO

The LT1794 is a high speed, 200MHz gain bandwidth

product, dual voltage feedback amplifier with high output

current drive capability, 500mA source and sink. The

LT1794 is ideal for use as a line driver in xDSL data

communication applications. The output voltage swing

has been optimized to provide sufficient headroom when

operating from

±12V power supplies in full-rate ADSL

applications. The LT1794 also allows for an adjustment of

the operating current to minimize power consumption. In

addition, the LT1794 is available in small footprint surface

mount packages to minimize PCB area in multiport central

office DSL cards.

To minimize signal distortion, the LT1794 amplifiers are

decompensated to provide very high open-loop gain at

high frequency. As a result each amplifier is frequency

stable with a closed-loop gain of 10 or more. If a closed-

loop gain of less than 10 is desired, external frequency

compensating components can be used.

Setting the Quiescent Operating Current

Power consumption and dissipation are critical concerns

in multiport xDSL applications. Two pins, Shutdown

(SHDN) and Shutdown Reference (SHDNREF), are pro-

vided to control quiescent power consumption and allow

for the complete shutdown of the driver. The quiescent

current should be set high enough to prevent distortion

induced errors in a particular application, but not so high

that power is wasted in the driver unnecessarily. A good

starting point to evaluate the LT1794 is to set the quiescent

current to 10mA per amplifier.

The internal biasing circuitry is shown in Figure 1. Ground-

ing the SHDNREF pin and directly driving the SHDN pin with

a voltage can control the operating current as seen in the

Typical Performance Characteristics. When the SHDN pin

is less than SHDNREF + 0.4V, the driver is shut down and

consumes typically only 100

µA of supply current and the

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