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LT1190

Ultrahigh Speed Operational Amplifier

Linear Technology 

LT1190

APPLICATIO S I FOR ATIO

Cable Terminations

The LT1190 operational amplifier has been optimized as a

low cost video cable driver. The ±50mA guaranteed output

current enables the LT1190 to easily deliver 7.5VP-P into

100Ω, while operating on ±5V supplies or 2.6VP-P on a

single 5V supply.

Double Terminated Cable Driver

5V

3+ 7 6

75Ω CABLE

2

LT1190

–4

RG

–5V

RFB

75Ω

Cable Driver Voltage Gain vs Frequency

10

AV = 2

8

RFB = 1k

6

RG = 330Ω

VS = ±5V

TA = 25°C

4

AV = 1

2

RFB = 1k

RG = 1k

0

–2

–4

–6

–8

–10

100k

1M

10M

FREQUENCY (Hz)

100M

LT1190 • TA08

Using the Shutdown Feature

The LT1190 has a unique feature that allows the amplifier

to be shut down for conserving power or for multiplexing

several amplifiers onto a common cable. The amplifier will

shut down by taking Pin 5 to V –. In shutdown, the

amplifier dissipates 15mW while maintaining a true high

impedance output state of 15kΩ in parallel with the

feedback resistors. The amplifiers must be used in a

noninverting configuration for MUX applications. In in-

verting configurations the input signal is fed to the output

through the feedback components. The following scope

photos show that with very high RL, the output is truly high

impedance; the output slowly decays toward ground.

Additionally, when the output is loaded with as little as 1kΩ

the amplifier shuts off in 400ns. This shutoff can be under

the control of HC CMOS operating between 0V and – 5V.

Output Shutdown

0V

VSHDN

– 5V

VOUT

When driving a cable it is important to terminate the cable

to avoid unwanted reflections. This can be done in one of

two ways: single termination or double termination. With

single termination, the cable must be terminated at the

receiving end (75Ω to ground) to absorb unwanted en-

ergy. The best performance can be obtained by double

termination (75Ω in series with the output of the amplifier,

and 75Ω to ground at the other end of the cable). This

termination is preferred because reflected energy is ab-

sorbed at each end of the cable. When using the double

termination technique it is important to note that the signal

is attenuated by a factor of 2, or 6dB. This can be compen-

sated for by taking a gain of 2, or 6dB in the amplifier. The

cable driver has a – 3dB bandwidth in excess of 30MHz

while driving the 150Ω load.

0V

VSHDN

– 5V

VOUT

LT1190 • TA09

1MHz SINE WAVE GATED OFF WITH

SHUTDOWN PIN, AV = 1, RL = SCOPE PROBE

Output Shutdown

1MHz SINE WAVE GATED OFF WITH

SHUTDOWN PIN, AV = 1, RL = 1kΩ

LT1190 • TA10

9

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