LT1191 データシートの表示（PDF） - Linear Technology

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LT1191

Ultrahigh Speed Operational Amplifier

Linear Technology 

LT1191

APPLICATIO S I FOR ATIO

Cable Terminations

The LT1191 operational amplifier has been optimized as a

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

current enables the LT1191 to easily deliver 7.5VP-P into

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

single 5V supply.

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 of 100MHz while

driving the 150Ω load. Note the response can be improved

by lowering the impedance of the feedback elements.

Double Terminated Cable Driver

5V

3+

7

6

75Ω CABLE

2

LT1191

–4

RG

–5V

RFB

75 Ω

Cable Driver Voltage Gain vs Frequency

10

AV = 2

RFB = 1k

RG = 330Ω

5

VS = ±5V

TA = 25°C

RFB= 1k

AV = 1 RG = 1k

0

RFB = 300Ω

RG = 300Ω

–5

Using the Shutdown Feature

The LT1191 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 resis-

tors. The amplifiers must be used in a noninverting con-

figuration for MUX applications. In inverting 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

0V

VSHDN

– 5V

VOUT

1MHz SINE WAVE GATED OFF WITH

SHUTDOWN PIN, AV = 1, RL = ∞

Output Shutdown

LT1191 • TA09

–10

100k

1M

10M

FREQUENCY (Hz)

100M

LT1191 • TA08

1MHz SINE WAVE GATED OFF WITH

SHUTDOWN PIN, AV = 1, RL = 1kΩ

LT1191 • TA10

9

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