9

LT1191

FREQUENCY (Hz)

10

LT1191 • TA08

100M

10M

1M

100k

R

= 1k

R = 1k

5

0

–5

–10

FB

G

CABLE

2

+

–

5V

7

LT1191

–5V

4

3

6

R

75

Ω

75

Ω

G

Cable Driver Voltage Gain vs Frequency

Double Terminated Cable Driver

Ω

Cable Terminations

The LT1191 operational amplifier has been optimized as a

low cost video cable driver. The

±50mAguaranteedoutput

100

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 (75

Ω to ground) to absorb unwanted en-

ergy. The best performance can be obtained by double

termination (75

Ω in series with the output of the amplifier,

and 75

Ω 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 150

Ω load. Note the response can be improved

by lowering the impedance of the feedback elements.

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

dissipates 15mW while maintaining a true high impedance

output state of 15k

Ω 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

output slowly decays toward ground. Additionally, when

the output is loaded with as little as 1k

Ωtheamplifiershuts

off in 400ns. This shutoff can be under the control of HC

CMOS operating between 0V and –5V.

APPLICATIO S I FOR ATIO

Output Shutdown

Output Shutdown

LT1191 • TA09

1MHz SINE WAVE GATED OFF WITH

∞

–5V

0V

LT1191 • TA10

1MHz SINE WAVE GATED OFF WITH

–5V

0V