LT1187

10

APPLICATIO S I FOR ATIO

LT1187 in Unity Gain

Using the Shutdown Feature

The LT1187 has a unique feature that allows the amplifier

to be shutdown 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 20k in parallel with the feedback resistors.

For MUX applications, the amplifiers may be configured

inverting, noninverting, or differential. When the output is

loaded with as little 1k from the amplifier’s feedback

resistors, the amplifier shuts off in 600ns. This shutoff can

be under the control of HC CMOS operating between 0V

and –5V.

The ability to maintain shutoff is shown on the curve

Shutdown Supply Current vs Temperature in the Typical

1MHz Sine Wave Gated Off with Shutdown Pin

Performance Characteristics section. At very high el-

evated temperature it is important to hold the shutdown

pin close to the negative supply to keep the supply current

from increasing.

Send Color Video Over Twisted-Pair

With an LT1187 it is possible to send and receive color

composite video signals more than 1000 feet on a low cost

twisted-pair. A bidirectional “video bus” consists of the

LT1195 op amp and the LT1187 video difference amplifier.

A pair of LT1195s at TRANSMIT 1, is used to generate

differential signals to drive the line which is back-termi-

nated in its characteristic impedance. The LT1187, twisted-

pair receiver, converts signals from differential to single-

ended. Topology of the LT1187 provides for cable com-

pensation at the amplifier’s feedback node as shown. In

this case, 1000 feet of twisted-pair is compensated with

1000pF and 50

Ω to boost the 3dB bandwidth of the

system from 750kHz to 4MHz. This bandwidth is adequate

to pass a 3.58MHz chroma subcarrier, and the 4.5MHz

sound subcarrier. Attenuation in the cable can be compen-

2, another pair of LT1195s serve the dual function to

provide cable termination via low output impedance, and

generate differential signals for TRANSMIT 2. Cable termi-

nation is made up of a 15

Ω and 33Ω attenuator to reduce

the differential input signal to the LT1187. Maximum input

1.5MHz Square Wave Input and Unequalized Response Through

1000 Feet of Twisted-Pair

LT1187 • AI06

C

B

A

LT1187 • AI07

SHUTDOWN

LT1187 • AI08