11

®

BUF600, 601

75

Ω

200

Ω

7

4

6

–5V

3

150

Ω

V

I

V

O

BUF601

68

Ω

120

Ω

470nF

2.2µF

+5V

1

5

8

470nF

2.2µF

–5V

4

V

O

BUF601

68

Ω

120

Ω

470nF

2.2µF

1

8

4

+5V

5

470nF

2.2µF

–5V

V

O

BUF601

68

Ω

120

Ω

470nF

2.2µF

1

8

4

+5V

2.2µF

470nF

5

–5V

100

Ω

2

250

Ω

+5V

OPA658

Z

O

BUF

Z

O

+5V

1

5

8

–5V

4

150

Ω

Z

O

V

O

V

I

Z

O

BUF

Z

O

+5V

1

5

8

–5V

4

150

Ω

Z

O

V

O

V

I

Direct Drive

A

Z

O

Z

R

A

Double Matched

R

OUT

FIGURE 4. Driving Cables.

DRIVING CABLES

The most obvious way is to connect the cable directly to the

output of the buffer. This results in a gain determined by the

buffer output resistance and the characteristic impedance of

the cable, assuming it is properly terminated.

Double termination of a cable is the cleanest way to drive it,

since reflections are absorbed on both ends of the cable. The

cable source resistor is equal to the characteristic impedance

less the output resistance of the buffer amplifiers. The gain

is –6dB excluding of the cable attenuation.

VIDEO DISTRIBUTION AMPLIFIER

In this broadcast quality circuit, the OPA623 provides a very

high input impedance so that it may be used with a wide

variety of signal sources including video DACs, CCD cam-

eras, video switches or 75

Ω cables. The OPA623 provides a

voltage gain of 2.5V/V, while the potentiometer of 200

Ω

allows the overall gain to be adjusted to drive the standard

signal levels into the back-terminated 75

Ω cables. Back

matching prevents multiple reflections in the event that the

remote end of the cable is not properly terminated.

FIGURE 5. Video Distribution Amplifier.