Two-Channel, Triple/Quad

RGB Video Switches and Buffers

_______________________________________________________________________________________

9

Driving Coaxial Cable

High-speed performance, excellent output current

capability, and an internally fixed gain of two make the

MAX465/MAX466/MAX469/MAX470 ideal for driving

50

Ω or 75Ω back-terminated coaxial cables. The

MAX465/MAX466/MAX469/MAX470 will drive a 150

Ω

load (75

Ω back-terminated cable) to ±2.5V.

The

Typical Operating Circuit shows the MAX465/MAX466

driving four back-terminated 75

Ω video cables. The

back-termination resistor (at each amplifier output) pro-

vides impedance matching at the driven end of the

cable to eliminate signal reflections. It forms a voltage

divider with the load impedance, which attenuates the

signal at the cable output by one-half. The amplifier

operates with an internal 2V/V closed-loop gain to pro-

vide unity gain at the cable’s output.

Driving Capacitive Loads

Driving large capacitive loads increases the likelihood

of oscillation in most amplifier circuits. This is especially

true for circuits with high loop-gains, like voltage follow-

ers. The amplifier’s output impedance and the capaci-

tive load form an RC filter that adds a pole to the loop

response. If the pole frequency is low enough, as

when driving a large capacitive load, the circuit phase

margin is degraded and oscillation may occur.

The MAX463–MAX470 phase margin and capacitive-

load driving performance are optimized by internal

compensation. When driving capacitive loads greater

than 50pF, connect an isolation resistor between the

amplifier output and the capacitive load, as shown in

Figure 3.

75

Ω CABLE

75

Ω CABLE

75

Ω CABLE

75

Ω CABLE

75

Ω

75

Ω

75

Ω

75

Ω

75

Ω

75

Ω CABLE

75

Ω

75

Ω

150

Ω

SOURCE:

TEKTRONIX

1910 DIGITAL GENERATOR

MEASUREMENT:

TEKTRONIX VM700

VIDEO MEASUREMENT

SET

MAX469/MAX470

MAX467/MAX468

DUT

DUT

(a)

(b)

Figure 2. Differential Phase and Gain Error Test Circuits (a) for the MAX469/MAX470 Gain-of-Two Amplifiers, (b) for the

MAX467/MAX468 Unity-Gain Amplifiers

MAX468

12

Ω

100pF

OUT_

IN_

Figure 3a. Using an Isolation Resistor with a Capacitive Load