The

Typical Operating Circuit shows the MAX442 dri-

ving a back-terminated 75

Ω video cable. The back-ter-

mination resistor (at the MAX442 output) is included to

match the impedance of the cable’s driven end to the

characteristic impedance of the cable itself. This, plus

the load-termination resistor, eliminates signal reflec-

tions from the cable’s ends. The back-termination resis-

tor forms a voltage divider with the load impedance,

which attenuates the signal at the cable output by one-

half. The amplifier is operated with a 2V/V closed-loop

gain to provide unity gain at the cable’s video output.

Capacitive-Load Driving

Driving large capacitive loads increases the likelihood

of oscillation in most amplifier circuits. This is especial-

ly true for circuits with high loop gains, like voltage fol-

lowers. The amplifier’s output impedance and the

capacitive 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.

With capacitive loads greater than approximately 50pF

and the MAX442 configured as a unity-gain buffer, use

an isolation resistor in series with the load, as shown in

Figure 2. The resistor removes the pole from the loop

response caused by the load capacitance.

Channel Switching Time/Transient

When the MAX442 multiplexer is switched from one

channel to another, a small glitch will appear at the out-

put. Figure 3 shows the results of putting a 0V to 5V

pulse 100ns wide into A0.

140MHz, 2-Channel

Video Multiplexer/Amplifier

6

_______________________________________________________________________________________

MAX442

22

Ω

OUT

IN

Figure 2. Capacitive-Load-Driving Circuit

A0 INPUT

5V/div

AMP

OUTPUT

200mV/div

GND

GND

Figure 3. Output Switching Transient when Switching Between

INPUT

1V/V

CABLE

OUTPUT

1V/V

GND

GND

INPUT

1V/div

CABLE

OUTPUT

500mV/div

GND

GND