140MHz, 2-Channel Video Multiplexer/Amplifier The Typical Operating Circuit shows the MAX442 dri- lowers. The amplifier’s output impedance and the ving a back-terminated 75Ω video cable. The back-ter- capacitive load form an RC filter that adds a pole to the mination resistor (at the MAX442 output) is included to loop response. If the pole frequency is low enough, as match the impedance of the cable’s driven end to the when driving a large capacitive load, the circuit phase characteristic impedance of the cable itself. This, plus margin is degraded and oscillation may occur. the load-termination resistor, eliminates signal reflec- With capacitive loads greater than approximately 50pF tions from the cable’s ends. The back-termination resis- and the MAX442 configured as a unity-gain buffer, use MAX442 tor forms a voltage divider with the load impedance, an isolation resistor in series with the load, as shown in which attenuates the signal at the cable output by one- Figure 2. The resistor removes the pole from the loop half. The amplifier is operated with a 2V/V closed-loop response caused by the load capacitance. gain to provide unity gain at the cable’s video output. Channel Switching Time/TransientCapacitive-Load Driving When the MAX442 multiplexer is switched from one Driving large capacitive loads increases the likelihood channel to another, a small glitch will appear at the out- of oscillation in most amplifier circuits. This is especial- put. Figure 3 shows the results of putting a 0V to 5V ly true for circuits with high loop gains, like voltage fol- pulse 100ns wide into A0. MAX442 INPUT GND IN 1V/div 22Ω OUT CLOAD > 50pF CABLE OUTPUT GND 500mV/div Figure 2. Capacitive-Load-Driving Circuit Figure 4. Pulse Response with RL = 100Ω (50Ω back-terminated cable), AVCL = +1V/V A0 INPUT GND 5V/div INPUT GND AMP 1V/V OUTPUT GND 200mV/div CABLE OUTPUT GND 1V/V Figure 3. Output Switching Transient when Switching Between Figure 5. Pulse Response with RL = 100Ω (50Ω back-terminated Two Grounded Inputs with RL = 100Ω cable), AVCL = +2V/V 6_______________________________________________________________________________________ Document Outline not-recomm-data-sheet-cover.pdf Not Recommended for New Designs
