Datasheet MAX442 (Analog Devices) - 6

140MHz, 2-Channel Video Multiplexer/Amplifier MAX442
To prevent oscillation and unwanted signal coupling,
_____________________Pin Description
minimize trace area at the circuit’s critical high-imped- ance nodes, especially the amplifier summing junction
PIN NAME FUNCTION
(the amplifier’s inverting input). Surround these critical 1 IN0 Analog Input, channel 0 nodes with a ground trace, and include ground traces between all signal traces to minimize parasitic coupling 2 GND Ground that can degrade crosstalk and/or amplifier stability. Keep signal paths as short as possible to minimize 3 IN1 Analog Input, channel 1 inductance, and keep all input channel traces at equal 4 V- Negative Power Supply, -5V lengths to maintain the phase relationship between the input channels. 5 IN- Amplifier Inverting Input Bypass all power-supply pins directly to the ground 6 VOUT Amplifier Output plane with 0.1µF ceramic capacitors, placed as close to the supply pins as possible. For high-current loads, 7 V+ Positive Power Supply, +5V it may be necessary to include 1µF tantalum or alu- Channel Address Input: minum-electrolytic capacitors in parallel with the 0.1µF 8 A0 A0 = logic 0 selects channel 0, ceramic bypass capacitors. Keep capacitor lead A0 = logic 1 selects channel 1 lengths as short as possible to minimize series induc- tance; surface-mount (chip) capacitors are ideal for this application.
__________Applications Information
The MAX442’s bipolar construction results in a typical
Differential Gain and Phase Errors
channel input capacitance of only 4pF, whether the In color video applications, lowest differential gain and channel is on or off. As with all ICs, the mux’s input phase errors are critical for an IC, because they cause capacitance forms a single-pole RC lowpass filter with changes in contrast and color of the displayed picture. the signal source’s output impedance. This filter can Typically, the MAX442’s multiplexer/amplifier combina- limit the system’s signal bandwidth if the RC product tion has a differential gain and phase error of only becomes too large. However, the MAX442’s low chan- 0.07% and 0.09°, respectively. This low differential nel input capacitance allows full AC performance of the gain and phase error makes the MAX442 ideal for use amplifier, even with source impedances as great as in broadcast-quality color video systems. 250Ω—a significant improvement over common mux or
Coaxial-Cable Drivers
switch alternatives. High-speed performance and excellent output current Feedback resistors should be limited to no more than capability make the MAX442 ideal for driving 50Ω or 500Ω to ensure that the RC time constant formed by the 75Ω coaxial cables. The MAX442 will drive 50Ω and resistors, the circuit board’s capacitance, and the 75Ω coaxial cables to ±3V. capacitance of the amplifier input pins does not limit the system’s high-speed performance. 75Ω
Power-Supply Bypassing
CABLE
and Board Layout
Realizing the full AC performance of high-speed ampli- 75Ω fiers requires careful attention to power-supply bypass- 75Ω MAX442 ing and board layout. Use a low-impedance ground 75Ω CABLE 75Ω plane with the MAX442. With multilayer boards, the 75Ω CABLE ground plane should be located on the PC board’s component side to minimize impedance between the 470Ω 75Ω components and the ground plane. For single-layer SOURCE: TEKTRONIX boards, components should be mounted on the board’s 1910 DIGITAL GENERATOR copper side and the ground plane should include the 470Ω MEASUREMENT: entire portion of the board that is not dedicated to a TEKTRONIX specific signal trace. VM700 VIDEO MEASUREMENT SET Figure 1. Differential Gain and Phase Error Test Circuit
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Document Outline not-recomm-data-sheet-cover.pdf Not Recommended for New Designs