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MAX469EWE Datasheet(PDF) 8 Page - Maxim Integrated Products |
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MAX469EWE Datasheet(HTML) 8 Page - Maxim Integrated Products |
8 / 16 page _______________Detailed Description The MAX463–MAX470 have a bipolar construction, which results in a typical channel input capacitance of only 5pF, whether the channel is on or off. This low input capacitance allows the amplifiers to realize full AC performance, even with source impedances as great as 250 Ω. It also minimizes switching transients because the driving source sees the same load whether the channel is on or off. Low input capaci- tance is critical, because it forms a single-pole RC low- pass filter with the output impedance of the signal source, and this filter can limit the system’s signal bandwidth if the RC product becomes too large. The MAX465/MAX466/MAX469/MAX470’s amplifiers are internally configured for a gain of two, resulting in an over- all gain of one at the cable output when driving back-ter- minated coaxial cable (see the section Driving Coaxial Cable). The MAX463/MAX464/MAX467/MAX468 are internally configured for unity gain. Power-Supply Bypassing and Board Layout To realize the full AC performance of high-speed ampli- fiers, pay careful attention to power-supply bypassing and board layout, and use a large, low-impedance ground plane. With multi-layer boards, the ground plane should be located on the layer that is not dedi- cated to a specific signal trace. To prevent unwanted signal coupling, minimize the trace area at the circuit's critical high-impedance nodes, and surround the analog inputs with an AC ground trace (analog ground, bypassed DC power supply, etc). The analog input pins to the MAX463–MAX470 have been separated with AC ground pins (GND, V+, V-, or a hard-wired logic input) to minimize parasitic coupling, which can degrade crosstalk and/or stability of the amplifier. Keep signal paths as short as possible to minimize inductance, and ensure that all input channel traces are of equal length to maintain the phase relationship between the R, G, and B signals. Connect the coaxial-cable shield to the ground side of the 75 Ω terminating resistor at the ground plane to further reduce crosstalk (see Figure 1). Bypass all power-supply pins directly to the ground plane with 0.1µF ceramic capacitors, placed as close to the supply pins as possible. For high-current loads, it may be necessary to include 10µF tantalum or alu- minum-electrolytic capacitors in parallel with the 0.1µF ceramics. Keep capacitor lead lengths as short as possible to minimize series inductance; surface-mount (chip) capacitors are ideal. Connect all V- pins to a large power plane. The V- pins conduct heat away from the internal die, aiding thermal dissipation. Differential Gain and Phase Errors Differential gain and phase errors are critical specifica- tions for an amplifier/buffer in color video applications, because these errors correspond directly to changes in the color of the displayed picture in composite video systems. The MAX467–MAX470 have low differential gain and phase errors, making them ideal in broadcast- quality composite color applications, as well as in RGB video systems where these errors are less significant. The MAX467–MAX470 differential gain and phase errors are measured with the Tektronix VM700 Video Measurement Set, with the input test signal provided by the Tektronix 1910 Digital Generator as shown in Figure 2. Measuring the differential gain and phase of the MAX469/MAX470 (Figure 2a) is straightforward because the output amplifiers are configured for a gain of two, allowing connection to the VM700 through a back-termi- nated coaxial cable. Since the MAX467/MAX468 are unity-gain devices, driving a back-terminated coax would result in a gain of 1/2 at the VM700. Figure 2b shows a test method to measure the differen- tial gain and phase for the MAX467/MAX468. First, measure and store the video signal with the device under test (DUT) removed and replaced with a short circuit, and the 150 Ω load resistor omitted. Then do another measurement with the DUT and load resistor in the circuit, and calculate the differential gain and phase errors by subtracting the results. Two-Channel, Triple/Quad RGB Video Switches and Buffers 8 _______________________________________________________________________________________ RT RT RETURN CURRENT RETURN CURRENT COAX COAX Figure 1. Low-Crosstalk Layout. Return current from the termination resistor does not flow through the ground plane. |
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