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50205Z Datasheet(PDF) 11 Page - Intersil Corporation |
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50205Z Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 23 page  11 FN2845.11 June 5, 2006 Referring to Figure 7, it can be seen that R6 will act as a pull-up resistor to +VCC if the DISABLE pin is left open. In those cases where the enable/disable function is not required on all circuits some circuits can be permanently enabled by letting the DISABLE pin float. If a driver is used to set the enable/disable level, be sure that the driver does not sink more than 20 µA when the DISABLE pin is at a high level. TTL gates, especially CMOS versions, do not violate this criteria so it is permissible to control the enable/disable function with TTL. Typical Applications Two Channel Video Multiplexer Referring to the amplifier U1A in Figure 8, R1 terminates the cable in its characteristic impedance of 75 Ω, and R 4 back terminates the cable in its characteristic impedance. The amplifier is set up in a gain configuration of +2 to yield an overall network gain of +1 when driving a double terminated cable. The value of R3 can be changed if a different network gain is desired. R5 holds the disable pin at ground thus inhibiting the amplifier until the switch, S1, is thrown to position 1. At position 1 the switch pulls the disable pin up to the plus supply rail thereby enabling the amplifier. Since all of the actual signal switching takes place within the amplifier, it’s differential gain and phase parameters, which are 0.03% and 0.03° respectively, determine the circuit’s performance. The other circuit, U1B, operates in a similar manner. When the plus supply rail is 5V the disable pin can be driven by a dedicated TTL gate as discussed earlier. If a multiplexer IC or its equivalent is used to select channels its logic must be break before make. When these conditions are satisfied the HA-5020 is often used as a remote video multiplexer, and the multiplexer may be extended by adding more amplifier ICs. Low Impedance Multiplexer Two common problems surface when you try to multiplex multiple high speed signals into a low impedance source such as an A/D converter. The first problem is the low source impedance which tends to make amplifiers oscillate and causes gain errors. The second problem is the multiplexer which supplies no gain, introduces all kinds of distortion and limits the frequency response. Using op amps which have an enable/disable function, such as the HA-5020, eliminates the multiplexer problems because the external mux chip is not needed, and the HA-5020 can drive low impedance (large capacitance) loads if a series isolation resistor is used. Referring to Figure 9, both inputs are terminated in their characteristic impedance; 75 Ω is typical for video applications. Since the drivers usually are terminated in their characteristic impedance the input gain is 0.5, thus the amplifiers, U2, are configured in a gain of +2 to set the circuit gain equal to one. Resistors R2 and R3 determine the amplifier gain, and if a different gain is desired R2 should be changed according to the equation G = (1 + R3/R2). R3 sets the frequency response of the amplifier so you should refer to the manufacturers data sheet before changing its value. R5, C1 and D1 are an asymmetrical charge/discharge time circuit which configures U1 as a break before make switch to prevent both amplifiers from being active simultaneously. If this design is extended to more channels the drive logic must be designed to be break before make. R4 is enclosed in the feedback loop of the amplifier so that the large open loop amplifier gain of U2 will present the load with a small closed loop output impedance while keeping the amplifier stable for all values of load capacitance. The circuit shown in Figure 9 was tested for the full range of capacitor values with no oscillations being observed; thus, problem one has been solved. The frequency and gain characteristics of the circuit are now those of the amplifier and independent of any multiplexing action; thus, problem two has been solved. The multiplexer transition time is approximately 15 µs with the component values shown. NOTES: 21. U1 is HA-5020. 22. All resistors in Ω. 23. S1 is break before make. 24. Use ground plane. VIDEO INPUT #1 VIDEO INPUT #2 R1 75 R3 681 R2 681 R4 75 R5 2000 +- U1A U1B R9 75 R10 2000 R7 681 R8 681 R6 75 +5V IN +5V 0.1 µF 10 µF -5V IN -5V 0.1 µF 10 µF + + 1 R11 100 VIDEO OUTPUT TO 75 Ω LOAD +5V S1 2 3 ALL OFF FIGURE 8. TWO CHANNEL HIGH IMPEDANCE MULTIPLEXER HA-5020 |
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