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EL2020CN Datasheet(PDF) 11 Page - Intersil Corporation |
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EL2020CN Datasheet(HTML) 11 Page - Intersil Corporation |
11 / 14 page 11 differential gain/phase of 0.1% and 0.1°, but these are too small to measure on single devices. Video Distribution Amplifier The distribution amplifier shown below features a difference input to reject common mode signals on the 75 Ω coax cable input. Common mode rejection is often necessary to help to eliminate 60Hz noise found in production environments. EL2020 Disable/Enable Operation The EL2020 has an enable/disable control input at pin 8. The device is enabled and operates normally when pin 8 is left open or returned to pin 7, VCC. When more than 250µA is pulled from pin 8, the EL2020 is disabled. The output becomes a high impedance, the inverting input is no longer driven to the positive input voltage, and the supply current is halved. To make it easy to use this feature, there is an internal resistor to limit the current to a safe level (~1.1mA) if pin 8 is grounded. To draw current out of pin 8 an “open collector output” logic gate or a discrete NPN transistor can be used. This logic interface method has the advantage of level shifting the logic signal from 5V supplies to whatever supply the EL2020 is operating on without any additional components. Using the EL2020 as a Multiplexer An interesting use of the enable feature is to combine several amplifiers in parallel with their outputs common. This combination then acts similar to a MUX in front of an amplifier. A typical circuit is shown. When the EL2020 is disabled, the DC output impedance is very high, over 10k Ω. However there is also an output capacitance that is non-linear. For signals of less than 5V peak to peak, the output capacitance looks like a simple 15pF capacitor. However, for larger signals the output capacitance becomes much larger and non-linear. The example multiplexer will switch between amplifiers in 5µs for signals of less than ±2V on the outputs. For full output signals of 20V peak to peak, the selection time becomes 25µs. The disabled outputs also present a capacitive load and therefore only three amplifiers can have their outputs shorted together. However an unlimited number can sum together if a small resistor (25 Ω) is inserted in series with each output to isolate it from the “bus”. There will be a small gain loss due to the resistors of course. DIFFERENTIAL PHASE OF 5 CASCADED GAIN-OF-TWO STAGES DIFFERENTIAL PHASE OF 5 CASCADED GAIN-OF-TWO STAGES VIDEO DISTRIBUTION AMPLIFIER WITH DIFFERENCE INPUT USING THE EL2020 AS A MULTIPLEXER EL2020 |
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