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EL4089C Datasheet(PDF) 7 Page - Elantec Semiconductor |
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EL4089C Datasheet(HTML) 7 Page - Elantec Semiconductor |
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7 / 8 page  EL4089C DC Restored Video Amplifier Typical Application The EL4089 can be used to DC-restore a video waveform (see Fig 1) The above circuit forces the cable driving video amplifier’s output to ground when the HOLD pin is at a logic low The ‘‘correction voltage’’ is stored on capacitor CX1 an external ceramic capacitor The capaci- tor value is chosen from the system require- ments The typical input bias current to the vid- eo amplifier is 1 mA so for a 62 ms hold time and a 001 mF capacitor the output voltage drift is 62 mV in one line The SH amplifier can provide a typical current of 300 mA to charge capacitor CX1 so with a 12 ms sampling time the output can be corrected by 36 mV in each line Using a smaller value of CX1 increases both the voltage that can be corrected and the drift while being held likewise using a larger value of CX1 reduces the voltages The RX1 resistor is in the circuit purely to simu- late some external source impedance and is not needed as a real component Likewise for RX2 The 75 X back terminating resistor RXT is rec- ommended when driving 75 X cables The board layout should have a ground plane un- derneath the EL4089 with the ground plane cut away from the vicinity of the VINb pin (pin 1) This helps to minimize the stray capacitance on pin 1 Power supply bypassing is important and a 01 mF ceramic capacitor from each power pin to ground placed very close to the power pins to- gether with a 47 mF tantalum bead capacitor is recommended When both digital and Analog grounds are on the same board the EL4089 should be on the Analog ground The digital ground can be connected to the Analog ground through a 100 X – 300X resis- tor near the EL4089 This allows the digital sig- nal a return path while preventing the digital noise from corrupting the analog ground 4089 – 19 Figure 1 7 |
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