EL4089 查看數據表（PDF） - Elantec -> Intersil

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EL4089

DC Restored Video Amplifier

Elantec -> Intersil 

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 0 01 mF capacitor the output voltage drift is

6 2 mV in one line

The S H amplifier can provide a typical current

of 300 mA to charge capacitor CX1 so with a

1 2 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 75X back terminating resistor RXT is rec-

ommended when driving 75X 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

0 1 mF ceramic capacitor from each power pin to

ground placed very close to the power pins to-

gether with a 4 7 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 100X –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

Figure 1

7

4089 – 19

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