EL4093C View Datasheet(PDF) - Elantec -> Intersil

Part Name

Description

Manufacturer

EL4093C

300 MHz DC restored video amplifier

Elantec -> Intersil 

EL4093C

300 MHz DC-Restored Video Amplifier

Applications Information Contd

A remedy for this situation is to attenuate the

colorburst before applying it to the S H input

Figure 6 below shows a 3 58 MHz chroma trap

which would notch out the colorburst while pre-

serving the video DC level

4093 – 42

Figure 6 Colorburst Trap for NTSC Applications

One may be tempted to use a RC lowpass filter to

suppress the colorburst as shown in Figure 7 be-

low This technique however poses several prob-

lems First to obtain enough attenuation we

need to set the pole frequency 10 to 20 times low-

er than 3 58 MHz This pole being close to the

auto zero loop pole would destabilize the system

and cause the loop to oscillate

4093 – 43

Figure 7 Caution Lowpass Filter Does Not

Work in NTSC Applications

Although we can cancel this pole by introducing

a zero the RC network introduces a time delay

between the CFA output and the S H input This

has undesirable effects in some NTSC applica-

tions as Figure 8 below illustrates There is only

0 6 ms from the rising edge of sync to the color-

burst If we are autozeroing over the back porch

the autozero period would begin somewhere in

this 0 6 ms interval Since the edge of sync is now

delayed by the RC network autozero begins be-

fore the video back porch reaches its final value

Consequently the autozero loop performs a cor-

rection on every line and never settles

4093 – 44

Figure 8 Lowpass Filter Delays Input to Sample and Hold

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