EL4093CS datasheet(14/16 Pages) ELANTEC

14 / 16 page

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 SH input

Figure 6 below shows a 358 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 358 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 SH input This

has undesirable effects in some NTSC applica-

tions as Figure 8 below illustrates There is only

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 06

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

EL4093CS Datasheet(PDF) 14 Page - Elantec Semiconductor