EL4093

FN7159 Rev 0.00

Page 11 of 12

January 1996

will further reduce the droop and hold step, but lengthen the

acquisition time. Since the droop and hold step are already

small to begin with, there is no apparent advantage to

droop and hold step but shorten the acquisition time. There is,

would cause the autozero loop to oscillate. The reason is that

when the S/H boost circuit turns on, the input stage gm

increases drastically and the circuit becomes nonlinear. A

linearity and force the loop to settle. For example, it has been

around 10MHz at the CFA output.

this value the loop remains stable over the entire operating

temperature range (-40°C to +85°C). The greatest instability

occurs at low temperatures, where we observe from the

performance curves that the S/H gm’s, and hence the GBWP,

are at their maximum. If the operating range is restricted to

room temperature or above, then 1.5nF is sufficient to keep the

reduces to about 1.5µs.

Video Performance and Application

Although the EL4093 is intended for high speed video

applications such as SVGA, it also offers excellent

performance for NTSC, with 0.04% dG and 0.02° dP at

3.58MHz. Some application considerations, however, are

required for handling NTSC signals.

Referring back to Figure 2, recall that typically, the autozero

interval lies in the back porch portion of video containing the

colorburst pulse. When the S/H compares the video to the

reference voltage during this period, the colorburst

autozero loop from settling.

A remedy for this situation is to attenuate the colorburst before

applying it to the S/H input. Figure 6 below shows a 3.58MHz

chroma trap which would notch out the colorburst while

preserving the video DC level.

One may be tempted to use a RC lowpass filter to suppress

the colorburst, as shown in Figure 7 below. This technique,

however, poses several problems. First, to obtain enough

attenuation, we need to set the pole frequency 10 to 20 times

lower than 3.58MHz. This pole, being close to the auto zero

loop pole, would destabilize the system and cause the loop to

oscillate.

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

applications, as Figure 8 illustrates. There is only 0.6µs from

the rising edge of sync to the colorburst. If we are autozeroing

over the back porch, the autozero period would begin

somewhere in this 0.6µs interval. Since the edge of sync is

now delayed by the RC network, autozero begins before the

video back porch reaches its final value. Consequently, the

autozero loop performs a correction on every line and never

settles.

FIGURE 5. AUTOZERO MECHANISM RESTORES

AMPLIFIER OUTPUT TO GROUND

AFTER +1V STEP AT INPUT

FIGURE 6. COLORBURST TRAP FOR NTSC

APPLICATIONS

FIGURE 7. CAUTION: LOWPASS FILTER DOES

NOT WORK IN NTSC APPLICATIONS