EL4093 Datasheet, PDF (11/12 Pages) Intersil Corporation – 300MHz DC-Restored Video Amplifier

EL4093
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.
FIGURE 5. AUTOZERO MECHANISM RESTORES
AMPLIFIER OUTPUT TO GROUND
AFTER +1V STEP AT INPUT
A natural question arises as to whether there are other
C_HOLD values that can be used. In one direction, increasing
C_HOLD 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 increasing C_HOLD.
In the other direction, decreasing C_HOLD would increase the
droop and hold step but shorten the acquisition time. There
is, however, a caveat to reducing C_HOLD: too small a C_HOLD
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
sufficiently large C_HOLD must be used to suppress the non-
linearity and force the loop to settle. For example, it has been
found that a C_HOLD of 470pF results in 1V_P-P oscillation
around 10MHz at the CFA output.
The minimum recommended value for C_HOLD is 2.2nF. With
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 loop stable. At this value of C_HOLD the
acquisition time 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
(40 IRE_P-P) triggers the S/H boost circuit and prevents the
autozero loop from settling.
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 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.
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 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.
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