EL2090C
100 MHz DC-Restored Video Amplifier
Typical Performance Curves
Maximum Power Dissipation
vs Ambient Temperature
14-Pin PDIP and 16-Pin SOL
Contd
This suggests that the largest applicable power
supply voltages be used so that the output swing
of the sample-and-hold can still correct for the
variations of DC offset in the video input with
large values of Raz The typical application cir-
cuit shown will allow correction of
g
1V inputs
with good isolation of the sample-and-hold out-
put Good isolation is defined as no video degra-
dation due to the insertion of the sample-and-
hold loop Lower supply voltages will require a
smaller value of DC feedback resistor to retain
correction of the full input DC variation The
EL2090 differential phase performance is opti-
mum at
g
9V supplies and differential gain only
marginally improves above this voltage Since all
video characteristics mildly degrade with increas-
ing die temperature the
g
9V levels are some-
what better than
g
15V supplies However
g
15V supplies are quite usable
Ultimate video performance especially in HDTV
applications can also be optimized by setting the
black-level reference such that the signal span at
the video amplifier’s output is set to its optimum
range For instance setting the span to
g
1V of
output is preferable to a span of 0V to
a
2V The
curves of differential gain and phase versus input
DC offset will serve as guides
The DC feedback resistor may be split so that a
bypass capacitor is added to reduce the initially
small sample-and-hold transients to even smaller
levels The corruption can be reduced to as low as
1 mV peak seen at the video amplifier output
The size of the capacitor should not be so large as
to de-stabilize the sample-and-hold feedback
loop nor so small as to reduce the video amplifi-
er’s gain flatness A resistor or some other video
isolation network should be inserted between the
video amplifier output and the sample-and-hold
input to prevent excessive video from bleeding
through the autozero section as well as prevent-
ing spurious DC correction due to video signals
confusing the sample-and-hold during autozero
events Figure 1 shows convenient component
values A full 3 58 MHz trap is not necessary for
suppressing NTSC chroma burst interaction with
the sample-and-hold input the simple R-C net-
work suggested in Figure 1 suffices
2090 – 10
Applications Information
The EL2090C is a general purpose component
and thus the video amplifier and sample-and-
hold pins are uncommitted Therefore much of
the ultimate performance as a DC-restored video
amplifier will be set by external component val-
ues and parasitics Some application considera-
tions will be offered here
The DC feedback from the sample-and-hold can
be applied to either positive or negative inputs of
the video amplifier (with appropriate phasing of
the sample-and-hold amplifier inputs) We will
consider feedback to the inverting video input
During a sample mode (the HOLD input at a log-
ic low) the sample-and-hold acts as a simple null-
ing op-amp
Ideally the DC feedback resistor Raz is a high
value so as not to couple a large amount of the
AC signal on the video input back to the sample-
and-hold amplifier output The sample-and-hold
output is a low impedance at high frequencies
but variations of the DC operating point will
change the output impedance somewhat No
more than a few ohms output impedance change
will occur but this can cause gain variations in
the 0 01% realm This DC-dependent gain
change is in fact a differential gain effect Some
small differential phase error will also be added
The best approach is to maximize the DC feed-
back resistor value so as to isolate the sample-
and-hold from the video path as much as possi-
ble Values of 1 kX or above for Raz will cause
little to no video degradation
7
ELANTEC [ ELANTEC SEMICONDUCTOR ]
