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EL2090CN

100MHz DC-restored video amplifier

Elantec -> Intersil 

EL2090C

100 MHz DC-Restored Video Amplifier

Typical Performance Curves

Maximum Power Dissipation

vs Ambient Temperature

14-Pin PDIP and 16-Pin SOL

Contd

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

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 g1V 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 g9V supplies and differential gain only

marginally improves above this voltage Since all

video characteristics mildly degrade with increas-

ing die temperature the g9V levels are some-

what better than g15V supplies However

g15V 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 g1V of

output is preferable to a span of 0V to a2V 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

7

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