EL2111CN 데이터 시트보기 (PDF) - Intersil

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EL2111CN

Low Cost, Gain of 2, Video Op Amp

Intersil 

EL2111

matching resistor to a cable and a 75Ω load to ground at the

end of the cable provide a 150Ω DC load. But output

capacitive coupling opens this DC path so an extra pulldown

resistor on the amplifier output to ground is required.

Figure 4 shows a 250Ω resistor. Capacitively coupling the

output will require that we shift the output offset voltage

higher than in the direct coupled case. Using RA = 43k and

RB = 4.7k will make the quiescent output offset voltage

about 1V. The output dynamic range will be 0.6V to 3V.

Input capacitive coupling will increase the needed dynamic

range of the amplifier. The standard NTSC video signal is 1V

peak to peak plus 143mV for the color AC peak. The video

signal is made up of the -286mV sync pulse plus the 714mV

picture signal which may very from 0V to 714mV. The video

signal average value for a black picture is about 28mV

(Figure 2) and with a white picture level is about 583mV

(Figure 3). This gives a maximum change in average value

of about 555mV. A direct coupled amplifier with an standard

NTSC video signal needs a dynamic range of 1.143V. But

with input capacitance coupling the dynamic range

requirements are the sum of the 1.143V video plus the

average picture value change of 0.555V or 1.698VP-P. At a

gain of two this doubles to 3.394V. These amplifiers do not

have this much dynamic range so a gain of less than 2 must

be used to avoid waveform compression under all

conditions.

Capacitively coupling the input and output is worse than a

capacitor only on the input. Without any special

compromises you can only take a gain of one. But if the

backmatch resistor is reduced to 36Ω, reducing the output

VIN

+ Video

– 1V

0.1µF 0.32VB 68K

CB

4.7K

RB

RA

+

0.64VB

V AMP 75

VOUT

1V

AMP

-

RO

1K

RF

+ V1

– 5V

FIGURE 1.

0.714V

0.0V

-0.286V

0.6

0.867

Ý10µs

53µs

0.583V

1.3µs

3.8µs

5.1µs

FIGURE 2. WHITE LEVEL VIDEO

0.714V

0.0V

-0.286V

53mV

Average BL

+45mV

0.686V +28mV

Average Picture Value Change — 555mV

FIGURE 3. BLACK LEVEL VIDEO

Gain = 2

1.428

1.166

1.8V offset

Amp

OutVolts

2.062

1.8

0

-0.572

+0.348

+0.062

Gain = 2

1.8V offset

Amp Out

Volts

+3.172

+56mV

+1.8

+1.172

5

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