el2111 Intersil Corporation, el2111 Datasheet - Page 5

• Manufacturer Part Number: el2111
• Description: Low Cost, Gain Of 2, Video Op Amp
• Manufacturer: Intersil Corporation

matching resistor to a cable and a 75: load to ground at the
end of the cable provide a 150: 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 250: resistor. Capacitively coupling the
output will require that we shift the output offset voltage
higher than in the direct coupled case. Using R
R
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
B
= 4.7k will make the quiescent output offset voltage
-0.286V
0.714V
-0.286V
0.714V
0.0V
0.0V
+
–
5
0.6
Video
1V
0.867
53mV
V
IN
Average Picture Value Change — 555mV
Ý10µs
A
0.1µF
4.7K
= 43k and
CB
RB
FIGURE 3. BLACK LEVEL VIDEO
FIGURE 2. WHITE LEVEL VIDEO
1.3µs
3.8µs
5.1µs
0.32VB
Average BL
53µs
+45mV
EL2111
+
-
FIGURE 1.
68K
RA
RF
1K
AMP
0.583V
0.64VB
(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 1.698V
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 36:, reducing the output
+
–
0.686V
V1
5V
V AMP
+28mV
RO
75
V
OUT
Gain = 2
-0.572
1.428
1.166
Gain = 2
+56mV
0
1.8V offset
1V
OutVolts
1.8V offset
+0.348
+0.062
Amp Out
2.062
Amp
+3.172
+1.172
1.8
Volts
+1.8
P-P
. At a