AD811
Operation as a Video Line Driver
The AD811 has been designed to offer outstanding perfor-
mance at closed-loop gains of one or greater, while driving
multiple reverse-terminated video loads. The lowest differential
gain and phase errors will be obtained when using ±15 volt
power supplies. With ±12 volt supplies, there will be an insig-
nificant increase in these errors and a slight improvement in
gain flatness. Due to power dissipation considerations, ±12 volt
supplies are recommended for optimum video performance.
Excellent performance can be achieved at much lower supplies
as well.
1V
10ns
100
90
V
IN
10
V
OUT
0%
The closed-loop gain vs. frequency at different supply voltages
is shown in Figure 36. Figure 37 is an oscilloscope photograph
of an AD811 line driver’s pulse response with ±15 volt supplies.
The differential gain and phase error vs. supply are plotted in
Figures 38 and 39, respectively.
1V
Figure 37. Small Signal Pulse Response, Gain = +2,
VS = ±15 V
Another important consideration when driving multiple cables
is the high frequency isolation between the outputs of the
cables. Due to its low output impedance, the AD811 achieves
better than 40 dB of output to output isolation at 5 MHz driv-
ing back terminated 75 Ω cables.
0.10
R = 649⍀
0.09
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
F
F
= 3.58MHz
C
100 IRE
MODULATED
RAMP
75⍀ CABLE
649⍀
649⍀
V
#1
OUT
75⍀
+V
75⍀
S
a. DRIVING A SINGLE, BACK TERMINATED,
75⍀ COAX CABLE
DRIVING TWO PARALLEL,
0.1F
b.
BACK TERMINATED, COAX CABLES
75⍀ CABLE
b
V
#2
OUT
AD811
75⍀
75⍀ CABLE
75⍀
V
IN
75⍀
a
5
6
7
8
9
10
11
12
13
14
15
SUPPLY VOLTAGE – ؎Volts
0.1F
Figure 38. Differential Gain Error vs. Supply Voltage for
the Video Line Driver of Figure 35
–V
S
Figure 35. A Video Line Driver Operating at a Gain of +2
0.20
12
R
= 649⍀
= 3.58MHz
F
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
0.02
F
C
G = +2
100 IRE
MODULATED
RAMP
V
R
= ؎15V
S
R
R
= 150⍀
= R
9
6
L
= 649⍀
FB
G
FB
a.DRIVING A SINGLE, BACK TERMINATED,
75⍀ COAX CABLE
DRIVING TWO PARALLEL,
b.
3
BACK TERMINATED, COAX CABLES
V
= ؎5V
S
R
= 562⍀
FB
b
0
–3
–6
a
5
6
7
8
9
10
11
12
13
14
15
1
10
FREQUENCY – MHz
100
SUPPLY VOLTAGE – ؎Volts
Figure 39. Differential Phase Error vs. Supply Voltage for
the Video Line Driver of Figure 35
Figure 36. Closed-Loop Gain vs. Frequency, Gain = +2
REV. D
–11–
ADI [ ADI ]
