Datasheet
AD811
Rev. E | Page 14 of 20
OPERATION AS A VIDEO LINE DRIVER
The AD811 has been designed to offer outstanding perform-
ance at closed-loop gains of +1 or greater, while driving
multiple reverse-terminated video loads. The lowest differential
gain and phase errors are obtained when using ±15 V power
supplies. With ±12 V supplies, there is an insignificant increase
in these errors and a slight improvement in gain flatness.
Due to power dissipation considerations, ±12 V supplies are
recommended for optimum video performance. Excellent
performance can be achieved at much lower supplies as well.
The closed-loop gain versus the frequency at different supply
voltages is shown in Figure 43. Figure 44 is an oscilloscope
photograph of an AD811 line driver’s pulse response with ±15 V
supplies. The differential gain and phase error versus the supply
are plotted in Figure 45 and Figure 46, respectively.
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
driving back-terminated 75 Ω cables.
V
IN
–V
S
+V
S
0.1µF
0.1µF
AD811
V
OUT
No. 2
V
OUT
No. 1
75Ω CABLE
75Ω CABLE
75Ω CABLE
75Ω
75Ω
649Ω649Ω
75Ω
75Ω
75Ω
+
–
7
6
4
3
2
00866-E-042
Figure 42. A Video Line Driver Operating at a Gain of +2
–6
–3
0
3
6
9
12
GAIN (dB)
FREQUENCY (MHz)
1 10 100
00866-E-043
V
S
= ±15V
R
FB
= 649Ω
V
S
= ±5V
R
FB
= 562Ω
G = +2
R
L
= 150Ω
R
G
= R
FB
Figure 43. Closed-Loop Gain vs. Frequency, Gain = +2
00866-E-044
10
90
100
0%
1V
1V
10ns
V
IN
V
OUT
Figure 44. Small Signal Pulse Response, Gain = +2, V
S
= ±15 V
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
0.08
0.09
0.10
DIFFERENTIAL GAIN (%)
56789101112131415
SUPPLY VOLTAGE (V)
00866-E-045
R
F
= 649Ω
F
C
= 3.58MHz
100 IRE
MODULATED RAMP
a. DRIVING A SINGLE, BACK-
TERMINATED, 75Ω COAX CABLE
b. DRIVING TWO PARALLEL, BACK-
TERMINATED, COAX CABLES
a
b
Figure 45. Differential Gain Error vs. Supply Voltage for
the Video Line Driver of Figure 42
0
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
DIFFERENTIAL PHASE (DEGREES)
56789101112131415
SUPPLY VOLTAGE (V)
00866-E-046
R
F
= 649Ω
F
C
= 3.58MHz
100 IRE
MODULATED RAMP
a. DRIVING A SINGLE, BACK-
TERMINATED, 75Ω COAX CABLE
b. DRIVING TWO PARALLEL, BACK-
TERMINATED, COAX CABLES
a
b
Figure 46. Differential Phase Error vs. Supply Voltage for
the Video Line Driver of Figure 42