AD8055/AD8056
APPLICATIONS
Four-Line Video Driver
The AD8055 is a useful low cost circuit for driving up to four
video lines. For such an application, the amplifier is configured
for a noninverting gain of 2 as shown in Figure 33. The input
video source is terminated in 75
Ω
and applied to the high
impedance noninverting input.
Each output cable is connected to the op amp output via a 75
Ω
series back termination resistor for proper cable termination.
The terminating resistors at the other ends of the lines will
divide the output signal by two, which is compensated for by
the gain-of-two of the op amp stage.
For a single load, the differential gain error of this circuit was
measured to be 0.01%, with a differential phase error of
0.02 degrees. The two load measurements were 0.02% and
0.03 degrees, respectively. For four loads, the differential gain
error is 0.02%, while the differential phase increases to 0.1
degrees.
75
+5V
402
75
V
OUT2
75
6
The gain of this circuit from the input to Amp 1 output is R
F
/R
I
,
while the gain to the output of Amp 2 is –R
F
/R
I
. The circuit
thus creates a balanced differential output signal from a single-
ended input. The advantage of this circuit is that the gain can be
changed by changing a single resistor and still maintain the
balanced differential outputs.
R
F
402
+5V
0.1 F
3
8
R
I
402
V
IN
10 F
49.9
AMP1
2
1
+V
OUT
402
402
AD8056
402
402
V
OUT1
75
6
49.9
AMP2
7
–V
OUT
402
2
7
0.1 F
10 F
5
4
75
0.1 F
–5V
10 F
AD8055
V
IN
75
3
4
75
0.1 F
10 F
V
OUT3
75
75
V
OUT4
75
Figure 34. Single-Ended to Differential Line Driver
Low Noise, Low Power Preamp
–5V
Figure 33. Four-Line Video Driver
Single-Ended to Differential Line Driver
The AD8055 makes a good low cost, low noise, low power
preamp. A gain of 10 preamp can be made with a feedback
resistor of 909 ohms and a gain resistor of 100 ohms as shown
in Figure 35. The circuit has a –3 dB bandwidth of 20 MHz.
909
+5V
+
10 F
V
OUT
Creating differential signals from single-ended signals is
required for driving balanced, twisted pair cables, differential
input A/D converters and other applications that require differen-
tial signals. This is sometimes accomplished by using an inverting
and a noninverting amplifier stage to create the complementary
signals.
The circuit shown in Figure 34 shows how an AD8056 can be
used to make a single-ended to differential converter that offers
some advantages over the architecture mentioned above. Each
op amp is configured for unity gain by the feedback resistors
from the outputs to the inverting inputs. In addition, each out-
put drives the opposite op amp with a gain of –1 by means of the
crossed resistors. The result of this is that the outputs are comple-
mentary and there is high gain in the overall configuration.
Feedback techniques similar to a conventional op amp are used
to control the gain of the circuit. From the noninverting input
of Amp 1 to the output of Amp 2, is an inverting gain. Between
these points a feedback resistor can be used to close the loop.
As in the case of a conventional op amp inverting gain stage, an
input resistor is added to vary the gain.
R
S
100
2
7
0.1 F
AD8055
3
4
6
0.1 F
–5V
10 F
Figure 35. Low Noise, Low Power Preamp with G = 10
and BW = 20 MHz
With a low source resistance (<approximately 100
Ω),
the major
contributors to the input referred noise of this circuit are the
input voltage noise of the amplifier and the noise of the 100
Ω
resistor. These are 6 nV/√Hz and 1.2 nV/√Hz, respectively.
These values yield a total input referred noise of 6.1 nV/√Hz.
REV. B
–9–
AD [ ANALOG DEVICES ]
