AD8055/AD8056
Rev. J | Page 12 of 16
APPLICATIONS
FOUR-LINE VIDEO DRIVER
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.
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 36. The input
video source is terminated in 75 Ω and is applied to the high
impedance noninverting input.
/R
The gain of this circuit from the input to AMP1 output is R
F I
,
while the gain to the output of AMP2 is −R
/R
F I
. The circuit
therefore 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, while still
maintaining the balanced differential outputs.
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 divide
the output signal by 2, which is compensated for by the gain of 2
of the op amp stage.
75Ω
+5V
–5V
AD8056
402Ω
402Ω
402Ω
402Ω
49.9Ω
49.9Ω
1
2
3
8
AMP1
5
6
7
4
AMP2
0.1µF
R
F
402Ω
R
I
402Ω
10µF
0.1µF 10µF
V
IN
+V
OUT
–V
OUT
01063-037
For a single load, the differential gain error of this circuit was
measured as 0.01%, with a differential phase error of 0.02°. The
two load measurements were 0.02% and 0.03°, respectively. For
four loads, the differential gain error is 0.02%, while the
differential phase increases to 0.1°.
V
OUT3
AD8055
+5V
–5V
6
7
2
3
4
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
75Ω
V
OUT1
V
OUT2
V
OUT4
0.1µF
0.1µF 10µF
10µF
75Ω
402Ω
402Ω
V
IN
01063-036
Figure 37. Single-Ended-to-Differential Line Driver
Figure 36. Four-Line Video Driver
LOW NOISE, LOW POWER PREAMP
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 Ω and a gain resistor of 100 Ω, as shown in
Creating differential signals from single-ended signals is
required for driving balanced, twisted pair cables, differential
input ADCs, and other applications that require differential
signals. This can be accomplished by using an inverting and a
noninverting amplifier stage to create the complementary
signals.
Figure 38. The circuit has a −3 dB bandwidth of 20 MHz.
0.1µF 10µF
0.1µF 10µF
+5V
–5V
+
AD8055
6
7
2
3
4
V
OUT
R
S
909Ω
100Ω
1063-038
The circuit shown in
Figure 37 shows how an AD8056 can be
used to make a single-ended-to-differential converter that offers
some advantages over the architecture previously mentioned.
Each op amp is configured for unity gain by the feedback
resistors from the outputs to the inverting inputs. In addition,
each output 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 complementary and there is high gain in the overall
configuration.
Figure 38. 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 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.
Feedback techniques similar to a conventional op amp are used
to control the gain of the circuit. From the noninverting input
of AMP1 to the output of AMP2 is an inverting gain.
