NCS2530, NCS2530A
http://onsemi.com
12
General Design Considerations
The current feedback amplifier is optimized for use in
high performance video and data acquisition systems. For
current feedback architecture, its closed−loop bandwidth
depends on the value of the feedback resistor. The
closed−loop bandwidth is not a strong function of gain, as is
for a voltage feedback amplifier, as shown in Figure 29.
Figure 29. Frequency Response vs. R
F
−20
−15
−10
−5
0
5
10
0.01 0.1 1.0 10 100 1000 10000
FREQUENCY (MHz)
GAIN (dB)
R
F
= 1.8 kW
R
F
= 1.2 kW
R
F
= 1 kW
A
V
= +2
V
CC
= +5 V
V
EE
= −5 V
The −3.0 dB bandwidth is, to some extent, dependent on
the power supply voltages. By using lower power supplies,
the bandwidth is reduced, because the internal capacitance
increases. Smaller values of feedback resistor can be used at
lower supply voltages, to compensate for this affect.
Feedback and Gain Resistor Selection for Optimum
Frequency Response
A current feedback operational amplifier’s key advantage
is the ability to maintain optimum frequency response
independent of gain by using appropriate values for the
feedback resistor. To obtain a very flat gain response, the
feedback resistor tolerance should be considered as well.
Resistor tolerance of 1% should be used for optimum
flatness. Normally, lowering RF resistor from its
recommended value will peak the frequency response and
extend the bandwidth while increasing the value of RF
resistor will cause the frequency response to roll off faster.
Reducing the value of RF resistor too far below its
recommended value will cause overshoot, ringing, and
eventually oscillation.
Since each application is slightly different, it is worth
some experimentation to find the optimal RF for a given
circuit. A value of the feedback resistor that produces
X0.1 dB of peaking is the best compromise between
stability and maximal bandwidth. It is not recommended to
use a current feedback amplifier with the output shorted
directly to the inverting input.
Printed Circuit Board Layout Techniques
Proper high speed PCB design rules should be used for all
wideband amplifiers as the PCB parasitics can affect the
overall performance. Most important are stray capacitances
at the output and inverting input nodes as it can effect
peaking and bandwidth. A space (3/16″ is plenty) should be
left around the signal lines to minimize coupling. Also,
signal lines connecting the feedback and gain resistors
should be short enough so that their associated inductance
does not cause high frequency gain errors. Line lengths less
than 1/4″ are recommended.
Video Performance
This device designed to provide good performance with
NTSC, PAL, and HDTV video signals. Best performance is
obtained with back terminated loads as performance is
degraded as the load is increased. The back termination
reduces reflections from the transmission line and
effectively masks transmission line and other parasitic
capacitances from the amplifier output stage.
Video Line Driver
NCS2530 can be used in video line driver applications.
Figure 30 shows a typical schematic for a video driver. In
some applications, two or more video loads have to be
driven simultaneously as shown in Figure 31. Figure 32
shows the typical performance of the op amp with single and
triple video load.
Figure 30. Video Driver Schematic
−
+
75 W
75 W
V
OUT
R
F
75 W
V
IN
R
G
Z = 75 W
Z = 75 W
Figure 31. Video Driver Schematic
for Three Video Loads
−
+
75 W
75 W
V
OUT2
R
F
75 W
V
IN
R
G
Z = 75 W
Z = 75 W
75 W
75 W
Z = 75 W
V
OUT3
75 W
75 W
Z = 75 W
V
OUT1
