NCS2535
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 28.
Figure 28. Frequency Response vs. R
F
−21
−18
−15
−6
0
18
21
10 k 1 M 10 M 100 M 1 G 10 G
FREQUENCY (Hz)
GAIN (dB)
R
F
= 450 W
R
F
= 500 W
R
F
= 400 W
A
V
= +2
V
S
= ±5 V
R
L
= 150 W
R
F
= 270 W
100 k
−12
−9
−3
9
15
3
6
12
R
F
= 330 W
R
F
= 200 W
R
F
= 100 W
R
F
= 150 W
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.
ESD Protection
All device pins have limited ESD protection using internal
diodes to power supplies as specified in the attributes table
(see Figure 29). These diodes provide moderate protection
to input overdrive voltages above the supplies. The ESD
diodes can support high input currents with current limiting
series resistors. Keep these resistor values as low as possible
since high values degrade both noise performance and
frequency response. Under closed−loop operation, the ESD
diodes have no effect on circuit performance. However,
under certain conditions the ESD diodes will be evident. If
the device is driven into a slewing condition, the ESD diodes
will clamp large differential voltages until the feedback loop
restores closed−loop operation. Also, if the device is
powered down and a large input signal is applied, the ESD
diodes will conduct.
NOTE: Human Body Model for +IN and –IN pins are
rated at 0.8 kV while all other pins are rated at
2.0 kV.
V
CC
Internal
Circuitry
V
EE
External
Pin
Figure 29. Internal ESD Protection
