NCS2530
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
−20
−15
−10
−5
0
5
0.01 0.1 1.0 10 100 1000 10000
FREQUENCY (MHz)
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.
ESD Protection
All device pins have limited ESD protection using internal
diodes to power supplies as specified in the attributes table
(See Figure 30). 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.
Figure 30. Internal ESD Protection
Internal
Circuitry
External
Pin
V
CC
V
EE
