8
LT1497
APPLICATIONS INFORMATION
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The LT1497 is a dual current feedback amplifier with high
output current drive capability. Bandwidth is maintained
over a wide range of voltage gains by the appropriate
choice of feedback resistor. These amplifiers will drive low
impedance loads such as cables with excellent linearity at
high frequencies.
Feedback Resistor Selection
The optimum value for the feedback resistor is a function
of the operating conditions of the device, the load imped-
ance and the desired flatness of frequency response. The
Small-Signal Bandwidth table gives the values which
result in the highest bandwidth with less than 1dB of
peaking for various gains, loads and supply voltages. If
this level of flatness is not required, a higher bandwidth
can be obtained by use of a lower feedback resistor. The
characteristic curves of Bandwidth vs Supply Voltage
indicate feedback resistors for peaking up to 5dB. These
curves use a solid line when the response has less than
1dB of peaking and a dashed line when the response has
1dB to 5dB of peaking. Note that in a gain of 10 peaking is
always under 1dB for the resistor ranges shown. Reducing
the feedback resistor further than 270Ω in a gain of 10 will
increase the bandwidth, but it also loads the amplifier and
reduces the maximum current available to drive the load.
Capacitive Loads
The LT1497 can drive capacitive loads directly when the
proper value of feedback resistor is used. The graph of
Maximum Capacitive Load vs Feedback Resistor should
be used to select the appropriate value. The graph shows
feedback resistor values for 5dB frequency peaking when
driving a 1k load at a gain of 2. This is a worst-case
condition. The amplifier is more stable at higher gains and
driving heavier loads (smaller load resistors). Alterna-
tively, a small resistor (10Ω to 20Ω) can be put in series
with the output to isolate the capacitive load from the
amplifier output. This has the advantage in that the ampli-
fier bandwidth is only reduced when the capacitive load is
present, and the disadvantage that the gain is a function of
the load resistance.
Capacitance on the Inverting Input
Current feedback amplifiers require resistive feedback
from the output to the inverting input for stable operation.
Take care to minimize the stray capacitance between the
output and the inverting input. Capacitance on the invert-
ing input to ground will cause peaking in the frequency
response (and overshoot in the transient response), but it
does not degrade the stability of the amplifier.
Power Supplies
The LT1497 will operate on single or split supplies from
±2V (4V total) to ±15V (30V total). It is not necessary to
use equal value split supplies, however, the offset voltage
and inverting input bias current will change. The offset
voltage changes about 1mV per volt of supply mismatch.
The inverting bias current can change as much as 10µA
per volt of supply mismatch, though typically the change
is less than 2.5µA per volt.
Thermal Considerations
The LT1497 contains a thermal shutdown feature that
protects against excessive internal (junction) tempera-
ture. If the junction temperature of the device exceeds the
protection threshold, the device will begin cycling
between normal operation and an off state. The cycling is
not harmful to the part. The thermal cycling occurs at a
slow rate, typically 10ms to several seconds, depending
upon the power dissipation and the thermal time con-
stants of the package and the amount of copper on the
board under the package. Raising the ambient tempera-
ture until the device begins thermal shutdown gives a
good indication of how much margin there is in the
thermal design.
For surface mount devices heat sinking is accomplished
by using the heat spreading capabilities of the PC board
and its copper traces. Experiments have shown that the
heat spreading copper layer does not need to be electri-
cally connected to the leads of the device. The PCB
material can be very effective at transmitting heat between
the pad area attached to V
–
pins of the device and a ground
