LT1259/LT1260
SI PLIFIED SCHE ATIC
, each amplifier
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+IN
–IN
OUT
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LT1259/60 • SS
APPLICATIO S I FOR ATIO
Feedback Resistor Selection
The small-signal bandwidth of the LT1259/ LT1260 are set
by the external feedback resistors and the internal junction
capacitors. As a result, the bandwidth is a function of the
supply voltage, the value of the feedback resistor, the
closed-loop gain and the load resistor. The LT1259/LT1260
have been optimized for
±5V
supply operation and have a
– 3dB bandwidth of 90MHz. See resistor selection guide in
Typical AC Performance table.
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). See
the section on Demo Board Information.
Capacitive Loads
The LT1259/LT1260 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 value
shown is for
≤
5dB peaking when driving a 150Ω load at a
gain of 2. This is a worst case condition. The amplifier is
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more stable at higher gains. Alternatively, 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 that the amplifier bandwidth is only reduced
when the capacitive load is present. The disadvantage is
that the gain is a function of the load resistance.
Power Supplies
The LT1259/LT1260 will operate from 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 500µV per volt of
supply mismatch. The inverting bias current can change
as much as 5µA per volt of supply mismatch though
typically, the change is about 0.1µA per volt.
Slew Rate
The slew rate of a current feedback amplifier is not
independent of the amplifier gain configuration the way
slew rate is in a traditional op amp. This is because both the
input stage and the output stage have slew rate limitations.
In the inverting mode, and for higher gains in the nonin-
verting mode, the signal amplitude between the input pins
is small and the overall slew rate is that of the output stage.
For gains less than ten in the noninverting mode, the
overall slew rate is limited by the input stage.
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