LT1396 Linear Technology Corporation, LT1396 Datasheet - Page 10

• Manufacturer Part Number: LT1396
• Description: Dual 400mhz Current Feedback Amplifier
• Manufacturer: Linear Technology Corporation

LT1395/LT1396/LT1397
Feedback Resistor Selection
The small-signal bandwidth of the LT1395/LT1396/LT1397
is set by the external feedback resistors and the internal
junction capacitors. As a result, the bandwidth is a func-
tion of the supply voltage, the value of the feedback
resistor, the closed-loop gain and the load resistor. The
LT1395/LT1396/LT1397 have been optimized for ±5V
supply operation and have a – 3dB bandwidth of 400MHz
at a gain of 1 and 350MHz at a gain of 2. Please refer to
the resistor selection guide in the Typical AC Perfor-
mance 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).
Capacitive Loads
The LT1395/LT1396/LT1397 can drive many capacitive
loads directly when the proper value of feedback resistor
is used. The required value for the feedback resistor will
increase as load capacitance increases and as closed-loop
gain decreases. Alternatively, a small resistor (5Ω to 35Ω)
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 capaci-
tive load is present. The disadvantage is that the gain is a
function of the load resistance. See the Typical Perfor-
mance Characteristics curves.
Power Supplies
The LT1395/LT1396/LT1397 will operate from single or
split supplies from ±2V (4V total) to ±6V (12V total). It
is not necessary to use equal value split supplies, how-
ever the offset voltage and inverting input bias current
will change. The offset voltage changes about 2.5mV per
volt of supply mismatch. The inverting bias current will
typically change about 10µA per volt of supply mismatch.
10
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Slew Rate
Unlike a traditional voltage feedback op amp, the slew rate
of a current feedback amplifier is not independent of the
amplifier gain configuration. In a current feedback ampli-
fier, both the input stage and the output stage have slew rate
limitations. In the inverting mode, and for gains of 2 or more
in the noninverting 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 2 in the noninverting mode,
the overall slew rate is limited by the input stage.
The input slew rate of the LT1395/LT1396/LT1397 is
approximately 600V/µs and is set by internal currents and
capacitances. The output slew rate is set by the value of
the feedback resistor and internal capacitance. At a gain
of 2 with 255Ω feedback and gain resistors and ±5V
supplies, the output slew rate is typically 800V/µs. Larger
feedback resistors will reduce the slew rate as will lower
supply voltages.
Enable/ Disable
The LT1395CS6 has a unique high impedance, zero
supply current mode which is controlled by the EN pin.
The LT1395CS6 is designed to operate with CMOS logic;
it draws virtually zero current when the EN pin is high. To
activate the amplifier, its EN pin is normally pulled to a
logic low. However, supply current will vary as the voltage
between the V
1, +I
voltage difference is less than 3V. For normal operation,
S
does vary with (V
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0
+
T
V
A
supply and EN is varied. As seen in Figure
+
= 25°C
= 5V
Figure 1. + I
1
2
V
–
V
+
= – 5V
+
3
– V
– V
S
EN
vs (V
4
EN
(V)
V
), particularly when the
+
–
5
= 0V
– V
EN
1395/6/7 F01
6
)
7
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