LT1397CS datasheet(10/20 Pages) LINER

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LT1395/LT1396/LT1397

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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 (5

Ω 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 10

µA per volt of supply mismatch.

APPLICATI

I FOR ATIO

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/

µsandissetbyinternalcurrentsand

capacitances. The output slew rate is set by the value of

the feedback resistor and internal capacitance. At a gain

of 2 with 255

Ω 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 + supply and EN is varied. As seen in Figure

1, +IS does vary with (V + – VEN), particularly when the

voltage difference is less than 3V. For normal operation,

Figure 1. + IS vs (V + – VEN)

V + – VEN (V)

0.5

1.5

2.0

2.5

5.0

3.5

1395/6/7 F01

1.0

4.0

4.5

3.0

TA = 25°C

V + = 5V

V – = – 5V

V – = 0V

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