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LT1398/LT1399/LT1399HV

APPLICATI

I FOR ATIO

Power Supplies

The LT1398/LT1399 will operate from single or split

supplies from

±2V (4V total) to ±6V (12V total). The

LT1399HV will operate from single or split supplies from

±2V (4V total) to ±7.5V (15V 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 600

µV per volt of supply mis-

match. The inverting bias current will typically change

about 2

µA per volt of supply mismatch.

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 LT1398/LT1399/LT1399HV 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 324

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

Each amplifier of the LT1398/LT1399/LT1399HV has a

unique high impedance, zero supply current mode which

is controlled by its own EN pin. These amplifiers are

designed to operate with CMOS logic; the amplifiers draw

zero current when these pins are high. To activate each

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, it is

important to keep the EN pin at least 3V below the V +

supply. If a V + of less than 3V is desired, and the amplifier

will remain enabled at all times, then the EN pin should be

tied to the V – supply. The enable pin current is approxi-

mately 30

µA when activated. If using CMOS open-drain

logic, an external 1k pull-up resistor is recommended to

ensure that the LT1399 remains disabled in spite of any

CMOS drain-leakage currents.

Figure 1. + IS vs (V

+ – VEN)

V + – VEN (V)

0.5

1.5

2.0

2.5

5.0

3.5

1398/99 F01

1.0

4.0

4.5

3.0

TA = 25°C

V + = 5V

V – = – 5V

V – = 0V

Figure 2. Amplifier Enable Time, AV = 2

VS = ±5V

VIN = 1V

RF = 324Ω

RG = 324Ω

RL = 100Ω

1398/99 F02

OUTPUT

Figure 3. Amplifier Disable Time, AV = 2

VS = ±5V

VIN = 1V

RF = 324Ω

RG = 324Ω

RL = 100Ω

1398/99 F03

OUTPUT

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