LM6181AMJSLASH883 datasheet(17/23 Pages) NSC

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Typical Applications (Continued)

SLEW RATE CONSIDERATIONS

The slew rate characteristics of current feedback amplifiers

are different than traditional voltage feedback amplifiers. In

voltage feedback amplifiers slew rate limiting or non-linear

amplifier behavior is dominated by the finite availability of the

1st stage tail current charging the compensation capacitor.

The slew rate of current feedback amplifiers, in contrast, is

not constant. Transient current at the inverting input deter-

mines slew rate for both inverting and non-inverting gains.

The non-inverting configuration slew rate is also determined

by input stage limitations. Accordingly, variations of slew

rates occur for different circuit topologies.

DRIVING CAPACITIVE LOADS

The LM6181 can drive significantly larger capacitive loads

than many current feedback amplifiers. Although the

LM6181 can directly drive as much as 100 pF without oscil-

lating, the resulting response will be a function of the feed-

back resistor value.

Figure 5 illustrates the small-signal

pulse response of the LM6181 while driving a 50 pF load.

Ringing persists for approximately 70 ns. To achieve pulse

responses with less ringing either the feedback resistor can

be increased (see typical curves Suggested R

f and Rs for

L), or resistive isolation can be used (10Ω–51Ω typically

works well). Either technique, however, results in lowering

the system bandwidth.

Figure 6 illustrates the improvement obtained with using a

Ω isolation resistor.

DS011328-15

3a: R

f = 820Ω

DS011328-16

3b: R

f = 1640Ω

FIGURE 3. Increasing Compensation

with Increasing R

DS011328-17

FIGURE 4. Reducing R

f for Large

Closed Loop Gains, R

f = 500Ω

DS011328-18

DS011328-19

FIGURE 5. A

V = −1, LM6181 Can Directly

Drive 50 pF of Load Capacitance with 70 ns

of Ringing Resulting in Pulse Response

LM6181AMJ/883 Datasheet(PDF) 17 Page - National Semiconductor (TI)

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