LMV710 datasheet(12/19 Pages) NSC

12 / 19 page

Application Note (Continued)

4.0 COMPENSATION OF INPUT CAPACITANCE

In the application (Figure 4) where a large feedback resistor

is used, the feedback resistor can react with the input ca-

pacitance of the op amp and introduce an additional pole to

the close loop frequency response.

This pole occurs at frequency f

p , where

Any stray capacitance due to external circuit board layout,

any source capacitance from transducer or photodiode con-

nected to the summing node will also be added to the input

capacitance. If f

p is less than or close to the unity-gain

bandwidth (5MHz) of the op amp, the phase margin of the

loop is reduced and can cause the system to be unstable.

To avoid this problem, make sure that f

p occurs at least 2

octaves beyond the expected −3dB frequency corner of the

close loop frequency response. If not, a feedback capacitor

F can be placed in parallel with RF such that

The paralleled R

F and CF introduce a zero, which cancels

the effect from the pole.

5.0 CAPACITIVE LOAD TOLERANCE

The LMV710, LMV711 and LMV715 can directly drive 200pF

in unity-gain without oscillation. The unity-gain follower is the

most sensitive configuration to capacitive loading. Direct

capacitive loading reduces the phase margin of amplifiers.

The combination of the amplifier’s output impedance and the

capacitive load induces phase lag. This results in either an

underdamped pulse response or oscillation. To drive a

heavier capacitive load, circuit in Figure 5 can be used.

In Figure 5, the isolation resistor R

ISO and the load capacitor

L form a pole to increase stability by adding more phase

margin to the overall system. The desired performance de-

pends on the value of R

ISO. The bigger the RISO resistor

value, the more stable V

OUT will be. But the DC accuracy is

not great when the R

ISO gets bigger. If there were a load

resistor in Figure 5, the output would be voltage divided by

ISO and the load resistor.

The circuit in Figure 6 is an improvement to the one in Figure

5 because it provides DC accuracy as well as AC stability. In

this circuit, R

F provides the DC accuracy by using feed-

forward techniques to connect V

IN to RL.CF and RISO serve

to counteract the loss of phase margin by feeding the high

frequency component of the output signal back to the ampli-

fier’s inverting input, thereby preserving phase margin in the

overall feedback loop. Increased capacitive drive is possible

by increasing the value of C

F . This in turn will slow down the

pulse response.

6.0 APPLICATION CIRCUITS

PEAK DETECTOR

Peak detectors are used in many applications, such as test

equipment, measurement instrumentation, ultrasonic alarm

systems, etc. Figure 7 shows the schematic diagram of a

peak detector using LMV710 or LMV711 or LMV715. This

peak detector basically consists of a clipper, a parallel RC

network, and a voltage follower.

10132518

FIGURE 4. Cancelling the Effect of Input Capacitance

10132521

FIGURE 5. Indirectly Driving A Capacitive Load using

Resistive Isolation

10132522

FIGURE 6. Indirectly Driving A Capacitive A Load with

DC Accuracy

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