Application Notes

1.0 BENEFITS OF THE LMV721/722 SIZE

The small footprints of the LMV721/722 packages save

space on printed circuit boards, and enable the design of

smaller electronic products, such as cellular phones, pagers,

or other portable systems. The low profile of the LMV721/

722 make them possible to use in PCMCIA type III cards.

Signal Integrity. Signals can pick up noise between the

signal source and the amplifier. By using a physically smaller

amplifier package, the LMV721/722 can be placed closer to

the signal source, reducing noise pickup and increasing

signal integrity.

Simplified Board Layout. These products help you to avoid

using long pc traces in your pc board layout. This means that

no additional components, such as capacitors and resistors,

are needed to filter out the unwanted signals due to the

interference between the long pc traces.

Low Supply Current. These devices will help you to maxi-

mize battery life. They are ideal for battery powered sys-

tems.

Low Supply Voltage. National provides guaranteed perfor-

mance at 2.2V and 5V. These guarantees ensure operation

throughout the battery lifetime.

Rail-to-Rail Output. Rail-to-rail output swing provides maxi-

mum possible dynamic range at the output. This is particu-

larly important when operating on low supply voltages.

Input Includes Ground. Allows direct sensing near GND in

single supply operation.

Protection should be provided to prevent the input voltages

from going negative more than −0.3V (at 25˚C). An input

clamp diode with a resistor to the IC input terminal can be

used.

2.0 CAPACITIVE LOAD TOLERANCE

The LMV721/722 can directly drive 4700pF in unity-gain

without oscillation. The unity-gain follower is the most sensi-

tive configuration to capacitive loading. Direct capacitive

loading reduces the phase margin of amplifiers. The combi-

nation of the amplifier’s output impedance and the capacitive

load induces phase lag. This results in either an under-

damped pulse response or oscillation. To drive a heavier

capacitive load, circuit in Figure 1 can be used.

In Figure 1, the isolation resistor R

C

margin to the overall system. the desired performance de-

pends on the value of R

value, the more stable V

waveform of Figure 1 using 100k

Ω for R

C

The circuit in Figure 3 is an improvement to the one in Figure

1 because it provides DC accuracy as well as AC stability. If

there were a load resistor in Figure 1, the output would be

voltage divided by R

Figure 3,R

F

provides the DC accuracy by using feed-

forward techniques to connect V

in choosing the value of R

the LMV721/722. C

phase margin by feeding the high frequency component of

the output signal back to the amplifier’s inverting input,

thereby preserving phase margin in the overall feedback

loop. Increased capacitive drive is possible by increasing the

value of C

3.0 INPUT BIAS CURRENT CANCELLATION

The LMV721/722 family has a bipolar input stage. The typi-

cal input bias current of LMV721/722 is 260nA with 5V

supply. Thus a 100k

Ω input resistor will cause 26mV of error

voltage. By balancing the resistor values at both inverting

and non-inverting inputs, the error caused by the amplifier’s

input bias current will be reduced. The circuit in Figure 4

shows how to cancel the error caused by input bias current.

10092218

FIGURE 1. Indirectly Driving A capacitive Load Using

Resistive Isolation

10092231

FIGURE 2. Pulse Response of the LMV721 Circuit in

Figure 1

10092219

FIGURE 3. Indirectly Driving A Capacitive Load with

DC Accuracy

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