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 sig-

nal 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 sig-

nal 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 inter-

ference 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 ca-

pacitive 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

ure 3,R

techniques to connect V

ing the value of R

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

Gain and Phase Margin vs.

Frequency (V

DS100922-16

Slew Rate vs.

Supply Voltage

DS100922-17

THD vs.

Frequency

DS100922-42

DS100922-18

FIGURE 1. Indirectly Driving A capacitive Load Using

Resistive Isolation

DS100922-31

FIGURE 2. Pulse Response of the LMV721 Circuit in

Figure 1

www.national.com

8