LMV921 datasheet(16/27 Pages) NSC

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Application Note (Continued)

This V

OS crossover point can create problems for both DC

and AC coupled signals if proper care is not taken. For large

input signals that include the V

OS crossover point in their dy-

namic range, this will cause distortion in the output signal.

One way to avoid such distortion is to keep the signal away

from the crossover. For example, in a unity gain buffer con-

figuration and with V

S = 5V, a 5V peak-to-peak signal will

contain input-crossover distortion while a 3V peak-to-peak

signal centered at 1.5V will not contain input-crossover dis-

tortion as it avoids the crossover point. Another way to avoid

large signal distortion is to use a gain of −1 circuit which

avoids any voltage excursions at the input terminals of the

amplifier. In that circuit, the common mode DC voltage can

be set at a level away from the V

OS cross-over point.

For small signals, this transition in V

OS shows up as a VCM

dependent spurious signal in series with the input signal and

can effectively degrade small signal parameters such as

gain and common mode rejection ratio. To resolve this prob-

lem, the small signal should be placed such that it avoids the

OS crossover point.

In addition to the rail-to-rail performance, the output stage

can provide enough output current to drive 600

Ω loads. Be-

cause of the high current capability, care should be taken not

to exceed the 150˚C maximum junction temperature specifi-

cation.

5.0 Power-Supply Considerations

The LMV921/LMV922/LMV924 are ideally suited for use with

most battery-powered systems. The LMV921/LMV922/

LMV924 operate from a single +1.8V to +5.0V supply and

consumes about 145µA of supply current per Amplifier. A

high power supply rejection ratio of 78dB allows the amplifier

to be powered directly off a decaying battery voltage extend-

ing battery life.

Table 1 lists a variety of typical battery types. Batteries have

different voltage ratings; operating voltage is the battery volt-

age under nominal load. End-of-Life voltage is defined as the

voltage at which 100% of the usable power of the battery is

consumed.

Table 1 also shows the typical operating time of

the LMV921.

6.0 Distortion

The two main contributors of distortion in LMV921/LMV922/

LMV924 family is:

1. Output crossover distortion occurs as the output transi-

tions from sourcing current to sinking current.

2. Input crossover distortion occurs as the input switches

from NPN to PNP transistor at the input stage.

To decrease crossover distortion:

1. Increase the load resistance. This lowers the output cross-

over distortion but has no effect on the input crossover dis-

tortion.

2. Operate from a single supply with the output always

sourcing current.

3. Limit the input voltage swing for large signals between

ground and one volt below the positive supply.

4. Operate in inverting configuration to eliminate common

mode induced distortion.

5. Avoid small input signal around the input crossover region.

The discontinuity in the offset voltage will effect the gain,

CMRR and PSRR.

TABLE 1. LMV921 Characteristics with Typical Battery Systems.

Battery Type

Operating

Voltage (V)

End-of-Life

Voltage (V)

Capacity AA

Size (mA -

LMV921

Operating

time (Hours)

Alkaline

1.5

0.9

1000

6802

Lithium

2.7

2.0

1000

6802

Ni - Cad

1.2

0.9

375

2551

NMH

1.2

1.0

500

3401

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