LM7131 datasheet(19/23 Pages) NSC

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Using the LM7131 (Continued)

The LM7131 can also supply +2.00V to a 50

Ω load to

ground, making it useful as driver in 50

Ω systems such as

portable test equipment.

Cable Driving with +3V Supplies

The LM7131 can drive 150

Ω to 2.00V when supplied by a 3V

supply. This 3V performance means that the LM7131 is use-

ful in battery powered video applications, such as camcord-

ers, portable video mixers, still video cameras, and portable

scanners.

Audio and High Frequency Signal Processing

The LM7131 is useful for high fidelity audio and signal pro-

cessing. A typical LM7131 is capable of driving 2V across

150

Ω (referenced to ground) at less than 0.1% distortion at

4 MHz when powered by a single 5V supply.

Use with 2.5V Virtual Ground Systems

with +5V Single Supply Power

Many analog systems which must work on a single +5V sup-

ply use a “virtual ground” - a reference voltage for the signal

processing which is usually between +5V and 0V. This virtual

ground is usually halfway between the top and bottom sup-

ply rails. This is usually +2.5V for +5V systems and +1.5V for

+3V systems.

The LM7131 can be used in single supply/virtual ground sys-

tems driving loads referenced to 2.5V. The output swing

specifications in the data sheet show the tested voltage lim-

its for driving a 150

Ω load to a virtual ground supply for +3V

and +5V. A look at the output swing specifications shows that

for heavy loads like 150 ohms, the output will swing as close

as one diode drop (roughly, 0.7V) to the supply rail. This

leaves a relatively wide range for +5V systems and a some-

what narrow range for +3V systems. One way to increase

this output range is to have the output load referenced to

ground — this will allow the output to swing lower. Another is

to use higher load impedances. The output swing specifica-

tions show typical numbers for swing with loads of 600

Ω to

ground. Note that these typical numbers are similar to those

for a 150

Ω load. These typical numbers are an indication of

the maximum DC performance of the LM7131.

The sinking output of the LM7131 is somewhat lower than

the amplifier’s sourcing capability. This means that the

LM7131 will not drive as much current into a load tied to 2.5

V as it will drive into a load tied to 0V.

Good AC performance will require keeping the output further

away from the supply rails. For a +5V supply and relatively

high impedance load (analog-to-digital converter input) the

following are suggested as an initial starting range for

achieving high (> 60 dB) AC accuracy

Upper output level —

Approximately 0.8V to 1V below the positive (V+) rail.

Lower output level —

Approximately 200 mV–300 mV above the negative rail.

The LM7131 very useful in virtual ground systems as an out-

put device for output loads which are referenced to 0V or the

lower rail. It is also useful as a driver for capacitive loads,

such as sample and hold circuits, and audio analog to digital

converters. If fast amplifiers with rail-to-rail output ranges are

needed, please see the National Semiconductor LM6142

datasheet.

DS012313-23

FIGURE 14. CCD Amplifier

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