LM7131ACMX datasheet(10/23 Pages) NSC

10 / 23 page

Application Information (Continued)

The LM7131 is available in two package types: SO-8 surface

mount package and the SOT23-5 Tiny package for space

and weight savings.

The LM7131 has been designed to meet some of the most

demanding

requirements

for

single

supply

amplifiers — driving analog to digital converters and video

cable driving. The output stage of the LM7131 has been spe-

cially designed for the dynamic load presented by analog to

digital converters. The LM7131 is capable of a 4V output

range with a +5V single supply. The LM7131’s drive capabil-

ity and good differential gain and phase make quality video

possible from a small package with only a +5V supply.

BENEFITS OF THE LM7131

The LM7131 can make it possible to amplify high speed sig-

nals with a single +5V or +3V supply, saving the cost of split

power supplies.

EASY DESIGN PATH FROM ±5V to +5V SYSTEMS

The SO-8 package and similar ±5V and single supply speci-

fications means the LM7131 may be able to replace many

more expensive or slower op amps, and then be used for an

easy transition to 5V single supply systems. This could pro-

vide a migration path to lower voltages for the amplifiers in

system designs, reducing the effort and expense of testing

and re-qualifying different op amps for each new design.

In addition to providing a design migration path, the

SOT23-5 Tiny surface mount package can save valuable

board space.

SPECIFIC ADVANTAGES OF SOT23-5 (TINY PACKAGE)

The SOT23-5 (Tiny) package can save board space and al-

low tighter layouts. The low profile can help height limited de-

signs, such as sub-notebook computers, consumer video

equipment, personal digital assistants, and some of the

thicker PCMCIA cards. The small size can improve signal in-

tegrity in noisy environments by placing the amplifier closer

to the signal source. The tiny amp can fit into tight spaces

and weighs little. This makes it possible to design the

LM7131 into places where amplifiers could not previously fit.

The LM7131 can be used to drive coils and transformers ref-

erenced to virtual ground, such as magnetic tape heads and

disk drive write heads. The small size of the SOT23-5 pack-

age can allow it to be placed with a pre-amp inside of some

rotating helical scan video head (VCR) assemblies. This

avoids long cable runs for low level video signals, and can

result in higher signal fidelity.

Additional space savings parts are available in tiny packages

from National Semiconductor, including low power amplifi-

ers, precision voltage references, and voltage regulators.

Notes on Performance Curves and

Datasheet Limits

Important:

Performance curves represent an average of parts, and are

not limits.

SUPPLY CURRENT vs SUPPLY VOLTAGE

Note that this curve is nearly straight, and rises slowly as the

supply voltage increases.

INPUT CURRENT vs INPUT VOLTAGE

This curve is relatively flat in the 200 mV to 4V input range,

where the LM7131 also has good common mode rejection.

COMMON MODE VOLTAGE REJECTION

Note that there are two parts to the CMRR specification of

the datasheet for 3V and 5V. The common mode rejection

ratio of the LM7131 has been maximized for signals near

ground (typical of the active part of video signals, such as

those which meet the RS-170 levels). This can help provide

rejection of unwanted noise pick-up by cables when a bal-

anced input is used with good input resistor matching. The

mid-level CMRR is similar to that of other single supply op

amps.

BODE PLOTS (GAIN vs FREQUENCY FOR A

V = +1)

The gain vs. frequency plots for a non-inverting gain of 1

show the three voltages with the 150

Ω load connected in two

ways. For the single supply graphs, the load is connected to

the most negative rail, which is ground. For the split supply

graphs, the load is connected to a voltage halfway between

the two supply rails.

DRIVING CABLES

Pulse response curves for driving 75

Ω back terminate cables

are shown for both 3V and 5V supplies. Note the good pulse

fidelity with straight 150 loads, five foot (1.5 meter) and 75

foot (22 meter) cable runs. The bandwidth is reduced when

used in a gain of ten (A

V = +10). Even in a gain of ten con-

figuration, the output settles to < 1% in about 100 ns, mak-

ing this useful for amplifying small signals at a sensor or sig-

nal source and driving a cable to the main electronics section

which may be located away from the signal source. This will

reduce noise pickup.

Please refer to

Figures 1, 2, 3, 4, 5 for schematics of test set-

ups for cable driving.

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