Application Hints

These devices are op amps with an internally trimmed input

offset voltage and JFET input devices (BI-FET II) These

JFETs have large reverse breakdown voltages from gate to

source and drain eliminating the need for clamps across the

inputs Therefore large differential input voltages can easily

be accommodated without a large increase in input current

The maximum differential input voltage is independent of

the supply voltages However neither of the input voltages

should be allowed to exceed the negative supply as this will

cause large currents to flow which can result in a destroyed

unit

Exceeding the negative common-mode limit with the non-in-

verting input or with both inputs will force the output to a

high state potentially causing a reversal of phase to the

output In neither case does a latch occur since raising the

input back within the common-mode range again puts the

input stage and thus the amplifier in a normal operating

mode

Exceeding the positive common-mode limit on a single input

will not change the phase of the output however if both

inputs exceed the limit the output of the amplifier will be

forced to a high state

The amplifiers will operate with a common-mode input volt-

age equal to the positive supply however the gain band-

width and slew rate may be decreased in this condition

When the negative common-mode voltage swings to within

3V of the negative supply an increase in input offset voltage

may occur

Each amplifier is individually biased by a zener reference

plies Supply voltages less than these may result in lower

gain bandwidth and slew rate

plifier is forced to drive heavier load currents however an

increase in input offset voltage may occur on the negative

voltage swing and finally reach an active current limit on

both positive and negative swings

Precautions should be taken to ensure that the power sup-

ply for the integrated circuit never becomes reversed in po-

larity or that the unit is not inadvertently installed backwards

in a socket as an unlimited current surge through the result-

ing forward diode within the IC could cause fusing of the

internal conductors and result in a destroyed unit

As with most amplifiers care should be taken with lead

dress component placement and supply decoupling in or-

der to ensure stability For example resistors from the out-

put to an input should be placed with the body close to the

input to minimize ‘‘pick-up’’ and maximize the frequency of

the feedback pole by minimizing the capacitance from the

input to ground

A feedback pole is created when the feedback around any

amplifier is resistiveThe parallel resistance and capacitance

from the input of the device (usually the inverting input) to

AC ground set the frequency of the pole In many instances

the frequency of this pole is much greater than the expected

3 dB frequency of the closed loop gain and consequently

there is negligible effect on stability margin However if the

feedback pole is less than approximately 6 times the ex-

pected 3 dB frequency a lead capacitor should be placed

from the output to the input of the op amp The value of the

added capacitor should be such that the RC time constant

of this capacitor and the resistance it parallels is greater

than or equal to the original feedback pole time constant

The benefit of the SO package results from its very small

size It follows however that the die inside the SO package

is less protected from external physical forces than a die in

a standard DIP would be because there is so much less

plastic in the SO Therefore not following certain precau-

tions when board mounting the LF453CM can put mechani-

cal stress on the die lead frame andor bond wires This

can cause shifts in the LF453CM’s parameters even caus-

ing them to exceed limits specified in the Electrical Charac-

teristics For recommended practices in LF453CM surface

mounting refer to Application Note AN450 ‘‘Surface Mount-

ing Methods and Their Effect on Product Reliability’’ and to

the section titled ‘‘Surface Mount’’ found in any Rev 1 Lin-

ear Databook volume

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