LF355 datasheet(17/47 Pages) TI2

17 / 47 page

LF155, LF156, LF256, LF257

LF355, LF356, LF357

www.ti.com

SNOSBH0D – MAY 2000 – REVISED NOVEMBER 2015

8 Application and Implementation

NOTE

Information in the following applications sections is not part of the TI component

specification, and TI does not warrant its accuracy or completeness. TI’s customers are

responsible for determining suitability of components for their purposes. Customers should

validate and test their design implementation to confirm system functionality.

8.1 Application Information

These are op amps with JFET input devices. 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 on either input will force the output to a high state, potentially

causing a reversal of phase to the output. Exceeding the negative common-mode limit on both inputs will force

the amplifier output to a high state. 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.

These amplifiers will operate with the common-mode input voltage equal to the positive supply. In fact, the

common-mode voltage can exceed the positive supply by approximately 100 mV independent of supply voltage

and over the full operating temperature range. The positive supply can therefore be used as a reference on an

input as, for example, in a supply current monitor and/or limiter.

Precautions should be taken to ensure that the power supply for the integrated circuit never becomes reversed in

polarity or that the unit is not inadvertently installed backwards in a socket as an unlimited current surge through

the resulting forward diode within the IC could cause fusing of the internal conductors and result in a destroyed

unit.

All of the bias currents in these amplifiers are set by FET current sources. The drain currents for the amplifiers

are therefore essentially independent of supply voltage.

As with most amplifiers, care should be taken with lead dress, component placement and supply decoupling in

order to ensure stability. For example, resistors from the output 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 resistive. The 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 six times the expected 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.

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