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.

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 feed-

back 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 feed-

back pole is less than approximately 6 times the expected 3

dB frequency a lead capacitor should be placed from the out-

put to the input of the op amp. The value of the added ca-

pacitor should be such that the RC time constant of this ca-

pacitor and the resistance it parallels is greater than or equal

to the original feedback pole time constant.

Typical Application

pH Probe Amplifier/Temperature Compensator

DS009156-10

To calibrate, insert probe in pH =7 solution. Set the “TEMPERATURE ADJUST” pot, R2, to correspond to the solution temperature: full clockwise for 0˚C, and

proportionately for intermediate temperatures, using a turns-counting dial. Then set “CALIBRATE” pot so output reads 7V.

Typical probe = Ingold Electrodes #465-35

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