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.
Application Hints (Continued)
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.
Typical Application
pH Probe Amplifier/Temperature Compensator
DS009156-10
=
=
***
±
1
%
For R2 50k, R4 330k
=
=
±
For R2 100k, R4 75k 1%
=
=
±
For R2 200k, R4 56k 1%
**
Polystyrene
*
Film resistor type RN60C
=
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
7
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NSC [ National Semiconductor ]
