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Instrumentation
frequency hum pickup may also be factors affecting precision. It pays to carefully read the manufacturers' spec ifications. Good-quality chart record ers, for example, usually have built-in hum-rejection filters tuned to the line frequency. To obtain the desired results from the modifier portion of your instru ment, you must consider the following: precision and temperature coefficient of circuit components, amplifier open-loop gain and frequency response, input cur rent, source impedance, common mode rejection, and noise. These factors are discussed below. At low frequencies (dc to 100 Hz), the open-loop gain of an op amp is normally high enough so that accuracy of a circuit is solely determined by how well you know the values of the components. The output of the cir cuit is given by the general equation Εa — —EiZf/Zi
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- EiZf/Zi
= - iiZf
·
(3)
The circuit components Zf and Z t may be composed of resistors, zener diodes, capacitances, etc. Be sure to choose parts that are adequate for your ap plication. Resistors, for example, are commercially available in ±0.05% tol erance and with temperature coefficients as low as ±0.0002%/° C. Precision capacitors are more difficult to obtain. Polystyrene units are available with ± 0 . 1 % tolerance and temperature co efficient as low as - 0 . 0 0 8 % / ° C. Zener diodes are available with temperature coefficients of voltage down to 0.0005%/° C. When you are working at higher frequencies, the component values will not be what you have at dc. A good-quality resistor, for example, can be made with very low inductance. Still, there is some inductance, and it will change the impedance of the re sistor at high frequency. These points are mentioned here only to make you aware of them. In some applications, they may be important. In most chem ical instrumentation, low frequencies or essentially dc signals are encountered, and you need not worry about such items as stray inductance and capaci tance. Just remember to use the proper values for your components at the fre quency at which you are working, since they may be different from the dc values. The simple equations above are only usable if amplifier open-loop gain, A, is very high. Ideally, A is infinite. As signal frequency increases, however,
Circle No. 41 on Readers' Service Card
96A
(2)
When the positive input is not used— i.e., it is grounded—the output is given by Equation 3 : E0=
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-fE+ (1 + Z,/Zt)
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
6
f i tie\A\ 3 2 / 0
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