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Frans_M_Everaerts_Isotachophoresis_378342.pdf

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146 DETECTION SYSTEMS<br />

Fig.6.17. Electronic circuit suitable for the determination of the conductivity of the various zones<br />

by the a.c. method.<br />

1<br />

or=- m<br />

If the ratio of the number of turns is unity, the impedance of the circuit between the<br />

inverting input and output of the operational amplifier of those signals which have a<br />

frequency or can be written as<br />

Impedance = - R"R<br />

R, +R<br />

The resistance R is the unknown resistance, for example between the micro-sensing<br />

electrodes of the conductivity cell.<br />

If vc = V, cos art (v = a.c. voltage, V= d.c. voltage) and we assume that the amplification<br />

of the operational amplifier is infinity and dl input currents are zero, we can write<br />

v1 =v2 (6.4)<br />

and<br />

If<br />

RZ =R1<br />

R3 R,<br />

then

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