Low Level Measurements Handbook

The disadvantage of this method is that the voltage across the unknownis a function of its resistance, so it cannot be easily controlled. Very highresistances tend to have large voltage coefficients; therefore, measurementsmade with a constant voltage are more meaningful. In addition, theresponse speed for resistances greater than 10GΩ will be rather slow. Thislimitation can be partially overcome by guarding.Electrometer Ohmmeter with Guarded Ohms ModeFigure 1-19 shows a modification of the circuit in Figure 1-18 in which theHI input node is surrounded with a guard voltage from the operationalamplifier output. The amplifier has unity gain, so this guard voltage is virtuallythe same potential as V 1 and the capacitance (C S ) of the input cable islargely neutralized, resulting in much faster measurements of resistancesgreater than 10GΩ.FIGURE 1-19: Electrometer Ohmmeter with Guarded OhmsBuilt-In Current SourceV SI = V S /RV 1 = IR XRI–A+R X C S GuardV 1V OThe guarded mode also significantly reduces the effect of input cableleakage resistance, as discussed in Section 2.4.2.Electrometer Voltmeter and External Current SourceIn this method, shown in Figure 1-20, a current source generates current(I), which flows through the unknown resistor (R X ). The resulting voltagedrop is measured with an electrometer voltmeter, and the value of R X is calculatedfrom the voltage and current.1-24 SECTION 1