Low Level Measurements Handbook

1.5.3 Coulombmeter CircuitThe coulombmeter measures electrical charge that has been stored in acapacitor or that might be produced by some charge generating process.For a charged capacitor, Q = CV, where Q is the charge in coulombs onthe capacitor, C is the capacitance in farads, and V is the potential across thecapacitor in volts. Using this relationship, the basic charge measuringscheme is to transfer the charge to be measured to a capacitor of knownvalue and then measure the voltage across the known capacitor; thus,Q = CV.The electrometer is ideal for charge measurements, because the low offsetcurrent won’t alter the transferred charge during short time intervalsand the high input resistance won’t allow the charge to bleed away.Electrometers use a feedback circuit to measure charge, as shown inFigure 1-16. The input capacitance of this configuration is AC F . Thus, largeeffective values of input capacitance are obtained using reasonably sizedcapacitors for C F .FIGURE 1-16: Feedback CoulombmeterC F–+AV O1.5.4 High Resistance Ohmmeter CircuitsElectrometer Picoammeter and Voltage SourceIn this configuration (Figure 1-17), a voltage source (V S ) is placed in serieswith an unknown resistor (R X ) and an electrometer picoammeter. The voltagedrop across the picoammeter is small, so essentially all the voltageappears across R X , and the unknown resistance can be computed from thesourced voltage and the measured current (I).The advantages of this method are that it’s fast and, depending on thepower supply voltage and insulating materials, it allows measuring extremelyhigh resistance. Also, with an adjustable voltage source, the voltagedependence of the resistance under test can be obtained directly.1-22 SECTION 1