Measuring circuit including switching means for charging a capacitance with alternate polarities in each switching cycle

Claim 1

Apparatus for measuring a parameter of an electrical circuit or component comprising two substantially identical circuit portions, each circuit portion including a condenser, a resistor and switching means, wherein the improvement comprises operating each of said switching means in an identical switching cycle during which it connects a charging voltage to the associated condenser until the condenser is substantially fully charged in one polarity sense and then connects the condenser to charge in the opposite polarity sense through its associated resistor until the condenser is substantially fully charged, the charging in opposite polarity senses being completed to such an extent that the mean voltages across the respective condensers over a switching cycle are directly proportional to a parameter of the respective circuit portion; there being further provided a differential operational amplifier responsive to said mean voltages across the respective condensers in the two circuit portions for producing an output proportional to the difference in said mean voltages over said switching cycle, and voltage measuring means responsive to the output of said differential operational amplifier, whereby, with one of said condensers connected to form at least a part of said electrical circuit or component, the voltage measured by said voltage measuring means is a function of the parameter of the electrical circuit or component to be measured.