US Patent 7225090 Method for measuring capacitance and equivalent parallel resistance

CX is measured in the presence of RP by charging CX with a constant current IC for a period time and taking four (time, voltage) pairs: (tA,VA), (tB,VB), (tC,VC) and (tD,VD). The first pair are sufficiently close together that the short straight line segment connecting (tA, VA) and (tB, VB) is not significantly different from the corresponding segment of a true graph of the capacitor's actual charging. A similar requirement is imposed on the time interval between tC and tD. The time interval tAtB need not be the same as tCtD. The interval tBtC should be long enough that if there is significant RP present then there is at least a discernable difference between the true graph and a straight line between the first pair and the second. CX and RP are computed as a function of the four pairs and IC, and CX may then be diminished by a previously measured value for a stray capacitance present in the absence of CX.