Patent ID: 8686744
Filing Date: 2014-04-01
Classification: G01R

Abstract:
1. A method of measuring the capacitance of a first capacitor relative to a second capacitor, comprising the steps of: connecting the first and second capacitors in series with one another between a first terminal at one plate of the first capacitor and a second terminal at one plate of the second capacitor; connecting a common node between the first and second capacitors to a gate of a first transistor; biasing source and drain nodes of the first transistor so that the first transistor conducts a substantially constant current in saturation; ramping an applied voltage at the first terminal relative to the second terminal; during the ramping step, maintaining a substantially constant drain-to-source voltage at the first transistor; and during the ramping step, measuring an output voltage at the source node of the first transistor, wherein the step of maintaining the substantially constant drain-to-source voltage comprises: determining an approximate capacitance ratio between the first and second capacitors; during the ramping step, applying a ramped voltage at the drain node of the first transistor at a time-rate-of-change corresponding to a product of a capacitance voltage divider ratio at the common node between the first and second capacitors and the time-rate-of-change of the applied voltage at the first terminal.