1. An apparatus, comprising: a charging switch coupled between a first supply voltage and a circuit node to selectively couple a sensing capacitor to the first supply voltage through the circuit node; a charge transfer switch coupled between the circuit node and a first terminal of a second capacitor to selectively couple the sensing capacitor through the circuit node to the second capacitor; and a first switch coupled between the circuit node and a second terminal of the second capacitor to selectively couple the second terminal to the sensing capacitor through the circuit node.