According to one general aspect, an apparatus includes a first resistor in a first current path of a resistor-capacitor (RC) circuit, the resistor connected to a power source. A variable capacitor is included in a second current path of the RC circuit and operably connected to the power source and a virtual ground generator. A comparison circuit is configured to make a determination regarding a voltage VR across the resistor to a ground relative to a voltage VC across the capacitor to a virtual ground from the virtual ground generator. A control circuit is configured to make an adjustment of a value of the variable capacitor, based on the determination.