An apparatus comprises: a first circuitry to charge first and second capacitors to a predetermined voltage level; a second circuitry to discharge the first capacitor through a diode at a first time; a third circuitry to discharge the second capacitor through the diode at a second time, wherein the second time is greater than the first time; a comparator to compare a first voltage of the first capacitor with a second voltage of the second capacitor; and logic to adjust a scaling factor applied to the second voltage according to an output of the comparator.