An apparatus for obtaining ion energy distribution measurements in a plasma processing system comprising a substrate for placement in the plasma processing system and exposure to the plasma, the substrate having an ion energy analyser disposed therein for measuring the ion energy distribution at the substrate surface during plasma processing, the analyser comprising a plurality of conductive grids, and a collection electrode, C, each grid separated by an insulating layer, a high voltage generating circuit within the substrate and configured to take the output voltage of a battery to power the high voltage generating circuit and apply a voltage to a first grid of the plurality of conductive grids, a high voltage switch configured to discharge the first grid to a floating ground of the apparatus and a resistor in parallel with the high voltage switch, wherein the ion energy analyser is configured to sample ion current during a first stage while a first voltage is being charged on the first grid from the floating ground potential to a plateau voltage and to sample ion current during a second stage while a second voltage applied to the first grid is discharging through the resistor from a predetermined voltage generated by the high voltage generating circuit to the plateau voltage.