To establish whether a precharged node remains isolated or alternatively is subject to discharge, the conventional circuit allows uncertainty. For a period after evaluation starts, the conventional circuit will give a tentative result that may subsequently turn out to be wrong. During evaluation power is dissipated. A differential offset dynamic comparator and timing circuit are used to evaluate whether the node is being discharged. Because the comparator has an offset, much smaller deviations from the precharge potential can be sensed: because it is dynamic, it does not consume steady state current. The timing circuit permits precise knowledge of when to look at the output: before the timing period has elapsed, the result is known to be invalid.