The electronic design of an active electrode for differential measurements in EIT is presented. EIT allows visualisation of the distribution of electrical properties within a body, based on the measurement and application of current or voltage patterns [2,3]. Besides the use of active electrodes [1,4,5], differential two-frequency imaging is often explored to improve system performance. In this type of imaging, data is gathered mainly sequentially, with the measured signals being converted into digital form, and subsequently subtracted. The resolution of the sequential measurement procedure, however, suffers from the finite digital word length. To avoid this limitation, an analogue subtraction of simultaneous measurements at two frequencies is proposed. The measurement properties of such a design are considered in this paper.