An active matrix electro-wetting on dielectric (AM-EWOD) device includes a plurality of array elements arranged in an array, each array element including array element circuitry, an element electrode, and a reference electrode. The array element circuitry includes an actuation circuit configured to apply actuation voltages to the electrodes, and an impedance sensor circuit configured to sense impedance at the array element electrode to determine a droplet property. The actuation circuitry includes a memory capacitor for storing voltage data corresponding to either an actuated state or an unactuated state of the array element, and an input applied to the memory capacitor operates to effect an operation of the impedance sensor circuit. Such input may isolate the array element from the actuation voltage during operation of the impedance sensor circuit, and the memory capacitor may operate as part of the impedance sensor circuit as a reference capacitor for determining the droplet property.