An active matrix electro-wetting on dielectric (AM-EWOD) device comprising: a plurality of array elements (100 - 120) arranged in an array of rows and columns, each of the array elements including array element circuitry, an element electrode, and a reference electrode; wherein the array element circuitry comprises: actuation circuitry configured to apply actuation voltages to the element and reference electrodes for actuating the array element; and impedance sensor circuitry configured to sense impedance at the array element electrode to determine a droplet or device property at the array element, the impedance sensor circuitry comprising a sensor capacitor and a sensor readout transistor that outputs an output current for sensing; and wherein: the sensor capacitor is electrically connected to a gate of the sensor readout transistor such that during a sensing phase a voltage perturbation is coupled through the sensor capacitor to the gate of the sensor readout transistor; and the impedance sensor circuitry further comprises a pre-charging element (T2,C3, T3) and a variable pre-charging voltage supply line (PC,VPIX), wherein the device is configured to provide, during the sensing phase and via the pre-charging voltage supply line (PC,VPIX), a pre-charging voltage supply to the gate of the sensor readout transistor that operates to turn on the sensor readout transistor during the sensing phase, thereby increasing the effect of the voltage perturbation on the output current. a plurality of array elements (100 - 120) arranged in an array of rows and columns, each of the array elements including array element circuitry, an element electrode, and a reference electrode; wherein the array element circuitry comprises: actuation circuitry configured to apply actuation voltages to the element and reference electrodes for actuating the array element; and impedance sensor circuitry configured to sense impedance at the array element electrode to determine a droplet or device property at the array element, the impedance sensor circuitry comprising a sensor capacitor and a sensor readout transistor that outputs an output current for sensing; and wherein: the sensor capacitor is electrically connected to a gate of the sensor readout transistor such that during a sensing phase a voltage perturbation is coupled through the sensor capacitor to the gate of the sensor readout transistor; and the impedance sensor circuitry further comprises a pre-charging element (T2,C3, T3) and a variable pre-charging voltage supply line (PC,VPIX), wherein the device is configured to provide, during the sensing phase and via the pre-charging voltage supply line (PC,VPIX), a pre-charging voltage supply to the gate of the sensor readout transistor that operates to turn on the sensor readout transistor during the sensing phase, thereby increasing the effect of the voltage perturbation on the output current. actuation circuitry configured to apply actuation voltages to the element and reference electrodes for actuating the array element; and impedance sensor circuitry configured to sense impedance at the array element electrode to determine a droplet or device property at the array element, the impedance sensor circuitry comprising a sensor capacitor and a sensor readout transistor that outputs an output current for sensing; and wherein: the sensor capacitor is electrically connected to a gate of the sensor readout transistor such that during a sensing phase a voltage perturbation is coupled through the sensor capacitor to the gate of the sensor readout transistor; and the impedance sensor circuitry further comprises a pre-charging element (T2,C3, T3) and a variable pre-charging voltage supply line (PC,VPIX), wherein the device is configured to provide, during the sensing phase and via the pre-charging voltage supply line (PC,VPIX), a pre-charging voltage supply to the gate of the sensor readout transistor that operates to turn on the sensor readout transistor during the sensing phase, thereby increasing the effect of the voltage perturbation on the output current. the sensor capacitor is electrically connected to a gate of the sensor readout transistor such that during a sensing phase a voltage perturbation is coupled through the sensor capacitor to the gate of the sensor readout transistor; and the impedance sensor circuitry further comprises a pre-charging element (T2,C3, T3) and a variable pre-charging voltage supply line (PC,VPIX), wherein the device is configured to provide, during the sensing phase and via the pre-charging voltage supply line (PC,VPIX), a pre-charging voltage supply to the gate of the sensor readout transistor that operates to turn on the sensor readout transistor during the sensing phase, thereby increasing the effect of the voltage perturbation on the output current.