Am-ewod array element circuitry with high sensitivity for small capacitance

Abstract

An AM-EWOD device includes a plurality of array elements arranged in an array of rows and columns, each of the array elements including array element circuitry, an element electrode, and a reference electrode. The array element circuitry includes actuation circuitry configured to apply actuation voltages to the element and/or 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. 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 (and possibly other circuit elements) to the gate of the sensor readout transistor. The impedance sensor circuitry further comprises a pre-charging element that operates to turn on the sensor readout transistor during the sensing phase in combination with coupling of the voltage perturbation, thereby increasing the effect of the voltage perturbation on the output current.