A method, comprising: generating an output voltage by a power supply; storing charge produced by the output voltage; superimposing an excitation signal onto the output voltage to produce a superimposed signal; connecting the superimposed signal to an electrode in a droplet actuator; suppressing the output voltage from the power supply when detecting an impedance at the electrode; supplying the charge to the droplet actuator to activate the electrode during the impedance; and measuring the impedance produced by the excitation signal while the output voltage is suppressed, wherein the impedance indicates presence of liquid at the electrode. Superimposing the excitation signal may comprise adding the excitation signal to the output voltage. Suppressing the output voltage may comprise at least one of stopping the output voltage generated by the power supply, disabling a switching action of the power supply, and disabling the power supply during the impedance measurement.