This disclosure provides systems, methods and apparatus for measuring capacitance of a display unit, such as an interferometric modulator (IMOD). In one example, a circuit may include an operational amplifier (op-amp), a voltage controlled current source, and feedback from an output of the op-amp as an input to the voltage controlled current source. An output of the voltage controlled current source may be provided to a display unit as well as an input of the op-amp. A second input of the op-amp may be provided a ramping reference voltage.