The present invention provides an electro-optical isolation system for coupling an electronic measuring device to a device under test for making accurate measurements of signals within a wide frequency range while the device under test is being subject to high power electrical disturbances. The invention provides an increased rejection of high common mode signals and reduction of undesired self-capacitance by implementing a shielded handheld transmitter unit having an integrated measurement probe. The transmitter unit converts the sensed signal to an optical signal which is transmitted through an optical medium to a receiver unit. Under control of a microprocessor, the level of the output signal from the transmitter unit is modulated by the signal received from the sensing probe. The microprocessor within the transmitter unit automatically controls the level of optical signal emitted by the optical converter, and further calibrates a driver circuit to maintain measurement accuracy. The modulated optical signal emitted by the electro-optical converter is communicated to the receiver unit by an optical fiber medium.