Optical fiber interface for distributed measurement and control in metrology setups: application to current sensing with fA resolution

Computer control of instrumentation employed in setups for precise electrical measurements is a challenging task because the interface bus collects surrounding noise (e.g., creating additional ground loops) and creates and distributes noise of digital origin. An acquisition and control system for sensitive instrumentation based on an optical fiber interface link is described. The system permits control of several instruments, each equipped with a remote unit, using a personal computer equipped with a computer unit, via an RS-232 port. The computer unit is extremely simple and acts only as an electrical-to-optical digital level translator and does not use any elaborate communication protocol. Each remote unit is based on a single-chip microcontroller which performs both communication and acquisition/control tasks. The microcontroller provides several digital I/O lines and can be interfaced directly with ADCs and DACs. An application of this optical link in a sensitive picoamperometer measurement system is presented. In this application, the remote unit consists of a transresistance amplifier (with a gain of 150 GΩ), an analog-to-digital converter (with a resolution of 14 bits) and the microcontroller, and is battery-operated. The equivalent white noise of the picoamperometer is 3.3 fA/√Hz, the linearity of 0.5% in the 1-5 pA range