Optical fibre 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 surroundings noise (e.g., creating additional ground loops) and create and distribute noise of digital origin. An acquisition and control system for sensitive instrumentation, based on a optical fibre interface link, is here described. The system permits control of several instruments, each one equipped with a remote unit, with a single personal computer, equipped with a computer unit, through RS-232 port. The computer unit is extremely simple and acts only as an electrical-to-optical digital level translator, without any communication protocol elaboration. 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 interface directly with ADCs and DACs. An application of the optical link to a sensitive picoamperometer is shown; this particular remote unit is made of a transresistance amplifier (with a gain of 150 GΩ), an analog to digital converter (with a resolution of 14 bits), and the microcontroller; it is completely 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