The design of neuro-stimulator has to respect tissue safety conditions. Especially, electrical charge-balancing of stimuli is of major concern, and electrode drivers have to ensure correct stimulation whatever the load. This paper describes a new front-end stimulation topology based on a charge metering closed-loop. The circuit was developed on a Field-Programmable-Analog-Array. Characterisation shows an equivalent leakage current under 1nA and the stimulator was successfully used on a rat's spinal-cord.