In this paper, we propose a time-based, programmable-gain A/D converter allowing for an easily-scalable, and power-efficient, implantable, biomedical recording system. The converter circuit is realized in a 90 nm CMOS technology, operates at 640 kS/s, occupy an area of 0.022 mm2, and consumes less than 2.7 μW corresponding to a figure of merit of 6.2 fJ/conversion-step.