This paper presents a 15-channel, 30-V, implantable current stimulator for restoring locomotion control after spinal cord injuries. The stimulator features performance specifications comparable to those of large desktop instrumentation: high linearity, high precision of the delivered currents, small channel-to-channel mismatches and a fast settling time of 0.3 μs. An ADC-based active charge balancing scheme using a digital PI (proportional-integral) controller was implemented in firmware.