Proposal of application of pulsed vision chip for retinal prosthesis

A vision chip based on pulse frequency modulation (PFM) is proposed as a retinal prosthesis device for use in subretinal implantation. A pixel circuit of PFM has been fabricated using standard CMOS technology. A dynamic range of 40dB is demonstrated at a low power supply voltage of I V. It is found that the competition between the charge Current to the photodiode and the photocurrent affects the output pulse characteristics under the conditionds of low power supply voltage and strong light illumination. For in vitro and in vivo experiments, two methods of improving the PFM circuits are considered. One is controlling the pulse frequency. By adding one transistor to the PFM circuit, the output pulse frequency can he experimentally controlled by a factor of two. The other is using a biphasic output, which is required to keep a charge balance in a living body. By introducing differential circuits, the biphasic output has been confirmed by simulation.