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Functional electrical stimulation of the retina has received increasing attention over the last several years [1] [2]. It restores basic vision through electrical nerve stimulation within the eyeball of blind people with retina degeneration. The system, known as an epiretinal prosthesis, is shown in Fig. 2.4.1.Electrical stimulation generates a nerve reaction upon the transfer of charge into the tissue via electrodes. For a retinal stimulator the integration of several hundreds of such stimulation sites is required in order to restore basic vision functions [1] [3]. Reliable operation of a nerve stimulator demands high ESD robustness at the electrodes. In addition, electrolysis caused by excess dc-current flow must be safely prevented, since it yields electrode and tissue destruction. The electrode impedance Re (Fig. 2.4.1) depends on the electrode size. For the retinal implant it is typically >10kΩ. This requires high voltage (HV) swing capabilities at the stimulation electrodes for feasible stimulation currents of up to 1mA [3]. Concurrently, the implanted system must be low power and the possible chip size is limited.Recent implementations of retina stimulators have focused on an increasing number of stimulation sites, and more than 100 have been achieved [1] [2]. Common architectures use global generation of stimulation currents and distribution over current switches, which allows only one single or a small number of electrodes to be activated at a time. Besides the usage of biphasic current pulses [1], charge balancing is either not covered [2] or implemented passively with electrode shorting resistors [3]. This is either power-ineffective, when permanently shorted, or requires switches to short only after stimulation. Moreover, ESD protection has not been addressed in recent publications, but it is of major concern due to the very large number of open contacts during surgery. Finally, the possible stimulation current is mostly limited by the output voltage swing capabilities; even with electrode reversal, only ±7V has been achieved [3].This design presents the concept of an array of fully digitally interfaced and programmable stimulation pad cells for a retinal implant in 0.35µm HVCMOS, which features a maximum voltage swing of ±15V, includes full custom ESD protection and an innovative active charge balancer [4]. Global functions (CDR, supply, ADC, controller, test) are shared. All local stimulation functions are concentrated in each stimulation pad cell. These pads are programmable over a bus by the global controller. The local functions include a 5b current steering DAC with 2 bias points and an output range of 0.8µA-32µA or 3.2µA-99.2µA. The DAC current is fed into a push-pull output current source (OCS), where a gain of 5 or 10 can be adjusted. This yields a stimulation current between 4µA and 992µA (DR≈50dB). The output node features custom ESD protection, HV swing capability and both polarities enabling biphasic, coarsely charge-balanced pulses. To account for biphasic pulse mismatch, an active desig...

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