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W. Liu, P. Singh, M. Humayun, J. Weiland; New Micro-Stimulator for High Resolution Retinal Prostheses . Invest. Ophthalmol. Vis. Sci. 2003;44(13):5088.
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Purpose: To develop new generation of micro-stimulator for retinal prostheses. The design should be able to meet area goal of placing 1000 output channels on a 5mmx5mm IC. Circuit should dissipate lower power and output stimulus should be designed to be better conformal with the medically determined requirements. Methods: Extrapolation of stimulus circuit area from our previous design (60 output IC in AMI 1.6µm) does not meet area goal. Estimates with deep submicron technology, TSMC 0.35µm, fulfilled the area goal. In TSMC 0.35µm, circuits are designed to meet high output voltage requirement with 1x8 demultiplexing of the stimulus circuit using embedded switching transistors. Power analysis using MATLAB is to understand and reduce power dissipation in the IC. Circuit using active feedback is designed to reduce power supply voltage and to attain better linearity. The circuit is designed to operate with variable power supply of ±3.5V to ±6.5V to optimize for power under variable stimulus requirements. Independent programmability of current scale for outputs is incorporated. A novel charge cancellation mechanism is provided to reduce the unwanted charge buildup. A 6-bit DAC is designed using combination of multi-bias, binary coding, and thermal coding to achieve good accuracy in small area. A test IC is fabricated in AMI 1.6µm. A test setup has been prepared and measurements are taken. Results: HSPICE simulations for circuits have been done. The layout of 8 outputs stimulus circuit is done in 525µmx92µm. Stimulus circuit for 1000 outputs accommodates in 25% of the set area goal if the digital circuit area extrapolation from previous designs area goals is met. The measurement results of the test chip has been as predicted by the simulation. Conclusions: A number of techniques to reduce area and power, and to improve performance are demonstrated. These tested techniques will greatly enhance the future design of a high-density micro-stimulator for retinal prostheses.
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