Pixel Array with 5x5 Spatial Highpass Filter for a Retinal Implant

Vision of patients with retinal implants is diminished by spatial lowpass behaviour of the interface between electrode and stimulated cells. Spatial highpass filtering can compensate the interface lowpass to a certain degree and thus improve visual perception. The retinal implant consists of an array of photodiodes, each with an amplification circuit and an electrode. The proposed spatial highpass filter is integrated into that topology as a system of current mirrors, connecting each pixel bidirectionally to 12 adjacent pixels. A fully functional prototype was fabricated in a 180 nm CMOS process. We present pixel based measurements and fixed pattern noise analysis (FPN) in various operation modes. For further verification a sharp edge was projected onto the array. The recorded data is visualised and indicates promising improvements regarding the perceptibility of patterns.