Large-scale Integration–Based Stimulus Electrodes for Retinal Prosthesis - Artificial Sight: Basic Research, Biomedical Engineering, and Clinical Advances - page 155

Table 8.1. Modifications of the PFM photosensor for
stimulating retinal cells.

Requirements Our work

Photosensor - Pulse frequency

modulation

Photo sensitivity High High

Stimulation Current Voltage

Pulse Pulse

Biphasic Monophasic

Frequency < 100Hz < 1MHz

Power supply - Required

< 1mC/cm 2

Injection charge Sufficient

The frequency limitation, however, causes a reduction in the range of input
light intensity. We have alleviated this problem by introducing a variable
sensitivity: the output frequency is divided into 2 − n with a frequency divider,
where n is the number of divisions. Note that the digital output of the PFM is
suitable for the introduction of such a logic function of the frequency divider.
Table 8.1 summarizes the photosensor requirements for a retinal prosthesis.
Based on the above modifications, we have designed and fabricated a pixel
circuitry using standard 06-m CMOS technology. Figure 8.3 shows a block
diagram of the pixel. The frequency limitation is achieved by a low pass
filter using switched capacitors. The biphasic current pulse is implemented by
switching the current source and sink alternatively.
Figure 8.4 demonstrates the experimental result of the variable photosensitivity
using the chip. The original output curve has a dynamic range of over 6-Log

Figure 8.3. Block diagram of the PFM photosensor with variable photosensitivity.