Retinal Prosthesis Dissertation - Student Home Pages

For the implemented/structural case of the 32 by 32 image the bit length of the
transmitted AER packet is composed of ten address bits and eighteen stimulus bits.
This corresponds to a bit rate determined by the AER packet frequency. The AER
frequency is derived from the partial_spike clock frequency. In the case of the test
setup described earlier using an FPGA clock of 25Mhz as the partial_spike
frequency the spike frequency would be a quarter of this i.e. 6.25MHz. As an
outerpulse (at maximum intensity) has a periodic time of five times spike time then
outerpulse frequency is 1.25MHz. As there are fifty outerpulses in a pixel then pixel
(AER) frequency is 25Khz.
6.5 Post Processing Interface
In medical experiments equivalent impedance for the retinal tissue has been
estimated at ≈ 10KΩ [53]. The post processing required to convert the signals from
the FPGA to those suitable for driving the electrodes of 10kΩ impedance [54] must
deliver sufficient charge to simulate the biological action potential (AP) within the
RGC of the optic nerve fibre. As there is only one interconnect lead per stimulation
site (to reduce the wiring through the surgical incision) the interface will require two
supply voltages to produce the biphasic current pulse. For a minimum supply voltage
of 5v [51] i.e. +5v and -5v the maximum current per electrode would be 500µA. The
instantaneous power would therefore be 2.5mw resulting in 2.5w if all electrodes
were continuously active. However for 50 active biphasic pulses within a second the
electrode is only active for 2ms of each pulse (1ms for each phase – 10% duty cycle)
and that’s at maximum intensity then the stimulation power requirement will reduce
to 0.25w. Presuming a 0.3 activity factor i.e. a scene is rarely fully white therefore
less than 50 pulses per second are common then power requirement would fall to
250/3 i.e. circa 83mw.
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