Fig.
6 compares the required discharge times of the conventional switch and
the proposed bootstrapped switch when a monophasic stimulation pulse is
applied to the circuit with the equivalent circuit model of the
electrode-electrolyte interface using CH=100nF,
RF=10MΩ, and RS=10kΩ as in [4]. The
monophasic pulse is programmed with an amplitude of 400µA and a duration
of 150µs, as shown in Fig. 6(a). Fig. 6(b) and (c) show the measured
voltage across CH when a conventional and
bootstrapped switch is used, respectively. The discharge time of the
conventional and bootstrapped switch is 86.9µs and 53.4µs, respectively,
under the condition that the time from the start of CB until the
residual charge of 1nC remains, which demonstrates that the bootstrapped
switch is 39% faster in discharge time.
Fig. 7 shows the voltage between working and reference electrodes when
the stimulation frequency is 5Hz with a bi-phasic current with 100µA
amplitude and 100µs pulse width. The total stimulation time is 30
seconds, and Fig. 7(a) is the waveform for 10 seconds in the second half
of the stimulation, and Fig. 7(b) is the zoomed waveform for 2 seconds
in the second half of the stimulus. The voltage fluctuation generated by
residual charges is at most 7.5mV as shown in Fig. 7(b), which
corresponds to the equivalent residual charge of 0.75nC, which shows the
proposed stimulation system is within 20 times less in the safe limit of
15nC [9].