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].