From (1), with constant Istim and dV, the blocking capacitor
value is proportional to the stimulation period dt. Thus, shorter
charging periods lead to a smaller blocking capacitor. The
basic idea is illustrated by the timing waveforms in Fig. 2,
where the complete active charging phase is obtained by the
summation of two high-frequency complementary currents, I1
and I2, which are generated by clock phases Φ1 and Φ2,respectively. Φ3 is the passive discharging phase in which
charge balance is achieved. The complete stimulus current
waveform, Istim, is shown at the bottom of Fig. 2. The
generation of Φ1 and Φ2 by an on-chip oscillator and logic
gates is shown in Fig 3. The D flip-flop in Fig 3 guarantees a
clock signal with a 50% duty cycle, regardless of the duty
cycle of the input signal. Thus, in the charging phase, Φ1 and
Φ2 are anti-phase and have the same “ON” time in total. The
frequency of the generated clock signal is one half of the
frequency of the oscillator.