An ultra-low power capacitance extrema and ratio detector for electrostatic energy harvesters

We present a novel ultra-low power capacitance extrema and modulation ratio detector for electrostatic micro vibration energy harvesters. The circuit signals the points in time where a varying harvester capacitance C_n(t) reaches its minimum and maximum values o min and o max in each oscillation period. A novel feature is that the circuit allows the capacitance modulation ratio a_c = Cmax/Cmin between two consecutive extrema to be determined digitally. Using a self-clocked charge pumping scheme implemented with two low-power comparators and a pulse generator, our detector circuit keeps the voltage across a dedicated sense capacitor within a predefined margin around a fixed reference level. Transitions through capacitance extrema are indicated by polarity reversals of the charge packets. The number of charge packets transferred between two polarity reversals is nonlinearly but uniquely related to a_c. The proposed circuit was implemented and fabricated in a commercial 0.35-/xm CMOS process. Measurements were performed using a rotating capacitor with C_max = 150 pF and adjustable a_c spinning with up to 110 Hz. Transitions through capacitance extrema were measured for a_c = 1.3 ... 4.4 and were signalled accurately within the systematic time resolution limits of the method. In the presented configuration, the circuit draws a current of 60 nA from a 3.8-V supply.