Low voltage charge-balanced capacitance-voltage conversion circuit for one-side-electrode-type fluid-based inclination sensor

A low voltage detection circuit for a capacitance sensor is important for connection to a low voltage digital circuit interface. We studied two different charge-balanced capacitance-voltage (C-V) conversion circuits configurations; the operational amplifier and the inverter amplifier. Both capacitance detection circuits were designed using 0.35 μm CMOS circuitry technology. Both amplifiers used in the detection circuits were not affected by offset voltage. The current consumption for capacitance detection circuit was reduced from 250 μA at V_dd 3.3 V to 38 μA at V_dd 1.3 V by switching from an operational amplifier to an inverter amplifier. These circuits were packaged with one-side-electrode-type fluid-based inclination sensors on ceramic substrates. The size of the sensor is ∅ 4.0 mm × 1.0 mm and pure propylene carbonate was used as electrolyte. Changes in temperature did not affect the output voltage of the sensor between -10 °C and 50 °C. This results show that the inverter amplifier used in the detection circuit was not affected by offset voltage and the output voltage V_m is depends only on capacitor ratio. The capacitance detection circuit using the inverter amplifier shows a high-sensitivity of about 7 mV/deg over the operational amplifier at V_dd 1.3 V. The response time, resolution and minimum moving angle of sensor were 0.7 s, 0.86° and 0.4°, respectively, at V_dd 1.3 V for the inverter amplifier type of capacitance detection circuit.