A 130 nm Event-Driven Voltage and Temperature Insensitive Capacitive ROC

This paper presents an 130 nm event-driven, all-digital, modular and scalable read-out circuit for capacitive sensors, whose operation is not influenced by either variation of the supply voltage or temperature. The sensing element of the ReadOut Circuit (ROC) is a voltage-controlled ring oscillator, designed to make the system robust to voltage and temperature variations with a dedicated calibration implemented to eliminate offset when no pressure is applied. The ring oscillator is used at the same time as a sensor and a clock signal for the entire event-driven unit. This, in turn, enables to further reduce the thermal drift of the measured capacitance. The sensitivity is 10 fF per LSB, considering a nominal sensing capacitance of 1 pF. The 8 bit output is asynchronously made available with a Parallel-In-Serial-Out register (PISO) after the ROC completes a measurement. The simulated average power consumption is 5.94 μ Wat 1.2V VDD on 1ms operation. The small active area (221×79 μm²) and power consumption make the circuit ideal to be replicated in an array in a cyber physical system, as capacitive pressure sensor in Humanoid Robots.