Wide Input Range Supply Voltage Tolerant Capacitive Sensor Readout Using On-Chip Solar Cell

In this paper, a wide input range supply voltage tolerant capacitive sensor readout circuit using on-chip solar cell is presented. Based on capacitance controlled oscillators (CCOs) for ultra-low voltage/power consumption, the sensor readout circuit is directly powered by the on-chip solar cell to improve the overall system energy efficiency. An extended sensing range with high sensing accuracy is achieved using a two-step successive approximation register (SAR) and delta-sigma (Delta Sigma) analog-to-digital (A/D) conversion (ADC) scheme. Digital controls are generated on-chip using a customized sub-threshold digital standard cell library. Systematic error analysis and optimization including the finite switch on-resistance, buffer input-dependent delay, and SAR quantization nonlinearity are also outlined. High power supply rejection ratio (PSRR) is ensured by using a pseudo-differential topology with ratiometric readout. The complete sensing system is implemented using a standard 0.18 mu m complementary metal-oxide-semiconductor (CMOS) process. Simulation results show that the readout circuit achieves a wide input range from 1.5 pF to 6.5 pF with a worst case PSRR of 0.5% from 0.3V to 0.42V (0.67% from 0.3V to 0.6 V). With a 3.5 pF input capacitance and a 0.3V supply, the Delta Sigma stage achieves a resolution of 7.1-bit (corresponding to a capacitance of 2.2 fF/LSB) with a conversion frequency of 371 Hz. With an average power consumption of 40nW and a sampling frequency of 47.5 kHz, a figure-of-merit (FoM) of 0.78 pJ/conv-step is achieved.