2020 Volume 17 Issue 18 Pages 20200268
A linear, high-precision capacitance-to-digital converter (CDC) for grounded-type capacitive sensors is presented in this paper. The designed CDC consists of an analog front-end (AFE) circuit that converts capacitance to voltage signal and a noise-shaping successive approximation register (NS-SAR) analog-to-digital (ADC) that converts voltage to digital output. Firstly, in order to measure very small capacitance with a grounded target electrode in the presence of a large offset capacitance, a variable capacitance multiplier circuit is introduced, which ensures that the final output is independent of the offset capacitance. Secondly, the designed CDC implements a fully differential operational amplifier (FDOA) with two T-networks to reduce non-linearity, which provides an output proportional to the variable capacitance. Accordingly, the design complexity of the analog front-end circuit can be minimized effectively. The interface is designed as an integrated circuit using a standard 0.18 µm CMOS process. The functionality of the proposed CDC is verified first using simulation results, showing that for a sensor capacitance ranging from 1 fF ∼ 1 pF, the minimum measurement accuracy can reach 0.1 fF with a parasitic capacitance up to 100 pF. The whole CDC consumes approximately 0.8 mA from a 1.8 V power supply.
