An accurate and power‐efficient period‐modulator‐based interface for grounded capacitive sensors

• Published in: International journal of circuit theory and applications 2019-08, Vol.47 (8), p.1211-1224
• Main Authors: Ahmadpour Bijargah, Arash, Heidary, Ali, Torkzadeh, Pooya, Nihtianov, Stoyan
• Format: Article
• Language: English
• Subjects: Capacitance; capacitance‐to‐period converter (CPC); capacitance‐to‐voltage converter (CVC); Circuits; CMOS; Cutting; Displacement measurement; Energy conversion efficiency; grounded capacitive sensor; Power consumption; Power supplies; Relaxation oscillators; Shielding; three‐signal autocalibration; Voltage converters; zoom‐in
• ISSN: 0098-9886; 1097-007X
• DOI: 10.1002/cta.2642

Summary A low‐power and high‐resolution capacitance‐to‐period converter (CPC) based on period modulation (PM) for subnanometer displacement measurement systems is proposed. The presented circuit employs the interface developed in a previous work, “a grounded capacitance‐to‐voltage converter (CVC) based on a zoom‐in structure,” further improving its performance through a symmetrical design of the applied autocalibration technique. The scheme is based on the use of a relaxation oscillator. To minimize the error contributed by the CPC circuitry, different precision techniques such as chopping, autocalibration, and active shielding are applied. The proposed CPC is realized in a 0.18‐μm complementary metal‐oxide‐semiconductor (CMOS) technology, occupies an area of 0.5 mm2, and consumes 135 μA from a 2‐V power supply. In order to achieve optimal performance and avoid overdesigning, a noise estimation of various parts of the CPC has been done. Accordingly, for a 10‐pF sensor capacitance, the overall CPC demonstrates a capacitance resolution of 0.5 fF for a latency of 128 microseconds, corresponding to an effective number of bits (ENOB) of 12.5 bits and an energy efficiency of 6 pJ/step. The nonlinearity error has been evaluated as well, resulting in a less than 0.03% full‐scale span (FSS). A power‐efficient capacitance‐to‐period converter (CPC) based on period modulation for grounded capacitive sensors is proposed. The performance of the CPC is significantly improved by applying zoom‐in, chopping, autocalibration, and active shielding techniques in a symmetrical structure. In order to achieve a power‐efficient CPC circuitry, a noise estimation of the CPC has been done based on transient noise simulations. Compared with recent designs, the proposed CPC represents a significant improvement in terms of resolution, power consumption, as well as a comparable energy efficiency.