This paper is a continuation of a previous work with regard to the direct connection of differential sensors to microcontrollers without using intermediate electronics between them. This paper focuses on the measurement of differential capacitive sensors, whereas the previous work dealt with the resistive counterparts. The proposed circuit is analyzed, and the main limitation seems to be the fact that the magnitude of the input parasitic capacitances of the microcontroller is similar to or even higher than the sensor capacitances. Methods to overcome this limitation are proposed, particularly when measuring low-value differential capacitive sensors such as microelectromechanical system (MEMS) sensors. Experimental tests of the circuit have been carried out by measuring a commercial capacitive accelerometer working as a tilt sensor. Although such a sensor has a low value (1.5 pF) and low sensitivity (0.105 pF/g), the measurement has shown a nonlinearity error of 1% full-scale span (FSS), which is a remarkable value considering the simplicity of the circuit.

Interfacing Differential Capacitive Sensors to Microcontrollers: A Direct Approach

A dual-slope capacitance-to-digital converter (CDC) that operates on the elements of a differential capacitive sensor and provides a digital output that is linearly proportional to the physical quantity being sensed by the sensor is presented and analyzed in this paper. The converter topology is so chosen that a linear digital output is obtained for not only a sensor possessing linear input-output characteristics but also a sensor possessing inverse characteristics. The digital output in the proposed converter is dependent only on, apart from the sensitivity of the sensor, a dc reference voltage. Hence, high accuracy and linearity are easily obtained by employing a precision dc reference. Since the proposed CDC is based on the popular dual-slope analog-to-digital converter structure, it possesses all the advantages (resolution, accuracy, and immunity to noise and component parameter variations) and limitations (requirement of auto-zero and low conversion speed) applicable to the dual-slope technique. A prototype built and tested for a typical differential capacitive sensor with a nominal capacitance value of 250 pF gave a worst-case error of less than 0.05%.

Analysis of the Switched-Capacitor Dual-Slope Capacitance-to-Digital Converter

The authors here propose an analog interface for differential capacitance estimation, able to reveal and quantify capacitive variations. Different from other works in the literature, the proposed interface can operate on a full-range scale. This solution takes inspiration from a preliminary work of the same authors, whose working principle is based on a modified De-Sauty AC bridge configuration; in particular, two (differential) capacitors and two resistors are employed, one of which is implemented by a voltage-controlled resistor (VCR). In this paper, the electronic interface has been redesigned to obtain full-range operation and a linear input–output characteristic. The implemented autobalancing mechanism controls the VCR generating a reference voltage following the sensor output. Simulation results in two different circuit configurations performed by OrCAD PSpice have shown a very good agreement with a theoretical model implemented in MATLAB/Simulink environment. In addition, an evaluation of the goodness of the experimental measurements at steady and unsteady conditions is given. Experimental tests on a discrete-component prototype have been carried out using an automated test equipment including National Instruments tools and a dedicated software in the LabVIEW environment. Measurements have shown satisfactory results, the standard deviation being on the order of 0.01, in the worst case. The linearity error has been evaluated as well, resulting in lower than 1% full scale.

Autobalancing Analog Front End for Full-Range Differential Capacitive Sensing

Summary
In this paper, an integrable novel fully analog Wheatstone bridge-based interface for differential capacitance estimation is presented. Its working principle takes advantage of the modified De-Sauty AC bridge configuration being employed only by two capacitors and two resistors. A feedback loop controls one of the resistors (e.g. a voltage-controlled resistor), to obtain an evaluation of the differential capacitance variation on a full range, thanks to a general but very simple formula that considers both the ‘auto-balancing’ and the bridge ‘out-of-equilibrium’ ranges. The proposed interface shows a satisfactory accuracy, being the percentage relative error within 0.45% for the experimental results. Copyright © 2016 John Wiley & Sons, Ltd.

Full range analog Wheatstone bridge-based automatic circuit for differential capacitance sensor evaluation

This study presents a multi-touch sensing circuit for large-sized (more than 12 inches) capacitive touch panel. A new AC sensing technique is developed to enable the touch sensing in a large-sized capacitive touch panel. This novel designs of multi-touch sensing circuit lies in the operation principles through a 4×4 ITO film sensor array, a low-disturbance array circuit, a capacitance to voltage converter circuit and the proposed design procedure of chip parameters by circuit simulation. Furthermore, the sequence control for the array is generated by an FPGA module to accomplish the operation of the whole circuit. Some of objectives are to reduce environmental disturbance of the panel and to distinguish multi-touching locations from the panel. The corresponding output voltages of multi-touch sensing circuit are output in proportional to location touched by a human finger. Finally, the whole circuit is implemented by using TSMC 0.35µm 2P4M process. The circuit output voltages are used to distinguish touch locations by a proposed algorithm.

https://ir.nctu.edu.tw:443/bitstream/11536/26798/1/000287982100066.pdf

A multi-touch interface circuit for a large-sized capacitive touch panel

A digital converter that directly translates variations in the capacitances of a differential-type capacitive sensor to a proportional digital value is described in this paper. A conventional dual-slope, analog-to-digital converter is suitably modified to obtain direct capacitance-to-digital conversion (CDC). Analysis of the proposed technique indicates that the effects of nonidealities and variations in circuit parameters on the performance of the CDC is either in the form of a gain error and/or an offset, both of which can be easily compensated. Simulation studies and experimental results obtained from a prototype built and tested prove the efficacy of the proposed scheme.

S. Kedarnath

Papers

Digital Converter for Differential Capacitive Sensors