A novel capacitance-to-digital converter (CDC) that performs an accurate measurement of the capacitance of a leaky capacitive sensor is presented. The conversion technique of this CDC is novel, which helps to achieve the highest interference rejection compared with the exiting CDC topologies. The digital output from the CDC is directly proportional to the sensor capacitance, Cx. It provides an accurate output even in the presence of a parasitic leakage resistance, Rp, parallel to the sensor. A prototype CDC has been developed and tested. Worst-case error was found to be 0.1%. The CDC employs a sinusoidal excitation for the sensor, and is suitable for a variety of applications such as proximity or touch detection, detection of ice, level sensing, humidity measurement, etc. This CDC is well suited for applications where the conventional schemes fail to work due to the extreme interference from other sources, e.g. power line. © The Institution of Engineering and Technology 2016.

Boby George

Department of Electrical Engineering

Prashanth Vooka

Capacitance

Frequency converters

Accurate measurement

CAPACITANCE-TO-DIGITAL CONVERTER

CONVENTIONAL SCHEMES

HUMIDITY MEASUREMENTS

INTERFERENCE REJECTION

PARASITIC LEAKAGES

SENSOR CAPACITANCE

Sinusoidal excitations

CAPACITIVE SENSORS

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Electronics Letters

A new Switched Capacitor (SC) Capacitance-to-Digital Converter (CDC) is presented in this paper. Its output exhibits a high-level of insensitivity to interference, by virtue of the conversion mechanism itself. The proposed scheme is a combination of an SC relaxation oscillator and an integrating type analog-to-digital converter (ADC). Its conversion time is independent of the measurand and hence can be set, such that it is an integral multiple of the interference period, unlike in a dual-slope ADC where the de-integration time is a function of the measurand. The final output is proportional to the number of transitions in the oscillator output, which is directly proportional to the value of the sensor capacitance and has negligible sensitivity to the presence of the interference signal. The proposed scheme has been realized and tested as a hardware prototype unit. The non-linearity of the scheme was found to be 0.4%, and the resolution, in terms of the effective number of bits (ENOB), was 11.13, when tested, with sensor capacitance varied from 10 to 32.5 pF. The studies conducted to evaluate the effect of capacitively coupled interference to the sensitive node on the CDC showed that it is negligible for a wide range of magnitude. © 2001-2012 IEEE.

IEEE Sensors Journal

An Extended Study on an Interference-Insensitive Switched Capacitor CDC

This paper presents a low-cost interface circuit for lossy capacitive sensors. Most of the capacitive sensors are lossy in nature, where the resistance value of the sensor significantly varies along with the capacitance value. Accurate measurement of those values of a lossy capacitive sensor is a challenging task. This paper deals with the development of an electronic interface circuit for such a capacitive sensor. The circuit is based on the measurement of quadrature and in-phase components of the signal. To extract the actual value of the capacitance and the resistance, an average of the quadrature and the in-phase components of output is required. This is implemented using a microcontroller and provides flexibility in averaging (e.g., adjustable length of data to be processed). A PCB of the circuit has been fabricated and tested in the laboratory using capacitors and resistors with known values. The accuracy of the circuit was better than 0.6% and-2.2% for the measurement of capacitance and resistance, respectively. Also, the circuit is highly linear (FS nonlinearity of 0.32%) for a wide range of capacitance measurement (162 pF-3.680 nF). To confirm the effectiveness of the circuit, it was interfaced to a lossy capacitive humidity sensor and another capacitive transducer to measure the dielectric constants of edible oils. The results were compared with a precision impedance analyzer (Agilent 4294A); the maximum relative error was noted as ±1.6% in the measurement of permittivity values of edible oil. The prototype developed has the option to easily adjust the sensitivity and operating range, depending on the application. © 1963-2012 IEEE.

IEEE Transactions on Instrumentation and Measurement

An Efficient Interface Circuit for Lossy Capacitive Sensors

A novel resistance-to-digital converter (RDC), based on the integrating type analogue-to-digital converter principle, is presented in this study. The conversion time of the proposed scheme is not a function of the current value of the parameter being measured. Thus, by suitably setting this parameter, the converter can be made to reject the effects of interference at a particular frequency, such as, that due to power-line at 50/60 Hz. Error analysis was conducted to ascertain the effects of non-idealities of various components of the circuit, on its output. Simulation studies were carried out in LTSPICE to verify the linearity and the interference rejection capability of the converter. Further, a prototype RDC was developed in the laboratory and tested to confirm the results of the simulation. © The Institution of Engineering and Technology.

IET Circuits, Devices and Systems

Resistance-to-digital converter designed for high power-line interference rejection capability

This paper presents a capacitance-to-digital converter (CDC) that employs a novel approach to perform an accurate measurement of capacitance of a leaky capacitive sensor. This CDC employs a sinusoidal source for excitation, which is advantageous for various sensing applications, including ice detection, liquid level measurement, humidity measurement, proximity sensing, and so on. Recently, an impedance-to-digital converter (IDC) based on the dual-slope conversion technique has been reported. It can measure the value of capacitance even when a parallel resistance is present, but requires a large number of sinusoidal excitation cycles to complete a conversion, leading to poor update rate. The CDC proposed in this paper gives much higher (about 125 times) update rate compared with the IDC as it requires only a few excitation cycles for the conversion, but still gives an accurate output due to the use of a specially designed clock, which helps to count the number of charge packets received by the integrator capacitor during the deintegration. Other than the operation of the CDC, this paper also describes an outcome of a thorough analysis conducted to quantify the effect of various circuit parameters on the output of the new CDC. A prototype of the improved CDC has been developed, and its performance parameters, such as accuracy (±0.27%), conversion time (24 ms), effect of parallel resistance, and so on, have been tested, and the results are reported in this paper. © 2015 IEEE.

An improved capacitance-to-digital converter for leaky capacitive sensors

IEEE Transactions on Consumer Electronics

Three dimensional touchless tracking of objects using integrated capacitive sensors

Measurement Science and Technology

A microcontroller-based interface circuit for lossy capacitive sensors

2007 IEEE Instrumentation & Measurement Technology Conference IMTC 2007

Susceptibility of Dual-Slope ADCs to Electromagnetic Interference: An Experimental Analysis

Capacitance-to-digital converter for leaky capacitive sensors

Journal	Electronics Letters
Publisher	Institution of Engineering and Technology
ISSN	00135194
Open Access	No