CCII-Based Linear Ratiometric Capacitive Sensing by Analog Read-Out Circuits (original) (raw)

A New Fully Closed-Loop, High-Precision, Class-AB CCII for Differential Capacitive Sensor Interfaces

Electronics

The use of capacitive sensors has advantages in different industrial applications due to their low cost and low-temperature dependence. In this sense, the current-mode approach by means of second-generation current conveyors (CCIIs) allows for improvements in key features, such as sensitivity and resolution. In this paper, a novel architecture of CCII for differential capacitive sensor interfaces is presented. The proposed topology shows a closed-loop configuration for both the voltage and the current buffer, thus leading to better interface impedances at terminals X and Z. Moreover, a low power consumption of 600 µW was obtained due to class-AB biasing of both buffers, and the inherent drawbacks in terms of linearity under the mismatch of class-AB buffering are overcome by its closed-loop configuration. The advantages of the novel architecture are demonstrated by circuit analysis and simulations; in particular, very good robustness under process, supply voltage and temperature vari...

A Current-Mode TransImpedance Amplifier for Capacitive Sensors

Proceedings

A Current-Mode (CM) TransImpedance Amplifier (TIA) based on Second Generation Current Conveyors (CCIIs) for capacitive microsensor measurements is presented. The designed electronic interface performs a capacitance-to-voltage conversion using 3 CCIIs and 3 resistors exploiting a synchronous-demodulation technique to improve the overall detection sensitivity and resolution of the system. A CM-TIA solution designed at transistor level in AMS0.35 µm integrated CMOS technology with a power consumption lower than 900 µW is proposed. Experimental results obtained with a board-level prototype show linear behavior of the proposed interface circuit with a resolution up to 34.5 fF and a sensitivity up to 223 mV/nF, confirming the theoretical expectations.

Linear Integrated Interface for Automatic Differential Capacitive Sensing

Proceedings

In this work, the authors introduce a new full-analog front-end for differential capacitance sensors which provides a DC output voltage, directly proportional to the measurand variations. The readout circuit architecture is based on a De Sauty bridge as core of the capacitive sensing whereas the feedback circuitry performs the bridge autobalancing operation by means of changes in a multiplier output. The circuit is designed in a standard CMOS technology (AMS 0.35 µm) so is suitable for portable systems. Simulated results have shown a good agreement with the theoretical model being the percentage relative error less than 2.5%. Interface sensitivity is constant and values around 0.055 V/mm for the considered application.

A Capacitive to Digital Converter with Automatic Range Adaptation for Readout Instrumentation

Capacitive sensors have profoundly found their way in everyday life. Devices and instrumentation ranging from specialty equipment to smartphones, all employ in one way or another a capacitive sensor and its associated readout circuit, making the presence of the latter ubiquitious. We present a capacitive readout system that automatically adapts its range to the unknown, measured capacitance, thereby extending its functional input range, as well as its application and instrumentation compatibility. The proposed system achieves a constant resolution for a range of input capacitance up to 690 pF.

A VCII-Based Stray Insensitive Analog Interface for Differential Capacitance Sensors

Sensors, 2019

In this paper, a novel approach to implement a stray insensitive CMOS interface for differential capacitive sensors is presented. The proposed circuit employs, for the first time, second-generation voltage conveyors (VCIIs) and produces an output voltage proportional to differential capacitor changes. Using VCIIs as active devices inherently allows the circuit to process the signal in the current domain, and hence, to benefit from its intrinsic advantages, such as high speed and simple implementation, while still being able to natively interface with voltage mode signal processing stages at necessity. The insensitiveness to the effects of parasitic capacitances is achieved through a simple feedback loop. In addition, the proposed circuit shows a very simple and switch-free structure (which can be used for both linear and hyperbolic sensors), improving its accuracy. The readout circuit was designed in a standard 0.35 μm CMOS technology under a supply voltage of ±1.65 V. Before the in...

A CMOS full-range linear integrated interface for differential capacitive sensor readout

Sensors and Actuators A: Physical, 2018

This manuscript presents the development of an integrated analog interface able to convert differential capacitive sensors variations into a DC voltage. The presented circuit exploits the architecture developed in previous works of the same authors, based on autobalancing bridge techniques, further improving its performances through the linearization of the input/output characteristic and the achievement of the full-range sensor variations capability. The interface takes advantage of Low Voltage and Low Power integrated circuit design techniques, optimizing performances for flux and force sensor. Simulated results with the AMS 0.35um standard CMOS technology have shown a very good agreement with the theoretical model developed, obtaining an error lower than 2.5% and a total power consumption of 5mW. Moreover, waiting for the chip production, a discrete element board has been also implemented and tested. Preliminary measurements on the test board have shown satisfactory input/output linearity characteristics with overall reduced percentage error (lower than 1.5%) in the measurand estimation.

Analysis of a Direct Interface Circuit for Capacitive Sensors

IEEE Transactions on Instrumentation and Measurement, 2000

We present the theoretical analysis and performance results of a direct microcontroller unit (MCU) interface circuit for capacitive sensors based on the charge-transfer method, when stray capacitances are considered. The interface circuit can implement two alternative two-point calibration techniques that reduce the effects of stray capacitance, temperature, and MCU parameters that depend on the power supply voltage. The best measurement deviation achieved from 0 • C to 50 • C and for power supply voltage from 4.0 to 5 V is below 0.01 full-scale range (FSR) for the two subranges from 10 to 100 pF and from 100 pF to 1 nF and 0.08 FSR for the subrange from 2 to 10 pF.

An accurate interface for capacitive sensors

IEEE Transactions on Instrumentation and Measurement, 2002

A new smart interface based on a first-order chargebalanced SC-oscillator is presented for capacitive sensors, which are shunted by a parasitic conductance. In the novel interface, the effect of shunting conductance is reduced by using the charge/discharge method. The effect of the stray capacitances is eliminated by using the two-port measurement. Moreover, all multiplicative and additive errors of the interface are also eliminated by using the auto-calibration technique and the chopping technique.

An Interface Circuit for &@-Accuracy Signal Processing of Differential-Capacitance Transducers

For high-accuracy signal processing of differential-capacitance transducers, an interface circuitry is developed. The architecture is based on the idea that the ratio of one of the transducer capacitances to its total capacitance represents the offset binary equivalent of the physical quantity under measurement. An opamp-based capacitance-to-voltage converter is commonly used for capacitance detection and an analog-to-digital converter is used for the ratiometric operation. A circuit analysis shows that the interface can detect the capacitance change as small as 0.01 % of the total capacitance. Experimental results are also given to confirm the analysis.

A Capacitance-to-Time Converter-Based Electronic Interface for Differential Capacitive Sensors

Electronics, 2019

In this paper we present an oscillating conditioning circuit, operating a capacitance-to-time conversion, which is suitable for the readout of differential capacitive sensors. The simple architecture, based on a multiple-feedbacks structure that avoids ground noise disturbs and system calibrations, employs only three Operational Amplifiers (OAs) and a mixer implementing a square wave oscillator that provides an AC sensor excitation voltage. It performs a Period Modulation (PM) and a Pulse Width Modulation (PWM) of the output signal proportionally to the sensor differential capacitance values. The sensor variation range and the detection sensitivity can be easily set through the additional resistors. Preliminary PSpice simulation results have shown a good agreement with theoretical calculations as well as a linear response with a high detection sensitivity of differential capacitive sensors having a baseline in the range [2.2 ÷ 180 pF]. Moreover, different experimental measurements h...