In this paper we present a suitable use of Wheatstone bridge-based circuits to employ a novel interface for both grounded and floating wide range resistive sensor estimation. The proposed topology is very simple and, through the use of only analog blocks in a feedback configuration, allows the automatic and continuous detection and quantification of the sensor resistance. Two circuit variations are proposed: both of them implement the commercial component analog multiplier AD633 as voltage controlled resistor placed in the feedback loop to compensate the sensor variations. Electrical measurement results, conducted on a prototype PCB, have shown that, even if the bridge autobalancing operation works in only 1.6 decades depending on the employment of the AD633, through a two-voltage-reading technique, it is possible to estimate up to 5-decades resistive variations, whose range is also settable, with a relative error within 1.5% and with a higher and supply-independent sensitivity, when compared to traditional bridge. This fact, together with its high resolution, has also allowed, in experimental tests performed by using an air quality resistive sensor, to reveal few ppm carbon monoxide particles.

Analog Wheatstone bridge-based automatic interface for grounded and floating wide-range resistive sensors

In this paper a novel fully-analog uncalibrated Wheatstone bridge-based topology suitable for wide-range resistive sensors estimation is presented. The proposed interface is an improvement of a work recently proposed by the authors, showing an increase of the operative dynamic range and both system accuracy and sensitivity. Thanks to the use of a simple closed feedback loop, based on an instrumentation amplifier and an inverting integrator, it is possible to easily set two voltage controlled resistor (VCR) values, so to maintain the bridge balancing condition in the case of sensor variation. The use of this configuration allows the sensor estimation for about 6 decades resistive variation (10 Ω–8 MΩ) with a very high accuracy for a wide-range sensor interface (absolute relative error within ±2%). Experimental tests, performed by using a commercial resistive sensor, have shown that, thanks to its high sensitivity value, the interface can reveal and correctly quantify both few ppm carbon monoxide (CO) particles and percentage relative humidity (RH%) variations.

A novel 6-decades fully-analog uncalibrated Wheatstone bridge-based resistive sensor interface

This paper reports a readout circuit for a resistive CO2 sensor, which operates by measuring the CO2-dependent thermal conductivity of air. A suspended hot-wire transducer, which acts both as a resistive heater and temperature sensor, exhibits a CO2-dependent heat loss to the surrounding air, allowing CO2 concentration to be derived from its temperature rise and power dissipation. The circuit employs a dual-mode incremental delta-sigma ADC to digitize these parameters relative to those of an identical, but isolated, reference transducer. This ratiometric approach results in a measurement that does not require precision voltage or power references. The readout circuit uses dynamically-swapped transducer pairs to cancel their baseline-resistance, so as to relax the required dynamic range of the ADC. In addition, dynamic element matching (DEM) is used to bias the transducer pairs at an accurate current ratio, making the measurement insensitive to the precise value of the bias current. The readout circuit has been implemented in a standard 0.16 $\mu \text {m}$ CMOS technology. With commercial resistive micro-heaters, a CO2 sensing resolution of about 200 ppm ( $1\sigma $ ) was achieved in a measurement time of 30 s. Similar results were obtained with CMOS-compatible tungsten-wire transducers, paving the way for fully-integrated CO2 sensors for air-quality monitoring.

/pdf/a-ratiometric-readout-circuit-for-thermal-conductivity-based-493aa2tidv.pdf

A Ratiometric Readout Circuit for Thermal-Conductivity-Based Resistive CO 2 Sensors

In this paper, a new configuration of operational amplifier -based square-wave oscillator is proposed. The circuit performs an impedance-to-period (Z–T) conversion that, instead of a voltage integration typically performed by other solutions presented in the literature, is based on a voltage differentiation. This solution is suitable as first analogue uncalibrated front-end for capacitive and resistive (e.g. relative humidity and gas) sensors, working also, in the case of capacitive devices, for wide variation ranges (up to six capacitive variation decades). Moreover, through the setting of passive components, its sensitivity can be easily regulated. Experimental measurements, conducted on a prototype printed circuit board, with sample passive components and using the commercial capacitive humidity sensor Honeywell HCH-1000, have shown good linearity and accuracy in the estimation of capacitances, having a baseline or reaching a value ranging in a wide interval [picofarads–microfarads], as well as, with a lower accuracy, in the evaluation of more reduced variations of resistances, ranging from kiloohms to megaohms, also when compared with other solutions presented in the literature.

Uncalibrated operational amplifier-based sensor interface for capacitive/resistive sensor applications

A method to implement a square-rooting circuit is described. The proposed circuit employs operational transconductance amplifiers (OTAs) as the only active elements. The implementation technique is based on an electronically variable resistor formed by the OTA, where the magnitude of the resistance is controlled by the output current, to provide the square-rooting function. The proposed scheme can operate with both a voltage input and a current input signal. The purpose of the circuit is emphasised in terms of simple configuration, high accuracy and low cost. Experimental results showing the circuit performance are presented.

Simple square-rooting circuit using OTAs

A CMOS controllable constant-power source for variable resistive loads is presented. Its operation is based on the resistive mirror which keeps the equality of the resistive original resistance and the resistive image resistance. Stability requirements are in accordance with the requirements of the basic approach itself enabling large load resistance dynamic range. Two designs are considered. Prototypes based on discrete components have been experimentally verified. Relative errors of the generated power with the discrete values in the range from 0.48 to 10.8 mW, for the load resistance range from 0.5 to 1.5 kQ, are smaller than 2.2%. Also, for the generated power of 6 mW, the load resistance range from 0.2 to 5.6 kQ with the relative error smaller than 0.7% has been achieved.

A CMOS Controllable Constant-Power Source for Variable Resistive Loads Using Resistive Mirror With Large Load Resistance Dynamic Range

Two types of self-balancing direct current bridge (SBDCB) circuits using resistive mirror-based voltage-controlled resistors (RMBVCRs) instead of the classical electromechanical trimming resistors have been designed. The proposed SBDCBs are the first designs reported in the literature that employ the semiconductor-controlled resistors. These bridge circuits have constant currents in their resistive branches. One of the SBDCBs has the variable supply voltage, whereas the other has the constant supply voltage. The resistive original and its mirror image within the RMBVCR have been realized using bipolar junction transistors. The bridges are automatically balanced with the output voltage directly proportional to the measured resistance. The conclusions derived by the error analysis have been verified by the experimental results. The maximal relative error of the proposed SBDCB circuit is less than 0.05% in the measured resistance range from 0.3 k/spl Omega/ to 300 k/spl Omega/.

Self-balancing linear bridge circuits with resistive mirrors for resistance measurement

A universal sensor interfacing circuit is presented in this paper. It is capable of giving a digital output directly proportional to various types of measured quantities, where those measured quantities are proportional to the square root of some other type of quantity. This sensor interfacing circuit is a square-rooting current-to-frequency converter. The design is based on two current-controlled resistors in CMOS technology with bisection of the input voltage used in place of the ordinary resistors within the relaxation oscillator. There is no need for highly stable voltage or current references in the proposed design. The temperature stability of the design is achieved without the need for any specific setting of the process parameters. Experimental results, obtained on a circuit implemented by discrete components, confirm the predictions of the analysis performed. The relative errors are less than 1% for the input current range from 12 µA to 650 µA, and for the output frequency range from 231 Hz to 2....

Smart sensor interfacing circuit using square-rooting current-to-frequency conversion

Two types of self-balancing DC bridge (SBDCB) circuits using resistive mirror-based voltage controlled resistors (RMBVCRs) instead of the classical electromechanical trimming resistors have been designed. The proposed SBDCBs are the first designs reported in the literature that employ the semiconductor controlled resistors. These bridge circuits have constant currents in their resistive branches. One of the SBDCBs has the variable supply voltage, whereas the other has the constant supply voltage. The bridges are automatically balanced with the output voltage directly proportional to the measured resistance. The behaviours of the proposed circuits have been verified by experiment. The maximal relative error of the proposed SBDCB circuit is less than 0.05% in the measured resistance range from 0.3 k/spl Omega/ to 300 k/spl Omega/.

Desa Gobovic

Papers

A CMOS Controllable Constant-Power Source for Variable Resistive Loads Using Resistive Mirror With Large Load Resistance Dynamic Range

Self-balancing linear bridge circuits with resistive mirrors for resistance measurement

Smart sensor interfacing circuit using square-rooting current-to-frequency conversion

Self-balancing DC bridge circuits using resistive mirror based voltage controlled resistors