Journal ArticleDOI

# A Linearizing Digitizer for Wheatstone Bridge Based Signal Conditioning of Resistive Sensors

15 Mar 2017-IEEE Sensors Journal (IEEE)-Vol. 17, Iss: 6, pp 1696-1705

TL;DR: A novel signal conditioning scheme, which provides a linear-digital output directly from the resistive sensor(s) that are connected in such bridge configurations, and drastically reduces the effect on the output due to the lead wires that connect the Wheatstone bridge and the DSADC.
Abstract: Output of a typical Wheatstone bridge, when it is connected to measure from a single or a dual resistive element, possesses non-linear characteristic. This paper presents a novel signal conditioning scheme, which provides a linear-digital output directly from the resistive sensor(s) that are connected in such bridge configurations. In the present scheme, the input stage of a dual-slope analog-to-digital converter (DSADC) is suitably augmented to incorporate the quarter-bridge and (or) half-bridge containing the resistive sensor as an integral part of the DSADC. A combination of the current mode excitation and wisely selected integration and de-integration operations of the DSADC enable to achieve linearization in the digitization process itself, leading to an overall reduction in the complexity level and number of blocks used keeping the high accuracy unaltered. A detailed analysis has been conducted to quantify the effect of various sources of errors in the output of the DSADC. The details are presented in the paper. The proposed method not only provides a linear digital output but also drastically reduces the effect on the output due to the lead wires that connect the Wheatstone bridge and the DSADC. Thus, the proposed scheme is well suited for the situations where the sensor(s) is (are) remotely located at a distance. Simulation studies as well as results from a prototype developed and tested establish the practicality of the proposed scheme. The inherent non-linearity of the Wheatstone bridge is reduced by nearly two orders of magnitude.
Topics: Wheatstone bridge (68%), Signal conditioning (53%)
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Journal ArticleDOI
TL;DR: This paper presents a review of different methods applied to linearize sensor characteristics reported in the literature and concludes that the digital methods combined with software techniques perform the job with better flexibility and efficiency.
Abstract: Today, the sensing devices play an important role for various system automation and monitoring of different physical and chemical parameters. Nonlinearity is an important long-time issue for most of the sensors, so to compensate nonlinearity, various linearization schemes are reported in the literature. The accuracy of linearization schemes depends on the type and the nonlinearity value of the sensor output. Since it is difficult to find an exact polynomial equation or other functions to represent the response curve; it gives more error when the measurement parameter is determined from the inverse approximation functions. As many sensors are used for different applications, the linearized characteristics will simplify the design, calibration, and accuracy of the measurement. This paper presents a review of different methods applied to linearize sensor characteristics reported in the literature. Due to availability of high-performance analog devices, analog methods are still popular among many researchers. However, due to the advancement of IC technologies, hardware implementation of the software methods can be done easily with reduced time, cost, and more accuracy, so the digital methods combined with software techniques perform the job with better flexibility and efficiency.

10 citations

### Cites background or methods from "A Linearizing Digitizer for Wheatst..."

• ...…necessary signal conditioning circuits have been employed to linearize the response of thermistor, Hall effect sensor, and single or double resistive element Wheatstone bridge (Mohan et al., 2008; Mohan et al., 2011; Sreekantan and George, 2014; Ramadoss and George, 2015; Nagarajan et al., 2017)....

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• ...Recently, conventional dual slope analog to digital converter with necessary signal conditioning circuits have been employed to linearize the response of thermistor, Hall effect sensor, and single or double resistive element Wheatstone bridge (Mohan et al., 2008; Mohan et al., 2011; Sreekantan and George, 2014; Ramadoss and George, 2015; Nagarajan et al., 2017)....

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• ...(Nagarajan et al., 2017) Dual slope ADC for direct interface to μC (quarter/half bridge resistive sensors) <0....

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Journal ArticleDOI
TL;DR: This article proposes simple relaxation-oscillator-based digital interfacing schemes for resistive sensors in single-element and quarter-bridge forms, equipped with novel compensation techniques, that render a direct-digital output proportional to sensor resistance.
Abstract: This article proposes simple relaxation-oscillator-based digital interfacing schemes for resistive sensors in single-element and quarter-bridge forms. The proposed interfaces, equipped with novel compensation techniques, render a direct-digital output proportional to sensor resistance. These interfaces offer many meritorious features, such as simplicity of design, nonrequirement of the reference voltage, lower execution time, and negligible influences from circuit nonidealities. The methodology of the interfaces and their design criteria and error analysis are described in this article. The first two interfaces are suitable for nonremote resistive sensors, while the third interface has been developed for remotely located resistive sensors. The functionality of the proposed interfaces has been verified using simulation as well as detailed experimental studies. The developed interfaces provide a linear direct-digital output, and the maximum experimental nonlinearity is merely 0.08%. Later, a representative sensor based on the giant magnetoresistance (GMR) phenomenon is selected, characterized, and tested with the developed interfaces. The complete instrumentation system is shown to act as a linear digital magnetometer. Finally, the performance of the developed interfaces is compared and shown to be better/comparable with respect to the existing works.

9 citations

### Cites background from "A Linearizing Digitizer for Wheatst..."

• ...However, these schemes require precision (matched) reference voltages and works in [11] need an additional phase to start the conversion....

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• ...Alternatively, the work in [11] makes the output independent of wire resistance using instrumentation amplifier (IA) and an involved switching logic....

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• ...Simple dual-slope-based circuits for different types of resistive sensors are reported in [10] and [11]....

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• ...A number of digital techniques, such as pulsewidth conversion [7]–[9], dual slope [10], [11], direct microcontroller interfacing [12]–[15], sigma-delta [16], and relaxation oscillator [17]–[21], are prevalent for resistive sensors....

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Journal ArticleDOI
TL;DR: A fully analog method has been introduced in order to implement a null measurement instrument for a resistive Wheatstone bridge with a single resistive sensor to evaluate the performance and usefulness of the proposed method.
Abstract: When high-speed measurements are not necessary, the Wheatstone bridge-based null measurement instruments surpass the Wheatstone Bridge-based instruments using deflection technique, due to the fact that the null method has some intrinsic advantages over the deflection method. In this particle, a fully analog method has been introduced in order to implement a null measurement instrument for a resistive Wheatstone bridge with a single resistive sensor. In order to implement the proposed circuit, only two operational amplifiers and some passive elements have been used. The proposed method can be constructed and implemented by discrete components without any special limitation. Since the suggested scheme has linear output, the required computation in order to measure and display the measurand is reduced dramatically. Finally, in order to evaluate the performance and usefulness of the proposed method, it has been tested experimentally as well as by using computer aided design software.

9 citations

Journal ArticleDOI
TL;DR: This paper presents a new and simple piecewise linearizing circuit to compensate the nonlinearity of highly nonlinear sensor and is CMOS compatible for ASIC implementation.
Abstract: To obtain the solutions to the problem of linearization of the sensor characteristics is a matter of investigations for a long time. This paper presents a new and simple piecewise linearizing circuit to compensate the nonlinearity of highly nonlinear sensor. It is having a mixed signal conditioning circuit, employing a 2-bit flash ADC, a ( 4 × 1 ) multiplexer and the Op amp-based signal conditioning units. The circuit was initially simulated using SPICE software and then hardware implemented for linearizing the nonlinear response characteristic of two sensors. One of the sensors is a graphene oxide capacitive humidity sensor with nearly 46% nonlinearity and the other is a thermistor-based temperature sensor with 16.4 % nonlinearity. The humidity sensor is fabricated and characterized to measure humidity in the range of 2–85% RH. The maximum nonlinearity of the humidity and temperature sensors of the hardware implemented circuit after compensation was 1% and 1.5 % respectively. The circuit is simple and requires few hardware components for implementation and applicable for any nonlinear sensor. The circuit is CMOS compatible for ASIC implementation.

6 citations

Journal ArticleDOI
Abstract: Analog signal conditioning circuit based on dual differential subtractor for wide range resistive measurement of single element resistive sensor is presented in this paper. The differential output from the sensor and a voltage-controlled reference resistor is followed by an auto-nulling technique, which helps to achieve high accuracy and wide dynamic range by generating a control voltage proportional to the sensor resistance. The presented automatic approach not only enhances the auto-nulling range but also ensures constant sensitivity. The interface design provides high accuracy, fast and linear response at the same time avoids drawbacks of lead wire variations. A prototype PCB of the proposed circuit is fabricated and tested. The experimental result shows that the circuit can measure a wide range of resistance (20 $\text{k}\Omega$ - 1.1 $\text{M}\Omega$ ) with relative error less than 0.78%, compared to other bridge-based circuits which are capable of measuring the maximum resistance of up-to 10 $\text{k}\Omega$ for the same accuracy. Multiple variations of the proposed circuit are designed and experimentally tested for verifying the tunability of the circuit in terms of the sensitivity and the range of resistance measurement. The prototype circuit is also interfaced with an in-house fabricated MEMS-based ammonia sensor, proving its efficacy in resistive sensor system development.

6 citations

### Cites background from "A Linearizing Digitizer for Wheatst..."

• ...To handle these dynamic properties robust signal conditioning circuits are required [8], [9]....

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• ...However, the Wheatstone bridge is not preferable for single elements resistive sensors due to high non-linearity [1], [9]....

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01 Jan 1999-

102 citations

### "A Linearizing Digitizer for Wheatst..." refers background in this paper

• ...1(b) is useful, when two sensing elements of same type are employed to achieve higher sensitivity [3]....

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• ...sensor may be located far from the measurement unit and the lead resistance of the wires will introduce appreciable errors in the measurement [3]....

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• ...Magneto Resistance (GMR) sensors, pressure sensors and flow meters [3]–[6]....

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• ...In (1), VB is the excitation for the bridge and kB is the bridge constant (kB = 4; quarter-bridge and kB = 2; half-bridge) Equation (1) clearly shows that the bridge output VoB will be a non-linear function of the measurand [3]–[6], and the transfer function of these bridge configurations will be a hyperbolic function [7]....

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• ...Various linearization techniques have been reported to obtain a final linear output, from the bridge [3], [8], [9]....

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Journal ArticleDOI
Boby George1, V.J. Kumar2Institutions (2)
TL;DR: A dual-slope capacitance-to-digital converter 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.
Abstract: 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%.

51 citations

### "A Linearizing Digitizer for Wheatst..." refers methods in this paper

• ...As in a typical dual slope technique, here too, to initiate a conversion, the digitizer has to invoke an auto-zero phase to set the output to zero [26]....

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Journal ArticleDOI
Abstract: This paper proposes the direct connection of different configurations of resistive sensor bridges to a microcontroller without any intermediate active component. Such a direct interface circuit relies on measuring the discharging time of a RC network that includes the resistances of the sensor bridge. For quarter-, half-, and full-bridge circuits, we combine the discharging times to estimate the fractional resistance change x of the bridge arms. Experimental results for half- and full-bridge circuits emulated by resistors yield a nonlinearity error below 0.3%FSR (full-scale range) for x between 0 and 0.1 and an effective resolution of 11 bit. Measurements on two commercial magnetoresistive sensors yield higher nonlinearity errors: 1.8%FSR for an AMR (Anisotropic Magnetoresistive) sensor and 5.8%FSR for a GMR (Giant Magnetoresistive) sensor, which are mainly due to the nonlinearity of the sensors themselves. Therefore, the nonlinearity of the measurement is limited by the sensors, not by the proposed interface circuit and linearisation algorithm.

50 citations

### "A Linearizing Digitizer for Wheatst..." refers methods in this paper

• ...Some of the methods proposed earlier for quarter-bridge configurations use an internal comparator and counter of a microcontroller and requires three charging and discharging periods [19], [20]....

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Journal ArticleDOI
Seyed Javad Azhari1, Hooman Kaabi1Institutions (1)
Abstract: This paper describes a new circuit topology based on duality of the well known voltage-mode Wheatstone bridge. This topology can implement ratiometric current measurement which is a vital aspect of current-mode instrumentation and signal processing. The theory of the proposed circuit is developed and its advantages and limitations are discussed. The new circuit (called AZKA cell) may be thought of as the current-mode alternative of traditional voltage-mode Wheatstone bridge. It presents the current-mode circuit designers and engineers their own relevant interface cell. The advantages of the new circuit are stated and some conditioning circuits for its output signal are proposed. Finally, a linearization circuit based on CCII+ is introduced.

46 citations

Journal ArticleDOI
TL;DR: A signal conditioning circuit for resistive sensors is presented based on a relaxation oscillator in which both the frequency and the duty-cycle of the square-wave output signal simultaneously carry information from a pair of different sensors.
Abstract: A signal conditioning circuit for resistive sensors is presented. It is based on a relaxation oscillator in which both the frequency and the duty-cycle of the square-wave output signal simultaneously carry information from a pair of different sensors. The output frequency is linearly related to the resistive unbalance of a Wheatstone bridge, while the duty-cycle is independently controlled by a second sensor. The latter can be a thermoresistor used for thermal compensation of the primary sensor. The design, analysis, and experimental characterization of the circuit and its application to a thick-film pressure sensor are reported. A method for compensating the accuracy degradation caused by the finite switching delays is illustrated, and results on its experimental validation are given.

44 citations

• ...half-bridge and full-bridge sensors [14]–[16]....

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