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.

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

A novel readout circuit for interfacing single element resistive sensors is presented in this paper. The proposed scheme is based on a new relaxation oscillator (RO). The RO, along with a timer counter provides a digital output proportional to the resistance of the sensor. The output characteristic of most of the existing readout circuits for resistive sensors suffers from gain, offset, and nonlinearity errors. The sources of errors include various nonideal circuit parameters and their drift. The output of the proposed readout circuit has a special feature that it is not a function of the circuit parameters such as: 1) offset voltages of the opamps and comparators; 2) bias currents of the opamps and comparators; 3) gain of various units employed; 4) ON-resistance of the switches; 5) value or mismatch in the magnitudes of the reference voltages employed; 6) delay of the switches and comparator; 7) leakage current of the switch; and 8) slew rate of the opamp. Such a scheme will be useful for high accuracy measurements, even when the parameters 1)–8) may vary or drift, due to variation in the measurement environment. A prototype of the proposed readout scheme has been developed in the laboratory and the performance has been evaluated under various conditions. The output was found to be linear with a worst-case nonlinearity of 0.05%. Test results from a prototype developed show that the proposed scheme possesses all the features, described above, as expected.

A Resistive Sensor Readout Circuit With Intrinsic Insensitivity to Circuit Parameters and Its Evaluation

Systems using microcontrollers make the interface of a transducer with the digital world much easier. They build smart, lucid, compact, cheap, and less power consuming electronic interfaces. Limited acquisition time is critical for many industrial applications. Single-element resistive sensors are used on a large scale. An attempt has been made to reduce the acquisition time of the measuring system compared to the existing work which converts the change in sensor resistance to the change in time with three charge–discharge cycles. Efforts have been made in this paper to reduce this to two charge–discharge cycles. The proposed work also compensates the errors due to lead wire resistance, the change in lead wire resistance, port pin resistances, and varying ambient temperature with reduced acquisition time. The mean of absolute errors, standard deviation, integral squared error (ISE), nonlinearity, and hysteresis of the proposed method have been computed. Simulation and experimentation study of the proposed system provided encouraging results. The mean of absolute errors and standard deviation of the proposed method in the simulation were 0.07 and 0.08 $\Omega $ , respectively. Furthermore, during experimentation, the values of the mean of absolute errors and standard deviation were 0.11 and 0.11 $\Omega $ , respectively. ISE was found to be 0.09 and 0.19 in simulation and experimentation, respectively. The proposed two-cycle method has an acquisition time of only 60% of the three-cycle method with no reduction in performance.

Enhanced Microcontroller Interface of Resistive Sensors Through Resistance-to-Time Converter

A novel direct resistive-sensor-to-digital readout circuit is presented, which achieves 16.1-bit ENOB while being very compact and robust. The highly digital time-based architecture employs a single voltage-controlled oscillator (VCO), counter, and digital feedback loop for the readout of an external single-ended highly nonlinear resistive sensor, such as an NTC thermistor. In addition to the inherent first-order noise shaping due to the oscillator, the second loop in SMASH configuration creates second-order noise shaping. Fabricated in 180-nm CMOS, the readout circuit achieves 16.1 bit of resolution for 1-ms conversion time and consumes only $171~\mu \text{W}$ , resulting in an excellent 2.4-pJ/c.s. FOMW for a resistive sensor interface while occupying only 0.064 mm 2. The specific closed-loop architecture tackles the VCO nonlinearity, achieving more than 14 bits of linearity. Multiple prototype chip samples have been measured in a temperature-controlled environment from −40 °C to 125 °C for the readout of commercial external NTC thermistors. A maximum temperature inaccuracy of 0.3 °C is achieved with only one-point trimming at room temperature. Since the circuit architecture decouples the sensor excitation from the feedback, high electromagnetic interference (EMI) immunity at the sensor node is demonstrated as well.

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A 16.1-bit Resolution 0.064-mm 2 Compact Highly Digital Closed-Loop Single-VCO-Based 1-1 Sturdy-MASH Resistance-to-Digital Converter With High Robustness in 180-nm CMOS

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.

/pdf/linearization-of-the-sensors-characteristics-a-review-4l71ua71m2.pdf

Linearization of the sensors characteristics: a review

Direct resistive sensor interface to a microcontroller has several advantages but has one prominent disadvantage, namely, the measurement is affected by the resistances of: 1) wires that connect the sensor to the port pins and 2) the internal resistances of the port pins of the microcontroller. A direct sensor-to-microcontroller interface scheme that compensates the effect due not only to resistances of lead wires but also the effect of microcontroller port pin’s internal resistance and any offset present in those pins is presented in this paper. Since the resistances of lead wires are compensated, automatic temperature compensation (temperature effect of lead wires) is also obtained. Simulation study and results obtained from a prototype built and tested establish the efficacy of the proposed method. A maximum error of 0.06% was observed from the prototype developed, when it was tested under room temperature, after interfacing it with the sensor Pt100, with a lead wire resistance $R_{\mathrm {LD}} = 21~\Omega $ . The error increased to a maximum of 0.08%, when the $R_{\mathrm {LD}}$ varied from 0 to $100~\Omega $ . When the same prototype was tested under elevated room temperature of 30 °C to 100 °C, the maximum error observed was 0.18%.

Improved Single-Element Resistive Sensor-to-Microcontroller Interface

An improved direct digital converter (IDDC) suitable for bridge-connected resistive sensors is presented in this paper. The input stage of a dual-slope analog-to-digital converter is altered to accommodate a resistive sensor bridge, as its integral part and the logic of conversion incorporate automatic compensation for offset, offset drift, and gain errors. Through a detailed error analysis of the DDC presented earlier, the effects of the non-ideal characteristics of practical components on the performance of the DDC are quantified. From the analysis, it is determined that mismatch between the bridge elements and the input offset of op-amps significantly affect the output. While the earlier version of the DDC requires offline correction for offset and gain errors, the proposed IDDC provides in situ correction for these and hence can also compensate drifts. Simulation studies and experimental results demonstrate the practicality of the proffered scheme and indicate that the worst case error of ±0.2% due to offset in op-amps and mismatch in nominal resistances of sensing elements reduces to ±0.05% with the use of the proposed compensation technique.

Ponnalagu Ramanathan Nagarajan

Papers

Improved Single-Element Resistive Sensor-to-Microcontroller Interface

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

An Improved Direct Digital Converter for Bridge-Connected Resistive Sensors