A Resistance-to-Digital Converter possessing exceptional insensitivity to circuit parameters

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

Abstract: 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.

Improved Calibration Method for Resistive Sensors Using Direct Interface Circuits

Abstract: Traditional calibration methods for measuring resistance values using a direct interface circuit, comprising a capacitor and some calibration resistors, offer a straightforward, economical option. However, such methods show greater inaccuracies as resistance values decrease. This article presents a modification of the method that ensures more accurate measurements and extends the range of resistance values that can be measured. Unlike the linear equation used in the traditional method, the proposed method obtains the power functions related to resistance value and measured discharge time. Since these circuits need calibration resistors, criteria are established to find their values. To reduce arithmetical complexity, two approximations are tested to calculate the exact value of the resistance. With this approach, the relative errors for low resistances are reduced and the measuring range is extended. Three measurement setups were carried out with a microcontroller and a field-programmable gate array (FPGA) to validate the results, showing the validity of the method independently of the device. Using the FPGA configured so that the maximum current that can be sink per output pin is 24 mA, the error in the estimation of a resistance of $9.9~\Omega $ is ten times less than that obtained using the classic two-point calibration method.

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

Abstract: 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.

Oscillator-Less Direct-Digital Front-End Realizing Ratiometric Linearization Schemes for TMR-Based Angle Sensor

Abstract: A simple oscillator-less direct-digital front end (ODF) for tunneling magnetoresistance (TMR) angle sensor is presented in this paper. The TMR sensor outputs vary as sine and cosine functions of the shaft angle to be measured. To these outputs, the ODF applies ratiometric-based techniques to obtain a linear output for the full-circle range. The first ratiometric linearization scheme (RLS-A) provides a linear output which is insensitive to the variation of sensor transformation constant. Alternatively, the second ratiometric linearization scheme (RLS-B) renders output with minimal nonlinearity (0.009%). The ODF does not require any sinusoidal oscillator for its operation, thereby eradicating one major error source. Furthermore, the proposed methodology uses dc excitation for the sensor. This makes the ODF insensitive to the sensor parasitic capacitances. The operation of the ODF and two linearizing schemes are explained in detail. Mathematical evaluation of the schemes is conducted to determine the effects of different sensor and circuit nonidealities. The static and dynamic performances of the ODF are studied using emulated TMR sensor outputs. The results establish a linear transfer-characteristic over 360° range. Tests are also conducted to quantify the effect of nonidealities like phase imbalance in sensor outputs and deviation in sensor transformation-constant. A close degree of agreement was observed between the emulation results and the mathematical analysis performed. Later, the ODF is interfaced and tested with a prototype TMR-based shaft-angle measurement unit. The interfacing results underline the fact that the developed ODF acts as an efficient linearizer for TMR angle sensors.

Quasi Single Point Calibration Method for High-Speed Measurements of Resistive Sensors.

Abstract: Direct interface circuits are a simple, inexpensive alternative for the digital conversion of a sensor reading, and in some of these circuits only passive calibration elements are required in order to carry out this conversion. In the case of resistive sensors, the most accurate methods of calibration, namely two-point calibration method (TPCM) and fast calibration methods I and II (FCMs I and II), require two calibration resistors to estimate the value of a sensor. However, although FCMs I and II considerably reduce the time necessary to estimate the value of the sensor, this may still be excessive in certain applications, such as when making repetitive readings of a sensor or readings of a large series of sensors. For these situations, this paper proposes a series of calibration methods that decrease the mean estimation time. Some of the proposed methods (quasi single-point calibration methods) are based on the TPCM, while others (fast quasi single-point calibration methods) make the most of the advantages of FCM. In general, the proposed methods significantly reduce estimation times in exchange for a small increase in errors. To validate the proposal, a circuit with a Xilinx XC3S50AN-4TQG144C FPGA has been designed and resistors in the range (267.56 Ω, 7464.5 Ω) have been measured. For 20 repetitive measurements, the proposed methods achieve time reductions of up to 61% with a relative error increase of only 0.1%.

A low-cost interface to high-value resistive sensors varying over a wide range

Abstract: This paper presents a low-cost interface for high-value resistive sensors varying over a wide range, from k/spl Omega/ to G/spl Omega/. The proposed circuit that acts as a "resistance to period converter" is suitable to be interfaced to a microcontroller or a counting device. The behavior of real electronic components that produce estimation errors, is taken into account. Experimental results obtained by the realized prototype show a good resolution and repeatability.

Interfacing Differential Capacitive Sensors to Microcontrollers: A Direct Approach

Abstract: This paper is a continuation of a previous work with regard to the direct connection of differential sensors to microcontrollers without using intermediate electronics between them. This paper focuses on the measurement of differential capacitive sensors, whereas the previous work dealt with the resistive counterparts. The proposed circuit is analyzed, and the main limitation seems to be the fact that the magnitude of the input parasitic capacitances of the microcontroller is similar to or even higher than the sensor capacitances. Methods to overcome this limitation are proposed, particularly when measuring low-value differential capacitive sensors such as microelectromechanical system (MEMS) sensors. Experimental tests of the circuit have been carried out by measuring a commercial capacitive accelerometer working as a tilt sensor. Although such a sensor has a low value (1.5 pF) and low sensitivity (0.105 pF/g), the measurement has shown a nonlinearity error of 1% full-scale span (FSS), which is a remarkable value considering the simplicity of the circuit.

A Linear Resistance-To-Time Converter with High

Abstract: A resistance-to-time converter employing a bridge amplifier, an integrator and a comparator is described. While possessing a resolution and linearity of the same order as that of a recently reported resistance-to-frequency converter, the present circuit has the advantage of grounded detecting resistance. Further, no compensation arrangement need be incorporated for maintaining wide-range linearity.

A linear resistance-to-time converter with high resolution

Abstract: A resistance-to-time converter employing a bridge amplifier, an integrator and a comparator is described. While possessing a resolution and linearity of the same order as that of a recently reported resistance-to-frequency converter, the present circuit has the advantage of grounded detecting resistance. Further, no compensation arrangement need be incorporated for maintaining wide-range linearity.

A Novel Dual-Slope Resistance-to-Digital Converter

Abstract: A dual-slope resistance-to-digital (DSRDC) converter that accepts the resistance of a single-element resistive-type sensor as input and provides a direct digital output proportional to the parameter being sensed by the resistive-type sensor is presented in this paper. A high level of linearity and accuracy is achieved since the output of the DSRDC is dictated only by the magnitudes of a pair of dc reference voltages and the transformation constant of the sensor. Sensitivity analysis shows that the effect of circuit parameter variations on the output is minimal. Simulation studies and test results obtained on a prototype establish the efficacy of the proposed scheme.