A new, simple and high-accuracy impedance-to-digital converter (IDC) is proposed in this paper. Conventionally, the parameters of sensors that can be modeled using a parallel combination of a capacitor ( ${C}$ ) and a resistor ( ${R}$ ) are measured using ac bridges, excited from a sinusoidal source. Recently, with the widespread use of digital systems in instrumentation, capacitance-to-digital converters and resistance-to-digital converters gained a lot of importance. An IDC that accepts sensors having ${C}$ and ${R}$ in parallel and provides digital outputs directly proportional to the ${C}$ and ${R}$ values is presented in this paper. This can be used not only for sensors whose ${C}$ and ${R}$ values vary with the measurand but also when ${C}$ or ${R}$ of a sensor needs to be measured keeping the output not affected by parasitic ${R}$ or ${C}$ present in parallel with the sensing element. Another application of the IDC is in the measurement of the dissipation factor of dielectric materials. The proposed IDC is based on a dual-slope technique, and hence provides high accuracy and immunity to noise and interference. A prototype of the proposed IDC has been developed and tested in the laboratory. Accuracy of the prototype IDC developed was 0.15% for the measurement of ${C}$ and 0.04% for the measurement of ${R}$ . The total conversion time of the prototype converter developed is 3 s, and its total power dissipation is 175.8 mW. The IDC was also interfaced with a polymer-based impedance humidity sensor, measured its ${C}$ and ${R}$ values for a range of humidity, computed the humidity, and compared its performance with another instrument, showing the practicality of the proposed IDC.

A Direct Digital Readout Circuit for Impedance 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

An improved digital interface circuit for resistive sensors is proposed in this paper. The circuit offers many positive features, such as (1) use of single-reference voltage in its architecture, (2) facility to tune/preset the sensor-excitation current, even for different values of resistive sensors, (3) minimal dependence on many circuit non-ideal parameters. The working of the circuit is mathematically described in this paper, followed by its extensive error evaluation. Detailed simulation and experimental studies are performed to verify the feasibility of the interface for three different types of sensors. These studies show that the circuit can efficiently be used in temperature-monitors, magnetometers, and displacement-sensing. The maximum experimental nonlinearity observed from all of these sensor configurations does not exceed 0.06 %. Then, the extensive studies are carried out by simulation as well as experimentation to show the effect of environmental temperature changes on the proposed circuit. Finally, the proposed circuit is compared with some of the recent resistive digital interfaces.

Performance Verification of a Digital Interface Suitable for a Broad Class of Resistive Sensors

This paper describes a high-speed delta-sigma modulator with 65-nm CMOS technology for ultrasound imaging systems. The delta-sigma modulator is based on a 4th-order single-loop switched-capacitor architecture with a 4-bit quantizer. The designed modulator has the advantages associated with input-feedforward architecture, such as the reduced output swing of the integrator, which relaxes the amplifiers' design requirements. Due to the power and area overheads and the timing constraint of the active adder in the conventional multibit input-feedforward modulator, we use an adder-less input-feedforward delta-sigma architecture. As a result, the designed architecture eliminates the extra power consumption and silicon area required by the adder. The designed architecture also relaxes the timing requirement for the quantizer and the dynamic element-matching block compared with the conventional delta-sigma modulator. The modulator achieves a dynamic range of 76dB and a peak signal-to-noise-plus-distortion ratio of 72.3 dB in a signal bandwidth of 6 MHz. The power consumption is 18.5 mW with 1.2-V supply voltage, and the chip core size is 0.25 mm2. The energy required per conversion step is 0.46 pJ/conv.

An Input-Feedforward Multibit Adder-Less $\Delta{-}\Sigma$ Modulator for Ultrasound Imaging Systems

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.

Simple and Efficient Relaxation-Oscillator-Based Digital Techniques for Resistive Sensors — Design and Performance Evaluation

A direct resistance-to-digital converter (RDC) that is suitable for differential resistive sensors is proposed and analyzed in this paper. The RDC presented here provides a digital output that is linearly proportional to the parameter being sensed by a differential resistive sensor possessing either linear or inverse characteristics. The RDC employs the sigma-delta analog-to-digital conversion (Sigma-Delta ADC) principle and, hence, possesses all the advantages and limitations of such an ADC. Analysis shows that the proposed RDC has negligible sensitivity to variations in circuit parameters. Experimental results on a prototype built and tested gave a worst-case error < 0.15%, establishing the efficacy of the proffered RDC.

V.J. Kumar

Papers

Analysis of a Sigma–Delta Resistance-to-Digital Converter for Differential Resistive Sensors