Calibration Resistors Research Articles

Two proposals to simplify resistive sensor readout based on Resistance-to-Time-to-Digital conversion

Direct Interface Circuits (DICs) are simple circuits used in readouts for all types of sensors. For resistive sensors, all DICs perform a resistance-to-time-to-digital conversion using just the sensor, some calibration resistors, one or two capacitors, and a Digital Processor. These circuits require a variable number of charging and discharging cycles of a capacitor to estimate the sensor resistance, Rx, increasing both acquisition time and power consumption. This paper presents two resistive DICs capable of estimating Rx by means of a single charging-discharging process, simplifying the readout process. Furthermore, this is achieved without increasing hardware requirements. Only two time measurements are used to obtain Rx. Despite the simplicity of the new circuits, the experimental results show that relative errors of estimating Rx can be below 0.8 %, and this in a wide range of resistances of over 40 dB. Moreover, acquisition time and energy consumption can be reduced by up to 75 %.

A CMOS Relaxation Oscillator with Process and Temperature Variation Compensation

In this paper, a CMOS relaxation oscillator with trimming and temperature compensation is presented for the on-chip multi-sensor systems which need MHz level frequency source. The proposed scheme uses a single current branch to charge the capacitor to generate the oscillation with voltage average feedback (VAF) circuit. Binary-weight current trimming array is adopted to reduce the frequency variation caused by the process variation under different process corners. A compensation calibration resistor array with Kelvin connection is utilized to improve the frequency variation with temperature. With the help of VAF, the frequency spread caused by the comparator delay is suppressed. This relaxation oscillator with a typical frequency of 13.4 MHz is implemented in a standard 180 nm CMOS process. Simulation results show that it achieves a frequency temperature coefficient of 28.3 ppm/∘C from [Formula: see text]C to 125∘C and a 0.074%/0.1 V frequency variation when supply voltage changes from 2.9 to 3.7 V.

A calibratable sensory neuron based on epitaxial VO2 for spike-based neuromorphic multisensory system

Neuromorphic perception systems inspired by biology have tremendous potential in efficiently processing multi-sensory signals from the physical world, but a highly efficient hardware element capable of sensing and encoding multiple physical signals is still lacking. Here, we report a spike-based neuromorphic perception system consisting of calibratable artificial sensory neurons based on epitaxial VO2, where the high crystalline quality of VO2 leads to significantly improved cycle-to-cycle uniformity. A calibration resistor is introduced to optimize device-to-device consistency, and to adapt the VO2 neuron to different sensors with varied resistance level, a scaling resistor is further incorporated, demonstrating cross-sensory neuromorphic perception component that can encode illuminance, temperature, pressure and curvature signals into spikes. These components are utilized to monitor the curvatures of fingers, thereby achieving hand gesture classification. This study addresses the fundamental cycle-to-cycle and device-to-device variation issues of sensory neurons, therefore promoting the construction of neuromorphic perception systems for e-skin and neurorobotics.

A Thermoelectric MEMS Microwave Power Sensor with Inline Self-Detection Function.

In this paper, the design, fabrication and measurement of a thermoelectric MEMS microwave power sensor with the terminal load inline self-detection function is proposed. The structure of the sensor mainly includes a coplanar waveguide, a thermopile, two terminal load resistors and two calibration resistors. In order to realize the inline self-detection function, the load and calibration resistors are designed to form a voltage divider circuit. The fabrication of this sensor is compatible with the GaAs MMIC technology. The on-chip performance is tested by using a microwave experimental platform. The measured reflection loss is less than −10 dB at 0.1–10 GHz. When the bias voltage is not applied, the sensitivity of the sensor is 47.39 μV/mW@5 GHz and 32.58 μV/mW@10 GHz, respectively, and when the bias voltage is applied, the sensitivity is 47.50 μV/mW@5 GHz and 32.73 μV/mW@10 GHz, respectively. The difference between the two cases is less than 0.5% at the same frequency, which indicates that whether or not to apply the bias voltage has little effect on the sensitivity. In addition, when the calibration resistance is increased from 50 to 100 Ω, the current flowing through the load resistance is decreased under the same bias voltage. Therefore, the DC power consumed on the load resistance will be significantly reduced. This makes the measured and theoretical results show better agreement, thus verifying the validity of the design.

Analog Frontend for Reliable Human Body Temperature Measurement for IoT Devices

In this paper an analog frontend for a reliable measurement of the human body temperature is presented. A new and novel temperature calibration technique using an on-chip resistor was developed specifically for the analog frontend. The discussed analog frontend consists of a bandgap current reference, a precision current source, a programmable gain amplifier, a voltage source proportional to absolute temperature and an on-chip calibration resistor. The developed calibration technique enables very high accuracy, even 0.1 °C, to be obtained in a very wide temperature range. This calibration method was elaborated for integrated circuits operating in temperatures from −40 °C to +125 °C. The presented analog frontend consumes no more than 185 μA and was designed and manufactured with United Microelectronics Corporation (UMC) CMOS 130 nm technology. The data presented in the paper were obtained from process corner and Monte Carlo simulations as well as from measurements. The measurements were taken using a manual wafer prober with climate-controlled microchamber, at temperatures ranging from −40 °C to +125 °C.

Wide Range Calibration Method for Direct Interface Circuits and Application to Resistive Force Sensors

A novel approach to interface resistive sensors directly to a Field Programmable Gate Array (FPGA) is presented in this paper. The circuit is based on Direct Interface Circuits (DICs), which make the sensor reading through a magnitude-to-time-to-digital conversion using just a few passive elements and a programmable digital device. Specifically, the system uses two calibration resistors of known value, two capacitors, and a new equation to estimate the sensor resistance in order to obtain an improved estimation in both the high and low parts of the resistance range. Furthermore, the new method can be used for reading resistive force sensors, with the advantage that the equations can be adapted for the output to directly provide the force exerted, thus avoiding the need for a resistance-to-force conversion. The circuit has been implemented using an FPGA as the programmable digital device to characterize the system, which allows both the reading of discrete resistors and those of a commercial force sensor. When discrete resistors are measured, estimation error in the low part of the range (a few hundred ohms) is reduced between twofold and fivefold compared to the results provided by other DICs. Moreover, the most significant errors produced by the new method when reading a force sensor come in the high part of the range (several tens of Newtons), standing at just 0.88%, compared to errors between 1.94% and 5.23% of other methods proposed in the literature.

Broadband linearity enhancement method for a 1.3 GHz–2.5 GHz digitally-assisted oscillator in a 55-nm CMOS technology

At the L/S band, the digital array radars with digital beam-forming characteristics require a compact RF module for a plane array. Such an RF module would require a small-sized local clock generation to synchronize with the central distributed clock signal. This paper proposes a 0.04 mm2, broadband local clock generation for the RF module in digital array radar. In such a multi-band digital array radar, the linearity of the clock generator is a bottleneck. A linearity enhanced algorithm for switched-capacitor digitally-assisted oscillators is proposed in this paper. In this linearity enhancement technique, a calibration resistor array is used to improve frequency-tuning characteristics. The digitally-assisted oscillator has a highly linearized frequency tuning curve without demanding much more power and silicon area. A prototype is fabricated using a 55-nm CMOS process. From measurement, it has a frequency tuning range from 1.3 GHz to 2.5 GHz while occupying 225 × 180 μm2. The measured coarse bank tuning maximum DNL is 0.39 LSB, and the maximum INL is 4.64 LSB, which has been reduced to 43% of the other state-of-art work. The maximum power consumption is 15.8 mW from a 1.2 V supply voltage.

Characterization and Modeling of Two-Port Wideband Resistance Calibration Standards

We report a single broadband on-wafer resistance calibration standard fabricated with a 15-nm-thick gold resistor on an alumina substrate. Because the resistor film is much smaller than the skin depth at microwave frequencies, this proposed standard demonstrates excellent stability over both frequency and temperature when compared to traditional calibration resistors. A variety of resistors were fabricated, demonstrating resistances of 41- 167 Ω in the frequency range of 10 MHz to 30 GHz. This resistance standard exhibits less than 2 K of heating when tested with 0 dBm of applied power and shows less than 0.2-dB difference in its resistance response across the measured frequency band. This work presents a significant step toward establishing the traceability of resistance standards over a wide frequency range.

Improved Calibration Method for Resistive Sensors Using Direct Interface Circuits

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.

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

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%.

Fast Calibration Methods for Resistive Sensor Readout Based on Direct Interface Circuits.

A simple method to measure the resistance of a sensor and convert it into digital information in a programmable digital device is by using a direct interface circuit. This type of circuit deduces the value of the resistor based on the discharge time through it for a capacitor of a known value. Moreover, the discharge times of this capacitor should be measured through one or two resistors with known values in order to ensure that the estimate is not dependent on certain parameters that change with time, temperature, or aging. This can slow down the conversion speed, especially for high resistance values. To overcome this problem, we propose a modified process in which part of the discharge, which was previously performed through the resistive sensor only, is only conducted with the smallest calibration resistor. Two variants of this operation method, which differ in the reduction of the total time necessary for evaluation and in the uncertainty of the measurements, are presented. Experiments carried out with a field programmable gate array (FPGA); using these methodologies achieved reductions in the resistance conversion time of up to 55%. These reductions may imply an increase in the uncertainty of the measurements; however, the tests carried out show that with a suitable choice of parameters, the increases in uncertainty, and therefore errors, may be negligible compared to the direct interface circuits described in the literature.

New Approaches for Increasing Accuracy in Readout of Resistive Sensor Arrays

One of the most efficient ways to connect a resistive sensor array is using columns and rows in a 2-D structure. The main problem in this implementation is crosstalk appearing due to non-idealities in the readout circuits. Crosstalk causes measurement errors which restrict the usable range in array size or resistive sensors values. This paper analyses the way in which these factors affect accuracy, setting out a new approach to eliminate or reduce their influence based on just an additional operational amplifier and some calibration resistors without a significant increase in readout time. The first method involves introducing an additional column in order to eliminate the combined effect of the output resistance buffers for row selection, offset voltages and bias current of the operational amplifier. In the second method, a row of resistors is added in order to reduce the effect of finite gains. Theoretical analysis has been performed and validated by simulation. Results show that an improvement in accuracy compared with other proposals in the literature is possible.

A 5-bit 3.2-GS/s Flash ADC With a Digital Offset Calibration Scheme

In high-speed Flash analog-to-digital converters (ADCs), preamplifiers are often placed in front of a comparator to reduce metastability errors and enhance comparison speed. The accuracy of a Flash ADC is mainly limited by the random offsets of preamplifiers and comparators. This paper presents a 5-b Flash ADC with a digital random offset calibration scheme. For calibration, programmable resistive devices are used as the loading devices of the second-stage preamplifiers. By adjusting the calibration resistors, the input-referred offset voltage of each comparator is reduced to be less than 1/2 LSB. Fabricated in a 0.13-¿m CMOS process, experimental results show that the ADC consumes 120 mW from a 1.2-V supply and occupies a 0.18- mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> active area. After calibration, the peak differential non-linearity (DNL) and integral non-linearity (INL) are 0.24 and 0.39 LSB, respectively. At 3.2-GS/s operation, the effective number of bits is 4.54 b, and the effective resolution bandwidth is 600 MHz. This ADC achieves figures of merit of 3.07 and 4.30 pJ/conversion-step at 2 and 3.2 GS/s, respectively.

Accuracy and resolution of direct resistive sensor-to-microcontroller interfaces

This paper analyses the accuracy and resolution of direct resistive sensor-to-microcontroller interfaces using theoretical and experimental methods. Three calibration techniques are evaluated: single-point, two-point and three-signal. This last method is a two-point calibration technique that needs a single calibration resistor. For each calibration formula, we analyse both the effects of the internal resistances of the microcontroller pins on the accuracy and the resolution, which is evaluated by the combined standard uncertainty of the calculated resistance. The experimental analysis was performed by measuring resistors in the range of Pt1000-type temperature sensors with two commercial microcontrollers (AVR AT90S2313 and PIC16F873). The experimental results were similar for both microcontrollers and agreed with theoretical predictions. For the AVR, the three-signal measurement method yielded a 0.01% relative systematic error and a 0.10 Ω resolution when averaging 10 calculated resistances.

Calibration of gypsum blocks and data loggers and their evaluation for monitoring soil water status

The use of commercial data loggers and gypsum blocks to monitor the matric potential of water in soil was evaluated in the laboratory and at 2 field sites. A set of calibration resistors that simulated a matric potential range of —20 to —2120 kPa was developed that showed close agreement with data that was originally used to develop algorithms for the loggers. Data loggers used in experiments were calibrated against the resistors so output values could be re-scaled to that of the resistor set. Close agreement was found between the matric potential of soil, which was estimated from filter papers, and re-scaled logged matric potential values for gypsum blocks embedded in containers of soil maintained at a range of water contents in the laboratory. Re-scaled logged soil matric potential recorded at 2 field sites also compared favorably with filter paper estimates for soil sampled adjacent to where gypsum blocks were buried. It was concluded that, provided individual data loggers were standardised to the same matric potential scale using either calibrated resistors or soil of known matric potential, the gypsum block technique provided an economical and accurate measure of soil water status.