Curvature Compensation Research Articles

Sampling moiré method: a tool for sensing quadratic phase distortion and its correction for accurate quantitative phase microscopy.

The advantages of quantitative phase microscopy (QPM) such as label-free imaging with high spatial sensitivity, live cell compatibility and high-speed imaging makes it viable for various biological applications. The measurement accuracy of QPM strongly relies on the shape of the recorded interferograms, whether straight or curved fringes are recorded during the data acquisition. Moreover, for a single shot phase recovery high fringe density is required. The wavefront curvature for the high-density fringes over the entire field of view is difficult to be discerned with the naked eye. As a consequence, there is a quadratic phase aberration in the recovered phase images due to curvature mismatch. In the present work, we have implemented sampling moiré method for real-time sensing of the wavefront curvature mismatch between the object and the reference wavefronts and further for its correction. By zooming out the interferogram, moiré fringes are generated which helps to easily identify the curvature of the fringes. The wavefront curvature mismatch correction accuracy of the method is tested with the help of low temporal coherent light source such as a white light (temporal coherence ∼ 1.6 µm). The proposed scheme is successfully demonstrated to remove the quadratic phase aberration caused due to wavefront mismatch from an USAF resolution target and the biological tissue samples. The phase recovery accuracy of the current scheme is further compared with and found to better than the standard method called principle component analysis. The proposed method enables recording of the corrected wavefront interferogram without needing any additional optical components or modification and also does not need any post-processing correction algorithms. The proposed method of curvature compensation paves the path for a high-throughput and accurate quantitative phase imaging.

Design of a Low Temperature Drift Bandgap Reference Circuit with Second-order Curvature Compensation

In order to improve the accuracy and stability of bandgap reference output voltage, a bandgap reference circuit with high power supply rejection ratio and low temperature drift is designed based on 0.18um BCD technology. Based on the traditional bandgap voltage reference structure, this paper designs a current mirror cascade and a prereference circuit to supply power to the bandgap reference, and achieves a high power rejection ratio. At the same time, a PTAT2 current generation circuit is designed to realize the second-order temperature compensation of the reference output voltage. The simulation results show that the power supply voltage is 5V, PSRR is - 80dB, the temperature changes from - 55°C to 125°C, and the reference voltage temperature coefficient is 4.27ppm /°C. The circuit is simple in structure and easy to integrate. It can be widely used in bandgap reference circuit design.

Reflection loss field visualization of curved RAS based on scanning free-space measurement and curvature compensation using perfect electric conductor

Electromagnetic properties of radar absorbing structures (RASs) are highly affected by their curvatures. Therefore, novel full-field curvature compensation methods in the reflection loss evaluation of curved RASs are proposed. These methods are commonly based on the scanning free-space measurement (SFM), whose curvature compensation method is based on the measurement of the actual perfect electric conductor (PEC) or the PEC reflection loss function-based prediction, which allows the analysis of the original reflection loss of a curved RAS. In the first method, an actual PEC specimen with the same shape as the RAS to be evaluated is used, and the full-field reflection loss of the PEC is obtained by the SFM. As a measure, when the PEC specimen is not available, the second method using the PEC reflection loss function-based prediction is also proposed to obtain the full-field reflection loss of the PEC. This method can predict the reflection loss map of the PEC quickly by defining the PEC reflection loss function according to the curvature and incident angle using the data from measurement. To determine the PEC reflection loss function, it is necessary to secure the reflection loss maps of the PEC specimens with constant curvatures as database, which is also prepared easily by the SFM. Therefore, once they are measured, it can be applied to RASs with various curvatures based on simple compensation. The database for the PEC reflection loss maps with various constant curvatures is ready in the SFM system, and this curvature compensation method can be performed in real time during the scanning. The two methods are validated by comparing each other in double-curvature and airfoil-shaped RASs.

Phase curvature compensation in digital holographic microscopy based on phase gradient fitting and optimization.

We propose a numerical method for phase curvature compensation in digital holographic microscopy, in which the phase curvature is compensated for by subtracting a numerical phase mask from the distorted phase. The parameters of the phase mask are obtained based on phase gradient fitting and optimization, in which the initial mask parameters are obtained by fitting the phase gradient, and then more accurate mask parameters are determined using a spectrum energy search. The compensation can be executed in a hologram without extra devices or any prior knowledge of the setup and specimen. A computer simulation and experimental results demonstrated the feasibility of the proposed method.

Wavefront curvature compensation in PFA for spotlight bistatic SAR with arbitrary platforms paths

Polar format algorithm (PFA) as is an efficient algorithm can be well applied to spotlight bistatic synthetic aperture radar (SAR). However, a planar wavefront assumption which leads to image distortion is made. In this study, a new method to compensate for the wavefront curvature of PFA image is presented and it can well correct the defocusing effect and geometry distortion even under arbitrary platforms paths. By projecting the coefficients of Taylor expansion from the azimuth-time domain to wavenumber domain, the new method does not need repeated interpolations to construct the compensation filters for all the errors, which can decrease computational expense in a way. Finally, the effectiveness of this method is verified by numerical simulation.

A 1-V Bandgap Reference in 7-nm FinFET With a Programmable Temperature Coefficient and Inaccuracy of ±0.2% From −45°C to 125°C

We present a 1-V precision voltage reference with a programmable temperature coefficient (“ temp-co ”). The voltage reference is based on a parasitic bipolar junction transistor (BJT) realized within the process steps available in the chosen 7-nm FinFET technology. Details of its characterization and SPICE modeling are presented to provide insight into key device–circuit optimization choices. The trimming techniques needed to cope with process spread together with two curvature compensation techniques are presented. The reference achieves a maximum inaccuracy of ±0.2% and a minimum temp-co of 6 ppm/°C from −45°C to 125°C. Furthermore, its temp-co is digitally programmable between −7 mV/100°C and +8 mV/100°C. Its line regulation is 0.1%/V, and it occupies 0.078 mm2.

An Improved Multi-Beam Trend Surface Imaging Method Based on Surface Curvature Compensation

An Improved Multi-Beam Trend Surface Imaging Method Based on Surface Curvature Compensation

A 0.5-V-supply, 37.8-nW, 17.6-ppm/°C switched-capacitor bandgap reference with second-order curvature compensation

This paper presents a switched-capacitor (SC) bandgap reference (BGR) with second-order curvature compensation for ultra-low power systems. The gate-source voltage of a sub-threshold MOSFET is proposed to implement the second-order curvature compensation voltage in this work. And then, a SC network is used, not only to add the second-order voltage and the first-order one together, but also to fulfill the summing weight effectively and precisely. In this way, the silicon area and power consumption can be reduced simultaneously. Furthermore, the design methodology of summing weight is investigated, and thus the temperature coefficient (TC) can be significantly reduced. The proposed SC-BGR was implemented and simulated in a CMOS 0.18 μm process, the average output voltage is 426.1 mV, achieving a TC of 17.6 ppm/°C from −20 to 100 °C under a 0.5 V minimum supply voltage. With the help of the SC architecture, a small chip area of 0.0975 mm2 is achieved with only 37.8-nW power consumption.

Increasing of Spectral Resolution of Multislit Imaging Spectrometer with Diffractive Grating

Registration of non-stationary processes, namely snapshot hyperspectral imaging that allows to capture data cube I ( x , y ,λ) in one measurement act, is of interest for imaging spectroscopy. The purpose of the work is increasing of spectral resolution of imaging spectrometers with spatial filtering of object image using multislit mask, where a diffractive grating is used as disperser (providing almost constant dispersion in working spectral range), and the data cube is projected on a detector as a set of local spectra from object fragments. An image is formed on detector by a two-objective telecentric system composed from two lenses focused on infinity so that their front focuses are matched. A diaphragm in the match point allows passing only for beams of needed diffraction order, so along with a bandpass filter near the system entrance they solve a typical problem of diffractive systems – elimination of beams of all orders but a needed one. The approach is implemented in two proposed designs of spectrometers: in the first a telecentric system is based on two multi-lens imaging objectives, in the second – is based on two reflective off-axis parabolic objectives. In this paper we proposed variants for optical design optimization: normalization of beam incidence on a mask and field curvature compensation; they allow to increase system resolution and to extend application area of multislit dispersive spectrometers; also a design being a synthesis of these two approaches is analyzed. According to simulation results, width on half-maximum in dispersion direction Δ l ≤ 10 µm, only for limited field points set Δ l ≤ 15 µm, that stands for spectral resolutio

Second-order compensation BGR with low TC and high performance for space applications

This paper presents a new Bandgap Voltage Reference (BGR) design, with second order curvature compensation, for space applications. The work focuses on the design of the different analog building blocks which integrate the BGR. The proposed design, also, includes the complete implementation of trimming networks, which is a post-fabrication technique that strongly improves the performance in all the special conditions that characterize the target environment. The circuit was designed in a 150 nm Silicon-on-Insulator CMOS technology node, using radiation-hardened components. Simulation results with typical models show a performance of 0.758 ppm/°C for temperature coefficient over a military temperature range, with a 1.25 V output voltage, which competes with the actual state-of-the-art solutions.

Wavelength-multiplexing surface plasmon holographic microscopy.

Surface plasmon holographic microscopy (SPHM), which combines surface plasmon microscopy with digital holographic microscopy, can be applied for amplitude- and phase-contrast surface plasmon resonance (SPR) imaging. In this paper, we propose an improved SPHM with the wavelength multiplexing technique based on two laser sources and a common-path hologram recording configuration. Through recording and reconstructing the SPR images at two wavelengths simultaneously employing the improved SPHM, tiny variation of dielectric refractive index in near field is quantitatively monitored with an extended measurement range while maintaining the high sensitivity. Moreover, imaging onion tissues is performed to demonstrate that the detection sensitivities of two wavelengths can compensate for each other in SPR imaging. The proposed wavelength-multiplexing SPHM presents simple structure, high temporal stability and inherent capability of phase curvature compensation, as well as shows great potentials for further applications in monitoring diverse dynamic processes related with refractive index variations and imaging biological tissues with low-contrast refractive index distributions in the near field.

A Sub-1ppm/°C Current-Mode CMOS Bandgap Reference With Piecewise Curvature Compensation

This paper presents a low temperature coefficient (TC) CMOS BGR for high-performance multi-channel analog-to-digital converter (ADC) working under wide temperature range. Besides the logarithmic compensation, both leakage and piecewise curvature compensation are implemented to extend its operating temperature range and keep its low TC. A $\beta $ -compensation technique is used to cancel the PTAT and non-PTAT spread of the output due to variation of limited $\beta $ in the Bipolar Junction Transistors. Moreover, the error-correcting resistors are implemented to eliminate the first-order inaccuracy of the output. The BGR designed in $0.18~\mu \text{m}$ 1.8-/5-V CMOS process has a minimum simulated TC of 0.42ppm/°C over a wide temperature of −60°C to 150 °C, making it appropriate to provide reference voltage for a high-precision ADC for sensor interface.

A radiation-hard curvature compensated bandgap voltage reference

In this work a radiation-hardened bandgap voltage reference circuit is presented. The circuit is targeted at precision applications, where trimming can be used to achieve a temperature coefficient smaller than $5~\mbox{ppm}/\mbox{K}$ . Curvature compensation is employed and trimming of the temperature coefficient and the curvature is possible. In order to achieve good performance several techniques were combined. Radiation hardening techniques on layout level were used along with design techniques to improve the robustness against total ionizing dose (TID) and process variations. The radiation hardness requirements were set after preliminary irradiation tests. At the layout level optimized transistors were used while at the topology level, a radiation-hardened trimming scheme was employed to mitigate the impact of leakage currents. Chopping techniques were required to ensure good performance over the process and temperature variations. The bandgap was realized in a standard 180 nm CMOS process and circuit performance was verified using extensive simulations.

A 1.2 V, 3.0 ppm/°C, 3.6 μA CMOS bandgap reference with novel 3-order curvature compensation

This paper presents a bandgap reference with a high-order curvature compensation circuit which can improve the temperature coefficient (TC) in a wide temperature range. The proposed compensation circuit includes a second-order and a third-order curvature current generators as well as an I-V converter. These two curvature currents are achieved by utilizing the exponential behavior of sub-threshold MOSFET and used for compensating the high-order temperature dependence of BJT base-emitter voltage via I-V converter. The proposed BGR is implemented in a CMOS 0.18 μm process with the active area of 0.056 mm2. Measurements on ten samples showed that at the minimum supply voltage 1.2 V, the TC varies from 1.7 to 6.9 ppm/°C over a temperature range of 170 °C (−45 °C–125 °C) with an average value of 3.0 ppm/°C and the total current consumption of 3.6 μA at room temperature. In the supply voltage range of 1.2–1.8 V, the line regulation (LR) is 0.025%/V.

A 0.55-V, 28-ppm/°C, 83-nW CMOS Sub-BGR With UltraLow Power Curvature Compensation

This paper proposes an ultralow power, high precision sub bandgap voltage reference (sub-BGR) for low-voltage self-powered devices. A novel ultralow power curvature compensation circuit is proposed to improve the temperature coefficient over a wide temperature range. A switch capacitor voltage divider with improved leakage current reduction switches is used to obtain a high accuracy and a low power. To minimize the clock feedthrough and charge injection in the switches, a clock scaling down circuit is proposed, that effectively improves the line sensitivity (LS) of the sub-BGR. The proposed sub-BGR is implemented in a 0.18- $\mu \text{m}$ standard CMOS process with a total area of 0.061 mm2. After measuring 30 chips, the average power consumption is 83 nW at 0.55 V of supply at 27 °C. In the supply voltage range of 0.55 to 1 V, the LS is 0.059%/V, and the error is ±0.75% ( $3\sigma $ ) after trimming.

A Low Power Bandgap Voltage Reference with Nonlinear Voltage Curvature Compensation

The reference voltage source is the main part both in the analog and mixed-signal integrated circuits. A good reference voltage source can provide a stable voltage for the circuit, so that the system can work stable accurately. This paper describes a low power bandgap voltage reference with second order compensated, which utilizes a Kujik bandgap reference as the first order curvature correction and adding second order non-linear voltage compensation. The presented circuit is implemented in a 0.18μm CMOS technology, and the voltage supply (VDD) is 1.8V. Piecewise curvature compensation is employed to reduce the temperature coefficient of the bandgap reference from 465ppm/°C to 5.5ppm/°C, with a temperature range –45~85°C. The circuit works under the condition of 27 °C, the output of the reference voltage is 1.207 V. The total current consumption of the whole bandgap is 11.69μA, the total area of the layout is 0.10132mm2 and the power supply rejection ratio is -63dB.

Common-path digital holographic microscopy for near-field phase imaging based on surface plasmon resonance.

We develop a common-path digital holographic microscopy based on prism-coupling surface plasmon resonance (SPR) for near-field phase imaging. A single beam splitter with specific configuration is introduced in an SPR imaging system to realize off-axis holographic recording. By measuring the phase shift difference of the reflected light at SPR exploiting the proposed holographic microscopy with high temporal stability, near-field characteristic measurement can be realized. With its simplicity, vibration isolation, and inherent capability of phase curvature compensation, the recommended system shows advanced performance in monitoring tiny refractive index variations and imaging biological tissues.

A sub-1V 78-nA bandgap reference with curvature compensation

This paper presents a low power bandgap reference (BGR) with 573mV average output voltage and 0.95-V minimum supply voltage. Here, the resistor ratio of two types of resistors that have opposite temperature coefficient (TC) realizes a second-order curvature compensation. In addition, to reduce the supply voltage we adopt a voltage divider, and we also employ proportional to absolute temperature (PTAT) voltage generators to reduce the resistor values for low power applications. The proposed BGR fabricated in 65-nm CMOS achieves an average TC of 57ppm/°C from 0 to 100°C with 8 samples. The power dissipation at room temperature is 78nA and the power supply ripple rejection (PSRR) is −67dB at DC with 1.2V supply.

Efficient Terahertz Wide-Angle NUFFT-Based Inverse Synthetic Aperture Imaging Considering Spherical Wavefront.

An efficient wide-angle inverse synthetic aperture imaging method considering the spherical wavefront effects and suitable for the terahertz band is presented. Firstly, the echo signal model under spherical wave assumption is established, and the detailed wavefront curvature compensation method accelerated by 1D fast Fourier transform (FFT) is discussed. Then, to speed up the reconstruction procedure, the fast Gaussian gridding (FGG)-based nonuniform FFT (NUFFT) is employed to focus the image. Finally, proof-of-principle experiments are carried out and the results are compared with the ones obtained by the convolution back-projection (CBP) algorithm. The results demonstrate the effectiveness and the efficiency of the presented method. This imaging method can be directly used in the field of nondestructive detection and can also be used to provide a solution for the calculation of the far-field RCSs (Radar Cross Section) of targets in the terahertz regime.

A 0.7 V, 2.7 μW, 12.9 ppm/° C over 180° C CMOS subthreshold voltage reference

SummaryAn all‐CMOS, low‐power, wide‐temperature‐range, curvature‐compensated voltage reference is presented. The proposed topology achieves a measured temperature coefficient of 12.9 ppm/°C for a wide temperature range of 180°C ( − 60 to 120°C) at a bias voltage of 0.7 V while consuming a mere 2.7 μW. The high‐order curvature compensation, which leads to a low‐temperature sensitivity of the reference voltage, is performed using a new, simple, but efficient methodology. The non‐linearities of an N‐type metal‐oxide‐semiconductor (NMOS) device operated in subthreshold are combined with the non‐linearities of two different kinds of polysilicon resistors, leading to the improved performance. The extended temperature range of this voltage reference gives it an important competitive advantage, especially at lower temperatures, where prior art designs' performance deteriorate abruptly. In addition, it utilizes an innovative trimming methodology whereby two trimmable resistors enable the tuning of both the overall slope and non‐linearities of the temperature sensitivity. The design was fabricated using TowerJazz Semiconductor's CMOS 0.18 μm technology, without using diodes or any external components such as compensating capacitors. It has an area of 0.023 mm2 and is suitable for high‐performance power‐aware applications as well as applications operating in extreme temperatures. Copyright © 2016 John Wiley & Sons, Ltd.