Digital cancellation of noise and offset for capacitive sensors

A digital noise and offset cancellation technique for use with charge-redistribution capacitance sense techniques is presented. It is insensitive to parasitic capacitances, and can cancel the effects of offsets, low-frequency noise sources, and sampled kT/C noise of the MOS switch used in the topology. It represents a significant improvement in capacitance resolution than previous methods. It is currently being used in the readout circuits of experimental pressure sensor chips containing 100 fF air-gap capacitors with a resolution in the 30 aF range at a sampling speed of 11 kHz     

New Fused-Silica-Dielectric 10-and 100-pF Capacitors and a System for Their Measurement

New instruments have been designed to bring to standards laboratories the improved accuracy of capacitor calibrations developed by the National Bureau of Standards. The new 10-and 100-pF reference standard capacitors, based upon an NBS design, use a fused-silica dielectric with gold electrodes to provide the time and voltage stability required for calibrations to parts in 107. To maintain the capacitor temperature constant to within 0.01 °C, one model of the capacitors is designed for use in an oil bath, the other model is fitted with a new, stable, thermostatically controlled air bath. The system to measure these capacitors with a precision of parts in 108 consists of a bridge, detector, and oscillator. For these special measurements and for the general calibration of a wide range of capacitors, the new transformer-ratio-arm bridge has 12 capacitance decades and a range of 10 MAF-10-7 pF, and five conductance decades and a range of 103110-10 tmho. The new phasesensitive detector and power oscillator provide high sensitivity to bridge imbalance over the frequency range 10 Hz-100 kHz.     

Dielectric properties of anodized Al-Ta alloy films with high Al content

At 1 mA cm^-2 the anodization voltage of sputtered Al-Ta alloy films with about 7 at.% Ta content rises linearly with time at a rate of 0.45 V s^-1. The Al-Ta thickness-anodization voltage ratio and the oxide growth constant have been determined, for 7–14 at.% Ta in the films with 1% citric acid electrolyte, to be 1.03-0.94 nm V^-1 and 1.42–1.5 nm V^-1, respectively. The permittivity was found to range from 10.0–11.4, corresponding to a capacitance density of about 330 pF mm^-2 for an anodization voltage of 200 V. The temperature coefficient of capacitance (t.c.c.) was 500 ppm K^-1. The capacitance between 400 Hz and 1 MHz remained constant within ±1%. At 1 kHz the disspation factor tan δ was 0.6%.Nearly all leakage currents were between 0.1 and 0.5 nA using 5.6 nF test capacitors and 50 V as the test voltage. This corresponds to an insulation resistance R of 10¹¹ Ω or more, or a time constant τ ( = RC) of approximately 1000 s. For the 50 V test the yield was about 90% and at least 80% of the capacitors had no short circuits after step stress tests up to 90 V.The current-voltage characteristics of capacitors made from Al-Ta (7 at.% Ta) films showed non-destructive breakdown voltages greater than 100 V. Only a slight deviation from symmetry was noticed whether the Al-Ta electrode was positive or negative, indicating that the capacitors were virtually non-polar. In some cases non-shorting breakdown was observed.     

A multi-nano-dot circuit and structure using thermal-noise-assisted tunneling for stochastic associative processing

The single-electron circuit and nanostructure described in this paper are designed for stochastic associative processing, which is an expanded version of ordinary associative memory processing. In stochastic associative processing, the association probability of each stored pattern depends on the similarity between the stored pattern and the input pattern. Such unique processing is useful for sequential stochastic association and for clustering for vector quantization. Conventional single-electron circuits operate only at very low temperature for practical junction capacitance (i.e., 30 K for 0.1 aF) because the charging energy in these circuits is directly related to the tunnel junction capacitance. Our multi-nano-dot circuit and structure operate at room temperature with a junction capacitance around 0.1 aF through tunneling processes assisted by thermal noise. We analyze the operation of this circuit in detail and propose for it a stochastic associative processing operation, where the detection timing of the electron position controls the association probability distribution.     

Application of electrolytic capacitors in measuring circuits

Conclusions Mass-produced electrolytic, and above all tantalum capacitors approach paper capacitors in many of their characteristics, and are superior to them with respect to their specific charges. In the range of –5 to +50C the electrolytic capacitors' values vary almost linearly with temperature. Moreover, capacitors designed for higher working voltages have a minimum instability. Aluminum capacitors type éM have the maximum instability, amounting approximately to 5–10% per 10C. Capacitors in hermetically-sealed casings types KéG and éGTs, groups OM and PM are 2–3 times more stable than the éM capacitors. Tantalum electrolytic capacitors have a stability 5–10 or more times greater than that of the éM capacitors.The storage of éM capacitors at room temperature for 10,000 h changes their values by 6% towards the end of that time. Aluminum capacitors in hermetically-sealed casings after 2000–4000 h of preliminary storage provide a more stable capacitance, and tantalum capacitors then obtain virtually constant values.It is possible to arrive at the conclusion from the measurement results that electrolytic capacitors, and above all tantalum capacitors, can be used successfully, in many instances, in various measuring circuits. Preliminarytesting and selection of capacitors, as well as the application of thermal compensating elements in measuring circuits with electrolytic capacitors will provide a higher precision of measurements.     

Determination of an Absolute Capacitance by a Horizontal Cross Capacitor

A cross capacitor is a calculable one whose capacitance is ideally determinable from one length measurement of its electrodes. In the Electrotechnical Laboratory of Japan (ETL), a horizontal cross capacitor was constructed. In this model, ease of checking alignment of the horizontal electrode bars, and a simple driving mechanism to move the guard electrode, were emphasized. Because the movable guard electrode rests on, and is guided by, the horizontal electrode bars no special supporting mechanism is necessary. The displacement, which defines the length of the capacitor, is observed by interference fringes using a Hg198 source. The sensitivity obtained for electrical measurement is better than 0.01 aF and for length measurement better than 1/30 fringe. The errors of this capacitor are examined in detail and confirmed by experimental tests. The absolute capacitance was determined as 0.1 pF with an accuracy better than 1 ppm.     

Ion-implanted capacitively coupled silicon strip detectors with integrated polysilicon bias resistors processed on a 100 mm wafer

Double-sided silicon strip detectors with integrated coupling capacitors and polysilicon resistors have been processed on a 100 mm wafer. A detector with an active area of 19 × 19 mm² was connected to LSI readout electronics and tested. The strip pitch of the detector is 25 μm on the p-side and 50 μm on the n-side. The readout pitch is 50 μm on both sides. The number of readout strips is 774 and the total number of strips is 1161. On the p-side a signal-to-noise of 35 has been measured using a ⁹⁰Sr β-source. The n-side has been studied using a laser.     

Microelectronic capacitance transducer for particle detection

This paper describes the CMOS circuit design of a sensor for detecting changes of capacitance due, for instance, to the incidence of particles or bubbles on the electrodes. The circuit is based on a simple design originating at the University of California, Berkeley, for measuring crosstalk on integrated circuits. The basic front-end sensor circuit comprises eight MOSFETs and has a sensitivity of 40 mV/fF. A differential amplifier receives the outputs from two sensor circuits each having 20-/spl mu/m square inter-digitated electrodes. The resulting sensitivity of the fabricated sensor is 1 V/fF with a noise level equivalent to 10 aF. Monte Carlo circuit simulations have been used to identify transistor dimensions to yield acceptable yield, and prototype custom silicon chips have been fabricated using a 0.8-/spl mu/m CMOS process. Static and dynamic tests, using polyamide particles as small as 10-/spl mu/m diameter, verify correct operation of the sensors. The sensor is now being developed for application in miniature electrical tomography systems.     

Capacitors with very low loss: cryogenic vacuum-gap capacitors

We report on measurements of capacitors with about 1 pF of capacitance, which have unmeasurably small leakage at very low frequencies, placing a lower bound of about 10¹⁹Ω on the parallel resistance at an effective frequency of 1 mHz. These measurements are made possible by two themes: the use of vacuum-gap capacitors (i.e., no dielectric material, operated in vacuum), and detection of leakage using single electron tunneling (SET) electrometers, which have very high input impedance. We also report on good achieved results in time stability and lack of frequency and voltage dependence     

Modified vacuum capacitor

The paper presents a brief account of the behaviour of a new type of vacuum capacitor, in which the conventional plate is replaced by a conducting film. It consists of two aluminized concentric glass tubes, outer one internally and inner one externally sealed off at both ends. Construction details, rating characteristics, effect of humidity and temperature changes are discussed. The fabricated experimental capacitors of value 50 pF were found to withstand 1 kV, above which vacuum breaks down as predicted by clump theory. The advantages and disadvantages of this capacitor over conventional vacuum capacitors are discussed.     

Noise optimised charge-sensitive CMOS amplifier for capacitive radiation detectors

A low-voltage, low-noise, charge-sensitive preamplifier (CSA) for particle tracking using a silicon strip detector was designed. The preamplifier was optimised in terms of the total output noise performance using a noise minimisation technique based on the MOSFET noise small signal equivalent circuit and readout front-end noise optimisation criteria valid in the strong inversion region. The preamplifier was designed and fabricated in a 0.35 m CMOS process by Austria Mikro Systeme for a specific silicon strip detector of 2 pF capacitance for X-ray spectroscopy. The circuit exhibits satisfactory performance compatible to the specific low-energy radiation detection application. Particularly, the CSA provides an equivalent noise charge of 254 e⁺13.5+e^-/pF, consumes 165 muW and achieves an output conversion gain equal to 2.81 mV/fC and a linearity <0.57 . Analysis is supported by extensive measurement results confirming the circuit characteristics and their flexibility to be used in a variety of readout applications.     

Capacitively-Coupled SQUID Bias for Time Division Multiplexing

The multiplexing scheme presented in this paper is part of the readout chain of the QUBIC instrument devoted to cosmic microwave background polarization observations. It is based on time domain multiplexing using superconducting quantum interference devices (SQUIDs) to read out a large array of superconducting bolometers. The originality of the multiplexer presented here lies in the use of capacitors for the SQUID addressing. Capacitive coupling allows us to bias many SQUIDs in parallel (in a 2D topology), with low crosstalk and low power dissipation of the cryogenic front-end readout. However, capacitors in series with the SQUID require a modification of the addressing strategy. This paper presents a bias reversal technique adopted to sequentially address the SQUIDs through capacitors using a cryogenic SiGe integrated circuit. We further present the different limitations of this technique and how to choose the proper capacitance for a given multiplexing frequency and current source compliance.     

Automatic Accurate Display of Capacitance-and Conductance-versus-Bias Characteristics at High Frequencies for Semiconductor Evaluation

An instrument is presented that permits the automatic and rapid measurement of the continuous capacitance-and conductance-versus-bias characteristics of semiconductor devices such as MOS capacitors, varactors and similar structures at 25, 50, or 100 MHz over a wide range of biases (-110to + 110V) and sweep speeds. It has a range of 10 pF and 4 mmho to 100 pF and 40 mmho full scale in 4 steps, an accuracy better than 2 percent and provides outputs for the capacitance C, the conductance G, and the bias Vb of the device under test for graphic display on an X-YY recorder. It is based on an accurate HF current-to-voltage converter in combination with two synchronous detectors. For accuracy reasons the high measuring frequency is converted to a suitable value (5 kHz) by means of a superheterodyne phase-lock loop system and two mixers. Provisions have been made to correct the errors due to parasitic effects. Representative experimental results obtained with the C, G(Vb) instrument are shown.     

Recent Results of a New Microwave SQUID Multiplexer

We present recent results of a prototype microwave SQUID multiplexer containing four SQUIDs coupled to GHz frequency resonant circuits and fed with a single microwave readout line. The system is operating at a readout frequency range of 8–10 GHz. All four SQUIDs share a common DC bias and modulation lines. A new modulation scheme is tested to eliminate the need for individual flux biasing of the SQUIDs, which extends the dynamic range of the readout. In this scheme a common modulation signal is imposed on each SQUID and the received signal is demodulated at one and two times the modulation frequency to maintain sensitivity at any flux state. We also demonstrated a microwave RF bias scheme eliminating the necessity of the DC current bias to the SQUID. Our preliminary performance tests at 4.2 K show that the input noise of the device is ∼5 pA/ .      

A digital X-ray imaging system based on silicon strip detectors working in edge-on configuration

We present the energy resolution and imaging performance of a digital X-ray imaging system based on a 512-strip silicon strip detector (SSD) working in the edge-on configuration. The SSDs tested in the system are 300 μm thick with 1 or 2-cm-long strips and 100 μm pitch. To ensure a very small dead area of the SSD working in edge-on configuration, the detector is cut perpendicular to the strips at a distance of only 20 μm from the end of the strips. The 512-strip silicon detector is read out by eight 64-channel integrated circuits called DEDIX [Grybos et al., IEEE Trans. Nucl. Sci. NS-54 (2007) 1207]. The DEDIX IC operates in a single photon counting mode with two independent amplitude discriminators per channel. The readout electronic channel connected to a detector with effective input capacitance of about 2 pF has an average equivalent noise charge (ENC) of about 163 el. rms and is able to count 1 Mcps of average rate of input pulses. The system consisting of 512 channels has an excellent channel-to-channel uniformity—the effective threshold spread calculated to the charge-sensitive amplifier inputs is 12 el. rms (at one sigma level). With this system a few test images of a phantom have been taken in the 10–30 keV energy range.     

基于峰值检测的MEMS器件微弱电容检测电路

针对MEMS器件研制中微弱信号的检测问题,提出了一种适用于电容式MEMS器件的微弱电容检测电路.此电路采用峰值检测技术,原理及结构简单;只检测待测电容的变化量,既可用于差分式检测,也可应用于单一待测电容的情况.首先利用正弦载波信号和微分电路对电容量进行载波调制,再通过减法电路得到幅值与电容变化量成比例的正弦信号,最后采用峰值检测方法解调信号,得到直流量输出.利用微小可调电容进行标定,结果表明检测电路的线性度良好,灵敏度约为3.631V/pF,精度达到0.2%.利用该检测电路检测MEMS陀螺上振动频率为2.85kHz的梳齿驱动器的电容量变化,输出信号频率为（2.85±0.02）kHz,误差低于0.7%,说明该电路能够应用于MEMS器件的微弱电容检测.     

The readout method for wire chambers

The method for readout of information from a wire chamber through a conventional conductive wire cathode is tested to study the possibility of its practical usage. It is observed that the value of the signal induced on an external readout electrode depends on the detector geometry and makes up approximately 5–10% of the anode signal value. The application of this method should permit one to separate a detector into an active device and an independent readout system as well as to make the readout system more flexible and to improve detector coordinate properties.     

Properties of thin film capacitors made with reactively sputtered Ta-Al

The dielectric properties of oxides formed on Ta-Al-N films were investigated. The capacitors made from these films exhibit excellent a.c. and d.c. properties and are comparable in performance with TaN capacitors. Owing to the high resistivity of Ta-Al-N films an aluminum underlayer was used for these capacitors to reduce losses at high frequencies. It is concluded that these capacitors are more desirable for applications in hybrid integrated circuits when used together with Ta-Al-N resistors.     

A Mixed-Voltage Sensor Readout Circuit With On-Chip Calibration and Built-In Self-Test

This paper reports a mixed-voltage mixed-signal chip for interfacing multiple capacitive transducers to embedded processors in integrated microsystems. A programmable switched-capacitor readout circuit accommodates capacitive sensors from 16 fF to 40 pF and allows self-test and online calibration. The 20 mm² chip has a sensitivity of 1.25 mV/fF and is realized in a 1 mum n-well BiCMOS 2 P/2 M process that permits high-voltage operation, large-value resistors, and nonvolatile on-chip memory. An on-chip charge pump generates voltages up to 30 V that permits transducer electrostatic self-test. In normal operation mode, the chip provides a fast sensor readout consuming only 90 nj of energy, making it suitable for portable applications.     

Evaluation of the frequency dependence of the resistance and capacitance standards in the BIPM quadrature bridge

The Bureau International des Poids et Mesures (BIPM) has established a measurement chain allowing calibration of capacitance standards in terms of the quantized Hall resistance (QHR). An important element in the chain is a quadrature bridge linking a pair of ac resistors of values 2R/sub K/ /spl ap/ 51.6 k/spl Omega/ to a pair of capacitance standards. The quadrature bridge can be operated at five different frequencies: 513, 1027, 1541, 3082, and 6164 Hz. For such measurements, we use different ratios (1/1, 4/1 and 1/4) for the main inductive voltage divider in the quadrature bridge and three different pairs of capacitors of values 3000, 2000, and 1000 pF. A calculable coaxial resistance of 1290.6 /spl Omega/ (R/sub K//20) is used as a reference to evaluate the frequency dependence of the 51.6-k/spl Omega/ resistances. This allows the calibration of capacitance standards at the five different frequencies. The measured frequency dependences of 10 and 100 pF capacitance standards are reported.