Electronic circuit

Details Electronic circuit Electronic circuit

1999-11-05

2002-03-05

Brown, Glenn W. (Department: 2858)

Electricity: measuring and testing

Electrostatic field

Reexamination Certificate

active

06353324

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to an electronic circuit and an array of such circuits for precisely measuring small amounts or small changes in the amount of charge, voltage, or electrical currents over a wide dynamic range.
BACKGROUND OF THE INVENTION
Often the measurement of a physical phenomena involves the creation of an electrical signal which is amplified and measured by an electrical circuit. A number of sensing media face limitations, imposed by the electronic measuring circuits, on their ability to measure physical phenomena. These media include solids, liquids, or gases in which the physical phenomena to be sensed causes the generation of mobile charges which then move under the influence of an electric field, or in which a charge separation is induced by the physical phenomena and/or dimensional changes in the sensing medium. Specific examples of sensing media are gases in the case of ionizing radiation, electrochemical mixtures, electret or capacitive microphones, variable capacitors, inductive pickups or coils, electric field measuring antennas, piezoelectric materials such as PVDF, semiconductors operated at temperatures where thermally-induced current generation is not dominant, a vacuum in the case of electron emissive surfaces or ion mobility instruments, and insulators. The limitations imposed by the measuring circuits include sensitivity, linearity, size, dynamic range, and operating voltages. These limitations arise from thermally-induced and/or bias currents in the components of the measuring circuit and current leakage paths over the surfaces of and/or through the components of the measuring circuit. The measuring circuit described herein aims to minimize these limitations.
One example of a physical phenomena is ionizing radiation. It presents a direct hazard to people; therefore, the measurement of radiation in various environmental settings is important. The type of radiation monitor or detector used to measure the radiation depends upon the type of radiation, e.g., beta, alpha, or X-ray and the environmental setting, e.g., an environmentally isolated laboratory, an open mine, or a waste dump holding potentially hazardous material. Different environments impose different requirements on the manner and sensitivity of the measurement; for example, the laboratory most likely requires a monitoring system with a continuous display and singular or multiple radiation detectors; the mine requires a moderately sensitive portable area detector; and the waste dump a relatively fast and sensitive directional detector.
D. A. Waechter et al. described in an article entitled “New Generation Low Power Radiation Survey Instruments,” a standard portable dosimeter (radiation monitor) system. The portable monitor consists of a Geiger-Muller tube (GM tube) with an event counter which records the number of ionizing events. There is a readout display and an audio alarm. The problem with the GM tube is that its response is not linear with the energy of the radiation so its accuracy varies with radiation photon energy, although it is useful for warning. In this instrument, the need to amplify the radiation signal in the GM tube limits the energy linearity and thus the accuracy of the instrument. An ion chamber made from tissue equivalent plastic and filled with a tissue equivalent gas gives a very accurate reading. However, at low doses and dose rates, the amount of charge generated per unit volume of gas is very small. For accurate measurement of the ionizing radiation, the signal current created by the ionizing radiation needs to be significant when compared to the leakage and/or noise currents in the electronic measuring circuit and the internal leakage currents in the gas sensing medium. It is preferred that the signal current be greater than the sum of all the leakage and noise currents. In general, the internal leakage currents of the sensing media, which are induced by physical phenomena other than the one to be measured, must be on the same order of magnitude or preferably less than the currents created by the physical phenomena to be measured in order for any measuring circuit, including the ones described herein, to obtain a measurement. Sensing media with low internal leakage currents are said to have high internal impedances. Gas is a very high impedance sensing medium and so does not contribute significant internal leakage currents. But, due to surface leakage currents on insulators and other circuit elements and due to other limitations within the implementing circuitry, prior art ion chambers tend to need a large volume of gas, and thus operate at high pressure or be inconveniently large, and need to employ high voltages to be sufficiently sensitive. By significantly reducing surface leakage currents and bias currents, this invention allows accurate ionizing radiation measurement with lower voltages and smaller chambers.
A second group of sensors that can benefit from the invention disclosed herein are charge inducing sensors such as capacitive sensors, where a charge is induced if the voltage difference is kept constant, or where a voltage is induced if the charge stays constant. Capacitive sensors can be used in many applications such as, microphones, pressure measurements, and accelerometers to name a few. Books like Capacitive Sensors Design and Application by Larry K. Baxter (ISBN 0-7803-1130-2) give many applications, some examples of which are given on pages 3-5. It is important to note that in capacitive sensors there is a high impedance material, often gas, but sometimes a liquid or a solid, between the capacitor electrodes. In this type of sensor the high impedance material is functioning as a separating medium rather than a sensing medium. The sensing medium is the capacitor plates themselves and the separation between them. There is not meant to be charge conduction through the separating medium. For this reason the term non-mobile charges can be used. The induced charges do move within the conductors connected to the sensing electrodes and the other electrodes, but they do not move through the separating medium. Most prior art capacitive sensors are limited to alternating current (AC) use because of the bias and leakage currents introduced by the prior art measurement electronics or within or over the surfaces of the capacitive sensors themselves. Among the limitations are: (i) response time, since it takes several cycles of the AC signal for a change to be noticed, (ii) power use, because the AC signal require an AC source and constant current draw, and (iii) sensitivity, because the measurement is a small change of a large AC signal.
Another group of charge sensors which can benefit from the invention is electrochemical sensors, where the phenomena to be measured interacts via a reversible or irreversible chemical reaction that causes the formation of charge in electrical dipole layers. The charges need not be mobile because the creation of an electric field by the induced charge distribution will result in a signal which can be measured. In some configurations, electrochemical sensors are low impedance sensors, with significant internal leakage currents inherent in the medium itself. Some water-based electrochemical cells are examples of this. Even in low internal impedance sensors, there may be a sensitivity or linearity benefit from using an electronic circuit which does not load or draw much current from the sensor. In some configurations, the current generated is the electrical signal which most directly relates to the physical phenomena. In other configurations, electrochemical sensors produce a voltage that is measured to represent the physical phenomena. The measurement of work function difference between two surfaces with an insulating medium as a separation material is an example of this. One prior art method for measuring work function difference is a moving plate or vibrating reed electrometer as described in pages 82-83 and pages 407-409 of Nuclear Radiation Detection by William J. Price (Second Edition, Library

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