Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – By integrating
Reexamination Certificate
2000-02-02
2001-09-25
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
By integrating
C327S336000, C330S007000, C330S273000
Reexamination Certificate
active
06294945
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the characteristics of capacitors, and more particularly to methods for reducing or eliminating dielectric absorption error in circuits using capacitors.
2. Description of the Relevant Art
A capacitor comprises two parallel conductive plates separated by a dielectric. The dielectric may be composed of various types of materials. Dielectric absorption refers to the small amount of excess charge absorbed or released by a dielectric after a capacitor has been charged or discharged. Dielectric absorption is a property that every capacitor exhibits to differing degrees. The amount of dielectric absorption for a particular capacitor depends primarily on the type of dielectric material used and the amount of dielectric material in the capacitor. It is desirable to compensate for the effects of dielectric absorption to achieve a more ideal and predictable capacitance.
The effects of dielectric absorption can be seen when a known amount of charge is deposited on a capacitor and the voltage across the capacitor is then observed. The voltage will decay at a constantly decreasing rate, causing the voltage at any given time to be slightly below the level of voltage initially observed across the capacitor. It is believed that this lost charge is absorbed by the dielectric and is, therefore, not apparent in a voltage reading across the capacitor.
The effects of dielectric absorption may be seen by following the following procedure. First a battery is used to charge a large-value tantalum capacitor to a level of 10 volts. Next, after the capacitor is fully charged and has reached a voltage level of 10 volts, the battery is removed. At this point, the voltage across the capacitor should be 10 volts. Now the capacitor may be rapidly discharged by momentarily putting a 100 ohm resistor across it. The resistor is then removed. When the voltage across the capacitor is measured again, the voltage across the capacitor may be observed increasing in voltage, even though the capacitor was previously discharged. The voltage across the capacitor may perhaps reach a volt or so after a few seconds. Dielectric absorption is the cause of the voltage across the capacitor increasing in voltage despite the fact the capacitor was discharged.
Dielectric absorption typically is less than 1% of the total charge applied to a capacitor. Some applications are not substantially affected by this degree of error, but most applications encourage the removal of as much error as possible. Because dielectric absorption varies greatly depending on the type of dielectric used in the capacitor, one way to minimize dielectric absorption is by choosing a dielectric that is less susceptible to this absorption. An air dielectric capacitor, for example, has a very small absorption rate, but is very impracticable in most applications. Air dielectric capacitors are physically much larger (by orders of magnitude) and much more expensive than lower quality capacitors with similar capacitive ratings. Another potential solution is to allow sufficient time between charging periods to allow the current associated with dielectric absorption to drop to an acceptable level. Other methods of partially compensating for dielectric absorption have been used such as providing an “equivalent and opposite impedance” to the dielectric absorption in a capacitive load (U.S. Pat. No. 5,557,242) and using a series of RC circuits to compensate for dielectric absorption (U.S. Pat. No. 5,519,328). However, improved methods are desired for compensating the dielectric absorption error of a capacitor.
SUMMARY OF THE INVENTION
A system and method are provided for compensating for the effects of dielectric absorption on a capacitor. The system may include a compensation circuit which connects to a circuit portion having a capacitor (main capacitor) which is desired to be compensated, wherein the compensation circuit compensates for dielectric absorption of the capacitor in the circuit portion.
In a particular embodiment, compensation techniques are used to substantially reduce or eliminate dielectric absorption in an integrator circuit. In this embodiment, the compensation circuit is coupled to an integration circuit configured to generate a triangle wave. An input signal voltage such as a square wave is input to the compensation circuit. The compensation circuit uses a second capacitor, referred to as the compensation capacitor, which may have substantially identical characteristics to the main capacitor desired to be compensated, e.g. the capacitor in the integration portion of the circuit. The compensation capacitor is initially charged to the level of the input signal voltage, and then the input voltage is removed. The measured voltage across the capacitor may then exhibit the effects of dielectric absorption by sagging as its dielectric material absorbs energy from the conducting plates. The compensation capacitor is used to produce a voltage, referred to as the modified input signal voltage, approximately equal to the input signal voltage less the voltage lost, referred to as the dielectric absorption voltage, caused by the dielectric absorption of the compensating capacitor. The voltage lost by the compensating capacitor is approximately equal to the dielectric absorption voltage exhibited by the main capacitor being compensated. The modified input signal voltage is then subtracted from twice the input signal voltage resulting in a compensated input signal voltage. The compensated input signal voltage approximately equals the input signal voltage plus the dielectric absorption voltage. The compensated input signal is then passed through the integration portion of the circuit. Because the integration portion of the circuit contains a capacitor with similar or identical characteristics to those of the compensating capacitor, the effects of the dielectric absorption on the circuit's output signal are minimized. This compensation creates, for example, a triangle wave output that is more linear for a square wave input.
In another particular embodiment, compensation circuitry is used to reduce or nullify the effects of dielectric absorption of any particular capacitor by choosing a compensation capacitor with a higher rate of dielectric absorption and a lower capacitance value than the main capacitor whose dielectric absorption effects are to be nullified. The dielectric absorption of the compensation capacitor is scaled according to the two resistors in the compensation circuitry. An amplifier in the compensation circuitry performs an impedance transformation on the compensation capacitor, thereby canceling the dielectric absorption of the main capacitor.
In another particular embodiment, the main capacitor is not tied to ground. The dielectric absorption of the main capacitor is cancelled by a compensation capacitor with similar dielectric absorption. The dielectric absorption of the compensation capacitor is scaled according to the two resistors in the compensation circuitry. The dielectric absorption of the compensation capacitor is then used to compensate the dielectric absorption of the main capacitor by having the amplifier in the compensation circuitry perform an impedance transformation on the compensation capacitor. The result is that the dielectric absorption of the main capacitor is cancelled.
REFERENCES:
patent: 3667055 (1972-05-01), Uchida
patent: 4211981 (1980-07-01), Lerma
patent: 4651032 (1987-03-01), Nobuta
patent: 5376892 (1994-12-01), Gata
patent: 5519328 (1996-05-01), Bennett
patent: 5557242 (1996-09-01), Wetherell
patent: 5585756 (1996-12-01), Wang
patent: 6064238 (2000-05-01), Wight et al.
Daigle Clayton
Regier Christopher G.
Callahan Timothy P.
Conley Rose & Tayon PC
Hood Jeffrey C.
Luu An T.
National Instruments Corporation
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