Capacitance measuring method and apparatus

• US 4,716,361 A
• Filed: 06/11/1985
• Issued: 12/29/1987
• Est. Priority Date: 11/05/1984
• Status: Expired due to Fees

• Current Assignee: John Fluke Mfg. Co., Inc. (Fortive Corp.)
• Original Assignee: John Fluke Mfg. Co., Inc. (Fortive Corp.)
• Inventors: Sheffer, Tzafrir
• Primary Examiner(s): Eisenzopf, Reinhard J.
• Assistant Examiner(s): Solis, Jose M.
• Application Number:
  US06/743,622
• Time in Patent Office: 931 Days
• Field of Search: 324/DIG. 1, 324/60 C, 324/61 R, 324/60 CD, 324/60 R, 340/347 C, 340/347 CC
• US Class Current: 324/679
• CPC Class Codes: G01R 17/06 Automatic balancing arrangements
• CPC Class Codes: G01R 27/02 Measuring real or complex resistance, reactance, impedance, or other two-pole characteristics derived therefrom, e.g. time constant by measuring phase angle only G01R25/00
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

An apparatus for performing ratiometric capacitance measurements by comparing the output of a digital-to-analog converter with the voltage at a tap between an unknown capacitance and a first inherent capacitance of the apparatus. A common input voltage is supplied to both the series circuit, including the first inherent and unknown capacitances, and to the digital-to-analog converter which accomplishes attenuation of the input voltage in response to a digital value. The digital value is generated by a successive approximation register supplying a digital output relating the unknown capacitance to the known capacitance. The apparatus is adapted to ratiometrically measure the first inherent capacitance by supplying the common input voltage to a series circuit, including a known reference capacitance and the first inherent capacitance, such that the digital output of the successive approximation register relates the first inherent capacitance to the reference capacitance. The apparatus is further adapted to ratiometrically measure a second inherent capacitance by supplying the common input voltage to a series circuit, including the first and second inherent capacitances, such that the digital output of the successive approximation register relates the second inherent capacitance to the first inherent capacitance. The measured value of the unknown capacitance is then corrected by the amount of the second inherent capacitance appearing in parallel with the unknown capacitance during measurement thereof with reference to the first inherent capacitance.

Claims

1. A method for measuring the magnitude of an unknown capacitance utilizing a ratiometric capacitance measurement apparatus adapted to measure the magnitude of a capacitor relative to a known capacitance coupled to said capacitor, the method comprising the steps of:

coupling said apparatus to said unknown capacitance; and

utilizing said apparatus to measure the magnitude of said unknown capacitance relative to said known capacitance wherein said known capacitance is the inherent capacitance coupling said capacitance mesurement device to a point of common reference potential.

10. The method according to claim 1 further comprising the steps of:

coupling a reference capacitance to said apparatus as said known capacitance, and

utilizing said apparatus to measure the magnitude of said inherent capacitance relative to said reference capacitance.

2. The method of determining the value of an unknown capacitance connectable between first and second terminals of a switching network respectively coupled by said switching network to first and second terminals of a measurement means adapted to measure an unknown impedance connected thereto ratiometrically by comparing the voltage drop across said unknown impedence with the voltage drop across a know reference impedance coupled between the second terminal of said measurement means and a point of common reference potential, there being a first inherent capacitance appearing between the first and second terminals of said switching network and a second inherent capacitance appearing between the second terminal of said switching network and the point of common reference potential, said method comprising:

a. measuring the value of said second inherent capacitance by disconnecting the first terminal of said switching network from the first terminal of said mesurement means, coupling an auxiliary reference capacitance across the terminals of said measurement means, applying a voltage to the first terminal of said measurement means, and comparing the voltage at said second terminal with the voltage between the first and second terminals of said measurement means to determine the value of said second inherent capacitance ratiometrically;

b. measuring the value of said first inherent capacitance by applying a voltage to said first terminal of said mesurement means with the last mentioned terminal coupled to the first terminal of said switching network, and comparing the voltage at the second terminal with the voltage between the first and second terminals of said mesurement means to determine the value of said first inherent capacitance ratiometrically; and

c. measuring the value of said unknown capacitance by coupling the unknown capacitance across the first and second terminals of the switching network, applying a voltage to said first terminal of said measurement means with the last mentioned terminal coupled to the first terminal of said switching network, comparing the voltage at the second terminal with the voltage between the first and second terminals of said measurement means to determine the value of the combination of said unknown capacitance and said first inherent capacitance ratiometrically, and subtracting the value of said first inherent capacitance wherein the second inherent capacitance determined in step a is employed as the ratiometric reference impedance in step b.

3. The method of determining the value of an unknown capacitance connectable between first and second terminals of a switching network respectively coupled by said switching network to first and second terminals of a measurement means adapted to measure an unknown impedance connected therto ratiometrically by comparing the voltage drop across said unknown impedance with the voltage drop across a known reference impedance coupled between the second terminal of said measurement means and a point of common reference potential, there being a first inherent capacitance appearing between the first and second terminals of said switching network and a second inherent capacitance appearing between the second terminal of said switching network and the point of common reference potential, said method comprising:

a. measuring the value of said second inherent capacitance by disconnecting the first terminal of said switching network from the first terminal of said measurement means, coupling an auxiliary reference capacitance across the terminals of said mesurement means, applying a voltage to the first terminal of said mesurement means, and comparing the voltage at said second terminal with the voltage between the first and second terminals of said measurement means to determine the value of said second inherent capacitance ratiometrically;

b. measuring the value of said first inherent capacitance by applying a voltage to said first terminal of said mesurement means with the last mentioned terminal coupled to the first terminal of said switching network, and comparing the voltage at the second terminal with the voltage between the first and second terminals of said measurement means to determine the value of said first inherent capacitance ratiometrically; and

c. measuring the value of said unknown capacitance by coupling the unknown capacitance across the first and second terminals of the switching network, applying a voltage to said first terminal of said measurement means with the last mentioned terminal coupled to the first terminal of said switching network, comparing the voltage at the second terminal with the voltage between the first and second terminals of said measurement means to determine the value of the combination of said unknown capacitance and said first inherent capacitance ratiometrically, and subtracting the value of said first inherent capacitance wherein the second inherent capacitance determined in step a is employed as the ratiometric reference impedance in step c.

4. The method of determining the ratio of a first capacitance and a second capacitance for ascertaining the value of one of them, wherein the first capacitance is adapted to be coupled via a first conductor and a second conductor to a measuring means, while said second capacitance is adapted to be coupled between the second conductor and a point of common reference potential, the conductors being characterized by inherent capacitance;

comprising;

disconnecting the first conductor, coupling an auxiliary reference capacitance across the measuring means, and measuring a first inherent capacitance between the second conductor and the point of common reference potential in proportion to said auxiliary reference capacitance;

reconnecting the first conductor and measuring a second inherent capacitance between the conductors in proportion to said first inherent capacitance used as a reference; and

coupling said first capacitance across said conductors and measuring the combination of said first capacitance and said second inherent capacitance in proportion to said first inherent capacitance.

5. An apparatus for determining the value of an unknown capacitance connectable between first and second terminals by a switching network, there being a first inherent capacitance appearing between the first and second terminals of said switching network and a second inherent capacitance appearing between the second terminal of said switching network and a point of common reference potential, said apparatus comprising:

means to selectively connect the unknown capacitance between the first and second terminals;

means to generate a reference voltage;

means to measure a second voltage at said second terminal;

a reference capacitance;

means to selectively couple the reference voltage to the second terminal through the reference capacitance, thereby allowing measurement of said second voltage at the second terminal according to the relative magnitudes of the reference capacitance and the second inherent capacitance when the unknown capacitance is not connected between said terminals; and

means to selectively apply the reference voltage to the first terminal thereby allowing measurement of the second voltage at the second terminal according to the relative magnitudes of the first and second inherent capacitances when the unknown capacitance is not connected between said terminals, and according to the relative magnitudes of the unknown, the first and second inherent capacitances when the unknown capacitance is connected between said terminals.

6. An apparatus as in claim 5 wherein the means to measure the second voltage comprises:

programmable attenuating means receiving the reference voltage as an input for attenuation, the programmable attenuating means having a plurality of attenuation determining inputs ranked in predetermined digital order;

a successive approximation register adapted to provide a succession of digital outputs having successive values wherein said digital outputs are coupled to provide the attenuation determining inputs for the programmable attenuation means; and

means to compare the output of the programmable attenuating means with the second voltage to supply a conversion determining input to the successive approximation register for bringing the voltage reference as attenuated into fixed relation with the second voltage so that the final digital output of the successive approximation register is indicative of the magnitude of the second voltage in relation to the magnitude of the reference voltage.

7. An apparatus for determining the value of an unknown capacitance comprising:

means providing a reference voltage;

programmable attenuating means receiving said voltage reference as an input for attenuation, said programmable attenuation means having a plurality of attenuation determining inputs ranked in predetermined digital order;

a successive approximation register adapted to provide a succession of digital outputs having successive values wherein said digital outputs are coupled to provide the attenuation determining inputs for said programmable attenuation means;

first and second conductors;

means to selectively apply said reference voltage to said first conductor;

means to selectively couple said unknown capacitance across said first and second conductors, said second conductor being coupled to a point of common reference potential through an inherent capacitance associated with said coupling means and said second conductor; and

means to compare the output of said programmable attenuating means with a voltage appearing on said second conductor, to supply a conversion determining input to said successive approximation register for bringing said voltage reference as attenuated into fixed relation with said voltage on said second conductor, so that when said reference voltage is applied to said first conductor and said unknown capacitance is coupled across said first and second conductors, the final digital output of said successive approximation register is indicative of the value of said unknown capacitance in relation to the value of said inherent capacitance.

8. The method of determining the ratio of a first capacitance and a second capacitance for ascertaining the value of one of them, wherein the first capacitance is adapted to be coupled via a first conductor and a second conductor to a measuring means, while said second capacitance is adapted to be coupled between the second conductor and a point of reference potential, the conductors being characterized by inherent capacitance;

comprising;

disconnecting the first conductor, coupling an auxiliary reference capacitance across the measuring means, and measuring an inherent capacitance between the second conductor and the point of common reference potential in proportion to said auxiliary reference capacitance; and

coupling said first capacitance across said conductors and measuring a capacitance in proportion to at least said inherent capacitance with said measuring means.

9. The method of claim 8 including adding a further reference capacitance in parallel with said inherent capacitance.