Circuit for measuring low capacitances

• US 4,295,091 A
• Filed: 12/04/1979
• Issued: 10/13/1981
• Est. Priority Date: 10/12/1978
• Status: Expired due to Term

• Current Assignee: Vaisala OY
• Original Assignee: Vaisala OY
• Inventors: Ponkala, Jorma
• Primary Examiner(s): Karlsen, Ernest F.
• Application Number:
  US06/100,200
• Time in Patent Office: 679 Days
• Field of Search: 324/60 CD, 324/60 C, 324/60 R, 324/61 R, 324/120, 340/870.37
• US Class Current: 324/611
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

A capacitance to be measured is connected between the input and output of an inverting amplifier. A bistable circuit has an input connected to the output of the inverting amplifier and an output impedance inversely proportional to the capacitance to be measured. A resistive feedback is connected from the output of the bistable circuit to the input of the inverting amplifier.

Claims

1. A circuit for measuring low capacitances with the elimination of the influence of stray capacitances, said circuit comprising

an inverting amplifier having an input and an output;

a capacitance to be measured connected between the input and the output of the inverting amplifier;

a bistable circuit having an input connected to the output of said inverting amplifier, an output and an output frequency inversely proportional to the capacitance to be measured; and

a resistive feedback connected from the output of the bistable circuit to the input of the inverting amplifier.

2. A circuit as claimed in claim 1, wherein the input of said amplifier functions as a low impedance generator having a low internal resistance and the output of said amplifier functions as a circuit measuring only current.

3. A circuit as claimed in claim 1, wherein bistable circuit comprises a bistable multivibrator.

4. A circuit as claimed in claim 1, wherein said bistable circuit comprises a Schmitt trigger.

5. A circuit as claimed in claim 1, wherein the capacitance to be measured is located remotely from said inverting amplifier and said bistable circuit and further comprising a pair of coaxial cables connecting said capacitance to be measured to the input and output of said inverting amplifier.

6. A circuit as claimed in claim 5, wherein said capacitance to be measured has a pair of spaced terminals and each of said coaxial cables has an inner conductor and an outer conductor, the outer conductor of each of said cables being connected to ground, the inner conductor of one of said cables connecting one of the terminals of said capacitance to be measured to the input of said inverting amplifier and the inner conductor of the other of said cables connecting the other of the terminals of said capacitance to be measured to the output of said inverting amplifier.