Capacitance measuring system

A system for generating a signal which is a linear function of the reciprocal of the capacitance of a sensor capacitor comprising:

• A. a sensor network including a reference capacitor and a sensor capacitor coupled in series between a first and second input terminal,
• B. a first switching network responsive to a switch control, said first switching network including means operative in a first state to connect said first input terminal to a first reference potential and said second input terminal to ground potential, and operative in a second state to connect said first input terminal to ground potential and said second input terminal to a feedback terminal,
• C. short term charge storage network including a capacitor having a first terminal coupled to the junction point of said reference and sensor capacitors,
• D. long term charge storage network including a capacitor having a first terminal connected to ground potential,
• E. second switching network responsive to said switch control, said second switching network including means operative in a first state to connect the second terminal of said short term charge storage capacitor to a second reference potential, and operative in a second state to connect said second terminal of said short term charge storage capacitor to the second terminal of said long term charge storage capacitor,
• F. means for generating a feedback signal and applying said feedback signal to said feedback terminal, said feedback signal being proportional to the integral of the difference between the voltage at said second terminal of said long term charge storage capacitor, and said second reference potential, and
• G. switch control means for cyclically controlling said first and second switching networks during a first portion of each cycle to be operative in said first states, and during a second portion of each cycle to be operative in said second states, whereby said feedback signal is proportional to the capacitive reactance of said sensor capacitor.

A system for generating a signal which is a linear function of the reciprocal of the capacitance of a sensor capacitor comprising:

• A. a sensor network including a reference capacitor and a sensor capacitor coupled in series between a first input terminal and ground potential, said sensor capacitor having one terminal connected to said ground potential,
• B. amplifier means having an inverting input terminal, a non-inverting input terminal and an output terminal, said non-inverting input terminal being coupled to the junction point of said reference and sensor capacitors, said amplifier means including means for generating a signal at said output terminal having a first additive component which is proportional to the difference in the a.c. signals at said inverting and non-inverting terminals, and a second additive component which is proportional to the a.c. signal at said inverting terminal,
• C. a first switching network responsive to a switch control, said first switching network being operative in a first state to couple said first input terminal of said sensor network and said inverting input terminal to a feedback terminal, and operative in a second state to connect said first input terminal of said sensor network to ground potential and said inverting input terminal to a reference potential,
• D. a short term charge storage network including a capacitor having a first terminal connected to said output terminal of said amplifier means,
• E. a long term charge storage network including a capacitor having a first terminal connected to ground potential,
• F. a second switching network responsive to a switch control, said second switching network being operative in a first state to couple the second terminal of said short term charge storage capacitor to said feedback terminal, and operative in a second state to couple said second terminal of said short term charge storage capacitor to the second terminal of said long term charge storage capacitor,
• G. means for generating a feedback signal and applying said feedback signal to said feedback terminal, said feedback signal being proportional to the integral of the difference between the voltage at said second terminal of said long term charge storage capacitor, and said reference potential, and
• H. switch control means for cyclically controlling said first and second switching networks during a first portion of each cycle to be operative in said first states, and during a second portion of each cycle to be operative in said second states, whereby said feedback signal is a linear function of the capacitance reactance of said sensor capacitor.