Capacitance measuring by comparison to reference capacitance

A capacitance measuring device for measuring the capacitance X of a first capacitor by means of the comparison of the charge and discharge periods of a second capacitor having a calibrated reference capacitor X.sub.E and said first capacitor during two successive reference and measuring phases of a comparison cycle, said device comprising:

• a relaxation oscillator including one of said first and second capacitors and having an oscillation period proportional to the capacitance of said included capacitor and a high stability,
• said relaxation oscillator also comprising means for neutrelizing said first and second capacitors during said reference and measuring phases respectively,
• switching means for including at the beginning of each of said reference and measuring phases said second and first capacitors in said relaxation oscillator respectively,
• means for counting a predetermined number N.sub.E of pulses delivered from said relaxation oscillator during said reference phase so as to define a determined duration of said reference phase and for counting a number N.sub.X of pulses delivered from said relaxation oscillator during said measuring phase, the ratio of said oscillation pulse numbers N.sub.E and N.sub.X being inversely proportional to the ratio of said capacitances X.sub.E, X of said second and first capacitors,
• timing means for generating pulses having at least a clock frequency,
• means for counting and down-counting said clock pulses during said reference and measuring phases respectively,
• means controlled by said counting means for changing the counting condition to the down-counting condition of said counting and down-counting means when the count of said counting means is equal to said predetermined number N.sub.E at the end of said reference phase,
• means controlled by said counting and down-counting means for stopping the counting of said counting means when the count of said counting and down-counting means is equal to zero at the end of said measuring phase so that the duration of said measuring phase is equal to or in a predetermined ratio R with said determined duration of said reference phase,
• means for computing a physical quantity proportional or inversely proportional to the value (N.sub.E X.sub.E)/(R N.sub.X) equal to said capacitance X of said first capacitor, and
• means for displaying the value of said physical quantity.

A capacitance measuring device for measuring the capacitance X of a first capacitor by means of the comparison of the charge and discharge periods of a second capacitor having a calibrated reference capacitance X.sub.E and said first capacitor during two successive reference and measuring phases of a comparison cycle, said device comprising:

• a relaxation oscillator including one of said first and second capacitors and having an oscillation period proportional to the capacitance of said included capacitor and a high stability,
• said relaxation oscillator also comprising means for inhibiting the effects of interference capacitances between the ground referenced voltage and the electrodes of said first and second capacitors,
• switching means for including at the beginning of each of said reference and measuring phases said second and first capacitors in said relaxation oscillator respectively,
• means for counting a predetermined number N.sub.E of pulses delivered from said relaxation oscillator during said reference phase so as to define a determined duration of said reference phase and for counting a number N.sub.X of pulses delivered from said relaxation oscillator during said measuring phase, the ratio of said oscillation pulse numbers N.sub.E and N.sub.X being inversely proportional to the ratio of said capacitances X.sub.E, X of said second and first capacitors,
• timing means for generating pulses having at least a clock frequency,
• means for counting and down-counting said clock pulses during said reference and measuring phases respectively,
• means controlled by said counting means for changing the counting condition to the down-counting condition of said counting and down-counting means when the count of said counting means is equal to said predetermined number N.sub.E at the end of said reference phase,
• means controlled by said counting and down-counting means for stopping the counting of said counting means when the count of said counting and down-counting means is equal to zero at the end of said measuring phase so that the duration of said measuring phase is equal to or in a predetermined ratio R with said determined duration of said reference phase,
• means for computing a physical quantity proportional or inversely proportional to the value (N.sub.E X.sub.E)/(R N.sub.X) equal to said capacitance X of said first capacitor, and
• means for displaying the value of said physical quantity.