A Simple, Linear Circuit for Measuring fF Range of Capacitances

There are numerous applications wherein capacitive sensing is performed such as, measurement of flow rates and environmental factors, dielectric testing, composition evaluation, proximity sensing, etc. Sometimes, the change brought on the capacitance of the sensor by the measurand may be in femto-Farad (fF) range. The circuitry required to measure changes this low can be very expensive and complex. So, a unique circuit was designed by the authors in one of their previous works that can detect such minute variations in capacitance without much complexity. This circuit does a differencing operation first, followed by a differential measurement to increase the sensitivity as well as to obtain linear outputs with respect to the capacitance measured. As a continuation, this paper presents the performance analysis of this circuit after some design modifications. The experimental results obtained were linear as per the theory of operation of this circuit and its sensitivity and repeatability were very good. However, a few parameters need improvement. But, as the results look promising on-the-whole, with suitable corrections, this circuit can be utilized for a wide variety of applications.