A Method for Reducing the Adverse Effects of Stray-Capacitance on Capacitive Sensor Circuits

We examine the increase in voltage noise in capacitive sensor circuits due to the stray-capacitance introduced by connecting cables. We have measured and modelled the voltage noise of various standard circuits, and we compare their performance against a benchmark without stray-capacitance that is optimised to have a high signal-to-noise ratio (SNR) for our application. We show that a factor limiting sensitivity is the so-called noise gain, which is not easily avoided. In our application the capacitive sensor is located in a metallic vessel and is therefore shielded to some extent from ambient noise at radio frequencies. It is therefore possible to compromise the shielding of the coaxial connecting cable by effectively electrically floating it. With a cable stray-capacitance of 1.8nF and at a modulation frequency of 100kHz, our circuit has an output voltage noise a factor of 3 larger than the benchmark.