A Method to Electrically Compensate Close Approach Effect for Calculable Capacitor

Abstract

Calculable capacitor (CC) is outstanding in its linear relationship between capacitance and its guard electrodes’ displacement. However, the relationship is no longer linear for capacitance below 0.1 pF level because the electrostatic field on guard electrode’s tip gets disordered, which is known as close approach effect. To rescue the linearity, we assemble an auxiliary electrode on the end of the movable guard electrode and load a scaled ac voltage with the same frequency as capacitance bridge by a ratio transformer to make the electrostatic field regular. The scaled ratio is carefully selected by simulation and experiments. We show that some specified ratios can perfectly compensate the close approach effect. As a result, the range of the linearity of the capacitance–distance curve is extended by 10 mm on our prototype by adopting optimal positive compensation voltage. Furthermore, the nonlinearity can be totally compensated even with capacitance less than 100 aF when adopting negative compensation voltage.