An enhanced method for measuring capacitance based on the direct interface circuit

An improved method is proposed to measure capacitances based on the direct interface circuit without using either a signal conditioner or an analog to digital converter. The direct interface circuit has some drawbacks causing the inaccuracy measurement of capacitance. For instance, the pins of microcontroller provide the high and low level. But the voltages of high and low level are not stable, so the fluctuating high and low threshold voltages influences the precise measurement of capacitance's discharging time; the high impendence state of pins do not completely isolate the discharging circuit and stray capacitances will appear in these pins. Therefore, some hardware circuits are introduced in direct interface circuit. The enhanced interface circuit uses constant voltage source to charge capacitance and voltage comparators to stabilize the fluctuating threshold voltages and analog switches to isolate completely the different discharging circuits. Experimental results show the enhanced interface circuit is more effective than the original interface circuit. When the values of capacitance to be measured vary from 100 pf to 2200 pt, the relative error of the original interface circuit is between 1.6 and 6 percentages, while the enhanced interface circuit is less than 2 percentages.