Engineering and Computer Science Faculty Publications

Improving the phase delay capacitive interface circuit technique using MOSFET switches

Document Type

Article

Publication Date

11-15-2019

Journal Title

Measurement Science and Technology

ISSN

1361-6501

Volume

31

Issue

2

DOI

10.1088/1361-6501/ab4a66

Abstract

Measuring the phase delay of an RC network with a known resistance and an unknown capacitance is a simple technique to determine the value of capacitance. In this technique, the state of an input square wave signal is buffered by CMOS inverters and delayed by an RC network which yields a pulse width modulated (PWM) signal according to the phase delay at the output. The duty cycle of the PWM signal is proportional to the unknown capacitance when the resistance is fixed. However, the response of this method becomes severely nonlinear if the phase delay is larger than approximately 45° due to incomplete charging and discharging processes of the unknown capacitor. A novel MOSFET interface circuit with a PMOS FET to charge the unknown capacitor and an NMOS FET to discharge it during each measurement cycle is presented. This method, which expands the range of linear operation, is demonstrated in simulation and experimentally validated using a fringing field capacitive sensor to measure the mass of added water. The novel circuit expands the linear operation to a phase delay of 140.04°, over which the R2 value achieves 0.9989.

Keywords

Capacitance measurement, sensor interface circuit, RC phase delay

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