Isolating Interface for Water Quality Sensors Based on Flying Capacitor Principle

Abstract

In a water quality monitoring project several sensors have electrical contact to each other due to water conductivity. Galvanic isolation is required to avoid electrical interference, which is established in this work by flying capacitor principle. Simultaneously this also provides channel multiplexing for use of a single analog-to-digital converter (ADC). By nature of the circuit, it is the task to measure a decaying voltage caused by capacitor discharge. An error analysis of this situation is given. Furthermore, the usage of an industrial ADC with finite input resistance, built-in lowpass filter, and serial communication with uncertain time behaviour results in additional measurement errors. A method is proposed how to get a measurement under this conditions with acceptable and predictable errors. Finally the analysis is verified with simulation and laboratory measurements.