A microcontroller uses a combination of several synchronized PWM outputs to generate a low distortion sine wave by summing the PWM outputs and filtering the summed signal. The sine wave is used as a guard voltage. The unknown impedance is measured by impinging the guard voltage on the sense electrode by a transistor connected in common base configuration and then transferring the sense current through the common base connected transistor to a transimpedance amplifier made out of a second transistor connected in common emitter configuration. The output voltage at the collector of the second transistor is measured by an ADC input of the microcontroller. The microcontroller translates the ADC output values into the unknown impedance to be measured by doing a software demodulation of the ADC output values. A reference impedance can be connected in parallel to the unknown impeder to eliminate gain errors of the signal sensing circuit.
Low-Cost Complex Impedance Measurement Circuit for Guard-Sense Capacitive Sensors Operated in Loading Mode
2017-12-14
Patent
Electronic Resource
English
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